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Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tid

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the earth's spin. It takes a lot of energy to lift a moon, relative to the amount of energy in the earth's spin, which is why it has such a dramatic effect on the day length.

redact207 4y ago
If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?
- yellowapple 4y ago
  Eventually, yeah; just as the Moon is tidally locked to Earth, Earth would be tidally locked to the Moon. I don't know offhand whether that would happen before the Moon drifts too far away to remain in Earth's orbit, however.
  - sliken 4y ago
    Moon is below escape velocity, so it's not leaving. In fact when 1 day = 1 lunar month the movement away will stop, and reverse. It will continue to get closer to earth until it hits the Roche limit and becomes a ring.
- aussieshibe 4y ago
  Fair warning: everything I know about orbital mechanics is from Kerbal Space Program. The moon cannot orbit at a higher speed while keeping the same semi-major axis (average distance to the orbital centre of mass). If you suddenly doubled the orbital speed of the moon right now, the apoapsis (the highest point in its orbit relative to the orbital centre of mass) would increase significantly. If you slowly accelerate the moon in the direction of its orbital velocity consistently over a long time period, the moon will slow down relative to the Earth, but it's semi major axis will increase. Actual scientists of HN: have at me.
  - incompatible 4y ago
    Yeah, the moon isn't "drifting away" on its own, its being forced away as it takes energy from the Earth.
  - robertlagrant 4y ago
    > Fair warning: everything I know about orbital mechanics is from Kerbal Space Program. Knowing what I know about that game I read this as: Warning; I actually know what I'm talking about here.
    - cwillu 4y ago
      It depends; there's actually a mod that adds realistic n-body gravity, and it's remarkable how much it affects.
- deleted 4y ago
  [deleted]
- devoutsalsa 4y ago
  The moon is drifting away because the Earth is rotating. Eventually the Earth will be tidally locked with the moon, the same side of the Earth facing the moon at all times. When this happens, the moon will (for the most part) stop moving away from the Earth. Well, that’s what would happen if we didn’t get eaten by the sun when it morphs into a red giant. For further research, check out… “Will the Moon ever leave the Earth’s orbit?” => https://youtu.be/IM_euz9PUiw https://youtu.be/IM_euz9PUiw
- dotancohen 4y ago
  It's already happened! More at the end. This is called tidal locking, and if the universe consisted of only the Earth and moon, this would in fact happen. However, the big heavy Sun also affects both the Earth and moon's rotations. So why do I say that it has happened? Because the moon, having significantly less mass than the Earth, is almost tidally locked to the Earth. That's why we always see the same side of the moon. So the Earth's rotation hasn't synchronized with the moon's revolution, but the moon's rotation has nearly synchronized with the Earth's revolution (actually both the Earth and moon revolve around their common barycenter).
- sliken 4y ago
  Yes, but it will take a long time. Then very slowly the moon will slow down, decrease it's orbit, until it hits the Roche limit and becomes a ring.
chowells 4y ago
If the moon's orbit is increasing in radius, doesn't that mean its orbital velocity is decreasing? Which way is energy being transferred? This stuff confuses me.
- ncallaway 4y ago
  I haven't done the math, but I strongly suspect that the potential energy (from moving away from the source of gravity) is increased more than the kinetic energy (from the reduced velocity) decreases. So, I would think when the moon moves away from the earth, its total energy increases. Thus, the earth's energy decreases (in the form of slightly reduced rotational kinetic energy)
  - onos 4y ago
    That’s right. The radius is set by the force mv^2/r = GMm/r^2. —> mv^2/2 = 0.5 GMm/r —> Kinetic plus potential = - 0.5 GMm/r This goes up with r.
  - sliken 4y ago
    Earths rotational velocity is decreased (longer days) and added to the moon which increases it's potential energy (distance relative to earth), increases it's orbital height, and increases the period of it's orbit (the lunar month). But once the lunar month = earth day the transfer will stop and the moon will slowly approach earth again, until it hits the roche limit and becomes a ring.
- sgtnoodle 4y ago
  Its relative velocity with respect to a Cartesian frame of reference fixed on the surface of the Earth may be dropping, but that's not particularly meaningful. Think of the moon being in free fall, without any external forces acting on it. It would be moving at a constant velocity in a straight line, except the space and time it is in is curved due to gravity. Because of that curved spacetime, the moon appears to accelerate relative to the Earth. It's not actually accelerating, though; it is moving in a straight line at a constant velocity, the straight line just happens to be curved completely around the Earth. The tidal forces are literal forces, and forces cause acceleration. So, the moon isn't quite moving at constant velocity. The change in velocity means the moon isn't quite travelling in a straight line through spacetime. The orbit changes, and in this case gets higher and slower relative to the Earth. Another way to think about it. If you're in a space ship at a point X1 in an orbit, you can steer the nose of the ship in the direction you're moving relative to the Earth, and fire your rocket engine. You're now going faster. The opposite end of your orbit, point Y1, will now be higher in altitude than it would have otherwise been. Your relative speed at Y1 will indeed be slower than where you would have been had you not fired your engine, but when you circle back to X1 again your speed will still be higher. When you get to Y1 again, you could fire your engine a second time and increase your speed even more. You'll no longer end up back at X1, but a new point X2 at a higher altitude than X1 was. Your relative velocity at X2 will be lower than it was at X1. In space, "speed" isn't really velocity, but acceleration. Big rocket engines make you go fast! In The Martian, the main character makes a comment to that effect when he talks about NASA convincing him to strap himself into a hodge podge death rocket, by claiming he'll be the "fastest" astronaut in history. In a future where humans practically travel to a distant star, a "fast enough" space ship would be one that can maintain constant non-trivial acceleration for many decades. You would accelerate to the halfway point, then turn around and decelerate the rest of the way. Assuming you got fast enough relative to the destination, weird relativistic effects would become obviously apparent and the travellers would perceive space and time compressing.

redact207 4y ago

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

yellowapple 4y ago
Eventually, yeah; just as the Moon is tidally locked to Earth, Earth would be tidally locked to the Moon. I don't know offhand whether that would happen before the Moon drifts too far away to remain in Earth's orbit, however.
- sliken 4y ago
  Moon is below escape velocity, so it's not leaving. In fact when 1 day = 1 lunar month the movement away will stop, and reverse. It will continue to get closer to earth until it hits the Roche limit and becomes a ring.
aussieshibe 4y ago
Fair warning: everything I know about orbital mechanics is from Kerbal Space Program. The moon cannot orbit at a higher speed while keeping the same semi-major axis (average distance to the orbital centre of mass). If you suddenly doubled the orbital speed of the moon right now, the apoapsis (the highest point in its orbit relative to the orbital centre of mass) would increase significantly. If you slowly accelerate the moon in the direction of its orbital velocity consistently over a long time period, the moon will slow down relative to the Earth, but it's semi major axis will increase. Actual scientists of HN: have at me.
- incompatible 4y ago
  Yeah, the moon isn't "drifting away" on its own, its being forced away as it takes energy from the Earth.
- robertlagrant 4y ago
  > Fair warning: everything I know about orbital mechanics is from Kerbal Space Program. Knowing what I know about that game I read this as: Warning; I actually know what I'm talking about here.
  - cwillu 4y ago
    It depends; there's actually a mod that adds realistic n-body gravity, and it's remarkable how much it affects.
deleted 4y ago
[deleted]
devoutsalsa 4y ago
The moon is drifting away because the Earth is rotating. Eventually the Earth will be tidally locked with the moon, the same side of the Earth facing the moon at all times. When this happens, the moon will (for the most part) stop moving away from the Earth. Well, that’s what would happen if we didn’t get eaten by the sun when it morphs into a red giant. For further research, check out… “Will the Moon ever leave the Earth’s orbit?” => https://youtu.be/IM_euz9PUiw https://youtu.be/IM_euz9PUiw
dotancohen 4y ago
It's already happened! More at the end. This is called tidal locking, and if the universe consisted of only the Earth and moon, this would in fact happen. However, the big heavy Sun also affects both the Earth and moon's rotations. So why do I say that it has happened? Because the moon, having significantly less mass than the Earth, is almost tidally locked to the Earth. That's why we always see the same side of the moon. So the Earth's rotation hasn't synchronized with the moon's revolution, but the moon's rotation has nearly synchronized with the Earth's revolution (actually both the Earth and moon revolve around their common barycenter).
sliken 4y ago
Yes, but it will take a long time. Then very slowly the moon will slow down, decrease it's orbit, until it hits the Roche limit and becomes a ring.

chowells 4y ago

If the moon's orbit is increasing in radius, doesn't that mean its orbital velocity is decreasing? Which way is energy being transferred? This stuff confuses me.

ncallaway 4y ago
I haven't done the math, but I strongly suspect that the potential energy (from moving away from the source of gravity) is increased more than the kinetic energy (from the reduced velocity) decreases. So, I would think when the moon moves away from the earth, its total energy increases. Thus, the earth's energy decreases (in the form of slightly reduced rotational kinetic energy)
- onos 4y ago
  That’s right. The radius is set by the force mv^2/r = GMm/r^2. —> mv^2/2 = 0.5 GMm/r —> Kinetic plus potential = - 0.5 GMm/r This goes up with r.
- sliken 4y ago
  Earths rotational velocity is decreased (longer days) and added to the moon which increases it's potential energy (distance relative to earth), increases it's orbital height, and increases the period of it's orbit (the lunar month). But once the lunar month = earth day the transfer will stop and the moon will slowly approach earth again, until it hits the roche limit and becomes a ring.
sgtnoodle 4y ago
Its relative velocity with respect to a Cartesian frame of reference fixed on the surface of the Earth may be dropping, but that's not particularly meaningful. Think of the moon being in free fall, without any external forces acting on it. It would be moving at a constant velocity in a straight line, except the space and time it is in is curved due to gravity. Because of that curved spacetime, the moon appears to accelerate relative to the Earth. It's not actually accelerating, though; it is moving in a straight line at a constant velocity, the straight line just happens to be curved completely around the Earth. The tidal forces are literal forces, and forces cause acceleration. So, the moon isn't quite moving at constant velocity. The change in velocity means the moon isn't quite travelling in a straight line through spacetime. The orbit changes, and in this case gets higher and slower relative to the Earth. Another way to think about it. If you're in a space ship at a point X1 in an orbit, you can steer the nose of the ship in the direction you're moving relative to the Earth, and fire your rocket engine. You're now going faster. The opposite end of your orbit, point Y1, will now be higher in altitude than it would have otherwise been. Your relative speed at Y1 will indeed be slower than where you would have been had you not fired your engine, but when you circle back to X1 again your speed will still be higher. When you get to Y1 again, you could fire your engine a second time and increase your speed even more. You'll no longer end up back at X1, but a new point X2 at a higher altitude than X1 was. Your relative velocity at X2 will be lower than it was at X1. In space, "speed" isn't really velocity, but acceleration. Big rocket engines make you go fast! In The Martian, the main character makes a comment to that effect when he talks about NASA convincing him to strap himself into a hodge podge death rocket, by claiming he'll be the "fastest" astronaut in history. In a future where humans practically travel to a distant star, a "fast enough" space ship would be one that can maintain constant non-trivial acceleration for many decades. You would accelerate to the halfway point, then turn around and decelerate the rest of the way. Assuming you got fast enough relative to the destination, weird relativistic effects would become obviously apparent and the travellers would perceive space and time compressing.