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LayMan
2006-Sep-04, 01:12 PM
Hello everyone, first time here...

Question: does the Moon really rotate?

I read a post somewhere where a person asked something, implying that the Moon didn’t rotate. The guy who answered his question pointed out that the Moon does rotate, but that its rotation and orbital movement were thus synchronized by ‘tidal lock’, that it always showed the same side towards the Earth. So 1 rotation per orbit.
He even gave an easy physical experiment to prove his point: imagine you're moving around an object (or another person), then you need to rotate to be able to keep your face pointing towards it (or him/her) all the time…

Now, here’s my problem: imagine a hammer thrower at the Olympics: as he/she spins and sways the ball around, which is attached to a rope, he/she always sees the same side of the ball. However, the ball is in my opinion unable to rotate, since it is fixed by means of the rope. I mean, if it was rotating, wouldn’t it start behaving like a yo-yo?
Or another example: if I’m driving on a roundabout, is my car rotating? I don’t think so, that way I would loose control over my vehicle each time I passed a roundabout!
Consider this: if I look out the driver’s window, I would never see the roundabout, only the surrounding scenery. That would lead me to believe I was in an orbiting object, not a rotating one, because if the car was rotating, at some point I would be able to see the roundabout, but I don’t, I only see the surroundings passing by in a regular fashion, i.e. after a certain while (depending on my speed), I see the whole scenery repeating itself… My passenger, looking out of the passenger seats window would continuously see the centre of the roundabout, thereby also concluding that he couldn’t be inside a rotating object, because then it would become inevitable at some point that he looses sight of it…

Isn’t it better than to just state that the Moon is not rotating, just orbiting and that it is for that reason we always see the same side, just like the ‘orbiting’ movement of the ball at the Olympics? Instead of having to come up with some technical ‘tidal lock’ or some inexplicable coincidence? Or am I making some huge logical error here? :doh:

One last example: take a yo-yo, wind it all the way up and simply let go: provided that you keep your arm and hand motionless, the following will happen: the yo-yo, under the influence of gravity, will start to rotate, say clockwise, as it makes his journey to the lowest point that it is allowed to reach given the length of the cord. At the ‘bottom’ of the cord, due to its momentum it will switch to a counter clockwise rotation, which will cause it to start moving upward till it reaches its peak. It will continue to switch between counter clockwise and clockwise rotation as it moves up and down, until it has eventually lost all of its momentum due to friction and stops rotating. As the yo-yo is hanging completely still, it is obviously not rotating anymore. If you now start to rotate yourself, your movement will cause the yo-yo to take an ‘orbital’ trajectory around you, just like the Moon orbits the Earth (with the difference that for the Moon – Earth system, the role of the rope is being taken by gravity). I’ve put the first orbital between parentheses, because I thought the term was only used to describe the movement of celestial bodies. In both cases, we see the same side – in the first case the same side of the yo-yo, in the second case the same side of the Moon. Now here comes: does the yo-yo in the first case rotate? I fail to see how it could without coming towards you. Besides, it it was rotating, you would be able to view its opposing side from time to time...

Can anyone clear up which point of view is the logically flawed: is the Moon rotating so that we need 'tidal lock' to explain why the same side of it is always visible, or the moon just being an orbital object that doesn't rotate and therefor always kees the same side towards the Earth? Preferably without getting to technical/mathematical about it?
Thanx...

P.S.: As the nick implies, I hold no higher degrees, just my high school diploma. I just like to think a lot about stuff :cool: . I’m also Dutch, so sorry for any linguistic errors…

grant hutchison
2006-Sep-04, 03:44 PM
Now, here’s my problem: imagine a hammer thrower at the Olympics: as he/she spins and sways the ball around, which is attached to a rope, he/she always sees the same side of the ball. However, the ball is in my opinion unable to rotate, since it is fixed by means of the rope. I mean, if it was rotating, wouldn’t it start behaving like a yo-yo?It's rotating. So is the rope, so is the hammer thrower. Because they're all rotating at the same rate, their relationship to each other doesn't change, but their relationship to the spectators (who're watching them spinning round and round) does.


Or another example: if I’m driving on a roundabout, is my car rotating? I don’t think so, that way I would loose control over my vehicle each time I passed a roundabout!Your car is rotating. If it didn't rotate, how could you end up travelling in a different direction when you left the roundabout?

Grant Hutchison

PS: If I remember my yo-yoing days correctly, the yo-yo doesn't switch rotation at the bottom of the string: it keeps turning clockwise and climbs the "other side" of the string.

Tog
2006-Sep-04, 03:51 PM
Hello everyone, first time here...
Welcome

Question: does the Moon really rotate?
Yes, it does.

I read a post somewhere where a person asked something, implying that the Moon didn’t rotate. The guy who answered his question pointed out that the Moon does rotate, but that its rotation and orbital movement were thus synchronized by ‘tidal lock’, that it always showed the same side towards the Earth. So 1 rotation per orbit.
He even gave an easy physical experiment to prove his point: imagine you're moving around an object (or another person), then you need to rotate to be able to keep your face pointing towards it (or him/her) all the time…

Now, here’s my problem: imagine a hammer thrower at the Olympics: as he/she spins and sways the ball around, which is attached to a rope, he/she always sees the same side of the ball. However, the ball is in my opinion unable to rotate, since it is fixed by means of the rope. I mean, if it was rotating, wouldn’t it start behaving like a yo-yo?

No, it wouldn't for two reasons. But first we have to make clear the meaning of revolution and rotation. Rotation happens when something spins about its own center of mass. Ths can be a like a skater, or a planet. A Revolution is one trip around another object. This can be one skater being swung around another, or a moon going around a planet. The two things are similar, but they are different. Using your hammer throw analogy, the ball on the hammer is rotating at a rate of one rotation per revolution. If it were to rotate faster, it would act like a yo-yo and 'climb' the rope. When the moon rotates, it just happens to revolve at the same time rate.

If you stand beside the hammer thrower, your view of the ball will be the rope side when his back is to you. As he spins toward you, your veiw of the ball will change until you can no longer see where the rope attatches. This means the ball is rotating.

Or another example: if I’m driving on a roundabout, is my car rotating? I don’t think so, that way I would loose control over my vehicle each time I passed a roundabout!
Consider this: if I look out the driver’s window, I would never see the roundabout, only the surrounding scenery. That would lead me to believe I was in an orbiting object, not a rotating one, because if the car was rotating, at some point I would be able to see the roundabout, but I don’t, I only see the surroundings passing by in a regular fashion, i.e. after a certain while (depending on my speed), I see the whole scenery repeating itself… My passenger, looking out of the passenger seats window would continuously see the centre of the roundabout, thereby also concluding that he couldn’t be inside a rotating object, because then it would become inevitable at some point that he looses sight of it…

But let's say there is a large mountain directly in front of you as you enter. As you begin to turn, you will see the mountain move around the car. At the mid point, it will be visible through the back window. As you continue around you will hav ethe mountin come back to the front. This would be the same as if you lost control of the car on an icy road, and spun in place.

Isn’t it better than to just state that the Moon is not rotating, just orbiting and that it is for that reason we always see the same side, just like the ‘orbiting’ movement of the ball at the Olympics? Instead of having to come up with some technical ‘tidal lock’ or some inexplicable coincidence? Or am I making some huge logical error here? :doh:

Bit of a logical error. Tidal locking is very real. If the moon did not rotate, we would see different parts of it at different times of the month, just as we see different stars at different times of the year.

One last example: take a yo-yo, wind it all the way up and simply let go: provided that you keep your arm and hand motionless, the following will happen: the yo-yo, under the influence of gravity, will start to rotate, say clockwise, as it makes his journey to the lowest point that it is allowed to reach given the length of the cord. At the ‘bottom’ of the cord, due to its momentum it will switch to a counter clockwise rotation, which will cause it to start moving upward till it reaches its peak. It will continue to switch between counter clockwise and clockwise rotation as it moves up and down, until it has eventually lost all of its momentum due to friction and stops rotating. As the yo-yo is hanging completely still, it is obviously not rotating anymore. If you now start to rotate yourself, your movement will cause the yo-yo to take an ‘orbital’ trajectory around you, just like the Moon orbits the Earth (with the difference that for the Moon – Earth system, the role of the rope is being taken by gravity). I’ve put the first orbital between parentheses, because I thought the term was only used to describe the movement of celestial bodies. In both cases, we see the same side – in the first case the same side of the yo-yo, in the second case the same side of the Moon. Now here comes: does the yo-yo in the first case rotate? I fail to see how it could without coming towards you. Besides, it it was rotating, you would be able to view its opposing side from time to time...

Well first of all, the yo-yo doesn't change directions at the bottom of the throw. It keeps spinning the same way. When you tug back on the string, the friction from the axle in the yo-yo maked it climb up the string and back into your hand. That's how you "walk the dog". When the yo-yo goes all the way out it's center of rotation changes from it's own center of mass, to the center of the user's.

Can anyone clear up which point of view is the logically flawed: is the Moon rotating so that we need 'tidal lock' to explain why the same side of it is always visible, or the moon just being an orbital object that doesn't rotate and therefor always kees the same side towards the Earth? Preferably without getting to technical/mathematical about it?
Thanx...

Something can rotate at a different rate that it revolves, but it can certainly do both at the same time. I hope this helps.

P.S.: As the nick implies, I hold no higher degrees, just my high school diploma. I just like to think a lot about stuff :cool: . I’m also Dutch, so sorry for any linguistic errors…

AstroSmurf
2006-Sep-04, 05:59 PM
[QUOTE=LayMan;818697]Now, here’s my problem: imagine a hammer thrower at the Olympics: as he/she spins and sways the ball around, which is attached to a rope, he/she always sees the same side of the ball. However, the ball is in my opinion unable to rotate, since it is fixed by means of the rope. I mean, if it was rotating, wouldn’t it start behaving like a yo-yo?

You mean, like this (http://en.wikipedia.org/wiki/Libration)? :)

hhEb09'1
2006-Sep-04, 06:58 PM
Hey, LayMan, welcome to the BAUT.

All of your examples are interesting ones. The only real error that you make, I think, is when you say that a yo-yo changes direction at the bottom of the string, but Tog_ has answered that already.

When we talk about rotation, we have to look far beyond our immediate surroundings. Like you say, in your roundabout example, the driver and passenger might even come to different conclusions. based upon what they see in front or beside them.

Imagine another situation, where you stand in the middle of the roundabout and actually rotate. That is, you spin slowly around, standing in about the same spot. That's what you would mean by "rotating", right? But while you did that, if you were to only focus your attention on the car going around the roundabout at the same time, all you would see in front of you is that car. Based upon that evidence, you might say you were not rotating, right? But we know that you are, so what is the solution?

Look beyond the car, at something that is so far away that we can't even measure its movement in a year. I'm talking about the stars. If we use the stars for our points of reference instead of cars or roundabouts, we find that yes you are rotating, and so is the moon (once per 27.3 days) and even more surprising the Earth is rotating once per 23 hours 56 minutes, not once per 24 hours.

The reason for the discrepancy in the case of the Earth is because the Earth is orbitting the Sun as well as rotating with respect to the Sun. It actually rotates 366 times per year, but we "lose" one of those rotations because of the orbit. That means that each day has to make up that lost time, and when you divide up 24 hours and distribute it to each of the other days, it's about four minutes per day. 24 hours is 24 x 60 = 1440 minutes, and divided by 365 it equals 3.95 minutes.

HTH

grapes

Wolverine
2006-Sep-04, 08:45 PM
Welcome to the forum, LayMan.

This discussion would be better served in our Questions & Answers section than in Against the Mainstream; as such I've moved the thread.

Jeff Root
2006-Sep-04, 10:29 PM
The title of LayMan's next book:

How to Make Something Which is Extremely Simple Seem to be
Terribly Complicated. 720 pages. Due out in November, 2008.

-- Jeff, in Minneapolis

Jens
2006-Sep-05, 02:03 AM
Layman,

I don't think there's any logical problem here, rather it seems like the difficulty is with what you mean by "rotate." You seem to mean, "rotate with respect to me." Using the example of the hammer, I think it should be quite obvious that to the athlete himself, the hammer doesn't seem to be rotating, because he always sees the same part of it. But what of the spectators? They see the hammer rotating. They also see the athlete rotating.

It's a little like the problem that happens when you're on a moving platform, like a train, for example. It doesn't seem to be moving to you, because you're moving along with it, but it seems to be moving to a person on the ground.

So you have to ask, rotating with respect to what?

In reality, it's a bit more complicated because unlike movement, there is an absolute reference frame for rotation, which is why Foucault's pendulum works. And the moon is actually rotating with respect to that absolute frame.

LayMan
2006-Sep-05, 10:39 AM
Thanks for the info, people, I guess I really do have some problems with the difference between rotating and revolving (or orbiting, or whatever...).

And indeed, as some have correctly pointed out, the yo-yo doesn't switch rotational direction, it just goes up the other side of the rope. Still, I find it pretty confusing stuff, especially if all movement is to be considered relative to certain reference frames like Jens indicates... Anyway, the thing about the yo-yo was not its rotational direction that interested me, the point was that if you let the yo-yo stop rotating (so it's hanging completely motionless on the cord - no rotation) and you would then start rotating yourself causing the yo-yo to revolve around you, then how can it start rotating at the same time without influencing the cord? I'm not sure wether I'm clear enough, I'll try another example: suppose I have a ball lying on the table. I can rotate it freely in any direction without changing it's central position on the table, right? Now I glue a stick onto the ball. I can still rotate the ball by turning it. The result now would be that the glued-on stick would follow the movement of the ball, indicating its rotational direction like the dials of a watch, since the other end of that stick is free to move. But what if I now take the free end of the stick in my hands and sway it around? I'm still having trouble grasping how the ball is able to rotate around its own axis without detaching itself from the stick, or without pulling the stick out of my hands?

@ Grant: you say the hammer and the rope and the athlete are all rotating from the spectators point of view, but I thought that from the spectators point of view, it would appear that only the athlete is actually rotating, and the hammer is revolving/orbiting around him. I mean, if the hammer is making a circle around the athlete and not its own central axis, I would call that motion an orbit, not a rotation...

Oh, and by the way, Jeff: good one... But I was thinking more in the lines of "How do I keep my head from spinning? - Or prevent the universe from revolving around it"... :razz:

One last thing: the Earth orbits around the Sun. On Earth, apart from our year (which results from the orbital movement), we also have a day and night cycle with regards to the Sun. We take that as prove that the Earth rotates, more specifically 1 rotation every 24 hours. I think I got that part right. Now, the Moon doesn't orbit the sun, it orbits the Earth and its resulting "orbital year" takes about 29 days. However, it does not have a day and night cycle with respect to the Earth. Viewed from the Moon, the Earth stands still in the sky, it doesn't rise or set. So if we take the day and night cycle of the Earth relative to the Sun as prove for its rotation, shouldn't we than take the absence of "the day and night cycle" of the Moon relative to the Earth as prove of absence of rotation? (Actually, as I'm typing this, I myself get the sneaky feeling there's a catch here, but I just can't put my finger on it).

Just for the record, personally, I couldn't care less wether or not the Moon rotates, relative or absolute, or with respect to any reference frame including. I'm not trying to build up to some wacky theory or something. It's just that when I first heard of the fact that the Moon not only orbits but also rotates, I just felt it to be very counter-intuitive to think of it like that... Or maybe I'm just a very poor intuitive thinker...:D

captain swoop
2006-Sep-05, 11:07 AM
If you were on the head of the hammer the athlete would look like he was stationary but the athetics field would certainly look like it was moving. Its the same with the Earth and the Moon.

Count Zero
2006-Sep-05, 11:15 AM
I’m also Dutch, so sorry for any linguistic errors…

Your english is flawless. Welcome LayMan.

LayMan
2006-Sep-05, 11:39 AM
Hey, LayMan, welcome to the BAUT.

All of your examples are interesting ones. The only real error that you make, I think, is when you say that a yo-yo changes direction at the bottom of the string, but Tog_ has answered that already.

When we talk about rotation, we have to look far beyond our immediate surroundings. Like you say, in your roundabout example, the driver and passenger might even come to different conclusions. based upon what they see in front or beside them.

Imagine another situation, where you stand in the middle of the roundabout and actually rotate. That is, you spin slowly around, standing in about the same spot. That's what you would mean by "rotating", right? But while you did that, if you were to only focus your attention on the car going around the roundabout at the same time, all you would see in front of you is that car. Based upon that evidence, you might say you were not rotating, right? But we know that you are, so what is the solution?

Look beyond the car, at something that is so far away that we can't even measure its movement in a year. I'm talking about the stars. If we use the stars for our points of reference instead of cars or roundabouts, we find that yes you are rotating, and so is the moon (once per 27.3 days) and even more surprising the Earth is rotating once per 23 hours 56 minutes, not once per 24 hours.

The reason for the discrepancy in the case of the Earth is because the Earth is orbitting the Sun as well as rotating with respect to the Sun. It actually rotates 366 times per year, but we "lose" one of those rotations because of the orbit. That means that each day has to make up that lost time, and when you divide up 24 hours and distribute it to each of the other days, it's about four minutes per day. 24 hours is 24 x 60 = 1440 minutes, and divided by 365 it equals 3.95 minutes.

HTH

grapes


Sorry, must have missed this post before the move... I think I'm beginning to see some light here: I believe I've found the source of my confusion and the reason why reference frames are so important in these kind of discussions!

We know the Earth is rotating around it's axis. But in my previous post, I distinguished between me standing on the North Pole and me standing on the Equator. Then, I stated that at the North Pole, I would be making a movement I defined as rotation with regards to the Earths axis, while on the other hand I described my movement at the Equator as orbit...
But that can't actually be corrrect because, if I define my motion at the Equator to be an orbital one, I would conclude the same thing about the ground I'm standing on, since it moves in exactly the same way as I do. But looking deeper into the Earths crust, that would be true for every underlying layer, correct? So therefor, by my own reasoning, I'm actually forcing myself to claim that the entire Earth itself is not rotating, but orbiting itself...?!?

So, let me see if I can get it right this time: an absolute rotation of an orbital body which is bigger than 1 time its orbital lenght produces the familiar behavior of the Earth as it continuously changes from day to night during it's orbital year (positive rotation), while an absolute rotation of less then 1 would yield an apparent retrograde or negative rotation. And in the case of the Moon, an absolute rotation equaling precisely 1 time the orbital lenght would lead to a relative rotation of 0, but that would be an apparent zero movement.

I think I got it: eventhough the Moon appears not to rotate, it absolutely does by one rotation per orbit, right?

Thx guys!

neilzero
2006-Sep-05, 11:45 AM
Hi layman, welcome. We seem to agree, all those border line examples are rotating. I wonder what our group would do with the idea that nothing is stationary, nearly everything is moving and rotating with respect to most everything else, the adjacent tetonic plate for instance? Neil

grant hutchison
2006-Sep-05, 11:47 AM
@ Grant: you say the hammer and the rope and the athlete are all rotating from the spectators point of view, but I thought that from the spectators point of view, it would appear that only the athlete is actually rotating, and the hammer is revolving/orbiting around him. I mean, if the hammer is making a circle around the athlete and not its own central axis, I would call that motion an orbit, not a rotation...Make a solid model of the athlete, rope and ball (with the rope a bit of rigid plastic, so the ball sticks out as if it were being swung around). Now rotate the model around the model athlete's feet, as if you were a kid pretending that the model was a real person. You'll see the model ball swing around in a way identical to the way it moves in real life.
But it's all one piece with the model athlete, so where can you draw the line saying "this is rotation" and "this is revolution"?
The sort of movement the hammer makes is just a rotation around a centre that's outside itself (in this case, inside the athlete). Likewise, you can think of the moon rotating around a centre that's inside the Earth, or your car rotating around a centre that inside the roundabout.

Grant Hutchison

LayMan
2006-Sep-05, 12:07 PM
Make a solid model of the athlete, rope and ball (with the rope a bit of rigid plastic, so the ball sticks out as if it were being swung around). Now rotate the model around the model athlete's feet, as if you were a kid pretending that the model was a real person. You'll see the model ball swing around in a way identical to the way it moves in real life.
But it's all one piece with the model athlete, so where can you draw the line saying "this is rotation" and "this is revolution"?
The sort of movement the hammer makes is just a rotation around a centre that's outside itself (in this case, inside the athlete). Likewise, you can think of the moon rotating around a centre that's inside the Earth, or your car rotating around a centre that inside the roundabout.

Grant Hutchison

Rats, just when I thought I got it... Now who's complicating things?

Actually, you don't need to create a solid model of the athlete, rope and ball, since it was already acting as one solid object to begin with. That was part of my problem, I guess: as the athlete spins around, so do his arms which are attached to his torso, his hands which are connected to his arms, the rope being held by those hands, and the ball which is fermly secured to the rope...

But what if I replaced the rope and ball by one of those globes of the Earth which are able to rotate between a crescend-shaped holder placed on a footstand? Or, just to stick with the athlete, what if the end of the rope wasn't connected directly to the ball, but to a net in which the ball was placed and able to move freely in relation to the rope (and athlete)?

And about drawing the line saying "this is rotation" and "this is revolution", maybe their both defining the same movement, just from different perspectives? But then we're back to square 1, no?

Jeff Root
2006-Sep-05, 12:16 PM
LayMan,

I wish that I could write Dutch even one-tenth of 1% as well as
you write English. All I know is a tiny handful of Dutch words.
Not enough to form a sentence. I studied German for three years
from a German native, but learned almost nothing.

Rotation appears to be one of those things which is so basic
that the only way to learn about it and understand it is through
observation and experience. It cannot be explained in simpler
terms because it is so simple. Yet it does have this subtlety
which you described. A whole object rotates, while each of its
parts circle the center of rotation. It doesn't matter whether
the center of rotation is inside the object.

-- Jeff, in Minneapolis

grant hutchison
2006-Sep-05, 12:23 PM
And about drawing the line saying "this is rotation" and "this is revolution", maybe their both defining the same movement, just from different perspectives?Only if the revolving object rotates to keep one side always towards the centre of revolution, as in your examples of the hammer-thrower and the car going around the roundabout, and in the case of the moon. Then we can look at the whole thing as if it is rigidly rotating around some distant centre.
If the moon didn't rotate, we wouldn't be able to make that construction. It would be as if the hammer thrower were spinning round with one of those Earth globes, as you say, and there was no friction in the globe's pivots to start it rotating with the hammer thrower. The thrower could then look at the globe and see different sides of it as he rotated (and the globe didn't). The spectators would see the thrower, the rope, and the globe's pivot rotating, but the globe not rotating while it revolved around the hammer thrower.

You're absolutely right in your conclusion that the moon revolves and rotates in the same time period, which is why it doesn't appear to rotate from the viewpoint of the Earth.

Grant Hutchison

Tog
2006-Sep-05, 12:26 PM
In number 1, the triangle is not rotating at all. It always points up. From the circle, it eems to rotate backwards because the circle will eventually see all sides of it.

In number 2, the trigangle rotates one time per revolution. This is like the Moon, the hammer or the car.

In number 3, the triangle rotates 2 times for every one revolution. It is pointing up in two different places.

Does that help any?

Jeff Root
2006-Sep-05, 12:29 PM
Layman,

It is possible for a body to go around in a circle without rotating.
Imagine yourself standing on a merry-go-round or carousel as it
rotates. There is a fabulously good-looking girl standing a little
distance away from the carousel. You can't take your eyes off
her. So as the carousel makes you go around in a circle, you
constantly turn to face the girl. As a result, you are always
facing in the same direction. You are not rotating.

A moon orbiting a planet could do that too, although there
would be no reason for it to happen except by chance. The
moon might always have the same side facing in the direction
of the constellation Orion as it orbits the planet.

-- Jeff, in Minneapolis

Jeff Root
2006-Sep-05, 12:43 PM
It would be as if the hammer thrower were spinning round with one
of those Earth globes, as you say, and there was no friction in
the globe's pivots to start it rotating with the hammer thrower.
Ah, good example. Likely more accessible would be a cup of
liquid with something floating on the surface to make motion
visible. How many times have I tried to stir something by
spinning around in the middle of my kitchen holding the cup?
Lots. I don't care that it doesn't work. And I haven't
spilled anything that way, yet.

-- Jeff, in Minneapolis

LayMan
2006-Sep-05, 02:49 PM
Only if the revolving object rotates to keep one side always towards the centre of revolution, as in your examples of the hammer-thrower and the car going around the roundabout, and in the case of the moon. Then we can look at the whole thing as if it is rigidly rotating around some distant centre.
If the moon didn't rotate, we wouldn't be able to make that construction. It would be as if the hammer thrower were spinning round with one of those Earth globes, as you say, and there was no friction in the globe's pivots to start it rotating with the hammer thrower. The thrower could then look at the globe and see different sides of it as he rotated (and the globe didn't). The spectators would see the thrower, the rope, and the globe's pivot rotating, but the globe not rotating while it revolved around the hammer thrower.

You're absolutely right in your conclusion that the moon revolves and rotates in the same time period, which is why it doesn't appear to rotate from the viewpoint of the Earth.

Grant Hutchison

Errr, are you now suggesting that the Earth doesn't rotate?!...



Just kidding, I'm going to stick to my reply to hhEb09'1: the trick seems to be to reckon with three relative rotations:

1) those that are smaller then the accompanying orbit (> 0 and < 1) --> apparent retrograde rotation

2) those that are exactly equal to the orbit (= 1) --> apparently no rotation

3) those that are bigger then the orbit (> 1) --> apparent "normal" or Earth-like positive rotation

From this point of view: absolute rotation isn't possible, since no object is otherwise completely motionless apart from that rotation, and even if it was, no object is compacted into one single point in space without any other moving body in its vicinity... Except perhaps the singularity of the Big Bang. I guess its safe to say that would be the only thing capable of absolute rotation. I think I got it.

You can really get a splitting headache trying to wrap your mind around things like this...

Kristophe
2006-Sep-05, 03:02 PM
From this point of view: absolute rotation isn't possible, since no object is otherwise completely motionless apart from that rotation, and even if it was, no object is compacted into one single point in space without any other moving body in its vicinity... Except perhaps the singularity of the Big Bang. I guess its safe to say that would be the only thing capable of absolute rotation. I think I got it.

You can really get a splitting headache trying to wrap your mind around things like this...

Someone please correct me if I'm wrong, but I was always under the impression that one could measure absolute rotation because acceleration is involved. I mean, if we assume that the Earth is the sole object in the universe, we could tell that it must be rotating because of its equitorial buldge, right?

You know, ignoring the fact that we'd never exist if that were the case...

LayMan
2006-Sep-05, 03:20 PM
Someone please correct me if I'm wrong, but I was always under the impression that one could measure absolute rotation because acceleration is involved. I mean, if we assume that the Earth is the sole object in the universe, we could tell that it must be rotating because of its equitorial buldge, right?

You know, ignoring the fact that we'd never exist if that were the case...

Maybe, but I on the other hand was always under the impression that the equatorial buldge was caused by the earths rotation, not the other way around... And even if the Earth was the only object in space, most of the atoms it consists of are not located in the exact centre, in fact not even the most central atom itself would be completely confined to the cental point... Therefor, none of the individual atoms would actually be rotating, they would all be orbiting the virtual central point of the total mass, no?

Here we go again...

Anyway, I'm off for now, maybe catch you all tomorrow.

grant hutchison
2006-Sep-05, 03:24 PM
Someone please correct me if I'm wrong, but I was always under the impression that one could measure absolute rotation because acceleration is involved. I mean, if we assume that the Earth is the sole object in the universe, we could tell that it must be rotating because of its equitorial buldge, right?Well ...
Some folks would suggest that if the Earth were alone in the Universe it couldn't be rotating, because it would have nothing to rotate relative to. That is, the Universe provides the local standard of "non-rotation", and if the Earth were the only occupant of the Universe it would, by definition, be non-rotating. :)
But in a Universe full of stuff, we find that reference frames which rotate relative to the background Universe experience pseudoforces (centrifugal, Coriolis), which don't appear in reference frames that aren't rotating relative to the Universe. So there's a reference frame for rotation, as you say.

Grant Hutchison

Jeff Root
2006-Sep-05, 05:59 PM
if we assume that the Earth is the sole object in the universe, we could
tell that it must be rotating because of its equitorial bulge, right?
Maybe, but I on the other hand was always under the impression that
the equatorial buldge was caused by the earths rotation, not the
other way around...
He is saying that we can deduce that the Earth must be rotating,
because we see that it has a bulge. The cause and effect of the
deduction are in the direction opposite that of the cause and
effect of the rotation and bulge!



And even if the Earth was the only object in space, most of the
atoms it consists of are not located in the exact centre, in fact
not even the most central atom itself would be completely
confined to the cental point... Therefor, none of the individual
atoms would actually be rotating, they would all be orbiting the
virtual central point of the total mass, no?
All the atoms are rotating, as well as going around in circles.
Essentially none of the atoms are rotating around their own
centers, but that fact is of no consequence to the fact that
they are rotating.

I say "going around in circles" rather than "orbiting" because
the term "orbit" has a technical meaning in astronomy which does
not apply to things in or on the Earth. The Moon orbits Earth.
A spacecraft orbits Earth. I do not orbit Earth. I'm not going
nearly fast enough. I'm doing less than 330 metres per second.
I would need to move at 7.9 kilometres per second to be in orbit
at my distance from Earth's center.

However, I am orbiting the Sun!

-- Jeff, in Minneapolis

LayMan
2006-Sep-06, 06:56 AM
He is saying that we can deduce that the Earth must be rotating,
because we see that it has a bulge. The cause and effect of the
deduction are in the direction opposite that of the cause and
effect of the rotation and bulge!


All the atoms are rotating, as well as going around in circles.
Essentially none of the atoms are rotating around their own
centers, but that fact is of no consequence to the fact that
they are rotating.

I say "going around in circles" rather than "orbiting" because
the term "orbit" has a technical meaning in astronomy which does
not apply to things in or on the Earth. The Moon orbits Earth.
A spacecraft orbits Earth. I do not orbit Earth. I'm not going
nearly fast enough. I'm doing less than 330 metres per second.
I would need to move at 7.9 kilometres per second to be in orbit
at my distance from Earth's center.

However, I am orbiting the Sun!

-- Jeff, in Minneapolis

You're right, I thought he was implying that the rotation of Earth was actually caused by the bulge, my mistake. Sorry 'bout that, Kristophe...

As for your remark about semantycs - the exact meaning of a given definition within a certain branch of science, you're right again (you know, I'm really starting to dislike you ;) )...

As far as the thread goes, I think that about wraps it up: my initial question was wether or not there was a logical flaw in saying that the moon rotates (in the sense that it spins around its own axis). Turns out there was one, but it was on my behalf. Don't worry, I'd rather be proven wrong, knowing why I was wrong, than to think I'm right without having a clue...

Thanks to all for taking the time to explain it.

But be warned, I still have some other questions, though! (And yes, I'm aware of the fact that an idiot can ask more questions then a thousand scientists can answer, but that's really their problem, isn't it?). Like for instance, when I was talking about the difference in motion when standing on the North Pole versus standing on the Equator: as you move from the Equator to the pole, you're "rotational" speed drops from approximately 1.666 km/h to 0 km/h in relation to the gravitational center of the Earth. Does that mean you actually become heavier during the trip?? Maybe I'll start a new thread, but for now, I think I'll first have a stroll around and look at some other threads.

ZaphodBeeblebrox
2006-Sep-06, 07:28 AM
You're right, I thought he was implying that the rotation of Earth was actually caused by the bulge, my mistake. Sorry 'bout that, Kristophe...

As for your remark about semantycs - the exact meaning of a given definition within a certain branch of science, you're right again (you know, I'm really starting to dislike you ;) )...

As far as the thread goes, I think that about wraps it up: my initial question was wether or not there was a logical flaw in saying that the moon rotates (in the sense that it spins around its own axis). Turns out there was one, but it was on my behalf. Don't worry, I'd rather be proven wrong, knowing why I was wrong, than to think I'm right without having a clue...

Thanks to all for taking the time to explain it.

But be warned, I still have some other questions, though! (And yes, I'm aware of the fact that an idiot can ask more questions then a thousand scientists can answer, but that's really their problem, isn't it?). Like for instance, when I was talking about the difference in motion when standing on the North Pole versus standing on the Equator: as you move from the Equator to the pole, you're "rotational" speed drops from approximately 1.666 km/h to 0 km/h in relation to the gravitational center of the Earth. Does that mean you actually become heavier during the trip?? Maybe I'll start a new thread, but for now, I think I'll first have a stroll around and look at some other threads.
Yes ...

Not ONLY Because of The Rotational Vector Mind you ...

Also, The Flattening Out at The Poles Actually Brings you Closer to The Earth's Center During the Journey, Anyone Know The Exact Parameters?

:think:

Jens
2006-Sep-06, 07:32 AM
Like for instance, when I was talking about the difference in motion when standing on the North Pole versus standing on the Equator: as you move from the Equator to the pole, you're "rotational" speed drops from approximately 1.666 km/h to 0 km/h in relation to the gravitational center of the Earth. Does that mean you actually become heavier during the trip??

Yes. It was discussed somewhere else. Although it's complicated, because the earth bulges at the equator, so you become slightly heavier because the bulge. But I'm pretty sure you still weight less at the equator than at the poles. Though not by very much.

LayMan
2006-Sep-06, 09:07 AM
I knew the diameter of the Earth is slightly shorter measured from pole to pole, then when measured from a point on the Equator to its opposing point, but the difference according to Wikipedia is only 43 km.
According to this (http://geography.about.com/od/learnabouttheearth/a/earthfacts.htm), Earths diameter at the Equator equals 12,756.1 km, while at the poles it's 12,713.5 km: now 12,756.1 - 12,713.5 = 42.6 km, so the difference at the poles is 42.6 / 12,756.1 = 0.003339... In other words, a difference in weight of approximately 0.33 %. An object weighing 1 kg at the equator would therefor - at the poles - weigh a third of one percent more then 1,000 grams, so 1,003 grams in total or 3 grams more then its weight at the Equator. Actually, I think that's not entirely correct, since the effect of gravity isn't linearly related to the distance, but exponentionally. So it would be a bit more then 3 grams.

However, that wasn't what I was thinking of: what I meant was, that the centrifugal force (generated at the Equator by the rotation of the Earth which tend to push you in the opposite direction to which gravity is trying to pull you) is apparently missing at the poles, since your "rotational speed" with respect to the Earths core is 0. If I'm going at around 1.666 km/h at the Equator, that force should be subtracted from the escape velocity, and it's the escape velocity which determines my weight. The escape velocity at the surface of the Earth equals 11.2 km/s, which is about 40,320 km/h. And 1,666 km/h divided by 40,320 km/h equals 0.041319... that's 4 % (and probably slightly more due to the exponentional function of gravity versus distance. But a total increase of around 4 % should be noticeable, no? In terms of our object of 1 kg, that would add up to more then 40 grams. The problem is that I've never heard of this effect, so I was wondering wether I made yet another mistake...:D

ZaphodBeeblebrox
2006-Sep-06, 09:11 AM
I knew the diameter of the Earth is slightly shorter measured from pole to pole, then when measured from a point on the Equator to its opposing point, but the difference according to Wikipedia is only 43 km.
According to this (http://geography.about.com/od/learnabouttheearth/a/earthfacts.htm) (http://geography.about.com/od/learnabouttheearth/a/earthfacts.htm%29), Earths diameter at the Equator equals 12,756.1 km, while at the poles it's 12,713.5 km: now 12,756.1 - 12,713.5 = 42.6 km, so the difference at the poles is 42.6 / 12,756.1 = 0.003339... In other words, a difference in weight of approximately 0.33 %. An object weighing 1 kg at the equator would therefor - at the poles - weigh a third of one percent more then 1,000 grams, so 1,003 grams in total or 3 grams more then its weight at the Equator. Actually, I think that's not entirely correct, since the effect of gravity isn't linearly related to the distance, but exponentionally. So it would be a bit more then 3 grams.

However, that wasn't what I was thinking of: what I meant was, that the centrifugal force (generated at the Equator by the rotation of the Earth which tend to push you in the opposite direction to which gravity is trying to pull you) is apparently missing at the poles, since your "rotational speed" with respect to the Earths core is 0. If I'm going at around 1.666 km/h at the Equator, that force should be subtracted from the escape velocity, and it's the escape velocity which determines my weight. The escape velocity at the surface of the Earth equals 11.2 km/s, which is about 40,320 km/h. And 1,666 km/h divided by 40,320 km/h equals 0.041319... that's 4 % (and probably slightly more due to the exponentional function of gravity versus distance. But a total increase of around 4 % should be noticeable, no? In terms of our object of 1 kg, that would add up to more then 40 grams. The problem is that I've never heard of this effect, so I was wondering wether I made yet another mistake...:D
No, you're Quiite RIGHT ...

In Fact, I Believe Those Two Effects Are Additive ...

I Thiink it Winds Up Being Roughly 50 g per kg, But I'm Suure Someone Here Knows The Correct Equation?

LayMan
2006-Sep-06, 09:26 AM
Right, that's it, I'm quiting my job. I'm off to buy some gold in Africa and sell it to the Eskimo's... :dance:

grant hutchison
2006-Sep-06, 11:30 AM
Yes. It was discussed somewhere else. Although it's complicated, because the earth bulges at the equator, so you become slightly heavier because the bulge. But I'm pretty sure you still weight less at the equator than at the poles. Though not by very much.Yes, we had a detailed discussion about this over on this thread (http://www.bautforum.com/showthread.php?t=42621).

Grant Hutchison

ZaphodBeeblebrox
2006-Sep-06, 12:13 PM
Yes, we had a detailed discussion about this over on this thread (http://www.bautforum.com/showthread.php?t=42621).

Grant Hutchison
Juust As I Thought ...

I Was OFF By a Whole Magnitude ...

However, That Still Leaves a Difference of 5g per kg Correct?

grant hutchison
2006-Sep-06, 02:51 PM
However, That Still Leaves a Difference of 5g per kg Correct?Pole to equator on the real, rotating Earth, yes.

Grant Hutchison

Kaptain K
2006-Sep-07, 06:29 AM
Right, that's it, I'm quiting my job. I'm off to buy some gold in Africa and sell it to the Eskimo's... :dance:
Be sure to use a "spring-type" scale! :whistle:

LayMan
2006-Sep-07, 11:05 AM
Layman,

It is possible for a body to go around in a circle without rotating.
Imagine yourself standing on a merry-go-round or carousel as it
rotates. There is a fabulously good-looking girl standing a little
distance away from the carousel. You can't take your eyes off
her. So as the carousel makes you go around in a circle, you
constantly turn to face the girl. As a result, you are always
facing in the same direction. You are not rotating.

A moon orbiting a planet could do that too, although there
would be no reason for it to happen except by chance. The
moon might always have the same side facing in the direction
of the constellation Orion as it orbits the planet.

-- Jeff, in Minneapolis


But wait a minute…

I know I shouldn’t be doing this…

Cause I wouldn’t like to look like a sore loser…

And I just know I’m going to regret this…

BUT…

Here’s an evil, little twist… :evil:

Consider this: 9 . Not the number, but the shape of it. It’s a curved line, ending into a circle, right? Look at the point where the circle and the curved line touch each other: let’s call that point X, ok? I’ll get back to this later on, first things first:

Let’s – for arguments sake – get rid of that quirky moon. In fact, let’s get rid of everything in the universe, except for the ‘absolute’ reference frame of an observer on Orion and another observer with his pitiful ‘relative’ reference frame here on good old Earth…

Now, our observer on Orion, let’s call him… Jeff, is very interested in a little thing called space-time continuum.. In fact, he’s so interested, he decides to conduct an experiment: he contacts his good old buddy – let’s call him Velikovsky :evil: , and asks him to build a moon. His buddy promptly delivers a full size moon, decorates it with beautiful craters and attaches one big shiny mirror (say, a couple of meters wide) onto its surface. He then promptly shoots it of the surface of Orion, in such a way that the side of the moon with the mirror is facing Orion with the mirror almost in the middle of the visible moon disk. Furthermore, an integrated (super)laser/detector, mounted on the surface of Orion, hits the mirror so, that the reflected beam neatly falls back onto the detector… I think you catch my drift.

Now, in order to make sure that the laser beam will continually be reflected off the mirror and back onto the detector during the moons journey, our ‘oriontal’ friends eject the moon off the surface of Orion in a perfect linear direction, perfectly vertical to the surface, making sure there is absolutely no rotational force being applied, because even the tiniest rotational movement along any axis would cause the mirror to move out of the reach of the laser beam, which would trigger the alarm of the detector, a sound so ghastly you would rather yank your ears of then to be exposed to it for even 5 seconds…

And off it goes. For many years, the alarm keeps silent, since it continually receives the laser beam as it is being reflected back by the mirror, which keeps its initial position perfectly since our friends so vigorously made sure there was no rotational momentum to spoil their fun. The detector, of course, was perfectly adapted to deal with the Doppler effect, which was what they wanted to study in the first place… Suddenly - after many, many years – the unthinkable happens: the alarm goes off! After that, it goes silent again for quite some time. Then, again for a short time, there’s that awful sound again. Then, again silence. Quickly, our friends realise this happens in a fixed pattern: a short burst of alarm, followed by a significantly longer period of silence.

What happened? Well, even though their moon started out in a linear direction, it soon got influenced by the Earths gravity, causing it to ‘bend’ off the righteous path and finally settling into a nice little circular orbit around it. Remember the shape of the 9 I mentioned earlier? Now, look at what happens as the moon, its “mirror side” facing backward towards Orion to be able to reflect the laser beam back, follows its “nine shaped” trajectory: as it ‘takes the bent’, just before arriving at point X for the first time, the mirror is no longer facing towards Orion, but away from it, right?

Aha, rotation! That’s making the alarm go off, right? Well…, ehm…, actually, not according to Einstein. According to Einstein, the Earth is bending space itself around it, causing the moon to follow the curvature of space. But that also goes for the pursuing laser beam! It follows the same curved path through space, makes the bent just like the moon, still hits the mirror and is reflected back along the exact same way, straight into the detector on the surface of Orion. Einstein will tell you the alarm isn’t being triggered by the moons rotation, but its orbit: each time the moon passes point X, the mirror is facing towards Orion again, but the incoming laser that also passes through point X, gets blocked for a short time as half the moons visible disk has to pass by before the mirror is back in its reflecting position again. It then continues to reflect. Actually, Einstein can’t observe this behaviour, he’ll deduce it from his mathematical calculations. The only observer capable of doing that needs to be on a point outside space, in other words God. He’ll confirm Einstein and tell you the path followed by the moon looks like a nine (even though the devil will claim it looks more like a six :evil: ). That is the reason why the alarm goes off on Orion, because of the moons orbit, not its rotation, since there was never any rotational momentum to begin with! This makes sence, because if it was caused by rotation, the alarm would behave in exactly the opposite way: it would sound alarm for a long period, as the mirror needs time to revolve back in reflecting position, and would only be silent for a very brief moment, since the mirror almost immediately goes out of synch again.

But we already agreed that the moon does rotate?! So Einstein's wrong and there is no God?



It cannot be explained in simpler
terms because it is so simple. Yet it does have this subtlety
which you described

-- Jeff, in Minneapolis

We've just debunked both Einstein and religion, and you call that subtle?? :D :D

hhEb09'1
2006-Sep-07, 03:20 PM
But we already agreed that the moon does rotate?! So Einstein's wrong and there is no God?

Welcome to the forum, LayMan.

This discussion would be better served in our Questions & Answers section than in Against the Mainstream; as such I've moved the thread.Maybe we might want to rethink that... :)

Jeff Root
2006-Sep-07, 04:36 PM
It is possible for a body to go around in a circle without rotating.
Imagine yourself standing on a merry-go-round or carousel as it
rotates. There is a fabulously good-looking girl standing a little
distance away from the carousel. You can't take your eyes off
her. So as the carousel makes you go around in a circle, you
constantly turn to face the girl. As a result, you are always
facing in the same direction. You are not rotating.

A moon orbiting a planet could do that too, although there
would be no reason for it to happen except by chance. The
moon might always have the same side facing in the direction
of the constellation Orion as it orbits the planet.
I know I shouldn’t be doing this...

Cause I wouldn’t like to look like a sore loser...
Not possible. You didn't lose anything.



And I just know I’m going to regret this...
Me, too.



Consider this: 9 . Not the number, but the shape of it. It’s a
curved line, ending into a circle, right? Look at the point where
the circle and the curved line touch each other: let’s call that
point X, ok? I’ll get back to this later on, first things first:

Let’s - for arguments sake - get rid of that quirky moon. In
fact, let’s get rid of everything in the universe, except for the
‘absolute’ reference frame of an observer on Orion and another
observer with his pitiful ‘relative’ reference frame here on good
old Earth...
You seem to think Orion is a planet. I chose Orion because the
constellation is familiar to almost everyone. However, this
peculiar error has no effect on your argument.



Now, our observer on Orion, let’s call him.. Jeff, [...] contacts
his good old buddy - let’s call him Velikovsky, and asks him to
build a moon. His buddy promptly delivers a full size moon, [...]
and attaches one big shiny mirror (say, a couple of meters wide)
onto its surface. He then promptly shoots it of the surface of
Orion, in such a way that the side of the moon with the mirror is
facing Orion with the mirror almost in the middle of the visible
moon disk. Furthermore, an integrated (super)laser/detector,
mounted on the surface of Orion, hits the mirror so, that the
reflected beam neatly falls back onto the detector...

even the tiniest rotational movement along any axis would cause
the mirror to move out of the reach of the laser beam, which would
trigger the alarm of the detector

Suddenly - after many, many years - the unthinkable happens: the
alarm goes off! After that, it goes silent again for quite some
time. Then, again for a short time, there’s that awful sound
again. Then, again silence. Quickly, our friends realise this
happens in a fixed pattern: a short burst of alarm, followed by
a significantly longer period of silence.

What happened? Well, even though their moon started out in a
linear direction, it soon got influenced by the Earths gravity,
causing it to ‘bend’ off the righteous path and finally settling
into a nice little circular orbit around it.
The moon would actually follow a hyperbolic trajectory past the
Earth. The degree of bending of the path depends on the speed
of the moon past the Earth and the distance between the center
of the Earth and the center of the moon. But circular orbit is
fine for this thought experiment.



Remember the shape of the 9 I mentioned earlier? Now, look at
what happens as the moon, its "mirror side" facing backward
towards Orion to be able to reflect the laser beam back, follows
its "nine shaped" trajectory:
A circular orbit is a circular orbit. However, a realistic
capture orbit would be highly elliptical, like that of Mars
Reconnaissance Orbiter when it first arrived at Mars.



as it ‘takes the bent’, just before arriving at point X for the
first time, the mirror is no longer facing towards Orion, but
away from it, right?
No, there is no reason for the moon to rotate as it goes into
orbit, or at any other time.



Aha, rotation! That’s making the alarm go off, right? Well...,
ehm..., actually, not according to Einstein. According to
Einstein, the Earth is bending space itself around it, causing
the moon to follow the curvature of space. But that also goes
for the pursuing laser beam! It follows the same curved path
through space, makes the bent just like the moon, still hits
the mirror and is reflected back along the exact same way,
straight into the detector on the surface of Orion.
No, light moves so fast that it is hardly affected by Earth's
gravity. The moon's path might be bent greatly. The light's
path will be bent so little that the bending would be quite
difficult to detect.



Einstein will tell you the alarm isn’t being triggered by the
moons rotation, but its orbit: each time the moon passes point X,
the mirror is facing towards Orion again, but the incoming laser
that also passes through point X, gets blocked for a short time
as half the moons visible disk has to pass by before the mirror
is back in its reflecting position again. It then continues to
reflect.
I gather that you studied under Rube Goldberg.



Actually, Einstein can’t observe this behaviour, he’ll deduce it
from his mathematical calculations. The only observer capable of
doing that needs to be on a point outside space,
If such behavior actually happened, any observer along the
light path could observe it.

-- Jeff, in Minneapolis

LayMan
2006-Sep-08, 08:02 AM
Not possible. You didn't lose anything.

-- Jeff, in Minneapolis

Well, I lost my splitting headache... But now I'm starting to hear voices.


You seem to think Orion is a planet.

-- Jeff, in Minneapolis

I knew Orion is a stellar constellation, I've read "The Orion mystery" - well, amongst other books. I actually own a copy of it... I also am the proud owner of 2 books by E. Von Däniken... I blame puberty for it. :shifty:

But like you said, it doesn't affect the argument, and I didn't want to make the post any longer then it already was going to be.


No, there is no reason for the moon to rotate as it goes into
orbit, or at any other time.

-- Jeff, in Minneapolis

Bit confused by this, are you actually agreeing with me here? :evil:


No, light moves so fast that it is hardly affected by Earth's
gravity. The moon's path might be bent greatly. The light's
path will be bent so little that the bending would be quite
difficult to detect.

-- Jeff, in Minneapolis

I know that. But that wasn't the point I was trying to make: if Einsteins theory (well, as I understand it) is correct and gravity bends space itself and any mass traveling in orbit around another mass does so because it follows that bent space, then I don't understand what speed has to do with it. Can light travel beyond space and end up in a 'lesser bent' part of it? Or are you implying that the amount of space-bending by a heavy mass is relative to the speed of any object that may pass by it? How would the Earth for instance know how much to bent its surroundings? Does it calculate the speed of the moon? Of course not, that was the whole point of the mental excercise, really: I know I'm wrong and you're right, I just like to know (understand) why... If Einstein says "Hey everybody, I've got an alternative way of looking at gravity which solves some of its mysterious characteristics.", then I'm intrigued by his proposal since I find gravity very fascinating. But to be honest, I feel more comfortable in the company of Isaac Newton then being in the same (bent) room with Einstein . Again, the real issue here is my apparent inability to correctly interpret things like relativity theory, quantum dynamics and the likes. Call it stuberness on my behalf, but I refuse to accept that inability, so I try to fine-tune my interpretation by asking questions, considering points of view from other people and generally having some fun with thought experiments... :lol:



I gather that you studied under Rube Goldberg.

-- Jeff, in Minneapolis

Never heard of the guy, but it's amazing what a little googling can do. You have no idea how much I regret not having done so. :doh:


If such behavior actually happened, any observer along the
light path could observe it.

-- Jeff, in Minneapolis

Yes, I know, but that actually raises another interesting issue: how would you be able to see the laser beam from aside? Here on Earth, I can see a laser beam that is being fired from one end of a room to the other end, wilst standing in the room next to it. I can try to explain that ability by saying that some of the light is being reflected of dust, or even atomic or sub-atomic particles. But how does it work in the vacuum of space? Do the laser photons that constitute the laser beam actually travel perpendicularly to the path of the beam itself? :confused:

Jeff Root
2006-Sep-09, 11:31 PM
No, there is no reason for the moon to rotate as it goes into
orbit, or at any other time.
Bit confused by this, are you actually agreeing with me here?
I don't know. I'll just talk about it a little and hope that
something I say will clarify it for you.

Interactions between bodies in Space which are purely
gravitational are different from the kinds of interactions that
we commonly experience. That is because friction plays a big
role in almost all interactions at or near Earth's surface,
while there is no friction in the scenario you described.

If you put a small object on a phonograph turnable, off to one
side, and then started it rotating at 45 rpm, the object would
begin to go around the center of the turntable, and it would
rotate as it did so. Then it would either slide or roll off of
the turntable and continue moving in a straight line while also
continuing to spin, for a short time, before it came to a stop.
Friction with the turntable makes the object rotate and revolve
around the center. When the centrifugal effect becomes greater
than the friction holding the object in place, it slides or
rolls away, inertia keeps it rotating and moving in a straight
line, and friction with the surface it is sliding or rolling
over stops the rotation and straight-line motion.

The only one of those things happening in your scenario is the
inertia, which keeps the moon moving in whatever direction it
has been pushed. The only force in your scenario is Earth's
gravity. Earth's gravity pulls on the moon, causing it to go
around the Earth. For this thought experiment, we can assume
that it goes into circular orbit, although that would actually
not be possible without another force, like friction, or a
rocket.

The force which makes the moon go round the Earth acts between
the centers of gravity of the Earth and the moon, assuming that
both are perfectly spherical, which I'm going to insist on
assuming! Since it does that, there is no component of the
force which could cause the moon to start rotating. So the
side of the moon which started out facing Orion would continue
to face Orion as it went into orbit around the Earth.



if Einsteins theory (well, as I understand it) is correct and
gravity bends space itself and any mass traveling in orbit around
another mass does so because it follows that bent space, then I
don't understand what speed has to do with it.
The curvature of space is a peculiar thing because it is real,
not just an analogy, yet the easiest way to visualize it is by
means of an analogy. And the analogy goes beyond merely being
an analogy in that it actually uses gravity and in some ways
is like gravity!

It's the rubber-sheet thing. Empty space is represented by
a huge rubber sheet. A big, heavy ball on the sheet represents
Earth, and makes the sheet curve around it which is something
like the way Earth's gravity curves the space around it. Of
course, the two are not curved in the same manner, but the
shapes of the curves are very similar, and have very similar
effects. When you roll a marble past the big, heavy ball, its
trajectory is affected by the curved surface it rolls over.
The closer the marble gets to the ball, the more its trajectory
is affected. The faster the ball is moving, the less its
trajectory is affected.

One noteable difference between gravity and the rubber-sheet
analogy is that the marble rolls along a surface. Of course
there is no such surface in space, and objects moving through
space do not roll on anything. A second major difference is
that the rubber sheet curves in the up/down direction, while
space doesn't do anything like that. It curves yet stays flat!

I hope that makes sense. If it doesn't, don't worry about it!



how would you be able to see the laser beam from aside?
You'd have to intercept a little bit of the beam. There would
be a lot of light in it, so nobody will notice if you take a
little sample once in a while. I suppose you were right that
looking at the beam in just one location wouldn't tell you what
direction it came from or how it was bending. You would need
to observe it in several different places. When the bending of
starlight past the Sun is observed, we can see that it is bent
by observing at several different times, as the Sun moves in
front of the distant stars.

-- Jeff, in Minneapolis

grav
2006-Sep-10, 08:39 PM
I believe I have now come to the personal opinion, at least, that the moon does not rotate. If it did, it should produce an equatorial bulge. I think this is a good definition of rotation. This definition would read that any rotating object experiences a centrifugal acceleration around the plane of its rotation. This would produce a centrifugal bulge in the moon's equator. Since this bulge would not be produced by tidal forces, since it is not turning in respect to the Earth, this should be easy to determine, unless some residual bulge in the equator is still present from a previous rotation in respect to the Earth. And the tidal bulge along this plane would add to it. A car going in a circle or a ball on a string does not experience this kind of centrifuge, as far as I know. If anything, it is just aimed away from the center of revolution. Even two large equal masses that orbit each other should not produce an equatorial bulge in either, so they are simply turning with the revolution, not rotating. It all depends on the precise definition of rotation.

hhEb09'1
2006-Sep-10, 08:48 PM
I believe I have now come to the personal opinion, at least, that the moon does not rotate. If it did, it should produce an equatorial bulge. I think this is a good definition of rotation. This definition would read that any rotating object experiences a centrifugal acceleration around the plane of its rotation. This would produce a centrifugal bulge in the moon's equator. ??

Not rotate? We just spent a few days over on the other thread showing that it did have an equatorial bulge (the "y" term)

grav
2006-Sep-10, 10:51 PM
??

Not rotate? We just spent a few days over on the other thread showing that it did have an equatorial bulge (the "y" term)
Oh, I hope we don't have to go through that again. :o You helped me to discover that the value for the centrifugal force applies to the entire surface of the moon, so that centrifugal force is not a consideration for tidal bulges. It was fun, and I learned a lot from that, by the way. Thanks. :) But the centrifugal force caused by rotation would still be completely separate from that of the revolution. Sure, they both have the same value because the period of revolution and rotation is the same, but that part that would be created by rotation would only cause a bulge around the equator, which is in addition to the uniform centrifugal bulge over its entire surface caused by its revolution.

So what I'm saying here is that if the moon is just turning with its revolution around the Earth, then it cannot truly be called a rotation unless the equatorial bulge is still present, since that becomes a local effect, independent of the Earth.

grav
2006-Sep-10, 11:05 PM
Now, if the moon actually did rotate once per revolution in the opposite direction of its direction of revolution in respect to the Earth, then it would appear stationary (non-rotating) to the rest of the universe. So in this case, it would appear it should have no equatorial bulge in respect to the universe, and is not rotating. Since it cannot not be rotating in both scenarios (this and in my earlier post), then I have a dilemna. It is easily resolved, however, by determining whether or not the moon has an equatorial bulge. If so, then it rotates. But at the same time, the tidal forces in this present scenario would cause the moon to have an equatorial bulge anyway. So I'm just not sure anymore. :confused: I guess I'll just ask the question. Does the moon have an equatorial bulge?

I guess an equatorial bulge might be difficult to determine with the tidal bulges in the way. It would be about one third (2 for front or back and 1 for lateral squeezing) of the apparent magnitude. If we look at the moon straight on from Earth, we shouldn't notice the tidal bulges, though (which is a good thing, since it makes this simpler). Also, the lateral tidal squeezing would act equally all the way around the circumference, so that isn't a factor either. So all I really need to know is if the moon as viewed from the Earth is noticably wider along its line of orbit than it is from top to bottom (perpendicular to its line of orbit).

PhantomWolf
2006-Sep-10, 11:43 PM
I would have thought that any equatorial bugle would have been dependant on the speed of rotation and make up of the moon as well, so that might not tell you anything. Surely the better idea is to determine what it would do if you removed the Earth altogther, so let's get rid of the pesky thing and observe the result.

Now we just have the moon orbiting the sun by itself and as it does, it rotates to give a year of 13 lunar days. Solved, the moon rotates.

Jeff Root
2006-Sep-10, 11:52 PM
I believe I have now come to the personal opinion, at least, that
the moon does not rotate. If it did, it should produce an equatorial
bulge. I think this is a good definition of rotation.
Does a phonograph turntable rotate? Does a thrown football
rotate? Do you ever turn from left-to-right or right-to-left?
Have you ever turned a mug or coffe cup around so the handle
would be on the other side? Have you ever indicated "yes" or
"no" by nodding or shaking your head? Have you ever flipped
a light switch on or off? Have you ever seen the hands on a
clock move?

-- Jeff, in Minneapolis

grav
2006-Sep-10, 11:54 PM
I would have thought that any equatorial bugle would have been dependant on the speed of rotation and make up of the moon as well, so that might not tell you anything. Surely the better idea is to determine what it would do if you removed the Earth altogther, so let's get rid of the pesky thing and observe the result.

Now we just have the moon orbiting the sun by itself and as it does, it rotates to give a year of 13 lunar days. Solved, the moon rotates.
The speed of rotation and make up of the moon are indeed factors to be considered. I have no idea how its make up would affect it, and that is why I cannot give a definitive value of the extra distance for an equatorial bulge, if it exists, except by comparing it to the tidal bulges, since they would be in proportion. If we took away the Earth, it still would not necessarily mean
the moon is rotating. It would be like a car going round in a circle. Would you then say the car is rotating, or revolving?

Jeff Root
2006-Sep-10, 11:56 PM
hhEb09'1,

I haven't been around here long enough to know... Are you
the poster formerly known as Grapes?

-- Jeff, in Minneapolis

grav
2006-Sep-11, 12:00 AM
Does a phonograph turntable rotate? Does a thrown football
rotate? Do you ever turn from left-to-right or right-to-left?
Have you ever turned a mug or coffe cup around so the handle
would be on the other side? Have you ever indicated "yes" or
"no" by nodding or shaking your head? Have you ever flipped
a light switch on or off? Have you ever seen the hands on a
clock move?

-- Jeff, in Minneapolis
Each of those things rotate, and they have a centrifugal force depending on the distance from the axis. The Earth also rotates. But if one were to stick a tack on the outer part of the record or football, would the tack be rotating or revolving?

PhantomWolf
2006-Sep-11, 12:02 AM
If we took away the Earth, it still would not necessarily mean
the moon is rotating.

If we remove the Earth from the picture, all that happens is that the Moon would settle into a more circular orbit about the Sun (The Sun has 2x the gravitational effect on the Moon as the Earth does.) It would indeed orbit the Sun, but a person sitting on the surface of the Moon would see the sun rise and set 13 times. Unless you are going to go Lunacentric on us, that means the Moon is rotating.

It would be like a car going round in a circle. Would you then say the car is rotating, or revolving?

If it wasn't then it wouldn't be able to do it. Consider this. Make the circle smaller until that circle is smaller than the car, now what does the motion look like?

grant hutchison
2006-Sep-11, 12:13 AM
It would be like a car going round in a circle. Would you then say the car is rotating, or revolving?It's rotating and revolving. If it's not rotating, how come it can end up pointing in a different direction when it stops?

Grant Hutchison

grav
2006-Sep-11, 12:13 AM
I could be wrong in all of this. I don't know. It all depends on whether or not the moon has an equatorial bulge. I don't think it should. Think of it this way. For an object to be rotating, its individual parts should revolve around each other and a common axis. Revolving around each other is the important concept here, however. Think about a rotating object. What do you visualize? I'm sure you see its individual parts revolving around each other over and over, right? Now visualize the moon revolving. What do you see now when you look at the individual parts. If it were rotating, its front and back should revolve around each other. But trace out the orbit of the moon with the front and back as individual points in space. The two never overlap. The back of the moon sweeps out a circle that is slightly larger than that of the front. The two paths never meet, and so the moon is not rotating. I'm still not through thinking about all of this, though.

grav
2006-Sep-11, 12:19 AM
It's rotating and revolving. If it's not rotating, how come it can end up pointing in a different direction when it stops?

Grant Hutchison
By revolving. Like I said, it all depends on our specific definitions. If what you stated is used as the definition, then revolution is rotation, although rotation would not be revolution, since a body could still have extra spin. I just think that bodies that produce a centrifugal force due to their rotation would be a good physical way to define it, and lose any potential confusion. If a centrifugal force around the equator is caused also by revolving, then your definition definitely sticks.

grant hutchison
2006-Sep-11, 12:23 AM
I could be wrong in all of this. I don't know. It all depends on whether or not the moon has an equatorial bulge.In theory, the moon would have an oblateness of around 0.0003 because of its slow rotation. That's a difference of around 500m between equator and pole, something you're not going to detect against the noise of surface relief.
Really, you just need to notice that a person standing on the moon would see the sky pass overhead once every 27.3 days: the moon points in a different direction from day to day. So it rotates.

Grant Hutchison

grant hutchison
2006-Sep-11, 12:26 AM
If a centrifugal force around the equator is caused also by revolving, then your definition definitely sticks.Didn't you just effectively work out that was the case on another thread (http://www.bautforum.com/showpost.php?p=822996&postcount=118)?

Grant Hutchison

grav
2006-Sep-11, 12:31 AM
In theory, the moon would have an oblateness of around 0.0003 because of its slow rotation. That's a difference of around 500m between equator and pole, something you're not going to detect against the noise of surface relief.
Really, you just need to notice that a person standing on the moon would see the sky pass overhead once every 29.5 days: the moon points in a different direction from day to day. So it rotates.

Grant Hutchison
If this oblateness really does exist, then I'm wrong. By my own definition, the moon rotates. If it does not, and the moon is still considered to rotate because of the definition you gave earlier, which appears common, then I guess I would just have to use a different definition other than rotation for what I consider to be its effects, such as centrifugal force from the center axis. Maybe spin?

grav
2006-Sep-11, 12:50 AM
An entire system rotates, not its constituent parts, I would think. The Earth-moon system is rotating about the barycenter. The Earth and moon are revolving. If the moon were considered to be rotating, then so is any small object placed on its surface. By the definition that is being used, since the solar system is rotating, then everything in it is rotating as well, which for some reason is considered the same as revolving, just because each object turns at the same rate as it revolves. In that case, you are rotating. I am rotating. Everything is rotating. The sun would also rotate as it revolves the galaxy. And so then would everything in the solar system. Everything is always rotating. What difference is this from revolving? I would just think a simpler definition (and more precise) would be one where rotation only exists for an entire system that revolves or rotates (the definition would be the same in this case) around its own central axis. And centrifugal force all the way around its equator would be evidence of this.

grant hutchison
2006-Sep-11, 12:51 AM
A way to visualize it.
Draw a circle representing the moon, some distance from the centre of a rotating disc. Draw a vector from the centre of gyration to the centre of the moon; draw a vector from the centre of gyration to some random point on its surface; draw a radius vector connecting the centre of the moon to the previously selected random point. You have a triangle, with one side a radius of the moon, one representing the distance of the moon's centre from the centre of gyration, and one the distance of the point on its surface from the centre of gyration.
Centrifugal force increases linearly with the distance from the centre of gyration. Therefore, the centrifugal force vectors at the centre of the moon and at the random point on its surface will be in the same ratio, and at the same angle, as the distance vectors we've already drawn. So the subtraction of the centre force vector from the surface force vector will draw a vector triangle that is similar to the distance vector triangle we've already drawn. The resultant will have the same direction as, and a magnitude proportional to, the moon's radius vector as we drew it in our distance triangle.
And that will apply for every point around the equator of the moon. The force vector subtraction will give a resultant vector that is directed radially from the centre of the moon, and is proportional to the distance from the centre of the moon.
So looking at the centrifugal forces generated by the moon's synchronous revolution around the Earth gives us an equatorial bulge which is identical to that produced by a rotating moon. It's just a change of coordinates: the moon rotates.

Grant Hutchison

Jeff Root
2006-Sep-11, 01:15 AM
Each of those things rotate, and they have a centrifugal force
depending on the distance from the axis. The Earth also rotates.
But if one were to stick a tack on the outer part of the record
or football, would the tack be rotating or revolving?
Both, obviously.



I could be wrong in all of this.
What an understatement!



I don't know.
Well, your first statement is correct, anyhow. You aren't
wrong about everything!



It all depends on whether or not the moon has an equatorial bulge.
No it doesn't. Whether the Moon has an equatorial bulge or not
doesn't tell you anything I don't already know. ! And again, !



I don't think it should.
Of course it should. It is rotating and it is not perfectly
rigid, so it deforms. Whether the bulge is measurable, I don't
know, but it must be very small.



Think of it this way. For an object to be rotating, its
individual parts should revolve around each other and a common
axis. Revolving around each other is the important concept here,
however. Think about a rotating object. What do you visualize?
I'm sure you see its individual parts revolving around each other
over and over, right?
No, at the moment I imagine my right arm -- which I'm using to
type -- rotating around the elbow joint, by maybe ten or twenty
degrees, and then rotating back to the starting position. And
now I'm imagining rotating my head by about 75 degrees, and
back again. I'm not Linda Blair.



Now visualize the moon revolving. What do you see now when you
look at the individual parts. If it were rotating, its front and
back should revolve around each other.
Which they do.



But trace out the orbit of the moon with the front and back as
individual points in space. The two never overlap. The back of
the moon sweeps out a circle that is slightly larger than that
of the front. The two paths never meet, and so the moon is not
rotating.
HaHaHaHaHaHa!!!!



I'm still not through thinking about all of this, though.
Have you even begun? Think about the two circles you just traced!

-- Jeff, in Minneapolis

grav
2006-Sep-11, 01:24 AM
A way to visualize it.
Draw a circle representing the moon, some distance from the centre of a rotating disc. Draw a vector from the centre of gyration to the centre of the moon; draw a vector from the centre of gyration to some random point on its surface; draw a radius vector connecting the centre of the moon to the previously selected random point. You have a triangle, with one side a radius of the moon, one representing the distance of the moon's centre from the centre of gyration, and one the distance of the point on its surface from the centre of gyration.
Centrifugal force increases linearly with the distance from the centre of gyration. Therefore, the centrifugal force vectors at the centre of the moon and at the random point on its surface will be in the same ratio, and at the same angle, as the distance vectors we've already drawn. So the subtraction of the centre force vector from the surface force vector will draw a vector triangle that is similar to the distance vector triangle we've already drawn. The resultant will have the same direction as, and a magnitude proportional to, the moon's radius vector as we drew it in our distance triangle.
And that will apply for every point around the equator of the moon. The force vector subtraction will give a resultant vector that is directed radially from the centre of the moon, and is proportional to the distance from the centre of the moon.
So looking at the centrifugal forces generated by the moon's synchronous revolution around the Earth gives us an equatorial bulge which is identical to that produced by a rotating moon. It's just a change of coordinates: the moon rotates.

Grant Hutchison
Actually, we just went through that in the Spinning Moon thread. The centrifugal force from revolution does not just apply along the equator, as to produce an equatorial bulge. Interestingly enough, it applies to every point on its surface equally, so that the entire moon will expand slightly and uniformly in all directions. The equatorial bulge, however, would only be created by a direct local rotation of the moon itself. Since the period of revolution is the same as its rotation, the centrifugal force around the equator would be the same magnitude as for the uniform expansion, but we would only observe that for the equatorial bulge. That is, if it really rotates. :)

PhantomWolf
2006-Sep-11, 01:29 AM
I could be wrong in all of this.

Personally I think you should have stopped there.

Look at it this way. If you go and stand on the north pole of the Earth and look at the stars, you'll see them describing a circle, right? This is because the Earth rotates about its axis. Now, what happens when we put a camera on the Moon's North Pole and take a time elaspe footage of the stars? How do they move?

If what you stated is used as the definition, then revolution is rotation

No, you can have an object that revolves and doesn't rotate, If I take a car and put it on a platform that sits on bearings so that it can rotate freely from the arm that it's attatched too, then I spin the arm about, the car will revolve about the central point, but because the platform and the arm are not locked togther, the platform won't rotate, so the car stays pointing in the same direction the entire time through out the revolution. only when I give the platform a push will the car turn and point in the opposite direction, showing the car has to rotate as well as revolve if it ends up pointing towards different places throughout it's reveloution.

I've illustrated this in the figures attached below. In the first, our plane is free of the revolution of the arm, meaning that it can remain pointing towards the top of the image throughout the entire revolution. Here we have revolution, but no rotatation. In the second I have given the plane a small push so that it is rotating at the same speed as it revolves meaning the tail is always pointing towards the centre of the arm. In the Third picture I have locked the plane onto the arm so it can't rotate freely. But note that the result is EXACTLY the same as if it had been given a slight rotation!!!!!

grav
2006-Sep-11, 01:31 AM
Originally Posted by grav
But trace out the orbit of the moon with the front and back as
individual points in space. The two never overlap. The back of
the moon sweeps out a circle that is slightly larger than that
of the front. The two paths never meet, and so the moon is not
rotating.

HaHaHaHaHaHa!!!!




Originally Posted by grav
I'm still not through thinking about all of this, though.

Have you even begun? Think about the two circles you just traced!

Having fun with this, are you? If the moon were rotating, the points on either side should trace out each other's paths. Each should meet up where the other one used to be after half the period. But with revolution, they don't.

PhantomWolf
2006-Sep-11, 01:38 AM
Having fun with this, are you? If the moon were rotating, the points on either side should trace out each other's paths. Each should meet up where the other one used to be after half the period. But with revolution, they don't.

You're still not getting it, thus isn't a case of either/or. It can do both, one or neither. In the moon's case it's doing both, as is the earth. By your own logic the Earth isn't rotating because the point on one side doesn't arrive at the same location in space as the point on the other side was 12 hours previous because the Earth is orbiting the Sun.

grant hutchison
2006-Sep-11, 01:51 AM
Actually, we just went through that in the Spinning Moon thread.I know. I pointed that out to you (http://www.bautforum.com/showpost.php?p=823126&postcount=55).

The centrifugal force from revolution does not just apply along the equator, as to produce an equatorial bulge. Interestingly enough, it applies to every point on its surface equally, so that the entire moon will expand slightly and uniformly in all directions.It won't. You explored in two dimensions and seem to have made the wrong assumption when extending to three. But recall that the centrifugal force has an "axis of origin" in 3D, rather than a point of origin. Now consider the moon's north and south poles, which are precisely as far from the axis of gyration as is the centre of the moon. The centrifugal force vector at the poles is therefore identical in direction and magnitude to the force at the moon's centre: no net force away from the centre.
You should now be able to extend this to latitudinal slices north and south of the equator and see where the equatorial bulge comes from.

Grant Hutchison

grav
2006-Sep-11, 01:53 AM
I could be wrong in all of this.

Personally I think you should have stopped there.

Look at it this way. If you go and stand on the north pole of the Earth and look at the stars, you'll see them describing a circle, right? This is because the Earth rotates about its axis. Now, what happens when we put a camera on the Moon's North Pole and take a time elaspe footage of the stars? How do they move?
You're probably right. Actually, I feel more at home in the ATM section anyway. I know how rotation is being defined, and I understand that all sides of the moon will be seen by a distant observer once per revolution. But then, this is the definition of revolution to begin with, at least when two bodies are connected or tidal locked. So what would be the difference between revolution and rotation otherwise in this case? Rotation is a convenient way to describe it, however. And I see I will no success attempting to get anyone to see it any differently. And it would be ridiculous to think I could change the definition as it is obviously described by so many people, even myself most of the time. So rather than subject myself to any more of this, although it's been fun, I guess :wall: , I will concede, and will call it spin from now on. Or if that also has the same definition, then I will just be forced to elaborate, and call it the rotation of a body around its center of mass, or central axis.


If what you stated is used as the definition, then revolution is rotation

No, you can have an object that revolves and doesn't rotate, If I take a car and put it on a platform that sits on bearings so that it can rotate freely from the arm that it's attatched too, then I spin the arm about, the car will revolve about the central point, but because the platform and the arm are not locked togther, the platform won't rotate, so the car stays pointing in the same direction the entire time through out the revolution. only when I give the platform a push will the car turn and point in the opposite direction, showing the car has to rotate as well as revolve if it ends up pointing towards different places throughout it's reveloution.


I meant the revolution is the same as rotation when they have the same period. That's why I said it doesn't apply the other way around. It's one of those logic things I should have studied up on. Rotation would not be the same as revolution, however, because, an extra rotation of the object itself could still exist. If the period of the extra rotation is exactly the same as the revolution but in the opposite direction, then a distant observer will see the revolution, but no rotation.

PhantomWolf
2006-Sep-11, 02:00 AM
If the period of the extra rotation is exactly the same as the revolution but in the opposite direction, then a distant observer will see the revolution, but no rotation.

I think you have this around the wrong way. If the rotation = revolution, then an observer in the middle will see revolution but no rotation, a distant observer will see both. If the rotation <> Revolution, then both observers will see both and if rotation = 0 then the observer in the middle will see both rotation and revolution, while the distant observer only see revolution.

Jeff Root
2006-Sep-11, 02:01 AM
Grav,

Have you ever seen a meat grinder, or a grindstone which is
turned by a handcrank? Or a wind-up Victrola record player
with a hand-crank? Or just about anything with a hand-crank
where you hold onto a handle and make the crank go around
and around in a circle? The handle is loosely attached to the
crank arm, so as you turn the crank, the arm rotates, but
the loose part that you are holding in your hand does not
rotate. It revolves around the center of rotation of the
crank arm, without rotating.

-- Jeff, in Minneapolis

grav
2006-Sep-11, 02:03 AM
Originally Posted by grav
The centrifugal force from revolution does not just apply along the equator, as to produce an equatorial bulge. Interestingly enough, it applies to every point on its surface equally, so that the entire moon will expand slightly and uniformly in all directions.


It won't. You explored in two dimensions and seem to have made the wrong assumption when extending to three. But recall that the centrifugal force has an "axis of origin" in 3D, rather than a point of origin. Now consider the moon's north and south poles, which are precisely as far from the axis of gyration as is the centre of the moon. The centrifugal force vector at the poles is therefore identical in direction and magnitude to the force at the moon's centre: no net force away from the centre.
You should now be able to extend this to latitudinal slices north and south of the equator and see where the equatorial bulge comes from.

Grant Hutchison
Well, I did that for points along the equator, you're right. But the points on either side of the equator that cross the plane through the center that is perpendicular to the Earth are symmetrical all the way around its circumference from top to bottom as well, as far as the amount of centrifugal force that is applied and its direction, so the same centrifugal force applies equally on that plane as well. So we have an equal centrifugal force applied all the way around the equator and all the way around the circumference perpendicular to it. Don't you think that means it applies to all points on the surface equally, or do you think I need to calculate for more points?

grav
2006-Sep-11, 02:24 AM
I think you are all missing the point of what I'm trying to say here. I have gotten a little away from it myself. What I am saying is that it should not matter where one is standing or how one perceives what is going on. It is all relative. And I am not saying that the moon is not rotating simply because we don't see it doing so from our perspective on Earth, or anything else like that. What I am saying is that if the moon is really rotating, then we should see the effects of that rotation, which is a centrifugal bulge around its equator. It all goes back to the thread, Is rotation absolute? (http://www.bautforum.com/showthread.php?t=43314). If the universe were completely void of all reference points, how would we know an object is rotating? The only way would be through the centrifugal force that is applied along its equator.

Jeff Root
2006-Sep-11, 03:13 AM
it should not matter where one is standing or how one perceives
what is going on. It is all relative.
Those two assertions contradict each other.

If things are measured relative to one another, then the
measurements depend on where one is and how one is moving.

-- Jeff, in Minneapolis

grav
2006-Sep-11, 03:49 AM
Those two assertions contradict each other.

If things are measured relative to one another, then the
measurements depend on where one is and how one is moving.

-- Jeff, in Minneapolis
It would seem so, wouldn't it? But then what is rotation relative to if we have no points of reference by which to compare? The exact, absolute rate of rotation of a body can be determined, however, by the centrifugal acceleration that is measured around its equator.

[EDIT- Oh, and I meant that how one perceives the situation is relative, but centrifugal acceleration is absolute. This makes rotation absolute as well.]

publius
2006-Sep-11, 04:29 AM
It would seem so, wouldn't it? But then what is rotation relative to if we have no points of reference by which to compare? The exact, absolute rate of rotation of a body can be determined, however, by the centrifugal acceleration that is measured around its equator.

[EDIT- Oh, and I meant that how one perceives the situation is relative, but centrifugal acceleration is absolute. This makes rotation absolute as well.]

Grav,

The notion of absolute rotation has clear meaning only in flat space-time, actually. In curved space, such as the Schwarzschild metric deep in a well, observers can rotate relative to each other with neither feeling any Coriolis frame forces.

And, even in a Newtonian picture, you can only measure the absolute rotation by centifugal forces if gravity is negligble. At the equator of the rotating earth, an accelerometer will not measure any centrigual force. It will only measure the "real force" required by the ground to keep the object moving in its "non-inertial orbit". And what it that? It is the difference between mg and mw^2r. If the earth were rotating at a rate so the surface speed at the equator was equal to orbital speed at that radius, one would be weightless, completely inertial and the accelerometer would measure nothing. And one would tend to rotate around one's axis once per day as well, being naturally in what NASA calls "stellar inertial" orbital mode. This is the ultimate limit to rotation speed. Gravity alone cannot hold anything together past that.

ETA: Rotate about your axis relative to the ground is what I meant. One would not be really rotating. One would have to "really rotate" once per day in order to keep the same side toward the ground. NASA has a term which I forget for that orbital mode where the spacecraft is set to rotating so that it keeps the same side toward the earth below at all times which is useful when you want to take pictures or keep other instruments fixed on the earth below. Stellar inertial mode is the non-rotating mode, where the spacecraft's axes remain fixed relative to the stellar background.


-Richard

grav
2006-Sep-11, 05:17 AM
Grav,

The notion of absolute rotation has clear meaning only in flat space-time, actually. In curved space, such as the Schwarzschild metric deep in a well, observers can rotate relative to each other with neither feeling any Coriolis frame forces.

And, even in a Newtonian picture, you can only measure the absolute rotation by centifugal forces if gravity is negligble. At the equator of the rotating earth, an accelerometer will not measure any centrigual force. It will only measure the "real force" required by the ground to keep the object moving in its "non-inertial orbit". And what it that? It is the difference between mg and mw^2r. If the earth were rotating at a rate so the surface speed at the equator was equal to orbital speed at that radius, one would be weightless, completely inertial and the accelerometer would measure nothing. And one would tend to rotate around one's axis once per day as well, being naturally in what NASA calls "stellar inertial" orbital mode. This is the ultimate limit to rotation speed. Gravity alone cannot hold anything together past that.

ETA: Rotate about your axis relative to the ground is what I meant. One would not be really rotating. One would have to "really rotate" once per day in order to keep the same side toward the ground. NASA has a term which I forget for that orbital mode where the spacecraft is set to rotating so that it keeps the same side toward the earth below at all times which is useful when you want to take pictures or keep other instruments fixed on the earth below. Stellar inertial mode is the non-rotating mode, where the spacecraft's axes remain fixed relative to the stellar background.


-Richard
I can't tell if you are agreeing with me or if this is intended as a rebuttal. As far as I can tell, everything you just said proves my point. Rotation is an absolute quantity that can be measured by its centrifugal acceleration. Even if slight, it still exists. If not, then what would this rotation be relative to? Now, of course, it must be relative to something, but what? Well, its axis of rotation, of course. But would that then be considered stationary or rotating as well? Obviously it must be relative to something also. You mentioned something about space-time. I believe that should be it. Whether you consider it to be related to Mach's principle, or relative to an ether, or a neutrino medium, or the fabric of space-time, it must be relative to the universe in general. Maybe not the entire universe, but at least the local region. In other words, there should be something about space (or space-time), at least in the immediate region of the universe, that causes the effect of centrifugal acceleration on a rotating object. But then, that would mean that space itself is absolute, which would go against the laws of relativity, I'm sure. Although, the speed of light could be considered an absolute limit, but it's really not. It's relative, as all speed is. Also, centrifugal acceleration is directly determined by the square of the speed divided by the distance from the center. It includes no universal constants that can be said to be determined by any local values of a medium or space-time. It does have a limit, however, and that is determined by the centripetal force, or gravity in this case. So it really just serves to lessen gravity. Do you consider this to be a GR effect of some kind?

astromark
2006-Sep-11, 06:06 AM
Does the moon rotate ?

Does the moon rotate ?

Does the . . . . . YES. It does.

The moon is orbiting the sun approximately once a year. In that year it turns around 13 times.
The moon is accompanied by the Earth in this annular dance. It turns 365 times in that same year. It is said that the moon is orbiting the Earth. This might well be so. It would appear that many Earth bound inhabitants are not seeing the other side of the moon. I could sagest that is simpler than this just to say, . . yes. or do I mean no,. No I mean yes. don't I?

grant hutchison
2006-Sep-11, 11:27 AM
Well, I did that for points along the equator, you're right. But the points on either side of the equator that cross the plane through the center that is perpendicular to the Earth are symmetrical all the way around its circumference from top to bottom as well, as far as the amount of centrifugal force that is applied and its direction, so the same centrifugal force applies equally on that plane as well.No, that's wrong. The centrifugal force vectors are always in a plane parallel to the equatorial plane, since they are always perpendicular to the axis of gyration. So the vertical section from pole to pole doesn't resemble the equatorial section at all, whereas each horizontal section through a line of latitude will be identical to the equatorial section we started with, except for its smaller radius.
In particular, how could centrifugal force have an expansive effect at the poles, as you suggest? It would need to be working parallel to the rotation axis.

Grant Hutchison

LayMan
2006-Sep-11, 12:03 PM
Wow, didn't know I was going to stir up so much commotion with my 'simple' question...

Let's see, there is off course the matter of semantics...

Okay, let's get semantical, shall we: what do I mean when I say that an object is rotating (versus revolving (versus orbiting))?

Let's put it like this: take an old vinyl record and put it on the table. Now drive a nail through the hole in the middle. Any force applied on the disc will have the effect (if any) that the record will 'move around' the nail, without having it's barycenter shifting with respect to the table, right? I mean, the nail fixes the center of the disc firmely into place on the table, right? If I was standing so far away from the table that I would still see the disc, but not the details like its label or the spiral groove, I would say that the disc was not moving, right? After all, I would see the disc lying "motionless" onto the table, since I would not be able to see/detect its spinning motion, not until I got a bit closer so I could make out the details on its surface. I'll call this rotation then...

Now, pick up the disc, attach a small clip on the nail and clip the record with the edge into the clip. Now, any force that will cause the disc to move, will make the disc 'turn around' the nail, right? Notice that now, the discs barycenter does move with respect to the table. We'll call that revolving.

Now, finally, remove the clip and tie a rope - say, 20 meters long - loosely around the nail and attach the other side of it to the record. Give the record a good swing. It will now 'move around' both the nail and the table, right? We'll call that orbiting.

Now, here's the point: I could call revolving just a special case of orbiting, no? I mean, every orbiting body has an orbital plane, right? What if I were to say that revolving is actually just a special case of orbiting, with an orbital plane of 0 meters diameter and a surface of 0 square meters? And what if we consider rotation as just a special case of revolving, were the center of revolving just happens to be in the middle of the object?

Doesn't really makes things easier, does it? Because, we would ultimately consider rotation as being a very special case of orbiting, where the orbital plane somehow got 'swallowed' by the barycenter of the object.

So semantics aren't going to solve it. I believe we need those three separate definitions of motion.

So I don't think that revolving invariably implies rotation, as some here have said. Let's put it like this: if we state that the Earth has three distinct movements, namely an orbit (around the sun), a revolution (around the gravitational point of equilibrium between the Earth and the Moon - which lies several kilometers below the Earths surface) and a rotation, caused by it spinning around its own gravitational center, then what about the Moon?

We know it orbits the Earth. No doubt about that. We also know it must be revolving around the shared point of equilibrium. Otherwise, that point wouldn't be there, right? Now, how about rotation then. Does the moon spin around its own axis? I used to think not when I started this thread, but I must say, my opinion has shifted somewhat towards saying: "Yes, it must be, otherwise it couldn't keep the same side facing the Earth all the time. After all, think back to my example with the 2 guys on Orion (or, better, on a planet in the constellation of Orion): I thought it proved that the moon could not be rotating, since no rotational energy was being applied. However, suppose now that we again shoot another moon away, but this time in such a way that the moon would not get stuck in an orbit, but would 'bent round' Earth, make a 180° turn and come back towards the planet in the Orion constellation where we are. At the beginning of its trajectory, the mirror was facing towards us, right? But after its return, the mirror is now on the other side of the moon. So somehow, the moon must have had a rotation of 180°. After all, if I place a ball with a label on it on the table, with the label facing the door, go out for a walk, return through the same door and notice the label now is on the opposite side of the ball, facing away from the door while the position of the ball itself didn't change with respect to the table, I would be inclined to say "Hey, somebody must have come in here while I was away and rotate the ball 180°", wouldn't I now?

The real reason why I posted this question in the first place, was this: I read somewhere that the Moon does not have a magnetic field. So I started to wonder: the planes on the surface which are generally called 'seas' were created by lava flows, right? So, what about the inner core of the Moon? Is it still liquid? Or a cold solid? IF it were liquid and hot, AND the Moon was rotating, shouldn't that rotation create an electromagnetic field, even if it would be very weak?

After all, the reason why Venus does not have a magnetic field (or is said not to have one, or one that is so weak, we wouldn't be able to detect it), is that it spins very slowly (about once every 243 Earth days). But if you're gonna say that the Moon rotates once per revolution, then that's once every 29 Earth days, right? That's almost 10 times as fast... Or would that still be insufficient to create such a field?

grant hutchison
2006-Sep-11, 12:18 PM
Don't you think that means it applies to all points on the surface equally, or do you think I need to calculate for more points?I think you're maybe doing too many calculations and not enough visualization of the problem. It doesn't require calculations to see what the vector field is going to look like, if you visualize things in the geometric way I suggested.
Over on another thread (http://www.bautforum.com/showpost.php?p=823354&postcount=130) I've just posted links to some diagrams which will perhaps make it clearer where you're going wrong when you generalize the equatorial situation to all possible great circle sections through the moon. What happens is that the equatorial situation generalizes instead to small latitudinal sections (parallel to the equator), which gives you a completely different picture when you assemble those latitudinal sections into a section through a great circle of longitude.

Grant Hutchison

LayMan
2006-Sep-11, 01:23 PM
I think you're maybe doing too many calculations and not enough visualization of the problem. It doesn't require calculations to see what the vector field is going to look like, if you visualize things in the geometric way I suggested.
Over on another thread (http://www.bautforum.com/showpost.php?p=823354&postcount=130) I've just posted links to some diagrams which will perhaps make it clearer where you're going wrong when you generalize the equatorial situation to all possible great circle sections through the moon. What happens is that the equatorial situation generalizes instead to small latitudinal sections (parallel to the equator), which gives you a completely different picture when you assemble those latitudinal sections into a section through a great circle of longitude.

Grant Hutchison

Maybe we are indeed overcalculating the darn thing...

So forget about semantics for a while and let's get back to our reference frames...

If we were to say that there is a difference between observing a rotation from within the objects orbital plane (the 'relative' reference frame, since the motion is put out against another motion from an observers point of view here on Earth), versus the observation from outside the orbital plane (for instance, as viewed from the constellation of Orion, which we would call the absolute reference frame since that observers point of view would be: "I don't care about orbits, I'm only interested in rotation"), would that make everybody happy?

(Actually, that still leaves us with the reference frame of someone standing on the surface of the Moon itself, off course... :shhh: ).

grant hutchison
2006-Sep-11, 01:47 PM
... would that make everybody happy?I don't think it's about making people happy, it's about how the world works.
And it's really simple.
Rotation and revolution are two different movements. An "orbit" is a just a gravitationally determined kind of revolution. An object may, in principle, revolve without rotating, or rotate without revolving.
Rotation generates pseudoforces within the rotating frame which tend to create an equatorial bulge. An object which revolves around another and turns one face always towards it must rotate in order to do that. That rotation will produce pseudoforces maximal around its equator, as previously described, which meets grav's desire to see an equatorial bulge.

The moon has a very small metallic core, if it has one at all, and that core may no longer be liquid. So there are a variety of reasons, including slow rotation, why the moon does not have a global magnetic field at present.
Some of the moon rocks brought back by Apollo appear to have cooled in a magnetic field, however, suggesting that the moon perhaps exhibited global magnetism 3-4 billion years ago. At that time a metallic core might still have been molten, and the rotation may have been faster.

Grant Hutchison

hhEb09'1
2006-Sep-11, 03:03 PM
Thanks. :) But the centrifugal force caused by rotation would still be completely separate from that of the revolution. Sure, they both have the same value because the period of revolution and rotation is the same, but that part that would be created by rotation would only cause a bulge around the equator, which is in addition to the uniform centrifugal bulge over its entire surface caused by its revolution.The short answer is: revolution does not cause a centrifugal bulge. By the way you set up the calculation, the points rotate about their common center and relative to that center they experience a centrifugal force. That's the the value "y" that you computed. There is not two different centrifugal forces--only one, the one you computed, and it's due to the rotation. It doesn't matter from which point you compute it.

A similar thing happens when you try to compute the torque on an object. It doesn't matter which point you choose to use as the center point--the calculation results in the same value. You can use the center, or either end point, or a point completely off the object--kinda like you did using the barycenter.

I haven't been around here long enough to know... Are you
the poster formerly known as Grapes?Sorta. It was a nickname. My former username have always been listed in my profile, and share the same avatar.
The real reason why I posted this question in the first place, was this: I read somewhere that the Moon does not have a magnetic field. So I started to wonder: the planes on the surface which are generally called 'seas' were created by lava flows, right? So, what about the inner core of the Moon? Is it still liquid? Or a cold solid? IF it were liquid and hot, AND the Moon was rotating, shouldn't that rotation create an electromagnetic field, even if it would be very weak?

After all, the reason why Venus does not have a magnetic field (or is said not to have one, or one that is so weak, we wouldn't be able to detect it), is that it spins very slowly (about once every 243 Earth days). But if you're gonna say that the Moon rotates once per revolution, then that's once every 29 Earth days, right? That's almost 10 times as fast... Or would that still be insufficient to create such a field?It's good that we get to the real reason by page five, that's a lot sooner than most. :)

The rotation doesn't really cause the magnetic field per se--a case in point is the Earth's magnectic field, which rotates with the Earth (I reset my car's compass just this morning, as it used to live in Texas). The rotation might influence the magnetic field but probably a necessary condition for a strong magnetic field is a fluid core, which the moon does not seem to have, as grant pointed out.

grav
2006-Sep-11, 05:14 PM
Let's see, there is off course the matter of semantics...

That's true. It's all a matter of semantics, and I never should have started in with that. But since I did, I guess we can all agree that the moon turns once per revolution. We can define that as rotation if we wish. So I guess what I am referring to is the extra rotation, which is the one that causes an equatorial bulge. So normal rotation is caused by the revolution. The extra rotation would be anything more or less than that which differs from the rate of revolution. If we give rotation a value of 1, which is equal to its revolution, which is also 1, then there is no extra rotation, as with the moon. If the extra rotation is -1, then there is no rotation to a distant observer, but only to an observer on Earth, and an equatorial bulge will form.




Originally Posted by grav
Don't you think that means it applies to all points on the surface equally, or do you think I need to calculate for more points?

I think you're maybe doing too many calculations and not enough visualization of the problem. It doesn't require calculations to see what the vector field is going to look like, if you visualize things in the geometric way I suggested.
Actually, that is exactly what I am doing, geometrically. I may have only calculated for points on the equator (which may be what is causing the confusion about where the centrifugal forces are aimed). Since all of these points are calculated using the revolutionary centrifugal force, it is initially aimed straight out away from the Earth and through the moon. In other words, it is aimed in such a way that it is pushing the moon straight away from Earth. The points I calculated are for the sine of the angle from the original direction of the force minus the cosine of the angle from the center of the moon. This gives the amount of the force aimed directly away from the center, which would in fact come out to be the same around the equator as if the moon were rotating instead of revolving, but that is only because the period would be the same.

With this much established, however, take those two points on either side of the equator on the circumference of the moon as viewed from Earth. This is purely geometrical. Since the initial centrifugal force which is caused by the revolution, which is the only one I am considering here, is aimed straight out from the barycenter, then those two opposite points on the equator are geometrically the same as all other points around the circumference of the moon as seen from Earth, and the force will be computed in exactly the same way with exactly the same result, and will have exactly the same force applied to them from the direction of the center of the moon as well. This circumference will cut through the equator, around the side of the moon, across the north pole, down the other side, and across the south pole as well, with the same force applied all the way around, just like around the equator. These two planes are perpendicular to each other and there is only one left. I figure that along this third circumference, the force is also the same where it crosses the equator and the north and south poles, so it should also be the same all the way around, and so all points on the surface of the moon would experience the same force, and the entire moon expands. All points within the moon will also experience this force, and will be proportional to the distance from its center. So it is not just an equatorial bulge we are considering here, it is the expansion of the entire moon, equally in all directions. And any centrifugal force caused by rotation hasn't even been considered yet. If it were, and the "extra" rotation is the same as the revolution, then the same quantity of force would be applied as well, but only along the equator, and will produce an equatorial bulge only, in addition to the initial expansion caused by the revolution.


The short answer is: revolution does not cause a centrifugal bulge. By the way you set up the calculation, the points rotate about their common center and relative to that center they experience a centrifugal force. That's the the value "y" that you computed. There is not two different centrifugal forces--only one, the one you computed, and it's due to the rotation. It doesn't matter from which point you compute it.
I'm afraid not. I only computed for revolution. The value for y only comes out the same if the period of revolution is the same for rotation. But that does not make them the same thing. They are additive. The revolution causes the entire moon to expand and an equal "extra" rotation would create an equatorial bulge. Since the moon has no "extra" rotation, the force is only that for revolution, which is 1y, the same for if the moon was rotating, that is true, but it is applied over the entire surface due to revolution. In other words, if the moon and Earth were not revolving around each other, but were just sitting at some distance away from each other, and a repulsive force was applied, as with, say, a negative gravity, that increases with distance as centrifugal force does, then the entire moon would expand in the same way. Of course, this means that gravity causes it to contract, although to a different degree since it decreases with the square of the distance. This means that an object sitting at rest in a gravitational field will contract slightly and reactions in the atoms themselves will occur more quickly because there is less distance to travel between them, causing an object to "age" faster than an object in free space, in accordance with GR. I will have to look into that some more.

hhEb09'1
2006-Sep-11, 05:31 PM
I'm afraid not. I only computed for revolution. The value for y only comes out the same if the period of revolution is the same for rotation.Which means, if you think about it, that the moon is rotating in your example :)

If you fixed your example so that the moon was not rotating, the "y" would disappear. Magic! :)

This means that an object sitting at rest in a gravitational field will contract slightlyYou computed that as well--a 2x expansion along the radial line, and an x contraction.
and reactions in the atoms themselves will occur more quickly because there is less distance to travel between them, causing an object to "age" faster than an object in free space, in accordance with GR. I will have to look into that some more.The GR effect has more to do with difference in potential, than the pure difference in force. For instance, an object in a constant gravity field (every point experiences the same gravitational force, in magnitude and direction), would have points at a lower potential and experiencing a different GR effect.

grant hutchison
2006-Sep-11, 06:04 PM
Since all of these points are calculated using the revolutionary centrifugal force, it is initially aimed straight out away from the Earth and through the moon.This is wrong. Centrifugal force is always at right angles to the rotation axis, not "straight away from the Earth". See my response on the other thread (http://www.bautforum.com/showpost.php?p=823529&postcount=135). Might I suggest that you and I restrict ourselves to an exchange on that thread? All this dodging back and forth saying the same thing twice is getting ridiculous.

Grant Hutchison

Jeff Root
2006-Sep-11, 06:12 PM
Okay, let's get semantical, shall we: what do I mean when I say
that an object is rotating (versus revolving (versus orbiting))?
Rotation is a change in orientation, or attitude.

Revolution is a certain kind of change in location, or position.

Orbiting is in general a synonym for "revolving", but in
astronomy it means being in a trajectory which is primarily
determined by a single gravitational source. It is possible
for a body to be in several different orbits simultaneously,
with those orbits determined by different gravity sources.
An Apollo spacecraft orbited the Moon; the spacecraft and the
Moon orbit the Earth; the spacecraft, the Moon, and the Earth
orbit the Sun; the spacecraft, the Moon, the Earth, and the
Sun orbit the center of mass of the Milky Way galaxy.

At considerable length, you described a phonograph record
spinning around its center hole, calling that rotation, and
the same record spinning around a point at its edge, calling
that revolution.

What if I drive a nail through the record partway between the
center and the edge, and then spin it? Would it be rotating
or revolving? If it is a combination of both, how much of
each? How much rotation and how much revolution?



So I don't think that revolving invariably implies rotation, as
some here have said.
I've lost track. Who are you referring to? Not me, I hope.



However, suppose now that we again shoot another moon away, but
this time in such a way that the moon would not get stuck in an
orbit, but would 'bent round' Earth, make a 180&#176; turn and come
back towards the planet in the Orion constellation where we are.
At the beginning of its trajectory, the mirror was facing towards
us, right? But after its return, the mirror is now on the other
side of the moon.
No, there is no reason that the moon would have to rotate.
The mirror could face Orion the whole time. There is no force
being applied to the moon which would change its orientation.



I read somewhere that the Moon does not have a magnetic field.
So I started to wonder: the planes on the surface which are
generally called 'seas' were created by lava flows, right?
So, what about the inner core of the Moon? Is it still liquid?
Or a cold solid? IF it were liquid and hot, AND the Moon was
rotating, shouldn't that rotation create an electromagnetic
field, even if it would be very weak?
The Moon probably has a small solid core.

Generation of planetary magnetic fields is complex, and mostly
beyond my understanding. But it seems to be caused by convection
currents continually separating electric charges and circulating
them. In the Earth, this takes place in the liquid outer core.

-- Jeff, in Minneapolis

PhantomWolf
2006-Sep-11, 09:03 PM
The real reason why I posted this question in the first place, was this: I read somewhere that the Moon does not have a magnetic field. So I started to wonder: the planes on the surface which are generally called 'seas' were created by lava flows, right? So, what about the inner core of the Moon? Is it still liquid? Or a cold solid? IF it were liquid and hot, AND the Moon was rotating, shouldn't that rotation create an electromagnetic field, even if it would be very weak?

After all, the reason why Venus does not have a magnetic field (or is said not to have one, or one that is so weak, we wouldn't be able to detect it), is that it spins very slowly (about once every 243 Earth days). But if you're gonna say that the Moon rotates once per revolution, then that's once every 29 Earth days, right? That's almost 10 times as fast... Or would that still be insufficient to create such a field?

Actaully the reason that both have small magnetic fields are that their cores are very small and nearly solid. The moon does have a very weak magnetic field, though several orders above what was expected. This is probably due to the tidal forces of the earth keeping its core more molten that it would have had without them. Without a moon, Venus, like Mars has cooled to nearly a solid ball and so both have small magnetic fields.

hhEb09'1
2006-Sep-11, 10:44 PM
Actaully the reason that both have small magnetic fields are that their cores are very small and nearly solid. The moon does have a very weak magnetic field, though several orders above what was expected. This is probably due to the tidal forces of the earth keeping its core more molten that it would have had without them. Without a moon, Venus, like Mars has cooled to nearly a solid ball and so both have small magnetic fields.No, I think the core of the moon has pretty much cooled out. The tides on the moon would be stronger, but since the moon doesn't rotate much with respect to the Earth (only librations) there's not much energy available there.

Isn't the lunar magnetic field just remnant magnetism?

PhantomWolf
2006-Sep-12, 12:26 AM
Possible, one of the Apollo missions did experiments and found it has one, and it's stronger than expected, but that's like finding you have three mls of water in your glass when you were predicting it was just half a drop.

hhEb09'1
2006-Sep-12, 04:44 AM
:) how many drops in a mL, again? :)

LayMan
2006-Sep-12, 09:07 AM
"Originally Posted by LayMan
So I don't think that revolving invariably implies rotation, as
some here have said."




I've lost track. Who are you referring to? Not me, I hope.

-- Jeff, in Minneapolis


Well, I may have misformulated that, I just got the impression that the main reason why people here were saying that the Moon does rotate, was because it always shows the same side towards the Earth as it is orbiting... My mistake. I think that basically defines the confusion between saying "the Moon does rotate, because if it wasn't, we would see different parts of it as it revolves" on the one hand and saying "the Moon can't be rotating, otherwise, it wouldn't be able to continually show the same side towards the Earth" on the other hand.

I think it all boils down to how you answer the following question: "If I were to slow down the Earths rotation, when would you say it actually stops rotating?" I'll explain: if I were to continually slow down the rotation of the Earth, without affecting its orbit, then the "earthly year" would continu to count ~365.25 days, while its day and night cycle would change. Right? It would mean that each day and each night would gradually get longer. When do I reach the point where you would say: "Now you've completely stopped the Earths rotation."? I hope you get my point here, I'm not sure I'm not actually adding to the confusion... :D

1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?

If you go for the first option, then the Earth is not rotating (we've agreed that it stopped doing that, right?), however, it does not continuously show the same side towards the Sun. In that case: YES, the Moon rotates.

If you're gonna go for the second option, then the Earth does continuously show the same side towards the Sun, even though it's no longer rotating. In that case: NO, the Moon does not rotate.

Van Rijn
2006-Sep-12, 09:30 AM
1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?


No.



2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?


No. In both cases, the earth continues to rotate relative to other planets and the distant stars.

LayMan
2006-Sep-12, 10:08 AM
Yes, but now you're talking relatively again, no? I thought that there was an absolute reference frame for rotation? :confused:


Layman,

I don't think there's any logical problem here, rather it seems like the difficulty is with what you mean by "rotate." You seem to mean, "rotate with respect to me." Using the example of the hammer, I think it should be quite obvious that to the athlete himself, the hammer doesn't seem to be rotating, because he always sees the same part of it. But what of the spectators? They see the hammer rotating. They also see the athlete rotating.

It's a little like the problem that happens when you're on a moving platform, like a train, for example. It doesn't seem to be moving to you, because you're moving along with it, but it seems to be moving to a person on the ground.

So you have to ask, rotating with respect to what?

In reality, it's a bit more complicated because unlike movement, there is an absolute reference frame for rotation, which is why Foucault's pendulum works. And the moon is actually rotating with respect to that absolute frame.

After all, if the Earths electro-magnetic field is caused by its rotation, what would happen to it when I didn't just slow it down, but would continue applying the "anti-rotational" force until it eventually started to rotate retrogradually (if that's a word...)? If the direction of the rotation would go from clockwise into anti-clockwise (or vice-versa), then at some point it would have stopped, no? I mean, take the dials on a clock, if I needed to adjust for daylight savings time, I would have to turn back the dial one hour. I couldn't possibly do that without stopping it at some point...??

Maybe this all may seem easy for someone well endowed in mathematics, but if you're stuck with just logic, it's a bit harder (yes, I know, logic doesn't cut the mustard...). :sad:

Tog
2006-Sep-12, 12:58 PM
[quote=LayMan;823952
1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?

If you go for the first option, then the Earth is not rotating (we've agreed that it stopped doing that, right?), however, it does not continuously show the same side towards the Sun. In that case: YES, the Moon rotates.

If you're gonna go for the second option, then the Earth does continuously show the same side towards the Sun, even though it's no longer rotating. In that case: NO, the Moon does not rotate.[/quote]

The Earth's rotation stops when I no longer need a drive on my telescope to keep a star in view anywhere in the sky. If the Earth stopped rotating, there would be six months of light followed by six months of dark, but the Sun would "rise" in the west. This is kind of what I tried to show with the diagram on this post (http://www.bautforum.com/showpost.php?p=819257&postcount=18). But, yes, the moon Does rotate in this example because while one side always faces the Earth, that same side does NOT always face the Sun. From the moon, the Sun will "rise once every 27 days or so.

Jeff Root
2006-Sep-12, 04:29 PM
1) Would you say the Earths rotation stops at the point where one
day would last for six months, followed by a night equally long?
No.
Actually, yes. If the Earth were not rotating, most of the
planet would see the daytime period last about half a year,
and the nighttime period last about half a year.

-- Jeff, in Minneapolis

Jeff Root
2006-Sep-12, 04:30 PM
I just got the impression that the main reason why people here
were saying that the Moon does rotate, was because it always
shows the same side towards the Earth as it is orbiting
There are many ways of expressing the idea. Some make use of
what we see; some make use of what an observer somewhere else
would see; some are more abstract and mathematical. They all
basically say the same thing.

We see the same face of the Moon as it goes around us, so we
know that it rotates. An observer on the Moon would see the
stars go around him, so he would know that it rotates. The
orientation of the Moon in space is constantly changing, so
we say that it rotates.



1) Would you say the Earths rotation stops at the point where one
day would last for six months, followed by a night equally long?
Yes.

Someone on one side of the Earth would see the Sun in the sky
for half a year, and the constellation Orion for the other half.
Someone on the opposite side of the Earth would see the Sun in
the sky for half a year, and the constellation Scorpius for the
other half.

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-12, 04:53 PM
Actually, yes. If the Earth were not rotating, most of the
planet would see the daytime period last about half a year,
and the nighttime period last about half a year.

-- Jeff, in Minneapolis

Then it is rotating relative to the sun. But true, it may not be rotating relative to the distant stars.

Jeff Root
2006-Sep-12, 05:26 PM
Then it is rotating relative to the sun.
No, it isn't.

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-12, 05:32 PM
No, it isn't.

-- Jeff, in Minneapolis

Then it is rotating relative to the stars.

Jeff Root
2006-Sep-12, 06:00 PM
Then it is rotating relative to the sun.
No, it isn't.
Then it is rotating relative to the stars.
We are talking about a planet which is not rotating.
It is not rotating relative to the Sun, and it is not
rotating relative to the stars. It is not rotating.

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-12, 06:08 PM
We are talking about a planet which is not rotating.
It is not rotating relative to the Sun, and it is not
rotating relative to the stars. It is not rotating.

-- Jeff, in Minneapolis


I must have missed where it was assumed that the earth had become a rogue planet (no longer in orbit around the sun).

Jeff Root
2006-Sep-12, 06:52 PM
We are talking about a planet which is not rotating.
It is not rotating relative to the Sun, and it is not
rotating relative to the stars. It is not rotating.
I must have missed where it was assumed that the earth had
become a rogue planet (no longer in orbit around the sun).
No, but you are confused about something.

If a planet orbits the Sun, but does not rotate, then the
distant stars will never rise or set, but the Sun will rise
and set once each year.

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-12, 07:51 PM
No, but you are confused about something.

If a planet orbits the Sun, but does not rotate, then the
distant stars will never rise or set, but the Sun will rise
and set once each year.

-- Jeff, in Minneapolis

No confusion. In that case, it is rotating relative to the sun but not rotating relative to the distant stars.

Jeff Root
2006-Sep-12, 08:23 PM
Weird science.

-- Jeff, in Minneapolis

LayMan
2006-Sep-13, 07:01 AM
The title of LayMan's next book:

How to Make Something Which is Extremely Simple Seem to be
Terribly Complicated. 720 pages. Due out in November, 2008.

-- Jeff, in Minneapolis

I think 720 pages ain't gonna cut it. And I may have to postpone the due date... :D

Anyway, if we agree that the Moon does rotate (one rotation per revolution around Earth (and stop saying that the Moon also orbits the Sun, I'm getting that splitting headache again (and no, Jeff Root, I'm not talking about you, I'm referring to the post of Astromark on the third page of this thread)... I know it moves around the Sun as it is bound to follow the Earths orbit, but I wouldn't say the Moon is orbitting the Sun. If it is, then call it a planet, not a moon...)) Oops, missed a bracket, ah, here it is -->), then shouldn't we say that Venus doesn't? Well, frankly, it does,i know... Its day and night cycle doesn't entirely coincide with its tropical year, but wouldn't it be more accurate to say that Venus almost does not rotate, that its actual rotation is closer to zero rotations per revolution?

Don't make me start a new thread called "Does Venus rotate?"!
Anything you'd say over there, could and would be used against you over here. :think:

And vice versa...

hhEb09'1
2006-Sep-13, 07:36 AM
but wouldn't it be more accurate to say that Venus almost does not rotate, that its actual rotation is closer to zero rotations per revolution?Venus is the slowest rotating object in the solar system (http://www.bautforum.com/showpost.php?p=444714&postcount=29), but it still rotates.

Van Rijn
2006-Sep-13, 07:53 AM
Its day and night cycle doesn't entirely coincide with its tropical year, but wouldn't it be more accurate to say that Venus almost does not rotate, that its actual rotation is closer to zero rotations per revolution?

Don't make me start a new thread called "Does Venus rotate?"!
Anything you'd say over there, could and would be used against you over here. :think:

And vice versa...

The question, always, is: Rotate relative to what? You won't have nearly so many problems if you define your terms.

LayMan
2006-Sep-13, 08:09 AM
The question, always, is: Rotate relative to what? You won't have nearly so many problems if you define your terms.

As stated before, when I say an object rotates, I mean rotate relative to that objects center, barycenter as some call it (I think). Not relative to the Sun, the stars or anyone standing anywhere doing anything. If science states that the Earth has a magnetic field with a North and South pole due to it's rotation, then I can hardly imagine it switching on and off just because some guy standing on a distant planet is blinking his eyes...

Does the Moon rotate? Yes... No... Yes... No... (I'll stop blinking now :lol: ).

grant hutchison
2006-Sep-13, 08:23 AM
As stated before, when I say an object rotates, I mean rotate relative to that objects center ...Not a great way to define things, I think.
Doesn't the centre rotate along with the object?
If it does, nothing could ever be said to rotate.
If it doesn't, you need a standard "non-rotating" reference frame for the centre to be stationary in.

Grant Hutchison

hhEb09'1
2006-Sep-13, 09:01 AM
If science states that the Earth has a magnetic field with a North and South pole due to it's rotation,It doesn't. The magnetic field is not due to the rotation of the earth. The magnetic field rotates with the earth.

LayMan
2006-Sep-13, 09:05 AM
It doesn't. The magnetic field is not due to the rotation of the earth. The magnetic field rotates with the earth.


I didn't know that, my mistake...

LayMan
2006-Sep-13, 09:12 AM
I know. I pointed that out to you (http://www.bautforum.com/showpost.php?p=823126&postcount=55).
It won't. You explored in two dimensions and seem to have made the wrong assumption when extending to three. But recall that the centrifugal force has an "axis of origin" in 3D, rather than a point of origin. Now consider the moon's north and south poles, which are precisely as far from the axis of gyration as is the centre of the moon. The centrifugal force vector at the poles is therefore identical in direction and magnitude to the force at the moon's centre: no net force away from the centre.
You should now be able to extend this to latitudinal slices north and south of the equator and see where the equatorial bulge comes from.

Grant Hutchison

You're right, I guess I should have said "relative to the polar axis", but then you're going to point out that that same axis wouldn't exist if the object wasn't rotating, aren't you?

However, I think it's not so accurate either to say that the center itself rotates... After all, a person standing in Belgium can hardly be said to revolve around the Earths center, can he? Not without assuming Belgium would have a tropical climate. Well, I guess if youre now going to draw a cone and call that a vector...

And I know, you didn't actually say that the center was rotating... And even if you had, that would say nothing about a person standing in Belgium... And I wouldn't actually mind if Belgium had a tropical climate (When I said I'm dutch, I meant dutch-speaking, I actually live in Belgium)... And I know, I don't know much about vectors... And...

Whatever, maybe I'll go take another look at the Moon tonight and this time, just simply enjoy the scenery without asking too many questions... :silenced: :o

Van Rijn
2006-Sep-13, 09:18 AM
It doesn't. The magnetic field is not due to the rotation of the earth. The magnetic field rotates with the earth.

It does rotate with the earth, but it isn't due to the earth's rotation (even in part)? That does seem to ignore dynamo models.

LayMan
2006-Sep-13, 09:28 AM
I didn't know that, my mistake...

But admit, science could have fooled me...

Quoted from this link (http://www.gi.alaska.edu/asahi/magfield01.htm):

"Earth's rotation causes its molten metal core to rotate. The movement of the core generates electricity which in turn generates Earth's magnetic field. The other magnetic planets in our solar system are Jupiter, Saturn, Uranus, and Neptune."

I mean, it would be nice if science could at least agree on its own terminology, no?

And yes, I know, they're not saying that the rotation of the Earth is causing the magnetic field, they're saying that the magnetic field is caused by electricity, which is caused by the rotation of the core (center?), which is caused by the rotation of the Earth... And I know, science isn't about being nice... Or making people happy... Or...

There's that splitting headache again... :sad:

ZaphodBeeblebrox
2006-Sep-13, 10:00 AM
You're right, I guess I should have said "relative to the polar axis", but then you're going to point out that that same axis wouldn't exist if the object wasn't rotating, aren't you?

However, I think it's not so accurate either to say that the center itself rotates... After all, a person standing in Belgium can hardly be said to revolve around the Earths center, can he? Not without assuming Belgium would have a tropical climate. Well, I guess if youre now going to draw a cone and call that a vector...

And I know, you didn't actually say that the center was rotating... And even if you had, that would say nothing about a person standing in Belgium... And I wouldn't actually mind if Belgium had a tropical climate (When I said I'm dutch, I meant dutch-speaking, I actually live in Belgium)... And I know, I don't know much about vectors... And...

Whatever, maybe I'll go take another look at the Moon tonight and this time, just simply enjoy the scenery without asking too many questions... :silenced: :o
Eh, Nuthin' Wrong wiith Asking Questions, But If you Want that Head Ache to Go Away, you Miight Wanna Thiink About it This Way ...

Rotation Is Truely One of The Very Few Absolute Motions, Under Certain Circumstances The Effects Can Even Emulate Gravity and Other Forms of Acceleration ...

Moreover, The ONLY Reference Frame Possible for Absolute Motions, Is In Reference to The Rest of The Universe; Thus, When Something Is Said to Rotate, it Does So Relative to The Rest of The Universe, as a Whole, Any Further Questions?

grant hutchison
2006-Sep-13, 10:05 AM
You're right, I guess I should have said "relative to the polar axis", but then you're going to point out that that same axis wouldn't exist if the object wasn't rotating, aren't you?I am. And I'm going to ask how you know the axis doesn't rotate with the body.

Fortunately the Universe gives us an absolute reference for rotation: if we're in a rotating reference frame, we experience pseudoforces (centrifugal, Coriolis). And that non-rotating reference frame turns out to be one that doesn't rotate relative to the Universe. So the natural reference frame is the rest of the Universe, which is why people are so interested in whether something rotates "relative to the stars".

Van Rijn's idea of relative rotation is useful, too, but it does lead to odd answers. In Jeff Root's example of a planet that does not rotate relative to the stars, Van Rijn maintains that it does rotate relative to the sun (because the direction from which the sun illuminates it changes as it moves around the sun, giving a six-month day and a six-month night). This would imply that a planet, not rotating relative to the stars, which went past the sun in a straight line would also be considered to "rotate relative to the sun", because the direction of illumination would change continuously.

Grant Hutchison

LayMan
2006-Sep-13, 10:26 AM
I am. And I'm going to ask how you know the axis doesn't rotate with the body.

Fortunately the Universe gives us an absolute reference for rotation: if we're in a rotating reference frame, we experience pseudoforces (centrifugal, Coriolis). And that non-rotating reference frame turns out to be one that doesn't rotate relative to the Universe. So the natural reference frame is the rest of the Universe, which is why people are so interested in whether something rotates "relative to the stars".

Van Rijn's idea of relative rotation is useful, too, but it does lead to odd answers. In Jeff Root's example of a planet that does not rotate relative to the stars, Van Rijn maintains that it does rotate relative to the sun (because the direction from which the sun illuminates it changes as it moves around the sun, giving a six-month day and a six-month night). This would imply that a planet, not rotating relative to the stars, which went past the sun in a straight line would also be considered to "rotate relative to the sun", because the direction of illumination would change continuously.

Grant Hutchison

Yes, but absolute rotation also implies absolute direction, no? IF it can be said from an object like Earth that it does absolutely rotate, we would be forced to admit that it definitely would do so either clockwise or counter clockwise. It can hardly be doing both. But if you're taking the Universe and the distant stars as point of reference, then any observer above the equatorial plane of our Solar system would claim that its direction of rotation is counter clock wise, while any observer from below would try to correct him?? Then, should we not just say that the safest way of looking at the Moon is from our point of view here on Earth? Actually, skip that, I just realized that my counterpart in Australia would try to correct me too, since to me, he's standing on his head???...

Maybe I'll try that next time I look at the Moon... :)

LayMan
2006-Sep-13, 10:32 AM
Please ignore my previous post, the sheer difference between absolute and relative motion just hit me like a ton of bricks... :D

grant hutchison
2006-Sep-13, 12:44 PM
Please ignore my previous post... No sooner said than done. Consider it ignored. :)

Grant Hutchison

LayMan
2006-Sep-13, 01:08 PM
Thanx :D


I am. And I'm going to ask how you know the axis doesn't rotate with the body.

Grant Hutchison

Because, in my opinion, it can't. An axis - that's to say, in my definition of it - is a line and therefor has only 1 dimension. Space has 3 dimensions (ok, perhaps more, according to GR, Super-string theory,...), a plane has 2, a line has 1 and a point has no dimensions. A line is an array of points, and each individual point itself is dimensionless. A line can therefor revolve around a give point on it, with relation to a 2-dimensional plane or a 3-D space (like the propeller of an airplane), but not rotate "along it's own lenght axis". Because that is not there, it's an axis itself, it can't posses one... In that definition, only 'spacious', 3-D objects can rotate.

Something tells me I'm going to regret having defined it like that... :doh:

grant hutchison
2006-Sep-13, 02:10 PM
A line can therefor revolve around a give point on it, with relation to a 2-dimensional plane or a 3-D space (like the propeller of an airplane), but not rotate "along it's own lenght axis".So if I lay the line on its side it can rotate around a central point? But the central point, which is part of the line, doesn't rotate?

Grant Hutchison

LayMan
2006-Sep-13, 02:14 PM
Actually, yes. If the Earth were not rotating, most of the
planet would see the daytime period last about half a year,
and the nighttime period last about half a year.

-- Jeff, in Minneapolis


Then it is rotating relative to the sun. But true, it may not be rotating relative to the distant stars.

And we're back to square one, which leaves us with the question whether or not this thread is rotating... :D

Actually, Van Rijn, this was my problem to begin with, the reason why I posted my inital question... If you're saying that an object which alternately shows its different hemispheres towards the object it is orbiting, is rotating, then the Moon is not rotating. It doesn't do that.

That was the reason why I posted those 2 questions:

"1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?"

- If you're going to answer both of them with NO, then nothing rotates.

- If you're going to answer the first one with YES, and the second with NO, then the Moon does NOT rotate.

- If you're going to answer the first one with NO, and the second with YES, then the Moon DOES rotate.

- If you're going to answer both with YES, then everything rotates.

Mind you, I am not telling you how you should answer them... That's still open for debate, apparently.

LayMan
2006-Sep-13, 02:38 PM
So if I lay the line on its side it can rotate around a central point? But the central point, which is part of the line, doesn't rotate?

Grant Hutchison

Maybe I wasn't completely clear on that, but I did use the word 'revolve' there, not 'rotate'. I'm not sure how to put that... Think of the propeller again: it can revolve around the nose of the airplane, but it can't rotate along its own length axis as long as it's 'stuck' on that nose. It's just an analogy, remember that before you're going to use that against me. I know you could detach the propeller and make it rotate along its axis. It's a 3-D object. I was merely using it to paint a visual picture. And if youre going to ask me now whether or not the central point of that 1 dimensional line is rotating, well, I'd hate to get semantical again, but according to me, that statement would be rather 'meaningless', since a point has no dimensions to rotate against - or within - or absolute/relative to...

And no, I'm not implying that anyone should accept my definitions. Or the way that I put them... But I can't really enter a discussion without using words and terms, can I? And since a lot of posts here stress the fact that it basically comes down to 'what you mean by what you're saying, when you're saying it', I thought I'd try to clear that up... Trust me, I really didn't mean to add to the confusion...

I just tried to better explain what it was that I understood as being 'rotation'. Not sure whether I succeeded in that, though. ;)

grant hutchison
2006-Sep-13, 02:47 PM
Don't worry, I'm just being light-hearted about this.
I'll stop hassling you. :)

Grant Hutchison

LayMan
2006-Sep-13, 02:48 PM
Just thought I'd extend this into the hypothetical "Does Venus rotate?" thread:

:evil:

"1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?"

- If you're going to answer both of them with NO, then nothing rotates.

- If you're going to answer the first one with YES, and the second with NO, then the Moon does NOT rotate and Venus DOES.

- If you're going to answer the first one with NO, and the second with YES, then the Moon DOES rotate and Venus does NOT.

- If you're going to answer both with YES, then everything rotates (Including, but not limited to, this thread...).

LayMan
2006-Sep-13, 02:57 PM
Don't worry, I'm just being light-hearted about this.
I'll stop hassling you. :)

Grant Hutchison

Deal! :)

(By the way, I'm off for today, see you all tomorrow...)

hhEb09'1
2006-Sep-13, 03:13 PM
It does rotate with the earth, but it isn't due to the earth's rotation (even in part)? That does seem to ignore dynamo models.The dynamo models do not need the earth to rotate, I think, even if there is a connection, it is not direct. The core is rotating with respect to the earth very slowly, but there is a smaller scale flow that is producing the magnetic field. It is considered a self-exciting dynamo--and it doesn't seem to be getting its energy from the rotation of the Earth.

ZaphodBeeblebrox
2006-Sep-13, 09:13 PM
Just thought I'd extend this into the hypothetical "Does Venus rotate?" thread:

:evil:

"1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?"

- If you're going to answer both of them with NO, then nothing rotates.

- If you're going to answer the first one with YES, and the second with NO, then the Moon does NOT rotate and Venus DOES.

- If you're going to answer the first one with NO, and the second with YES, then the Moon DOES rotate and Venus does NOT.

- If you're going to answer both with YES, then everything rotates (Including, but not limited to, this thread...).
I'm Going to HAFTA Go wiith Conclusion 2, wiith a Caveat ...

Venus Does Rotate ...

Backwards!

:eek:

PhantomWolf
2006-Sep-13, 09:14 PM
Well sorry to increase the headache, but the moon is actually attracted to the sun by twice the gravitational force of the Earth. Without the Earth, the moon would just have a more circular orbit. Currently we perterb its orbit back and forth across ours so at some stages it is between us and the sun and so takes less time to get to the crossing point and passing infront of us, and at other times it's outside our orbit thus takes longer to hit the crossing point and does so behind us. They could be considered a Twin Planet system, though generally aren't because their mutual oribital point (Barycentre) is inside the Earth.

As to Rotation and the Magnetic Field, well this is from BA's review on The Core. (Who would have thunk that movie would be useful for more than being a doorstop?)


Again, in the movie, they got something right: they said that a spinning ball of hot metal will create a magnetic field. This is a very basic property of magnetism: it's caused by moving charges. An electron zipping past you has a magnetic field associated with it, because it is charged and moving. The core of the Earth is tremendously hot, so hot that the electrons in the iron have been stripped from the nuclei of the atoms. Since it's spinning, the charges are moving, so you get a magnetic field.

The real situation is far more complicated, as life, in general, is messier than an oversimplified movie review. The Earth's core isn't just spinning; there are currents inside it, and other factors which influence the Earth's magnetic field. If the writers had been clever, instead of saying the Earth's core had stopped, they could have said that the currents of molten and ionized iron inside the core had become chaotic. It takes a relatively stable flow to make a magnetic field, so the chaotic motion could collapse the Earth's magnetic field.

Not only is this plausible, it appears to be true: the magnetic field of the Earth is not constant. In fact, for reasons still not well-understood, the magnetic polarity of the field sometimes reverses, with the north magnetic pole becoming the south, and vice-versa. This happens every few hundreds of thousands of years (and note that the Earth's surface doesn't boil when the field drops to zero during a reversal!).

Jeff Root
2006-Sep-13, 09:34 PM
1) Would you say the Earths rotation stops at the point were one
day would last for six months, followed by a night equally long?

or:

2) would you say this happens at the point were one hemisphere
would be continually in daytime, while the other would experience
an everlasting night?

- If you're going to answer both of them with NO, then nothing rotates.

- If you're going to answer the first one with YES, and the second
with NO, then the Moon does NOT rotate.

- If you're going to answer both with YES, then everything rotates.
You got those last three statements backward.

An answer of "NO" to both questions could imply that everything
always rotates, or it could mean that you just didn't offer the
right answer as one of the choices.

An answer "YES" to the first question and "NO" to the second
question -- which is the answer I gave -- implies that the
Moon does rotate.

An answer of "YES" to both questions could imply that nothing
rotates, or -- more reasonably -- that the person answering
didn't understand the questions.

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-14, 02:12 AM
The dynamo models do not need the earth to rotate, I think, even if there is a connection, it is not direct. The core is rotating with respect to the earth very slowly, but there is a smaller scale flow that is producing the magnetic field. It is considered a self-exciting dynamo--and it doesn't seem to be getting its energy from the rotation of the Earth.

Is that the general consensus? My recollection (and a quick google review seems to confirm it) is that while there are issues and limitations to current dynamo models and a "simple" dynamo won't fit, rotation is considered a large factor in sustaining the earth's magnetosphere.

Van Rijn
2006-Sep-14, 02:21 AM
And we're back to square one, which leaves us with the question whether or not this thread is rotating... :D

Actually, Van Rijn, this was my problem to begin with, the reason why I posted my inital question... If you're saying that an object which alternately shows its different hemispheres towards the object it is orbiting, is rotating, then the Moon is not rotating. It doesn't do that.

That was the reason why I posted those 2 questions:

"1) Would you say the Earths rotation stops at the point were one day would last for six months, followed by a night equally long?



You left out a vital piece of the question: Rotating relative to what?



2) would you say this happens at the point were one hemisphere would be continually in daytime, while the other would experience an everlasting night?"


And again, rotating relative to what?

PhantomWolf
2006-Sep-14, 02:24 AM
rotation is considered a large factor in sustaining the earth's magnetosphere.

If this is so, explain how the magnetic field fades and switches every couple thousand years?

Van Rijn
2006-Sep-14, 02:36 AM
rotation is considered a large factor in sustaining the earth's magnetosphere.

If this is so, explain how the magnetic field fades and switches every couple thousand years?

I believe it is usually just a tad more than a couple thousand years. And did you read the piece you quoted from the BA?

PhantomWolf
2006-Sep-14, 02:45 AM
Yes I did (I'll note the years Thing I didn't check back up to make sure it was entirely right) and he seems to be stating that there are currents that are independant of the cores rotation, that these switch in direction and cause the pole reversals. How does that mesh with "Rotation of the core causes the magnetic feild"? From what I uderstood of the quote I posted, he's disagreeing that this is the major cause.

Jeff Root
2006-Sep-14, 02:59 AM
Van Rijn,

Stand somewhere that you won't knock anything over, let your
arms hang at your sides, and spin around in place rapidly.
What happens to your arms? Why does it happen?

-- Jeff, in Minneapolis

Van Rijn
2006-Sep-14, 03:12 AM
Yes I did (I'll note the years Thing I didn't check back up to make sure it was entirely right) and he seems to be stating that there are currents that are independant of the cores rotation, that these switch in direction and cause the pole reversals. How does that mesh with "Rotation of the core causes the magnetic feild"? From what I uderstood of the quote I posted, he's disagreeing that this is the major cause.

I suspect you are misinterpreting what both he and I are saying. As I read it, he is saying that rotation is a major factor in the earth's magnetosphere, but not the only factor, and the system is somewhat chaotic. Of course, you can ask him.

hhEb09'1
2006-Sep-14, 03:31 AM
Is that the general consensus? My recollection (and a quick google review seems to confirm it) is that while there are issues and limitations to current dynamo models and a "simple" dynamo won't fit, rotation is considered a large factor in sustaining the earth's magnetosphere.The rotation seems to stabilize the field, as near as I can tell. Which links were you reading from google?

PhantomWolf
2006-Sep-14, 04:00 AM
I suspect you are misinterpreting what both he and I are saying. As I read it, he is saying that rotation is a major factor in the earth's magnetosphere, but not the only factor, and the system is somewhat chaotic. Of course, you can ask him.

If the rotation is the major factor, it doesn't explain the pole reversals, unless the core starts spinning the other way suddenly. If the currents are the major factor and can become chaotic and then reverse against the spin, that
would explain it.

publius
2006-Sep-14, 04:12 AM
The "geo-dynamo" is pretty complex thing from what I gather, and the exact mechanism in not really know, although they have some complex models that predict the pole-flipping.

A "dynamo" properly is an electrical generator, self-excited -- it's own EMF supplies the field current which makes the magnetic field, motion through which generates the EMF. Now, the problem is getting one started -- you need residual magnetism, or some current source to get started. But once you get it started, it is self-sustaining as long as you provide mechanical input.

The primary energy input for the internal dynamo is thought to be internal heat. Molten metal moves through the existing magnetic field, inducing an EMF, which drives currents, which make the field. THe heat is the source that drives it against ohmic dissipation (which makes more heat. :) The thing couldn't be perpetual motion of course, and has to depend on internal heat flowing out -- a heat engine).

Rotation plays a role in this, but the energy (or not much) doesn't seem to be coming from stored rotational energy, it's from whatever the internal heat sources are.

As far as the chaotic behavior, we can rig a self-excited DC generator to do such a thing. Let's wire the field backwards so the current driven by the EMF *opposes* the original field. :) Residual magnetism will get a little voltage started. But when a current flows, it reduces the field, lowering, rather than sustaning the EMF. That sucker will be very chaotic, if makes much field and current at all. We can tweak things around, adding combinations of series and parallel field windings with various relative polarities that can make such a thing behave in all sorts of complex ways. And one such behavior is building a strong field, peaking, decaying, and flipping and starting over in the opposite direction.

The earth's dynamo is something like that.

-Richard

Van Rijn
2006-Sep-14, 05:03 AM
Van Rijn,

Stand somewhere that you won't knock anything over, let your
arms hang at your sides, and spin around in place rapidly.
What happens to your arms? Why does it happen?

-- Jeff, in Minneapolis

The arms will feel inertial forces due to changing motion.

Van Rijn
2006-Sep-14, 05:13 AM
The rotation seems to stabilize the field, as near as I can tell. Which links were you reading from google?

For example, from here:

http://www.es.ucsc.edu/~glatz/geodynamo.html
[snip]

These buoyancy forces cause fluid to rise and the Coriolis forces, due to the Earth's rotation, cause the fluid flows to be helical. Presumably this fluid motion twists and shears magnetic field, generating new magnetic field to replace that which diffuses away.
[snip]
The resulting three-dimensional numerical simulation of the geodynamo, run on parallel supercomputers at the Pittsburgh Supercomputing Center and the Los Alamos National Laboratory, now spans more than 300,000 years. The simulated magnetic field has an intensity and a dipole dominated structure that is very similar to the Earth's (Figure 2) and a westward drift of the non-dipolar structures of the field at the surface that is essentially the same as the 0.2 degrees/year measured on the Earth. Our solution illustrates how the influence of the Earth's rotation on convection in the fluid outer core is responsible for this magnetic field structure and time dependence [1].


would seem to indicate that rotation is a major factor in the geodynamo.

Van Rijn
2006-Sep-14, 05:23 AM
I suspect you are misinterpreting what both he and I are saying. As I read it, he is saying that rotation is a major factor in the earth's magnetosphere, but not the only factor, and the system is somewhat chaotic. Of course, you can ask him.

If the rotation is the major factor, it doesn't explain the pole reversals, unless the core starts spinning the other way suddenly. If the currents are the major factor and can become chaotic and then reverse against the spin, that
would explain it.

I'll let the BA speak for himself. As for me:

If I said that rotation was the major factor, instead of a major factor, or a large factor (as I said elsewhere) in the Geodynamo, you might have an argument.

My reading of the subject is that rotation can hardly be ignored or shoved under the rug as a minor or trivial issue in the operation of the Geodynamo. However, I never said it was the primary factor.

Jeff Root
2006-Sep-14, 08:56 AM
Stand somewhere that you won't knock anything over, let your
arms hang at your sides, and spin around in place rapidly.
What happens to your arms? Why does it happen?
The arms will feel inertial forces due to changing motion.
Be specific. Describe what happens. Explain why it happens.

-- Jeff, in Minneapolis

LayMan
2006-Sep-15, 08:57 AM
You left out a vital piece of the question: Rotating relative to what?



And again, rotating relative to what?

I mean rotating relative to the effect of that rotation.

* Disclaimer -

Be warned:
yes, my next post is going te be a long one... Again...
And no, it does not - for all practical purposes - claim to be complete or logically correct... Hence the disclaimer.
And yes, I've got a list of smileys, and I'm not afraid of using them.

:D

*

hhEb09'1
2006-Sep-15, 09:01 AM
And yes, I've got a list of smileys, and I'm not afraid of using them.There should be a limit on smileys per post :)

LayMan
2006-Sep-15, 09:03 AM
Hold on to your pantyhose…

OK, here goes…

Regarding rotation and orbit, relative or absolute, whatever your frame of reference, imagine the following: we’ll take a hypothetical observer on a hypothetical Earth. It’s not quite unlike Earth as we know it, and the observer is not quite unlike any observer on the real Earth.

There are, however, three moons, clearly visible in the sky, in all respects not unlike our own Moon, that’s to say, they each have an orbit of about 1 month (how you’re going to get that in a working cosmological model is not the issue, remember, we’re talking hypothetically).

I think we can safely assume that, as stated before, ‘absence of proof’ does NOT explicitly imply ‘proof of absence’, but it also does keep the option open, right? For instance, even though some distant celestial objects (like the outer planets, quasars, etc.) were unknown to our ancestors, we do now know that they exist. So, non-observation can mean non-existence, but doesn’t exclude possible existence. I mean, if you we’re to go back to the 13th century and tried to prove to our predecessors that Pluto exists, using their technology, how would you be able to do that?

However, observation of facts does necessarily mean existence of the observed, right? The reason why our hypothetical observer would say that his planet has 3 moons, is mainly because he can clearly see them, right? Now, since he wants to know more about these moons, our observer plants an array of telescopes around the entire globe so he can track the moons as they make their monthly orbit.

Now, suppose the following is being observed on this hypothetical planet: 1 of the 3 moons, call that moon A, can clearly be seen to show its different hemispheres during that 1 month, that is to say, after one month a clearly visible ‘marker’ - say, a large mountain range at the equator- would be visible about half the month, invisible about the other half, would have made a complete turn and ended up at the same spot after the entire month, as the video footage of the telescope array would prove.

The second moon, call that moon B, also orbits the planet in about the same time as moon A. However, it always keeps the same side with its own specific ‘marker’ continuously facing the planet. But when closely examined, it does not stay ‘fixed’: it can be clearly observed that the marker - in this case, say a large crater located near one of the poles – in the course of one month, slowly moves, like the dials of a watch, from the south pole to the north pole and back again. So after one quarter month, it would have moved from the ‘six position’ to the ‘nine position’, over the ‘twelfth position’, on to the ‘three position’ and back to the ‘six position’… Again, this behavior shows up on the video footage.

Finally, the third moon, much like the second (moon B), always shows the same side, but its visible disk remains fixed (like our own Moon does here on real Earth).

Patience, please, we’re getting there…

Now, our observer isn’t really satisfied with just the video footage, or what his eyes are telling him (perhaps he feels that video tapes can be forged or something, maybe he heard something about optical illusions,…), since this still doesn’t really answer all his questions. So he decides to do the following: he builds three train tracks, which encompass the entire hypothetical planet. He places three train carts on each track. Then, he jumps into his space shuttle, visits all three moons and attaches a strong, very long cable to them. He returns to his planet, and attaches the other end of each cable to each train cart (call them carts A, B and C) that is waiting on its track.

What happens? All three carts will be towed along the surface of the planets surface, as the three moons are making their orbit. Our observer observes this and takes it as undeniable evidence (proof) that the three moons are actually orbiting the planet and do so in a time span of 1 month. Just like the videos suggested. He also notices something else, something ‘weird’: while this is the only, primary thing happening to cart A, carts B and C show another, extra-ordinary behavior: apart from being dragged along the tracks across the planets, cart C would also leave its tracks and be lifted upward, and continue to ‘foollow’ those tracks, al be it now from several meters of the ground. The upper part of cart B would rotate around (if it were free to do so, otherwise, due to the tension on the cord, it would also be lifted upwards, just at a much slower rate then cart A).

Cart C however would show no secondary movement or effect at all…

So the secondary effect can not be caused by the orbits, since they were said to be 'the same'.

To summarize:

IF you then would accept another force, call that rotation, as responsible for the secondary effect,
AND you would accept the difference in direction of that rotation between moons A and B, as responsible for the difference in behavior of carts A and B with respect to that secondary force,
THEN I believe you should also accept that the absence of the secondary effect in cart C is best explained by the absence of the force in moon C.

LayMan
2006-Sep-15, 09:15 AM
To summarize the summary:

I feel good about saying that the Earth is rotating counter clockwise at around +24 rotations per year (read: 'orbit'). That's why the Sun comes up in the east and sets in the west around 12 hours later (on average, depending on you're position here on Earth).
I also have no problem in stating that the Earth could be said to rotate clockwise at -24 rotations per year, if the Sun were to raise in the west and set in the east.

I don't see a problem in saying that Venus is rotating clockwise at almost -1 rotation per 'Venus-year" (again, read 'orbit'). And I wouldn't subject to saying that it would be rotating counter clockwise at +1 rotation per Venus-year if it were rotating in the other direction.

However, I do feel uneasy trying to get the Moon into this all...

Let me put it this way:

... -599.5 -24 -1 0 1 24 599.5 ...

Spot the odd one out...

:D

hhEb09'1
2006-Sep-15, 09:16 AM
cart C would also leave its tracks and be lifted upward,

::snip::

Cart C however would show no secondary movement or effect at all…You have your carts mixed up.
IF you then would accept another force, call that rotation, as responsible for the secondary effect,
AND you would accept the difference in direction of that rotation between moons A and B, as responsible for the difference in behavior of carts A and B with respect to that secondary force,
THEN I believe you should also accept that the absence of the secondary effect in cart C is best explained by the absence of the force in moon C.First, rotation is not a force, and moon B is pretty much non-physical because its rotation axis seems to point to earth all the way around its orbit, a huge precession.

I'll grant that the difference in the two scenarios (let's ignore B, once they're properly labeled) is that one is rotating--but it's the one that is showing the same face to the earth.

LayMan
2006-Sep-15, 09:19 AM
There should be a limit on smileys per post :)

I'll trade you 1 of my smileys for 4 of yours.. :D

LayMan
2006-Sep-15, 09:28 AM
You have your carts mixed up..

I don't think so: since there is no change in the relation between the point where the cable is attached to moon C and the other end that is attached to the cart. However, if that same attachment on moon A is facing away from the planet at some point, then the cart would be lifted the equivalent of that moons diameter per rotation.


First, rotation is not a force, and moon B is pretty much non-physical because its rotation axis seems to point to earth all the way around its orbit, a huge precession..

You mean, like Uranus? ;) But I agree, moon B was only added for completeness... As far as saying that rotation isn't a force, well, beat's me, but I believe there will be people here that are going to disagree with that...


I'll grant that the difference in the two scenarios (let's ignore B, once they're properly labeled) is that one is rotating--but it's the one that is showing the same face to the earth.

I'm inclined to disagree here... (or is that inclination being caused by my inherent rotation?).

LayMan
2006-Sep-15, 09:30 AM
You have your carts mixed up.First, rotation is not a force, and moon B is pretty much non-physical because its rotation axis seems to point to earth all the way around its orbit, a huge precession.

I'll grant that the difference in the two scenarios (let's ignore B, once they're properly labeled) is that one is rotating--but it's the one that is showing the same face to the earth.


And once again, reality strikes like ton of bricks, I really should take the time to read before replying...

:wall: :eek:

LayMan
2006-Sep-15, 09:36 AM
"Originally Posted by LayMan
cart C would also leave its tracks and be lifted upward,

::snip::

Cart C however would show no secondary movement or effect at all… "

I was going for "Cart A would also leave its tracks..."

Please let the disclaimer include that I can't be held responsible for my on stupidity... :D

hhEb09'1
2006-Sep-15, 09:36 AM
I don't think so: since there is no change in the relation between the point where the cable is attached to moon C and the other end that is attached to the cart. However, if that same attachment on moon A is facing away from the planet at some point, then the cart would be lifted the equivalent of that moons diameter per rotation.The two sentences that I quoted both said "Cart C"
You mean, like Uranus? ;) No, Uranus's axis does not point towards the sun for its full revolution. It points in the same direction, but that is sometimes away, sometimes not.
But I agree, moon B was only added for completeness... As far as saying that rotation isn't a force, well, beat's me, but I believe there will be people here that are going to disagree with that...Start a poll :)

Tog
2006-Sep-15, 09:39 AM
There should be a limit on smileys per post :)

"You have included 25 images in your message. You are limited to using 8 images so please go back and correct the problem and then continue again."

There is.:lol:

LayMan
2006-Sep-15, 10:00 AM
"Originally Posted by LayMan

Please let the disclaimer include that I can't be held responsible for my on stupidity... :D

Or my own inability to spell correctly...

Anyway, has anyone spotted the odd one out yet? I wonder what the visual effect would be if the Moon had a retrograde rotation...?

hhEb09'1
2006-Sep-15, 10:10 AM
I wonder what the visual effect would be if the Moon had a retrograde rotation...?Is that a rhetorical question, or are you having difficulty imagining it?

Attach a cable! :)

LayMan
2006-Sep-15, 10:34 AM
Is that a rhetorical question, or are you having difficulty imagining it?

Attach a cable! :)


I can't reach it... :lol:

And it's not a rhetorical question, but a Zen one:

"NO WATER, NO MOON

The original source for the following koan was translated into English from a book called the Shaseki-shu (Collection of Stone and Sand), written late in the thirteenth century by the Japanese Zen teacher Muju (the "non-dweller"), and from anecdotes of Zen monks taken from various books published in Japan around the turn of the 20th century.

The nun Chiyono (Mugai Nyodai, 1223-1298) studied and meditated for years, most notably under the venerated Zen master Wu-hsueh Tsu-yuan (Bukko, 1226-1286, founder of Engakuji temple, arrived in Japan from China in 1280), on the ultimate question of existence, but was unable to reach the far shore.
The more she longed for Enlightenment the further off it seemed. But one moonlit night she was carrying an old bucket filled with water from the well that eventually came to bear her name, and as she walked she noticed the full moon reflected in the pail of water. As she continued along the path the bamboo strip that held the pail staves broke.

The pail began to come apart, the bottom broke through, and the water disappeared into the soil beneath her feet, the moon's reflection disappearing along with it. In that moment Chiyono realized that the moon she had been looking at was just a reflection of the real thing...just as her whole life had been...she turned to look at the moon in all it's silent glory, and ...that was it. Like the moonlight driven event surrounding the Enlightenment of the mysterious wandering monk Totapuri, or similar moonlight driven event foretold by the Wanderling's Zen Mentor and described in Dark Luminosity, Chiyono herself disappeared. She was NOT ----- and what IS, was.

Afterwards she wrote the following

"This way and that way
I tried to keep the pail of water together,
hoping the weak bamboos
would never break
But suddenly the bottom fell out:
no more water
no more moon in the water
and emptiness in my hand!"

Taken from this site http://www.angelfire.com/realm/bodhisattva/chiyono.html

(DISCLAIMER: No religious discussion intended, I've read the rules on this forum!).

;)

hhEb09'1
2006-Sep-15, 03:25 PM
I can't reach it... :lol:Use your imagination! like this guy (http://www.bautforum.com/showthread.php?p=826205#post826205) :)

grav
2006-Sep-15, 09:22 PM
Please ignore my previous post, the sheer difference between absolute and relative motion just hit me like a ton of bricks... :D
I think that statement pretty much says it all. hhEb09'1 and Grant Hutchinson helped me to realize that the synodic rotation also creates the exact same centrifugal force of spin from the center of the moon as it would if the moon were spinning on its own, according to its absolute rotation. So it appears that there are different types of rotation, and things would be much simpler if we determine which one we are referring to. For instance, there is the relative rotation, which I just consider to be that rotation relative to the axis of rotation (and to the Earth), since it is absolute for anything outside that system. For the moon, it is zero, since it is tidal locked. Then there is synodic rotation, which is the same as the revolution, and so is always one (when using the revolution as a reference). The absolute rotation, then, is simply the sum of the two, which comes to 0+1=1 for the moon. So as hhEb09'1 has pointed out to me, in the case of the moon, since it is tidal locked, which automatically defines the relative rotation as zero, the synodic rotation, therefore, is exactly the same as the absolute rotation.

What I would find fascinating now is to determine how we would define the difference between revolution and rotation when the axis of rotation is extremely close to that for the revolution. For example, the barycenter of the Earth-moon system is very close to the center of mass of the Earth. So if the moon were not present, we might very well have 13 less days in our year, due to the synodic rotation of the Earth and moon. So do we say that this adds to the rotation of the Earth itself, or is this still considered revolution? The mass of the sun is so large that its axis of rotation and the barycenter between any particular planet is practically the same. For an object this large, the extremely small distance between the barycenter and its own center of mass would act to make it appear to rotate in this respect while it may still just be due to its revolution. If hhEb09'1 were to have his way, he would probably say that there really no such thing as a true revolution anyway. It is all just rotation, which is just the absolute rotation of an individual mass and/or that of an entire system. So the moon isn't really revolving around the Earth, or vice versa. The entire system is rotating. And when I think about it, it does make sense, and I would probably have to agree. :think: And that does seem to be a much easier way to think about it. So since it seems to be the absolute rotation we are all referring to after all, then if we can all agree on that, maybe all we really need to define is what is rotating? Are we considering the absolute rotation of the moon itself or that of the Earth-moon system? But in terms of absolute rotation only, it really doesn't even matter which one we are referring to in this case, since the moon is tidal locked. They are both the same in magnitude, so yes, the moon is rotating, and at exactly the same rate as that of the system. :)

grant hutchison
2006-Sep-15, 09:58 PM
So if the moon were not present, we might very well have 13 less days in our year, due to the synodic rotation of the Earth and moon.How would that work? Why would the Earth rotate faster in the absence of the moon?

Grant Hutchison

ZaphodBeeblebrox
2006-Sep-15, 10:06 PM
How would that work? Why would the Earth rotate faster in the absence of the moon?

Grant Hutchison
It Wouldn't Rotate Faster, he Means it Would REVOLVE Faster ...

I Don't Thiink I Agree Though, at Any Raate ...

Doesn't Kepler's Law Come Into Effect, And Mean That The Earth's Orbit Averages Out to a Siimple Ellipse, Anyway?

grant hutchison
2006-Sep-15, 11:19 PM
It Wouldn't Rotate Faster, he Means it Would REVOLVE Faster ...Or maybe rotate slower. Which was what I meant to write. :o

Grant Hutchison

ZaphodBeeblebrox
2006-Sep-15, 11:47 PM
Or maybe rotate slower. Which was what I meant to write. :o

Grant Hutchison
No Matter ...

It Does However, Beg a Question About The Proper Application of Kepler's Laws ...

Does The Earth, Even though it Perceptibly Wobbles Due to The Influence of The Moon, Have The SAME Orbital Period it Would Have, If it Were Simply Orbiting On its Own?

grav
2006-Sep-16, 12:03 AM
How would that work? Why would the Earth rotate faster in the absence of the moon?

Grant Hutchison
Well, first, yes, it would rotate slower. I'm thinking about this in the same way that the revolution around the sun adds one extra day to our year because of its synodic rotation. Relative to the sun, it is not observed, but only with its absolute rotation (relative to the stars). We've had this discussion before, however, when you mentioned that there would be many factors to consider within the initial development of the solar system as to what the rotation of the Earth might be if the moon were not present, and I agree, which is why I said "might very well have" instead of "would have". It would not revolve, that much is clear, since there is nothing to revolve around (except the sun), so the extra 13 or so days (per year) of synodic rotation attributed by the revolution of the Earth and moon around each other might not have existed if that were the case. On the other hand, one might argue that that is the rotation the Earth would have had anyway, and that the moon just "transforms" some of it into its revolution, but the overall (absolute) rotation of the Earth remains the same. Anyway, just something to think about. :)

LayMan
2006-Sep-20, 02:01 PM
Sorry for the delay, but I was away for the weekend, without Internet access and the last two days I was very busy, so I could only browse a little while through the thread…

Well, there may off course be different forms of rotation… And of revolution… And of orbit… The thing is, I use these three definitions like this: the Earth orbits the Sun. It causes us here on this planet to have what we call a year. The combined effect of having an orbit and the fact that the polar axis is not perfectly perpendicular to the orbital plane causes that year to be divided into four seasons. The Earth also revolves around the shared gravitational point of equilibrium with the Moon. And it also rotates (almost) along its polar axis, although it probably isn’t as straight forward as that… But I still can’t take the logical hurdle…

I mean, if there is such a thing as absolute rotation, there also has to be something like absolute non-rotation. Not relative to anything. After all, it doesn’t matter whether you look at it from our perspective or from an observer in the vicinity of Orion. Both observers in the hypothetical example I gave will notice 1 of the carts having an extra movement, and both will come to the conclusion that the movement across the surface is being caused by 1 distinct force, namely the orbit of both respective moons, while only 1 of both carts displays a second effect, which they will both attribute to the rotation of 1 moon, while the other doesn’t cause this effect and therefore doesn’t rotate. The difference in behaviour between the 2 carts is clearly observable and can’t be denied, regardless of where you’re standing.

This is actually where I went wrong in trying to understand your objections that the Moon does rotate:



Just kidding, I'm going to stick to my reply to hhEb09'1: the trick seems to be to reckon with three relative rotations:

1) those that are smaller then the accompanying orbit (> 0 and < 1) --> apparent retrograde rotation

2) those that are exactly equal to the orbit (= 1) --> apparently no rotation

3) those that are bigger then the orbit (> 1) --> apparent "normal" or Earth-like positive rotation

From this point of view: absolute rotation isn't possible, since no object is otherwise completely motionless apart from that rotation, and even if it was, no object is compacted into one single point in space without any other moving body in its vicinity... Except perhaps the singularity of the Big Bang. I guess its safe to say that would be the only thing capable of absolute rotation. I think I got it.

You can really get a splitting headache trying to wrap your mind around things like this...

The fractional rotation doesn’t cause retrograde rotation: if the Earth were rotating at 0.25 % of its current rotation, the Sun would still rise in the East. It would take a great deal of time before it would set again, due to the slow rotation, but it would still set in the West. The only way that enables us to speak of retrograde rotation with respect to the Earth, is to speak of negative rotation: a rotation of +24 hours causes the familiar effect of having app. 365 days a year with the Sun rising in the East, while a rotation of –24 hours would have the exact same effect, but this time with the Sun rising in the west. A “rotation” of 0 hours would – in my book – result in the Earth not rotation, while one same side of it would be continuously facing the Sun. Just like the Moon always keeps the same side facing the Earth.

hhEb09'1
2006-Sep-20, 02:41 PM
The only way that enables us to speak of retrograde rotation with respect to the Earth, is to speak of negative rotation:A positive retrograde rotation is the same thing as a negative prograde rotation. That's true.
a rotation of +24 hours causes the familiar effect of having app. 365 days a year with the Sun rising in the East,It's a little less than 24 hours, about 23h 56m, and it's a little more than 365 days per year, around 365.25 (plus or minus a few minutes depending on which definition of the year one is using :) )
while a rotation of –24 hours would have the exact same effect, but this time with the Sun rising in the west.Not quite. The number of days per year would be different. The actual number would depend upon whether you would use -24hr or -23h56m
A “rotation” of 0 hours would – in my book – result in the Earth not rotation,I'd agree with that but I don't agree with the next clause:
while one same side of it would be continuously facing the Sun. Just like the Moon always keeps the same side facing the Earth.If the earth were not rotating, each side would have a turn facing the sun as it orbitted the sun.

LayMan
2006-Sep-20, 02:57 PM
I'd agree with that but I don't agree with the next clause:If the earth were not rotating, each side would have a turn facing the sun as it orbitted the sun.

Well, I guess this basically sums up the whole point of the disagreement: that way, the Earth would have 1 day AND 1 night per year, with the Sun rising in the east. but I don't view that as the 'zero'- point, since there would be a negative counterpart if the Earth did exactly the same thing, but this time retrograde style... Then there would also be 1 day and 1 night per year, but with the Sun rising in the west.

Maybe it's my twisted sence of logic, but it still 'sticks' better to me to view the Earth having 1 day OR 1 night per year, that way, there's no negative - or retrograde - counterpart... :eh:

Anyway, I hope I have some more time tomorrow, I have to go now... :sad:

hhEb09'1
2006-Sep-20, 04:16 PM
Well, I guess this basically sums up the whole point of the disagreement: that way, the Earth would have 1 day AND 1 night per year, with the Sun rising in the east. but I don't view that as the 'zero'- point, since there would be a negative counterpart if the Earth did exactly the same thing, but this time retrograde style... Then there would also be 1 day and 1 night per year, but with the Sun rising in the west.I'm not sure what you mean by "exactly the same thing". Since it's not rotating, it's rotation is zero, and retrograde would mean negative zero rotation, which yes is exactly the same thing, but it would result in the same thing: one day and one night per year with the Sun rising in the west of course.

Rotation and revolution are two different things.

Thought problem: if you move a queen piece on a chessboard from QB3 to QB5, then to K5, then to K3, and back to QB3, all the time facing forward, has the queen rotated? If the king is at Q4, has the queen revolved around the king?

LayMan
2006-Sep-21, 12:11 PM
I'm not sure what you mean by "exactly the same thing". Since it's not rotating, it's rotation is zero, and retrograde would mean negative zero rotation, which yes is exactly the same thing, but it would result in the same thing: one day and one night per year with the Sun rising in the west of course.

Rotation and revolution are two different things.

Thought problem: if you move a queen piece on a chessboard from QB3 to QB5, then to K5, then to K3, and back to QB3, all the time facing forward, has the queen rotated? If the king is at Q4, has the queen revolved around the king?


Well, I - finally - took the logical hurdle, so there's some good news and some bad news... the good news is: my splitting headache's gone!! The bad news is that it's 'checkmate' for me... So, I guess this is it, then: 'the last post'.

You guys have been right all along, haven't you - why didn't anybody explain that to me?? :D :D :D

See, the thing is that I honestly thought I had a good point in considering rotation as an absolute motion, you know, detached from orbits, etc... But that was basically the reason for my error and my difficulty in understanding it... I thought I was discussing rotation on it's own, not relative to orbit. But I wasn't, the 'reversed' orbit escaped and (d)eluded me:


Not quite. The number of days per year would be different. The actual number would depend upon whether you would use -24hr or -23h56mI'd agree with that but I don't agree with the next clause:If the earth were not rotating, each side would have a turn facing the sun as it orbitted the sun.

As hhEb09'1 explained 2 posts ago, the number of days would be different. At first, I didn't grasp this... I mean, if I were to reverse the rotational direction of the Earth into a retrograde one, then surely I was only meddling with its rotation, right, so the number of days should remain the same, no?

And this is where all my gorillas disappeared into the mist: I was going to be right about this, but only if I also reversed the Earths orbit around the Sun, not just its rotation...

And then it hit me: if I included both directions of rotation, as well as both directions of orbit, you were all right about the 1 day/1 night thing. The only way I would have been right, was to eliminate both orbital movements from the equation, which would have left me, well... basically, with 2 objects in complete rest with respect towards one another. That's when same face means no rotation works...(Well off course it would, since then you end up with probably the easiest 2-object system in the entire universe... :doh: )

Oh well, I'm glad I finally understand it now... Thanks, everyone, for your patient explanations! I sure hope I didn't waste too much of your time, explaining such a basic concept...

Sorry 'bout that, but could you do me one more favor, please? I know now that it's logically sound to say that the Moon rotates, but couldn't we just call it a bit counter-intuitive??? That way, I don't loose too much face... :D ;)

Thx again! :clap:

hhEb09'1
2006-Sep-21, 01:14 PM
Sorry 'bout that, but could you do me one more favor, please? I know now that it's logically sound to say that the Moon rotates, but couldn't we just call it a bit counter-intuitive??? That way, I don't loose too much face... :D ;)

Thx again! :clap:yw :)

I'd go so far as to say that it confuses a lot of people. There's no denying that! :) But that's why we have science and math, to disconfuse us

SeanF
2006-Sep-21, 02:15 PM
Thought problem: if you move a queen piece on a chessboard from QB3 to QB5, then to K5, then to K3, and back to QB3, all the time facing forward, has the queen rotated?
Define "facing forward." If it's relative to the chessboard, then we can't answer whether or not the queen has rotated until we know whether or not the chessboard is rotating.

:D

hhEb09'1
2006-Sep-21, 02:24 PM
Define "facing forward." If it's relative to the chessboard, then we can't answer whether or not the queen has rotated until we know whether or not the chessboard is rotatingWe know it's rotating, because we're earthlings and the earth rotates. :)

But the gedanken is performed on a chess board universe in which nothing else exists except the players and their playthings. Oh, and they have clothes on. ;)