Why Does Earth Keep Spinning Around the Sun?

[D H]

It is often easier to look at these kinds of problems using conservation laws. The Moon raises tides (water and rock) on the Earth. If the Earth were rotating at the same rate as the Earth-Moon orbital rate, the tidal bulge would line up with the Earth-Moon line. However, the Earth's rotation advances the tidal bulge so the bulge on the Moon side of the Earth leads the Moon. This leading bulge exerts a slightly greater gravitational force on the Moon than does the trailing bulge on the opposite of the Earth. The Moon receives a small prograde boost, increasing its angular momentum. Conservation of momentum dictates that the Earth lose angular momentum.

 

[pivoxa15]

It is often easier to look at these kinds of problems using conservation laws. The Moon raises tides (water and rock) on the Earth. If the Earth were rotating at the same rate as the Earth-Moon orbital rate, the tidal bulge would line up with the Earth-Moon line. However, the Earth's rotation advances the tidal bulge so the bulge on the Moon side of the Earth leads the Moon. This leading bulge exerts a slightly greater gravitational force on the Moon than does the trailing bulge on the opposite of the Earth. The Moon receives a small prograde boost, increasing its angular momentum. Conservation of momentum dictates that the Earth lose angular momentum.

But that dosen't explain why the moon is moving away from the earth? It only says the moon is spinning faster.

 

[D H]

But that dosen't explain why the moon is moving away from the earth? It only says the moon is spinning faster.

I guess I wasn't clear enough. I said

The Moon receives a small prograde boost, increasing its angular momentum.

This increase in angular momentum is the Moon's orbital orbital angular momentum, not its angular momentum due to rotation about its axis. That increase in orbital angular momentum means that the Moon is moving away from the Earth.

In fact, the Moon's angular momentum due to its rotation will decrease over time. The Earth exerts a similar torque on the Moon. If the Moon rotates at the same rate as the Moon's orbital rate, the Moon's tidal bulge lines up with the Moon-Earth line and is fixed on the lunar surface. If the rotation rate and orbital rate differ, the bulge will be out of alignment and it moves, just as the tides move on the Earth's surface. Friction forces act against this motion of the tidal bulge, slowing or speeding up the Moon's rotation until it is once again in synch with the Moon's orbital rate.

 

[zennocobra]

yups.. i am wondering the same problem... and why does not it stop? or when does our Earth stop going around the sun? and spinning itself? It sure take "gigaterabillion" of energy to make such move... where does those energy coming from?

 

[DaveC426913]

yups.. i am wondering the same problem... and why does not it stop? or when does our Earth stop going around the sun? and spinning itself? It sure take "gigaterabillion" of energy to make such move... where does those energy coming from?

Newton's First Law: an object that is already in motion will tend to stay in motion (it requires no energy to do so).

It got the initial kick from the Big Bang, which gave everything a lot of kinetic energy. All that has happened since them is that everything has been transferring its kinetic and potential energy which, under gravity's influence, has everything tend toward elliptical paths.