4 ms·
That's the key part. We actually have good reasons (conservation of momentum) to believe that the revolution period does not change significantly over that time
by jofer 7y ago
That's the key part. We actually have good reasons (conservation of momentum) to believe that the revolution period does not change significantly over that timescale. There's nothing that should significantly change the momentum of the orbit around the Sun by that amount/time.
Contrast that to the Earth/Moon's rotational period, which we expect to slow over time due to energy consumed by tides "sloshing around".
- TimSchumann 7y ago> Contrast that to the Earth/Moon's rotational period, which we expect to slow over time due to energy consumed by tides "sloshing around". Not consumed, literally flung out into space via the moon. It’s speeding up, we’re slowing down. No violation of thermodynamics required.
- jofer 7y agoI simplified. Some of it actually is consumed by friction (and therefore heating), too. You're correct that most is transferred. However, the angular rate of the Moon's rotation is staying the same over time (it's tidally locked at one rotation per revolution). It's not exactly speeding up. Instead it's getting further away, which increases the moment of inertia and therefore transfers momentum.
- fhars 7y agoThat‘s what “speeding up” in a 1/r potential means, you move to a higher energy orbit, which happens to be longer and slower. Classical physics can be weird, too.
- jofer 7y agoYes, absolutely, but must people will misunderstand "speeding up" as increasing rate of rotation, which is why it's useful to clarify.