3 ms·

> The length of a year has been constant over Earth’s history, because Earth’s orbit around the Sun does not change. That is decidedly not true. At the very l

> The length of a year has been constant over Earth’s history, because Earth’s orbit around the Sun does not change. That is decidedly not true. At the very least the eccentricity of the Earth's orbit around the Sun is known to change[0]: "The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ±0.012)". Moreover, I seem to recall reading that over the 4.5 billion year scale the distance of various planets to the Sun has varied as well, though I don't have a reference for that right now. [0] https://en.wikipedia.org/wiki/Milankovitch_cycles https://en.wikipedia.org/wiki/Milankovitch_cycles

saagarjha 7y ago
From your link: > The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis. Which makes sense, because the semi-major axis depends on the energy of the orbit, and there's not really much that would be altering that in the short timeframe of a Milankovitch cycle.
- simcop2387 7y ago
  It should be mostly invariant, but tugs and changes from other bodies in the system (Jupiter and Saturn particularly) can change the energy of the orbit. It's one of things that makes n-Body solutions to orbital mechanics nearly impossible to make. I doubt it's change too significantly for something as massive as earth though, but the collision with the mars sized body that created the moon early on in Earth's history definitely would have been able to change the orbital energy.
- cryptonector 7y ago
  On the scales of the Milankovitch cycles, yes. But on galactic time scales, IIUC, no, because the gas giants' gravity does affect the orbits of each other and the smaller planets.
- ChrisLomont 7y ago
  >because according to Kepler's third law, it .... Kepler's law is an approximation, and it most certainly does not hold over large time scales. Heck, it fails on short time frames too - one of the early successes of relativity was to explain the precession of Mercury's perihelion from Kepler style theory to observation. Heck, even restricting to classical physics, Kepler's law fails as soon as you have three bodies, since the derivation is only for a two body problem. A third (or more) body makes his laws fail. Our solar system has well over thousands of bodies all interacting. So this is only an argument for the invariance of the (incomplete) model of Kepler, but fails in real life. We have plenty of better models (i.e., that agree better with observation) that are not invariant.
uoaei 7y ago
Theory says that the moon is sapping energy from the Earth's rotation because it's moving away from the Earth -- angular momentum decreases as moment of inertia increases, all else being equal. I suspect the friction between water and Earth's crust also plays a role, since the tides are drawn to and fro by the moon.
- Sharlin 7y ago
  Tidal drag is not an additional cause, it's exactly the mechanism of how momentum is transferred from Earth to the moon. However as far as I know, oceans play a relatively minor part, with the deformation of Earth's crust and mantle having the greatest effect.
  - uoaei 7y ago
    Isn't there a difference between "tidal forces" in the gravitational sense and the forces induced by the momentum of the fluid? Or are they the same?
    - Sharlin 7y ago
      Tidal force is the force that causes an object to stretch in a nonuniform gravitational field. If the object is rotating relative to the field, tidal forces induce dynamic stresses on the object, heating it up and slowing down its rotation (which means something else in the system has to speed up because momentum is conserved).

saagarjha 7y ago

From your link: > The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis. Which makes sense, because the semi-major axis depends on the energy of the orbit, and there's not really much that would be altering that in the short timeframe of a Milankovitch cycle.

simcop2387 7y ago
It should be mostly invariant, but tugs and changes from other bodies in the system (Jupiter and Saturn particularly) can change the energy of the orbit. It's one of things that makes n-Body solutions to orbital mechanics nearly impossible to make. I doubt it's change too significantly for something as massive as earth though, but the collision with the mars sized body that created the moon early on in Earth's history definitely would have been able to change the orbital energy.
cryptonector 7y ago
On the scales of the Milankovitch cycles, yes. But on galactic time scales, IIUC, no, because the gas giants' gravity does affect the orbits of each other and the smaller planets.
ChrisLomont 7y ago
>because according to Kepler's third law, it .... Kepler's law is an approximation, and it most certainly does not hold over large time scales. Heck, it fails on short time frames too - one of the early successes of relativity was to explain the precession of Mercury's perihelion from Kepler style theory to observation. Heck, even restricting to classical physics, Kepler's law fails as soon as you have three bodies, since the derivation is only for a two body problem. A third (or more) body makes his laws fail. Our solar system has well over thousands of bodies all interacting. So this is only an argument for the invariance of the (incomplete) model of Kepler, but fails in real life. We have plenty of better models (i.e., that agree better with observation) that are not invariant.

uoaei 7y ago

Theory says that the moon is sapping energy from the Earth's rotation because it's moving away from the Earth -- angular momentum decreases as moment of inertia increases, all else being equal. I suspect the friction between water and Earth's crust also plays a role, since the tides are drawn to and fro by the moon.

Sharlin 7y ago
Tidal drag is not an additional cause, it's exactly the mechanism of how momentum is transferred from Earth to the moon. However as far as I know, oceans play a relatively minor part, with the deformation of Earth's crust and mantle having the greatest effect.
- uoaei 7y ago
  Isn't there a difference between "tidal forces" in the gravitational sense and the forces induced by the momentum of the fluid? Or are they the same?
  - Sharlin 7y ago
    Tidal force is the force that causes an object to stretch in a nonuniform gravitational field. If the object is rotating relative to the field, tidal forces induce dynamic stresses on the object, heating it up and slowing down its rotation (which means something else in the system has to speed up because momentum is conserved).