Asked by David Doran
Astronomy books are fond of quoting the fact that the Moon is gradually spiralling away from Earth. This is the flipside of the tidal forces that our satellite imposes on Earth – tidal bulges raised in our planet’s oceans by the Moon’s gravity pull back at the Moon and cause it to speed up, which in turn raises it into a higher orbit. At present, the Moon gets 3.8 cm further away from Earth every year, and it was much closer to Earth in prehistoric times.
But the Moon’s outward spiral is dwindling as its distance from Earth decreases and its tidal forces get weaker. This alone should be enough to prevent our satellite from ever leaving orbit around Earth completely without intervention from some outside force. Another factor to consider is that the Moon’s satellite’s tidal pull slows down Earth’s rotation by 2 milliseconds per century. Given enough time, will eventually slow it so that Earth takes a month to rotate (however long a month may be by that time). At this point, Earth will be fixed with one side facing towards the Moon, just as the Moon is already fixed with one side facing towards Earth. At this point, Earth’s tidal bulges will become ‘frozen’ is place, and incapable of influencing either Earth or Moon any longer.
Answered by astronomy and space author Giles Sparrow