|MadSci Network: Astronomy|
Dear Bill, You're right, the Earth's gravity does have a big effect on the Moon...and the other way round. We only see one side of the Moon because the gravitational forces of the Earth have gradually slowed it down and locked it into a `synchronous' orbit, so that its rotational period matches its orbital period around the Earth (incidentally, this is quite common for other satellites in the solar system). It rotates exactly once for each time it orbits the Earth, meaning the same side of the Moon always faces us. But the Moon's gravity is also acting on the Earth...the most obvious effects being the tides. The bulges raised in the surface of the ocean get dragged along by the Earth's rotation, meaning that the force between the Earth and the Moon is no longer pointing directly along the line connecting them. This results in a torque which causes the Earth's rotation to slow slightly and also moves the Moon farther away (by a few centimetres a year). Eventually, the Earth's rotation will match the Moon's period, and only one side of the Earth will be visible from the Moon. The timescale for this, however, is likely to be several tens of billions of years (by which time the Sun will long since have died!) But the Sun's gravity influences the Earth as well, of course...this is why the tides are strongest when the Sun, Moon, and Earth are aligned. But to bring some math into, tidal forces decrease as the cube of the distance between the objects. That's why although the Sun is much more massive than the Moon, the tides raised by the Moon are much stronger, because it's so much closer. So while the Sun may produce a similar torque on the Earth to try to force it into synchronous rotataion, my guess is that the timescale for this is much, much too long to make much of a difference! A nice website with lots of information on the solar system is http://www.seds.org/nineplanets/nineplanets/ Cheers, Meghan
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