# Moon is getting away from earth

1. May 11, 2009

### Mahbod|Druid

Hello

why is moon getting away from earth about 2-3 centimeter/year ?

2. May 11, 2009

### Dr.D

Because the Martians are pulling harder!

3. May 11, 2009

### chroot

Staff Emeritus
The Moon generates tides, which involve the movement of vast quantities of water. The movement of all that water causes friction, which leads to the release of energy as heat. The energy actually comes from the Earth's rotational inertia. The tides are literally slowing down the Earth's rotation.

Because the Earth's rotation is slowing, the Moon is moving further away. It must do so to converse angular momentum.

- Warren

4. May 11, 2009

### cosmicpencil

can we estimate when the moon will have flown out of its orbit?

5. May 11, 2009

### Count Iblis

The friction mentioned by chroot causes the tidal bulges to move forward relative to the Moon. The gravitational force exerted on the Moon is then a bit larger because of these bulges that are propelled a bit ahead of the Moon. This causes the Moon's velocity to be a bit higher than it should be, causing the Moon to move further away from the Earth.

Note that by action is minus reaction, the Moon pulls has hard on the bulges as the bulges pull on the Moon. This means that in addition to the friction forces exerted by the Earth on the bulges there are the tidal force exerted by the Moon on the bulges as well. Because the bulges are moving at constant velocity with the Moon, there is equilibrium: The tangential component of the gravitational force exerted by the Moon on the bulges is the negative of the friction force exerted by the Earth.

6. May 11, 2009

### Janus

Staff Emeritus
We wouldn't to ever do so. Before that, the Earth's period of rotation will match the period of the Moon's orbit. The Earth will keep one face pointing at the Moon, The tidal friction that causes the Earth's slowing will no longer be there (The tides no longer move across the face of the Earth), and the Moon's recession will stop.

7. May 12, 2009

### Chronos

The sun will enter its red giant phase before the moon escapes earth orbit. After that, all bets are off.

8. May 13, 2009

### cosmicpencil

it seems the sum will die before lots of interesting events occure.

9. May 16, 2010

### blank.black

I heard that dark matter is supposed to keep everything together and dark energy is supposed to separate everything. Is any of that true?

If its true then how does the dark matter and dark energy affect our solar system, if it does in anyway? Also if its true, then is it possible that dark energy is what is pulling/separating the moon away from the earth?

10. May 17, 2010

### phizo

It won't occur on a monday as monday is named after the moon!!

11. May 17, 2010

### phizo

Tidal energy systems would also add to the problem I beleive?

12. May 20, 2010

### Matterwave

The moon does not move faster in higher orbits, it moves slower. The circular orbit equation: $$\frac{v^2}{r}=\frac{GM_e}{r^2}$$ therefore $$v=\sqrt{\frac{GM_e}{r}$$ clearly shows this.

This is because of the virial theorem. The energy boost given to the moon by the tidal forces increases its total energy (remember total energy is negative). According to the virial theorem: <V>=-2<T> so that E=<V>+<T>=-<T>. As E increases (gets less negative), <T> must decrease - i.e. it gets slower. Twice the energy boost is going to the potential energy so the kinetic energy must decrease by one times the energy boost for energy to be conserved.

13. May 20, 2010

### Count Iblis

That's right, the Moon is indeed slowing down as it moves away from Earth, but that happens because of the extra energy added which leads to the Moon always moving a tiny bit faster than the speed it should move at if it were not for the tidal bulges.

14. May 21, 2010

### Matterwave

I suppose on short enough time scales, the moon may not be in virial equilibrium and "speed-up" a little bit before being slowed back down again. On longer time scales, though, the moon should be slowing.

15. May 21, 2010

### Count Iblis

The Moon is slowing down, not speeding up. But this is because the Moon is always moving at a tiny, tiny, higher speed than it should move at if you ignore the tidal bulges pulling on the Moon.

16. May 21, 2010

### D H

Staff Emeritus
That's not quite right. Yes, the Moon is moving a bit faster than it would if the bulges were not present. However, that is not what is causing the Moon to recede. Suppose the Earth was tidally locked to the Moon and was covered with an all-encompassing ocean. The tidal bulges would be perfectly aligned with the line between the Earth and Moon. The Moon would still be moving a bit faster than it would were the bulges not present -- and the Moon would not be receding.

The reason the Moon is receding is because the bulges lead the line between the Earth and Moon. This gives the Moon a transverse component to the acceleration vector. It is this transverse acceleration that makes the Moon recede.

17. May 21, 2010

### Count Iblis

Yes, I agree, it is caused by the tangential acceleration (I did mention this in post nr. 5)

18. May 24, 2010

### HeLiXe

:rofl:

19. May 24, 2010

### HeLiXe

This might be for another thread, but what were the effects on the moon's "drift", if any, caused by the LCROSS and LRO missions? Was the libration of the moon affected at all? If you have an answer for any of these questions (whether it be yes or no or whatever) can you please give me a detailed answer, including any magnitude of force and important equations which may be involved. Thx!

20. Jun 2, 2010

### cahill8

I'll leave a better explanation to someone else, but by simply looking at the size of the craters on the moon and thinking about the size of the objects that caused them, do you think LCROSS would have a noticeable effect?