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plant
2015-Oct-26, 10:08 AM
Hi all,
i thought i was being 'clever' when i explained to my daughter that the moon actually does fall towards the earth, it's just that it falls in a circle... i.e. orbiting..
but then i was thinking... if you assume the earth and moon could exist for billions of years... what would eventually happen to the moon's orbit?

* is there any 'friction' along the moon's orbit to slow it down? solar wind? small asteroids etc?
* does the moon have a magnetic field that would interact with earths and slow it down?
* does an orbiting body emit gravity waves and eventually slow down?

* do tidal forces mean the moon is actually speeding up and receding?

i read somewhere that the moon would move faster/further away due to tidal forces until a day = a month = 49d approx... and then the moon would radiate gravity waves and eventually crash into the earth.... is that true??

thanks in advance!

Noclevername
2015-Oct-26, 10:14 AM
No
No.
No.

In the case of the Moon, it's actually speeding up, as the Earth's rotation drags at it. This transfers energy from our rotation to the Moon's orbital speed.

No, the Moon will remain distant until the Sun expands, at which point Solar gravity will probably disrupt its orbit. It has equal odds of crashing into molten Earth, eaten by the Sun, or being flung away into the Solar System. At no point will it "radiate gravitational waves".

JohnD
2015-Oct-26, 10:36 AM
Yes, the tides are causing the Moon to slow down and recede from the Earth.
See: http://physics.stackexchange.com/questions/9290/why-does-the-moon-drift-away-from-earth
And: https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment

Newtonian mechanics are counter-intuitive! Larry Niven wrote, in the Integral Tree stories, ""East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back".
This gnomic statement is explained here: http://larryniven.net/physics/img34.shtml where it explains that you move West in relation to your previous closer orbit when you move Out.

The future of this development will be that the Earth will continue to slow its rotation until it too is tidally locked to the Moon, which will then be much further away.
The Earth Day will last a month (47 present days) and the Moon will be nearly half as far away again.

But this will take billions of years, during which the Sun will mature into a red giant, expand and introduce far more drag from the Solar Wind, which will further slow the Moon, moving it further from the Earth before they are both consumed in the exapanding Sun.

Makes it all seem a pointless, doesn't it?
"Life? Don't talk to me about Life. The first ten million years were the worst. And the second ten million... they were the worst too. The third ten million I didn't enjoy at all. After that, I went into a bit of a decline." Marvin, THHGTTU.
John

plant
2015-Oct-26, 11:22 AM
JohnD - don't you mean the moon is speeding up (in least in linear velocity?) and therefore receding?
Wouldn't the angular velocity stay the same?

now this linear vs angular velocity is confusing!

Noclevername
2015-Oct-26, 11:24 AM
JohnD - don't you mean the moon is speeding up (in least in linear velocity?) and therefore receding?
Wouldn't the angular velocity stay the same?

now this linear vs angular velocity is confusing!

A larger (higher) orbit is a slower orbit.

WaxRubiks
2015-Oct-26, 11:26 AM
So the energy from the drag just lifts the moon? Does it keep the same speed?

Noclevername
2015-Oct-26, 11:33 AM
So the energy from the drag just lifts the moon? Does it keep the same speed?

No, the added net energy pushes it into a higher orbit, so counter-intuitively, the more kinetic energy you add, the slower the Moon actually gets, as it spreads out its momentum over a much larger circle of motion

Fiery Phoenix
2015-Oct-26, 04:06 PM
So the energy from the drag just lifts the moon? Does it keep the same speed?
The excess mass from the Earth's bulge facing the Moon causes a drag that expands the Moon's orbit, which in turn reduces its orbital velocity (as per Kepler's 3rd law).

It's pretty counter-intuitive like Noclevername said. The truth is there is a lot of physics going on at once, so you end up getting a bunch of intermediate scenarios that are seemingly unrelated.

It's worth noting that if, for instance, the Earth rotated more slowly than the Moon orbits (or if the Moon orbited in the opposite direction), the opposite would happen, and the Moon would move towards us as opposed to away from us. Again, same principle but different conditions. I understand this is currently happening with Triton and Neptune as well as Phobos and Mars.

Ken G
2015-Oct-26, 04:18 PM
It might help to consider what happens to a bullet on a diagonal trajectory-- it is going higher, but slowing down. Now give the bullet a rocket engine, but not enough of one for it to speed up-- it is still slowing down as it rises, just not slowing down as fast as if it had no rocket engine. That's what is in effect happening to the Moon-- it is on a slightly "diagonal" trajectory, because it is always getting (very slowly) farther from Earth, and it has a "rocket engine" (coming from the pull from Earth's tidal bulges), but that "rocket engine" is not strong enough to speed the Moon up, so instead the Moon slows down. In that case, it is the presence of the "rocket engine" that is the reason the Moon is on a diagonal path in the first place-- that's where the orbital mechanics comes in, but seems less counterintuitive if you say it this way.

Grey
2015-Oct-26, 05:07 PM
At no point will it "radiate gravitational waves".Tiny nitpick: the Earth and the Moon orbiting each other definitely constitute a system that should radiate gravitational waves, and that should indeed gradually reduce the energy of their orbit, bringing them closer together. It's just that, for objects as relatively small as the Earth and Moon, which are orbiting at pretty sedate speeds, these gravitational waves would be completely insignificant compared to all the other effects people have brought up here, and not even close to measurable with current technology (or even technology rather more advanced than ours). The only cases where we've actually measured the energy loss from gravitational waves is much more extreme situations, like close neutron star binaries (and in those cases, it matches very well with the prediction from general relativity), but in principle, it should still be there.

But it's absolutely true that this won't have a measurable effect on the Moon's orbit unless both the Moon and the Earth are somehow still here and orbiting each other trillions of years after all these other effects have played out.

grapes
2015-Oct-26, 07:17 PM
Tiny nitpick: the Earth and the Moon orbiting each other definitely constitute a system that should radiate gravitational waves, and that should indeed gradually reduce the energy of their orbit, bringing them closer together. It's just that, for objects as relatively small as the Earth and Moon, which are orbiting at pretty sedate speeds, these gravitational waves would be completely insignificant compared to all the other effects people have brought up here, and not even close to measurable with current technology (or even technology rather more advanced than ours). The only cases where we've actually measured the energy loss from gravitational waves is much more extreme situations, like close neutron star binaries (and in those cases, it matches very well with the prediction from general relativity), but in principle, it should still be there.

But it's absolutely true that this won't have a measurable effect on the Moon's orbit unless both the Moon and the Earth are somehow still here and orbiting each other trillions of years after all these other effects have played out.
Tidal forces from the sun are much greater than that though (although a lot less than the lunar ones), and when the moon-earth locks, it will slowly spiral in--but the death of the sun will probably occur before they collide. Again :)

Fiery Phoenix
2015-Oct-26, 07:31 PM
Tidal forces from the sun are much greater than that though (although a lot less than the lunar ones), and when the moon-earth locks, it will slowly spiral in--but the death of the sun will probably occur before they collide. Again :)
Are you saying once the Earth and the Moon are both tidally locked to each other, they will start moving towards each other and collide again? (Regardless of the whole Sun becoming a red giant thing.)

Interesting if so. I didn't know that.

Hornblower
2015-Oct-26, 08:16 PM
Are you saying once the Earth and the Moon are both tidally locked to each other, they will start moving towards each other and collide again? (Regardless of the whole Sun becoming a red giant thing.)

Interesting if so. I didn't know that.

When the Earth's spin slows down to being synchronized to the Moon's orbital motion, tidal interaction with the Moon becomes zero for the moment. However the Sun is still raising tides which will continue to slow the Earth's rotation. Now the Moon will overrun it and start spinning it up at the expense of the Moon's own orbital energy. This will make it spiral inward. This is assuming they don't get consumed by the bloated Sun's tenuous envelope.

Fiery Phoenix
2015-Oct-26, 09:18 PM
When the Earth's spin slows down to being synchronized to the Moon's orbital motion, tidal interaction with the Moon becomes zero for the moment. However the Sun is still raising tides which will continue to slow the Earth's rotation. Now the Moon will overrun it and start spinning it up at the expense of the Moon's own orbital energy. This will make it spiral inward. This is assuming they don't get consumed by the bloated Sun's tenuous envelope.
Very interesting, thanks. I never thought of it that way.

Celestial mechanics really is crazy, but that's exactly how I love it!

WaxRubiks
2015-Oct-26, 10:31 PM
If the moon gradually spiralled in, when it got to the roach limit, would it just break up and form a ring?

Noclevername
2015-Oct-26, 10:32 PM
If the moon gradually spiralled in, when it got to the roach limit, would it just break up and form a ring?

Roche, and probably not; the Roche limit applies to rubble piles mostly, it doesn't work as well on solid objects.

Fiery Phoenix
2015-Oct-26, 10:36 PM
If the moon gradually spiralled in, when it got to the roach limit, would it just break up and form a ring?
Theoretically, yes. Assuming it is held together only by its own self-gravity.

As I noted in an earlier comment, this is actually already underway with Neptune-Triton and Mars-Phobos. Both moons are expected to end up breaking apart and forming rings around their respective planets.

Jens
2015-Oct-27, 04:57 AM
But this will take billions of years, during which the Sun will mature into a red giant, expand and introduce far more drag from the Solar Wind, which will further slow the Moon, moving it further from the Earth before they are both consumed in the exapanding Sun.


Somehow that seems wrong to me. I thought that the drag would slow the moon, making it fall inward, but that it would pick up speed while falling so that it would come to rest at a lower altitude, though the drag would eventually fall inward. If what you say is true, then why doesn't the drag from the earth's atmosphere cause the ISS to go into a further orbit? AFAIK they have to push it back up, not back down...

JohnD
2015-Oct-27, 10:55 AM
Actually Fiery Phoenix, it was me that described Newtonian mechanics as "counter-intuitive", as the post above by Jens, usually so well informed, shows.

Is it worth repeating the Larry Niven lesson, "East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back"?
It refers to what happens if while in orbit you apply thrust in the named directions, in a remarkably concise and memorable way that can make it more intuitive.

John

Fiery Phoenix
2015-Oct-27, 12:30 PM
Actually Fiery Phoenix, it was me that described Newtonian mechanics as "counter-intuitive", as the post above by Jens, usually so well informed, shows.

Is it worth repeating the Larry Niven lesson, "East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back"?
It refers to what happens if while in orbit you apply thrust in the named directions, in a remarkably concise and memorable way that can make it more intuitive.

John
I chuckled :p

And yes, apologies for the mistake on my part!

Ken G
2015-Oct-27, 01:46 PM
Somehow that seems wrong to me. I thought that the drag would slow the moon, making it fall inward, but that it would pick up speed while falling so that it would come to rest at a lower altitude, though the drag would eventually fall inward. If what you say is true, then why doesn't the drag from the earth's atmosphere cause the ISS to go into a further orbit? AFAIK they have to push it back up, not back down...Yes, I'm not sure just what JohnD meant, or how it relates to the Larry Niven quote, but drag from the solar wind would cause the Moon to spiral in toward the Earth, even as it also caused the Earth-Moon system to spiral inward toward the Sun. That's what happens when both energy and angular momentum are removed by drag.

Jens
2015-Oct-27, 01:53 PM
Actually Fiery Phoenix, it was me that described Newtonian mechanics as "counter-intuitive", as the post above by Jens, usually so well informed, shows.

Is it worth repeating the Larry Niven lesson, "East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back"?
It refers to what happens if while in orbit you apply thrust in the named directions, in a remarkably concise and memorable way that can make it more intuitive.


But I didn't think you were talking about applying thrust. I thought you were thinking of applying dust!

grapes
2015-Oct-27, 01:55 PM
Actually Fiery Phoenix, it was me that described Newtonian mechanics as "counter-intuitive", as the post above by Jens, usually so well informed, shows.

Is it worth repeating the Larry Niven lesson, "East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back"?
It refers to what happens if while in orbit you apply thrust in the named directions, in a remarkably concise and memorable way that can make it more intuitive.

Are you saying Jens is wrong?

loosecannon
2015-Oct-27, 02:28 PM
Somehow that seems wrong to me. I thought that the drag would slow the moon, making it fall inward, but that it would pick up speed while falling so that it would come to rest at a lower altitude, though the drag would eventually fall inward. If what you say is true, then why doesn't the drag from the earth's atmosphere cause the ISS to go into a further orbit? AFAIK they have to push it back up, not back down...

In the realm of circular orbits, "more energy" ==> "higher orbit" ==> "slower speed". The result is less kinetic energy, but more potential energy; the potential energy dominates. The kinetic energy of an object in a circular orbit is one half of what is needed to have it escape orbit completely.

If we start with the moon in a circular orbit, then slowing it down (let's say, firing a rocket in the direction opposite its motion), then the orbit will become elliptical. The point where the thrust was applied will remain in the new orbit. The speed will be different at different points in the orbit. It will be lower at the high point, but it may well be substantially higher at the low point. So has the moon sped up or slowed down? Well, both, really. Now suppose at the low point of the orbit, you slow the moon down again, once again firing a rocket in the opposite direction of the moon's motion. Further suppose you apply just the right amount of thrust to restore a circular orbit, but at the new, lower level. Since the moon is once again in a circular orbit, but at a lower altitude, it will be moving faster. So you have slowed it down twice, and the result is that it is moving faster.

The forces that are changing the moon's orbit are adding energy to it, not taking energy away. Specifically, the rotational energy of the earth is being transferred into the orbital motion of the moon. So if we try to keep it simple and pretend that this happens in two discrete bursts (instead of happening continuously), it would be the opposite of the situation described above. Transfer of energy to the moon increases its speed, so it enters an elliptical orbit, and reaches a higher altitude. Before letting it fall back down again, we transfer some more energy, restoring a circular orbit, but at a higher altitude. It is now moving slower. We have added energy, speeding it up twice, and its moving slower than it was before - this is because it slowed down as it went up to the higher altitude.

Assuming this occurs in two discrete bursts is of course a silly oversimplification, as the process is really continuous. Energy is being added, which, by itself, would add kinetic energy to the moon (speed it up). But this causes it to move to a higher altitude, slowing down in the process. Energy is being added, and simultaneously, kinetic energy is being converted into potential energy. The amount of kinetic energy converted into potential energy is greater than the amount of kinetic energy added to the system, so the result is a slowing down, even though the moon is in a higher energy state.

And if you want to make it still more complicated, the moon is not really orbiting the earth; both are orbiting a centre of gravity. So it's a bit more complicated even than that.

George
2015-Oct-27, 04:31 PM
Yes, I'm not sure just what JohnD meant, or how it relates to the Larry Niven quote, but drag from the solar wind would cause the Moon to spiral in toward the Earth, even as it also caused the Earth-Moon system to spiral inward toward the Sun. That's what happens when both energy and angular momentum are removed by drag. Yet only half the orbit is a drag; the other half would be more fun, so to speak, right? Greater eccentricity would make sense, but I am unclear how a net drag is a result, regardless of the net solar wind vector (to include Earth's transverse motion into the wind).

Ken G
2015-Oct-27, 04:46 PM
Yet only half the orbit is a drag; the other half would be more fun, so to speak, right? Greater eccentricity would make sense, but I am unclear how a net drag is a result, regardless of the net solar wind vector (to include Earth's transverse motion into the wind).
I would say the way to think of this is, the motion of the gas itself doesn't matter, because it produces an outward force that is the same all the time, and all it does it to slightly offset gravity. So it's like the Sun's gravity is slightly weaker, but so what? A gravity that is weaker by a fixed amount won't cause long-term orbital evolution. So it is only the force that correlates with the motion of the orbiting object, i.e., that changes direction when the object's motion changes direction, that matters. Such a correlation allows there to be "net work" done on the object. That correlation will tend to produce a force against the direction of motion ("drag"), so that does negative net work, and produces a negative net torque. That's what causes the inspiral.

grapes
2015-Oct-27, 04:55 PM
In the realm of circular orbits, "more energy" ==> "higher orbit" ==> "slower speed". The result is less kinetic energy, but more potential energy; the potential energy dominates. The kinetic energy of an object in a circular orbit is one half of what is needed to have it escape orbit completely.

If we start with the moon in a circular orbit, then slowing it down (let's say, firing a rocket in the direction opposite its motion), then the orbit will become elliptical. The point where the thrust was applied will remain in the new orbit. The speed will be different at different points in the orbit. It will be lower at the high point, but it may well be substantially higher at the low point. So has the moon sped up or slowed down? Well, both, really. Now suppose at the low point of the orbit, you slow the moon down again, once again firing a rocket in the opposite direction of the moon's motion. Further suppose you apply just the right amount of thrust to restore a circular orbit, but at the new, lower level. Since the moon is once again in a circular orbit, but at a lower altitude, it will be moving faster. So you have slowed it down twice, and the result is that it is moving faster.

The forces that are changing the moon's orbit are adding energy to it, not taking energy away. Specifically, the rotational energy of the earth is being transferred into the orbital motion of the moon. So if we try to keep it simple and pretend that this happens in two discrete bursts (instead of happening continuously), it would be the opposite of the situation described above. Transfer of energy to the moon increases its speed, so it enters an elliptical orbit, and reaches a higher altitude. Before letting it fall back down again, we transfer some more energy, restoring a circular orbit, but at a higher altitude. It is now moving slower. We have added energy, speeding it up twice, and its moving slower than it was before - this is because it slowed down as it went up to the higher altitude.

Assuming this occurs in two discrete bursts is of course a silly oversimplification, as the process is really continuous. Energy is being added, which, by itself, would add kinetic energy to the moon (speed it up). But this causes it to move to a higher altitude, slowing down in the process. Energy is being added, and simultaneously, kinetic energy is being converted into potential energy. The amount of kinetic energy converted into potential energy is greater than the amount of kinetic energy added to the system, so the result is a slowing down, even though the moon is in a higher energy state.

And if you want to make it still more complicated, the moon is not really orbiting the earth; both are orbiting a centre of gravity. So it's a bit more complicated even than that.
But the comment of Jens is in regards to drag, removing energy from the system, no?

WaxRubiks
2015-Oct-27, 05:08 PM
I suppose the force isn't the same for the moon going towards the Sun and away from it, as the relative speed of the solar wind is less when the moon is moving away from the Sun?

George
2015-Oct-27, 05:16 PM
I would say the way to think of this is, the motion of the gas itself doesn't matter, because it produces an outward force that is the same all the time, and all it does it to slightly offset gravity. So it's like the Sun's gravity is slightly weaker,... That is a clever way to see it. Regardless of the Moon's position in the orbit, the Sun's gravity will feel weaker.


... So it is only the force that correlates with the motion of the orbiting object, i.e., that changes direction when the object's motion changes direction, that matters. Such a correlation allows there to be "net work" done on the object. That correlation will tend to produce a force against the direction of motion ("drag"), so that does negative net work, and produces a negative net torque. That's what causes the inspiral. Isn't work being done to decelerate the orbit for half the period and accelerate it by an equal amount for the other half? Work is being done so I can see where you are going, but I'm having trouble seeing its net effect.

George
2015-Oct-27, 05:18 PM
I suppose the force isn't the same for the moon going towards the Sun and away from it, as the relative speed of the solar wind is less when the moon is moving away from the Sun?

If that is true, it would explain things, but why is one side of the orbit different (in net acceleration/deceleration)?

Jeff Root
2015-Oct-27, 05:33 PM
The planet's orbit around the Sun will become more circular,
not more elongated. The orbits of Earth satellites circularize
just before they re-enter. Dramatization down the page on
the right. If you have changed your browser's zoom level,
temporarily put it back to 100% to view the gif animation
without nasty artifacts. I didn't know about that at the time
I made it. Also screens were generally smaller back then,
so it is tiny.

http://www.freemars.org/jeff/speed/index.htm

-- Jeff, in Minneapolis

George
2015-Oct-27, 05:55 PM
The planet's orbit around the Sun will become more circular,
not more elongated. The orbits of Earth satellites circularize
just before they re-enter. Dramatization down the page on
the right. If you have changed your browser's zoom level,
temporarily put it back to 100% to view the gif animation
without nasty artifacts. I didn't know about that at the time
I made it. Also screens were generally smaller back then,
so it is tiny.

http://www.freemars.org/jeff/speed/index.htm The Solar wind has a push-pull effect on the Moon, very much unlike the push-only effect caused by atmospheric drag for crashing satellites.

WaxRubiks
2015-Oct-27, 06:10 PM
But I didn't think you were talking about applying thrust. I thought you were thinking of applying dust!


I think the drag that was talked about was drag from the sea tide. The bulge of the sea pulling the moon in the direction of its orbit...I think....

Buttercup
2015-Oct-27, 06:48 PM
Kryptonite? :)

Jeff Root
2015-Oct-27, 07:16 PM
Pull? You'll have to explain that. There might be a very
miniscule outward push from solar wind, but the main
drag is when the Sun's atmosphere gets big enough to
engulf the Earth and Moon.

I'm pretty sure the Sun's loss of mass to solar wind has
a far greater effect than the direct impact of the wind on
the Moon.

-- Jeff, in Minneapolis

George
2015-Oct-27, 09:11 PM
Pull? You'll have to explain that. There might be a very
miniscule outward push from solar wind, but the main
drag is when the Sun's atmosphere gets big enough to
engulf the Earth and Moon.

I'm pretty sure the Sun's loss of mass to solar wind has
a far greater effect than the direct impact of the wind on
the Moon. I think I see how you see it, namely that the solar wind acts like our atmosphere does on satellites, which contribute greatly to orbital decay. This is probably what Ken is saying as well, but I envision two separate push-pull effects: one radial and one tangential. As the Moon moves radially toward the Sun, the wind will act ("push") to slow it down, but as it moves radially away from the Sun, the wind acts ("pulls") on the Moon to speed it up. As the Moon swings more behind the Earth, it will turn tangentially in its orbit whereupon the wind will push to slow it down, but 180 deg. later, cause it to speed back up. Or we could combine the two using a vector of about 4 to 5 degrees [sine(67k mph/895k mph)].

cjameshuff
2015-Oct-27, 10:35 PM
I'm pretty sure the Sun's loss of mass to solar wind has
a far greater effect than the direct impact of the wind on
the Moon.

Also, the force exerted by sunlight is about 1000 times greater.



I think I see how you see it, namely that the solar wind acts like our atmosphere does on satellites, which contribute greatly to orbital decay. This is probably what Ken is saying as well, but I envision two separate push-pull effects: one radial and one tangential. As the Moon moves radially toward the Sun, the wind will act ("push") to slow it down, but as it moves radially away from the Sun, the wind acts ("pulls") on the Moon to speed it up. As the Moon swings more behind the Earth, it will turn tangentially in its orbit whereupon the wind will push to slow it down, but 180 deg. later, cause it to speed back up. Or we could combine the two using a vector of about 4 to 5 degrees [sine(67k mph/895k mph)].

I don't see how the force exerted by the solar wind (or sunlight) is ever in any sense a "pull". How are the speed and direction of motion of the moon relevant? The only things that matter are the motion of the wind and direction of the force it applies. Sometimes it accelerates the moon and sometimes it decelerates the moon, but it is always a push outward from the sun.

JohnD
2015-Oct-27, 11:30 PM
Are you saying Jens is wrong?

YOU may say that, I could not possibly comment.

But yes, I was extrapolating to fast. Atmospheric drag would bring them in.
I think in Niven's Smoke Ring, "West" was in the direction of travel, so thrusting West would have the same effect as drag.
And "West takes you in"
""East takes you out, out takes you west, west takes you in, in takes you east, port and starboard bring you back".

John

Jeff Root
2015-Oct-28, 04:45 AM
JohnD,

I think you were NOT suggesting that Jens was wrong,
but that was how grapes interpreted what you wrote,
and it puzzled me for a moment, too.

* * * *

I haven't read the story, but from the quotes, I'd say that
the direction of travel must be from west to east. And I'd
say that means "East" was in the direction of travel, to use
your phrasing.

When you say "thrusting West" do you mean that your
thrust pushes you toward the west, or that your exhaust
goes toward the west? I'd think it would be the former,
in which case your statement that thrusting West would
have the same effect as drag is correct -- If you are
moving from west to east.

If you think in terms of wind directions, God help you.

-- Jeff, in Minneapolis

Ken G
2015-Oct-28, 05:19 AM
As the Moon moves radially toward the Sun, the wind will act ("push") to slow it down, but as it moves radially away from the Sun, the wind acts ("pulls") on the Moon to speed it up. As the Moon swings more behind the Earth, it will turn tangentially in its orbit whereupon the wind will push to slow it down, but 180 deg. later, cause it to speed back up. Or we could combine the two using a vector of about 4 to 5 degrees [sine(67k mph/895k mph)].How you express what is happening is all in how you break up the problem. I'm just saying one good way to break up the problem is to first look at the effect the solar wind would have if the Moon was not moving at all, and that's an outward push, but it only has the effect of weakening the inward pull of gravity a little-- so has no long-term evolutionary effect. Then once we've removed the effects of the solar wind motion, we can put in the Moon's motion, and see how it interacts with a stationary gas. That's the drag effect, that only causes inspiral, so is not changing sign. Then we put the two back together again to get the combined effect, and it all looks like drag for the long-term evolutionary changes in the orbit. Now that's just one way to "slice" what is happening, but it seems like a good way for seeing what does, and what does not, affect the long-term evolution.

And it is a good point that the mass lost by the Sun will affect its gravity more than the direct push on the Moon, and possibly even more than the drag effect, given that the loss of mass by the Sun will force the Earth-Moon system to move outward just to keep its angular momentum and still be in a circular orbit around a weaker gravity source, which will likely mean that the Earth-Moon system is never engulfed in the star, and may never experience significant drag.

George
2015-Oct-28, 01:28 PM
Also, the force exerted by sunlight is about 1000 times greater.




I don't see how the force exerted by the solar wind (or sunlight) is ever in any sense a "pull". How are the speed and direction of motion of the moon relevant? The only things that matter are the motion of the wind and direction of the force it applies. Sometimes it accelerates the moon and sometimes it decelerates the moon, but it is always a push outward from the sun. I was being artistic to illustrate the difference between the constant pressure on a satellite when entering our atmosphere (Jeff's example) vs. the "push" that decelerates a satellite for half an orbit, then accelerates it for the other half (ie "pull"). A push and pull has a tendency to get you back to where you were with little or no change, which was my point.

George
2015-Oct-28, 02:24 PM
How you express what is happening is all in how you break up the problem. I'm just saying one good way to break up the problem is to first look at the effect the solar wind would have if the Moon was not moving at all,... Right, and I imagined the wind pushing it first one way then another, but I failed to see that the force (orbital tangential component) would always act to slow the Moon, as in Jeff's decaying satellite example. Dang! Thanks.

cjameshuff
2015-Oct-28, 10:13 PM
I was being artistic to illustrate the difference between the constant pressure on a satellite when entering our atmosphere (Jeff's example) vs. the "push" that decelerates a satellite for half an orbit, then accelerates it for the other half (ie "pull"). A push and pull has a tendency to get you back to where you were with little or no change, which was my point.

I'm just not seeing how pushing from behind, from the front, or from any other direction is anything even remotely like pulling.

JohnD
2015-Oct-28, 10:54 PM
A push and pull has a tendency to get you back to where you were with little or no change, which was my point.

I don't think that's so either, George! Again it's counterintuitive.
I remember school problems that involved trains or cars performing return journeys, with and against a wind. The point is that with the wind against you, you spend more time being resisted, and less when it's in your favour, than if there were no wind.
The effect is that the advantage of the favourable wind cannot make up for the delay caused by the wind in your face.
Bit like the Second Law of Thermodynamics: You can't win. You can't break even, even on on a very windy day.

John

plant
2015-Oct-29, 01:10 AM
... also i was thinking.. is the moon technically falling in a straight line... through curved spacetime??.. rather than falling in a 'circular' orbit...?

plant
2015-Oct-29, 01:40 AM
I would say the way to think of this is, the motion of the gas itself doesn't matter, because it produces an outward force that is the same all the time, and all it does it to slightly offset gravity. So it's like the Sun's gravity is slightly weaker, but so what? A gravity that is weaker by a fixed amount won't cause long-term orbital evolution.

..but would the cumulative collisions with solar wind obey the same power law as gravity? (i was thinking if the particles are distributed through an ever increasing volume of space it should reduce by a power of 3?)
... also the interactions with an object would be proprtional to it's surface area rather than it's mass.... and since a larger object has more mass / surface area (if made of the same material).. presumably the change in momentum would be less when it is hit by the solar wind?

is this correct?
..or is it insignificant?

cjameshuff
2015-Oct-29, 02:20 AM
The interactions are proportional to the cross-sectional area, not the surface area, and fall off with the square of distance, as the force is distributed over the surface area of a sphere, not its volume. It would be less significant for larger objects because of their greater volume and mass, yes. And dependent on surface properties in a few odd ways: a surface that traps the solar wind or absorbs much of their energy when they rebound will receive less force than one that the particles elastically collide from while keeping most of their energy.

As I mentioned previously, it would be far smaller than light pressure effects, and also thermal effects. The latter is due to the fact that the surface absorbs light but then radiates it again as infrared, and if the body is rotating, it doesn't get radiated in the same direction the light came from. Thermal effects can do weird things, including acceleration or deceleration and changing the rotation, in some cases eventually spinning small bodies up to the point where they break apart (the YORP effect: https://en.wikipedia.org/wiki/Yarkovsky%E2%80%93O%27Keefe%E2%80%93Radzievskii%E2 %80%93Paddack_effect).

Ken G
2015-Oct-29, 07:47 AM
..but would the cumulative collisions with solar wind obey the same power law as gravity? (i was thinking if the particles are distributed through an ever increasing volume of space it should reduce by a power of 3?)Actually, the power is 2, just like light, if the speed is constant (and the solar windspeed is pretty constant by the time it gets to Earth.) The reason it's the same power is you can think of gravity as being caused by something like a "wind" of gravitons.


... also the interactions with an object would be proprtional to it's surface area rather than it's mass....Doesn't matter if neither are changing.

and since a larger object has more mass / surface area (if made of the same material).. presumably the change in momentum would be less when it is hit by the solar wind?
The solar wind force is certainly very tiny, that's why it isn't worth thinking about except in terms of long-time drag effects. Even those are small with today's solar wind.

George
2015-Oct-29, 02:21 PM
I'm just not seeing how pushing from behind, from the front, or from any other direction is anything even remotely like pulling. It is a tensionless pull. Amateur heliochromoloigsts often attempt colorful expressions, especially if they are oxymoronic. Most attempts fail, unfortunately.

George
2015-Oct-29, 03:32 PM
I don't think that's so either, George! Again it's counterintuitive.
I remember school problems that involved trains or cars performing return journeys, with and against a wind. The point is that with the wind against you, you spend more time being resisted, and less when it's in your favour, than if there were no wind. I don't think that the radial solar wind component will have a time difference, but I could be wrong. If the Earth was fixed, to address only the radial component, the Moon following its full moon phase would begin to decelerate until the new moon phase, giving it a little more time to decelerate. From the new moon phase, the now slower Moon will have a little more time in this period to accelerate. I am guessing that there is no time difference, but curious if this is the case.


The effect is that the advantage of the favourable wind cannot make up for the delay caused by the wind in your face. The train analogy would apply [if modified] if we had a coasting train (frictionless wheels) that had an initial speed against the wind of say, 100 kph. If it reached its destination at say, 80 kph, then what would be the time difference if we turn it around with the wind with the initial speed of 80 kph? The boat analogy is another analogy that uses only the velocity difference to and from, so time difference can be significant, but is not applicable to our Solar wind story.

A better analogy is looking at the difference in time between the upward travel time of a thrown ball vs. the downward time. In a vacuum, there is none. Here the force of gravity is analogous to the force of the Solar wind (radial only).

Given my prior gross error, I may stand corrected, though I may choose to lie down for a while instead.

Grey
2015-Oct-29, 03:40 PM
... also i was thinking.. is the moon technically falling in a straight line... through curved spacetime??.. rather than falling in a 'circular' orbit...?Yes, sort of (at least, assuming that general relativity is an accurate way of looking at gravity). The term usually used is "geodesic (https://en.wikipedia.org/wiki/Geodesic)", which is the straightest possible line between two points in a curved space. For example, on the curved surface of the Earth, geodesics are great circles. So a geodesic isn't quite a straight line, but it's the closest thing you can get to it in curved space. And yes, if an object is in freefall (as orbiting objects are), then it is travelling along a geodesic.