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poorleno
2004-Apr-01, 07:48 PM
Well, my question is pretty simple -
How fast is the earth moving in the absolute sense?

There are several factors to consider - motion around the sun, motion with the sun, motion with our galaxy, local group, etc. Could there be a simple answer? Should there be a point of reference to consider the relativistic effects, or aren't there any worth mentionable ones?

Is there such thing as absolute velocity? I would imagine so, as nothing can go faster than the speed of light. So the absolute velocity should be 0<v<c.

How off am I?

Ut
2004-Apr-01, 08:14 PM
What do you mean by "absolute"? Motion is defined as being with respect to something else...

MKR
2004-Apr-01, 08:21 PM
What do you mean by "absolute"? Motion is defined as being with respect to something else...

Hypothetical situation:

The sun and all objects in the solar system other than the earth were to disapear, leaving the earth hurtling in whatever direction it was in before the disapearence. Assume that another object randomly appeared directly in the earth's path(the object is too small to have an effect on the earth); how long would it take the earth to get from that point to this mystery object's location?

JohnW
2004-Apr-01, 08:28 PM
What do you mean by "absolute"? Motion is defined as being with respect to something else...

Hypothetical situation:

The sun and all objects in the solar system other than the earth were to disapear, leaving the earth hurtling in whatever direction it was in before the disapearence. Assume that another object randomly appeared directly in the earth's path(the object is too small to have an effect on the earth); how long would it take the earth to get from that point to this mystery object's location?

It depends. How far away is the object, and how fast is it moving relative to the Earth?

NubiWan
2004-Apr-01, 08:34 PM
Compared to what?

Grand Vizier
2004-Apr-01, 08:48 PM
It depends. How far away is the object, and how fast is it moving relative to the Earth?

Not only that, it also depends on what time of year it is. Earth orbital speed is 30km/s roughly. The orbital speed of the sun around the centre of the Galaxy is about 225 km/s.

So, if the Sun disappears and the Earth is thrown off at a tangent at one time of the year (must figure out exactly when), it would be going at 225-~30=~195 km/s relative to the centre of the Galaxy. Six months later it would be flung off at ~255 km/s. (Yeah - I'm assuming coplanar here - in fact the plane of the Galaxy and the ecliptic plane are not coplanar, so a correction is needed but I'm being lazy just to make the case. I'd actually guess at 205-245 km/s range.)

And that's without even looking at motion of the Galaxy with respect to the barycentre of the Local Group (whereever that is).

JohnW
2004-Apr-01, 08:58 PM
It depends. How far away is the object, and how fast is it moving relative to the Earth?

Not only that, it also depends on what time of year it is. Earth orbital speed is 30km/s roughly. The orbital speed of the sun around the centre of the Galaxy is about 225 km/s.

So, if the Sun disappears and the Earth is thrown off at a tangent at one time of the year (must figure out exactly when), it would be going at 225-~30=~195 km/s relative to the centre of the Galaxy. Six months later it would be flung off at ~255 km/s. (Yeah - I'm assuming coplanar here - in fact the plane of the Galaxy and the ecliptic plane are not coplanar, so a correction is needed but I'm being lazy just to make the case. I'd actually guess at 205-245 km/s range.)

And that's without even looking at motion of the Galaxy with respect to the barycentre of the Local Group (whereever that is).

If we ignore the effects of gravity, wouldn't it depend entirely on the object's distance and velocity relative to the Earth?

Grand Vizier
2004-Apr-01, 09:10 PM
If we ignore the effects of gravity, wouldn't it depend entirely on the object's distance and velocity relative to the Earth?

Entirely? - well only if you choose to go geocentric and make the Earth your fixed point of reference - you can do that, but it's hardly the easiest approach if you want to do a calculation. The original idea was what happened if the Sun disappeared - and that still depends on the time of year. It doesn't matter whether you hold the Earth fixed, or whether you hold the distant object fixed, the relative velocities are still determined by Earth's vectors relative to the (now non-existent) Sun.

JohnW
2004-Apr-01, 09:39 PM
My paraphrasing may get it wrong, but I think MKR's question meant "how fast is the Earth moving relative to an object at rest?" I was trying to say that there's no such thing as "at rest", and we need to know the velocity of this object relative to the Earth in order to determine how long it will take for the Earth to reach it, if it ever does.

The Earth's speed around the galactic nucleus does of course vary throughout the year, as Grand Vizier points out.

poorleno
2004-Apr-01, 09:42 PM
What do you mean what do I mean?
And what are you all on about?
Let's see.

Can there be such a thing as absolute velocity relative to nothing?

Let's assume that the maximum velocity we can have is c and the minimum is 0. Since we can't reach c, due to something some guy postulated a while back, that must mean, that any momentum must be within the scope of 0<v<c. Which is a finite number. Good. Hence if a force was to accelerate something somewhere for no reason and that something would have at least some Ek, it should mean that this Ek is in fact in direct relation to the phantom absolute velocity. v = sqrt(2Ek/m).

However, relativity. Ek, it says, is equal to [mc2 / sqrt( 1 – v2/c2)], and therefore only apparent, or potential, so to speak. So a perceived v gives a perceived Ek. Then, my ineffable phantom absolute velocity wouldn’t exist, it seems. No, good point.

So the answer is no, and ultimately my very question is invalid. Unless I put it into some perspective – let’s say: What’s the Earths speed relative to a celestial object moving so, that this earth relative speed against it, would be as big as possible.

Now I’m confusing myself.

Ut
2004-Apr-01, 09:45 PM
Can there be such a thing as absolute velocity relative to nothing?

No. Velocity is, by definition, the change in position over time of an object relative to some other object.

Grand Vizier
2004-Apr-01, 10:34 PM
Can there be such a thing as absolute velocity relative to nothing?
[...]


In a word, no. And remember that velocity is a vector, too - it has a direction or can be broken down into 3d components, which breakdown depends on your arbitrary coordinate system (hence my stress on time of year and problems with calculations).

'Speed' is the term for non-directional ('scalar') motion - and speed is only really useful if we are in a toy universe with only two objects ('The truck went past me at a speed of 80mph' is a quite valid statement.)

But, if you really want to get into reference frameworks, it is not velocity, but acceleration, that causes the most problems in understanding. The treatment of acceleration is at the heart of general relativity and the 'twin paradox' (Why does one twin age more than the other? One twin is subject to acceleration, the other isn't. Acceleration relative to what? Something called the 'Inertial Reference Frame of the Universe'. Then, before you know it, it's Mach's Principle time, I get a headache and have to go and lie on the sofa and watch mindless TV.)

MKR
2004-Apr-02, 01:11 AM
I wrote up that complex hypothetical and forgot one of the most crutial details:

The object is 8,000 miles from earth's present position(where it was at in orbit when the solar system vanished).

Ut
2004-Apr-02, 01:47 AM
I think you've still left out the most important part. What about the motion of this hypothetical object? If it's moving at the same velocity as the Earth, or greater, they'll never collide.

skrap1r0n
2004-Apr-02, 05:01 AM
hmm ok I think a good reference point would be the center of our galaxy, from there you can determine how fast the sun is moving in relation to the Galaxy Center, and then you can figure out how fast the Earth moves in relation to the sun. here's the kicker though, sometimes the earth will be travelling faster than the sun, sometimes it will be moving slower than the sun, Assuming we are rotating on the same ecliptic as the sun is rotating within the galaxy.

Here's my question though, If we are traveling on the same ecliptic, rotating counter clockwise around the sun, while the sun is rotating counterclock wise in the galaxy, why would we not observe some type of whiplash effect when we (earth) are farthest from the galaxy center, getting ready to head back toward the center. I am assumoing this would be the same time each year where the sun essntially puts earth in an "eclipse", Directly between us and the center of the galaxy.

milli360
2004-Apr-02, 09:18 AM
Not only that, it also depends on what time of year it is. Earth orbital speed is 30km/s roughly. The orbital speed of the sun around the centre of the Galaxy is about 225 km/s.
And what about the motion in the local group, mentioned in the OP?

The OP didn't mention our own rotational motion, but I will: at the equator the surface of the Earth is rotating approx. (2pi x 6378km)/sidereal day, or a little less than half a kilometer per second.

Here's my question though, If we are traveling on the same ecliptic, rotating counter clockwise around the sun, while the sun is rotating counterclock wise in the galaxy, why would we not observe some type of whiplash effect when we (earth) are farthest from the galaxy center, getting ready to head back toward the center. I am assumoing this would be the same time each year where the sun essntially puts earth in an "eclipse", Directly between us and the center of the galaxy.
Because we are in freefall, essentially. It all balances out.

AstroSmurf
2004-Apr-02, 09:25 AM
Some points:

1) The center of the galaxy (at R.A 17h 45m, declination -28° 56') never passes behind the sun from our perspective, though the sunlight naturally drowns it out when it's closest; the difference in declination is something like 40° when they're at the same R.A.

2) The Sun's current motion relative to the CoG is towards R.A 21h 12m, declination 48° 19'. I don't know if this direction is in the plane of the Milky Way (I don't have a planetarium software here) but at any rate, the angular difference measured against the ecliptic seems fairly large.

http://seds.lpl.arizona.edu/messier/more/mw.html

milli360
2004-Apr-02, 09:56 AM
1) The center of the galaxy (at R.A 17h 45m, declination -28° 56') never passes behind the sun from our perspective, though the sunlight naturally drowns it out when it's closest; the difference in declination is something like 40° when they're at the same R.A.
The center of the galaxy is actually pretty close to the ecliptic--I think we had a thread a while ago where the OP asked if that were something more than a coincidence. Regardless, I see the Sun at about that RA on Dec. 17, near the winter solstice, so it's only about five degrees away from the galactic center then.

MKR
2004-Apr-02, 01:14 PM
I think you've still left out the most important part. What about the motion of this hypothetical object? If it's moving at the same velocity as the Earth, or greater, they'll never collide.

The mystery object is immobile.

Celestial Mechanic
2004-Apr-02, 01:31 PM
The mystery object is immobile.
With respect to what?

MKR
2004-Apr-02, 02:59 PM
The mystery object is immobile.
With respect to what?

What do you mean?

Ut
2004-Apr-02, 03:05 PM
The speed or velocity of an object is always measured with respect to some other object. To say something is immobile is to say that its speed is 0. So, with respect to what is the speed of the object zero?

eburacum45
2004-Apr-02, 03:17 PM
The local group of galaxies is moving at 600km/s relative to the Cosmic Microwave Background;

this is as close to the concept of absolute motion as it is possible to get.

http://csep10.phys.utk.edu/astr162/lect/cosmology/cbr.html

tofu
2004-Apr-02, 03:18 PM
What’s the Earths speed relative to a celestial object moving so, that this earth relative speed against it, would be as big as possible.
If I can pick any celestial object to use as a frame of reference, my choice is easy, I'll pick another galaxy – specifically one as far away as possible.

The apparent velocity of the Earth as seen from the Sun, or the velocity of our solar system as seen from a fixed point at the center of the galaxy is nothing compared to how fast we are moving away from the farthest galaxies.

It's like if you are standing on the side of the road and a car goes by at 100 kph. Inside the car you see a person who just happens to be moving the car seat forward. He's also holding a tennis ball and just as he passes you, he tosses the ball up on the dash board.

Your question is, how fast is the tennis ball going? The answer is "really close to 100 kph." On that scale, the other relative motions don't even matter. If you want to be a stickler, you can say "yeah, but he's moving his seat forward, so the correct answer is 100.001 kph." You can say that if you want but it doesn't change the answer by more than a hair.

It's the same with all these relative motions. Yes, the Earth is going around the Sun. Yes, the Sun is going around the Milky Way. But those motions are minuscule compared to how fast we are moving away from distant galaxies. We are moving away from some galaxies at speeds of tens of thousands of kilometers per second!

JohnOwens
2004-Apr-02, 04:51 PM
2) The Sun's current motion relative to the CoG is towards R.A 21h 12m, declination 48° 19'. I don't know if this direction is in the plane of the Milky Way (I don't have a planetarium software here) but at any rate, the angular difference measured against the ecliptic seems fairly large.
I've just pulled it up in XEphem, and that's dead on the galactic plane. It's also helpful to know that now, thanks! (wanders off muttering calculations to himself....)

informant
2004-Apr-02, 07:24 PM
The speed or velocity of an object is always measured with respect to some other object.
Question: object or reference frame? (Is there a difference?)

Ut
2004-Apr-02, 07:38 PM
I suppose technically there is. I usually relate reference frames in my mind to things, though, so... You know, your milage may vary.

ToSeek
2004-Apr-02, 08:29 PM
How can you have a reference frame without something that defines it?

Ut
2004-Apr-02, 10:21 PM
You can chose a random point in space and attach a reference frame to it. Unfortunately, you'd have no way of knowing how you're moving relative to that point, or the frame it rests in, if there's nothing there. In the presented example, say that point lies at the centre of the sun. When the sun disappears, that point in space is still there. You just can't really keep track of where it is.

poorleno
2004-Apr-02, 10:42 PM
Can there be an absolute velocity, such that is defined between
0<V<c; with respect to another object, assumed to be "ground zero" - velocity wise?

And if so, can we assume a hypothetical ground zero on the time axis? And use that as a reference point to calculate the numinous absolute velocity?