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I can see how counting the rings can tell you that there were 372 rotations per revolution vs today's 365 rotations per revolution. What I don't get is how you

I can see how counting the rings can tell you that there were 372 rotations per revolution vs today's 365 rotations per revolution. What I don't get is how you correlate that to shorter days. Wasn't the Earth's revolution on a different period back then too? If you assume the revolution period was same back then as it is now... sure, half an hour difference I get it. Or are we assuming that the rotation is changing faster than the revolution?

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 ago
    I 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 ago
      That‘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 ago
        Yes, absolutely, but must people will misunderstand "speeding up" as increasing rate of rotation, which is why it's useful to clarify.
stan_rogers 7y ago
There is no force comparable to lunar tidal forces affecting the period of revolution. The Earth's slower rotation is coupled to the Moon's recession. The closest thing we've got with our orbit is resonance with Jupiter, and that's an awfully long way away.
- TimSchumann 7y ago
  Yeah, I was trying to come up with a way to address the parent comment’s first slight inaccuracy — that the revolution period (year timescale) of our planet is being slowly changed by some other force. It’s essentially not. Or, stated another way, anything that could account for that large of time variation in our year over that short of a time period... imagine ocean tides but with the Earth’s mantle instead. Then we’re not here to have this debate. For all practical purposes, the rotational period of the earth around the sun can be considered a constant.
- londons_explore 7y ago
  > There is no force comparable to lunar tidal forces affecting the period of revolution. But there are... The earth orbiting causes tiny tides on the sun. They might only be a few millimeters, but they're non-zero. Over time, tidal drag will tend to make years longer. Anyone have the time and skill to do a ballpark guess the magnitude of this effect?
  - jbay808 7y ago
    Wouldn't tidal drag make years shorter? Longer years means a more distant, high energy orbit.
    - TimSchumann 7y ago
      The sidereal year is what we're talking about here, the time it takes the earth to orbit the sun and come back to the same position. What's changing isn't that, it's the sidereal day. Earth spins slower > Moon Speeds Up Less Rotations per Orbit > More Hours per Day Number of days is changing because the day is going from 23.X hours to 24.X hours due to the Earth rotating slower. Hence, same length year if you measure it in absolute time, just less days in relative time.

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 ago
  I 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 ago
    That‘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 ago
      Yes, absolutely, but must people will misunderstand "speeding up" as increasing rate of rotation, which is why it's useful to clarify.

stan_rogers 7y ago

There is no force comparable to lunar tidal forces affecting the period of revolution. The Earth's slower rotation is coupled to the Moon's recession. The closest thing we've got with our orbit is resonance with Jupiter, and that's an awfully long way away.

TimSchumann 7y ago
Yeah, I was trying to come up with a way to address the parent comment’s first slight inaccuracy — that the revolution period (year timescale) of our planet is being slowly changed by some other force. It’s essentially not. Or, stated another way, anything that could account for that large of time variation in our year over that short of a time period... imagine ocean tides but with the Earth’s mantle instead. Then we’re not here to have this debate. For all practical purposes, the rotational period of the earth around the sun can be considered a constant.
londons_explore 7y ago
> There is no force comparable to lunar tidal forces affecting the period of revolution. But there are... The earth orbiting causes tiny tides on the sun. They might only be a few millimeters, but they're non-zero. Over time, tidal drag will tend to make years longer. Anyone have the time and skill to do a ballpark guess the magnitude of this effect?
- jbay808 7y ago
  Wouldn't tidal drag make years shorter? Longer years means a more distant, high energy orbit.
  - TimSchumann 7y ago
    The sidereal year is what we're talking about here, the time it takes the earth to orbit the sun and come back to the same position. What's changing isn't that, it's the sidereal day. Earth spins slower > Moon Speeds Up Less Rotations per Orbit > More Hours per Day Number of days is changing because the day is going from 23.X hours to 24.X hours due to the Earth rotating slower. Hence, same length year if you measure it in absolute time, just less days in relative time.