If the Sun's gravitational pull on the Moon is greater than the Earth's, why does the Moon go around the Earth? Why is the Moon tidally locked to the Earth, but not to the Sun?

You are right that the Sun's gravitational pull on the Moon is greater
than the Earth's. The Moon goes around the Sun because of it as does
the Earth. But that doesn't mean the force of gravity between the
Earth and the Moon can be ignored. The Moon, being lighter, therefore
orbits the Earth. An analogy might be two people going round and
round on a carousel. Even though they have an overall big circular
motion, they will also react if they push one another. Spin-locking
is caused by tidal forces which are raised by gravity and which get
rid of energy. For example the Moon raises tides on the Earth. The
energy that goes into raising high tides is changed into useless
energy in the form of turbulence in the ocean. Over time, enough
energy has extracted from the spin of the Moon, so that it is tidally
locked. However, tidal forces depend drop off more steeply with
distance than gravity, so the Sun's tidal force on the Moon is less
than the Earth's, I think. Also, for the Sun to tidally lock the
Moon, it would also have to get rid of the orbital energy of the Moon
and this would take a long time. Mercury is the only planet that is
almost tidally locked to the Sun. I hope this helps!
Jennifer Johnson

Back to the Ask An Astronomer page.