Is the moon escaping?
Both John Hughes and I have separately been puzzled by an article in New Scientist
in which it is reported that the moon is gradually (very) getting further and further from the earth.
This is measured using the reflectors left on the moon by Apollo astronauts, and also one by a French(?) robot.
The explanation given is that this is a result of energy absorption by the tides.
However, when a low-orbit satellite loses energy owing to atmospheric friction, its altitude is reduced,
not increased.
How do we reconcile these two observations?
Explanation due to John
The error is to take an anthropocentric view in which the earth is the frame of reference (i.e. still).
Look “down” on the solar system, i.e. looking down on the earth’s north pole.
The earth is rotating anti-clockwise once every 24 hours,
and the moon goes round its orbit every 28 days (approx).
As the earth spins its oceans spin with it, but they experience the moon’s gravity,
which drags them “back”.
There is an equal and opposite force on the moon, which actually slightly increases its orbital velocity,
and so increases the height of its orbit.
So, yes, the moon is escaping, but very, very slowly.
Furthermore, the earth’s day is being lengthened by this mechanism, as the oceans then exert a drag on the
earth’s spin.
In time, lots of it,
this effect will eventually stop when the earth no longer spins reletave to the moon.
A similar mechanism, acting at a time when the moon was less solid than it is now,
explains why the Man in the Moon is always looking at us.