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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 close

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.
    - jbay808 7y ago
      Assuming that revolution means sidereal year, this discussion thread is speculating about how minute the changes in the sidereal year would be, and in what direction they would have been. I don't think there's confusion about the fact that the orders of magnitude larger change as discussed in the article is in the sidereal day, except perhaps for GGGGP's comment that started the thread. >>>>> Wasn't the Earth's revolution on a different period back then too? >>>> 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 [...] Over time, tidal drag will tend to make years longer. >> Wouldn't tidal drag make years shorter? > What's changing isn't that, it's the sidereal day.
- stan_rogers 7y ago
  That would explain a change in the rotational speed of the sun. Our orbit would then be affected by the tidal bulge on the sun leading us, and millimetres (or less) over the distance involved isn't going to do it, at least not to any degree comparable to the tidal interactions between the Earth and Moon. Also, we're not the only body that would have significant tidal effects on the mass distribution of the sun - we're not even at the top. Venus would have a larger effect pushing us one way; Jupiter a larger effect pulling us the other way. We're a bit of fluff, a dust mote. We're as likely to lose kinetic energy as to gain it, making the year shorter and bringing us closer.

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.
  - jbay808 7y ago
    Assuming that revolution means sidereal year, this discussion thread is speculating about how minute the changes in the sidereal year would be, and in what direction they would have been. I don't think there's confusion about the fact that the orders of magnitude larger change as discussed in the article is in the sidereal day, except perhaps for GGGGP's comment that started the thread. >>>>> Wasn't the Earth's revolution on a different period back then too? >>>> 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 [...] Over time, tidal drag will tend to make years longer. >> Wouldn't tidal drag make years shorter? > What's changing isn't that, it's the sidereal day.
stan_rogers 7y ago
That would explain a change in the rotational speed of the sun. Our orbit would then be affected by the tidal bulge on the sun leading us, and millimetres (or less) over the distance involved isn't going to do it, at least not to any degree comparable to the tidal interactions between the Earth and Moon. Also, we're not the only body that would have significant tidal effects on the mass distribution of the sun - we're not even at the top. Venus would have a larger effect pushing us one way; Jupiter a larger effect pulling us the other way. We're a bit of fluff, a dust mote. We're as likely to lose kinetic energy as to gain it, making the year shorter and bringing us closer.