Just Hanging There

We are so used to the daily rising and setting of the Moon that we hardly notice it. But the Earth has other "moons" which never rise or set. These are the "hovering" satellites, called geosynchronous (geo-sin-crow-nus), those satellites we use for television and for long distance telephone calls. They just seem to hang in the sky.

Look at a satellite antenna. It never moves, except to aim at a different satellite. It's pointed to a satellite twenty-two thousand miles above the Earth, traveling at six thousand miles an hour. At that speed the satellite travels once around the Earth in just twenty-four hours. But of course, we also turn once in twenty-four hours beneath it. Since we're turning at the same rates, it appears fixed. Like children on a merry-go-round: our friends, turning as fast as we do, seems not to move at all.

Every satellite -- the Moon, the International Space Station, as well as these communication satellites -- are moving so fast that there is a balance between Earth's gravity pulling them and their own centrifugal force pushing them outwards. So they don't come crashing down. What's special about these hovering satellites is that, in addition, they revolve around the Earth in exactly twenty-four hours.

The astronauts on the Moon saw something like that, something really awe-inspiring: an Earth that never rose or set, an Earth fixed in the sky. We knew they would see that. Wherever we are on the Earth, we always see the same side of the Moon: that familiar pattern of craters and "seas". If you lived in the center of that pattern, the Earth would be exactly overhead. Always. The Earth would never rise or set.

The Moon revolves around the Earth once in twenty-eight days. From the point of view of the Moon, itıs the Earth that revolves around it in twenty-eight days. The Moon also rotates once in twenty-eight days. This is not a coincidence.

In the same way that the Moon causes tides on the Earth, the Earth causes tides on the Moon. No matter that there is almost no water on the Moon. The tides we cause are in the rocks themselves, the rocks in line with the Earth, distorting them. Long ago, as the Moon turned, this distortion moved from rock to rock. Over eons the friction caused by this motion has slowed the Moon until it turns just once in twenty-eight days. The distortion is now in line with the Earth. It is now fixed, and now the Moon always shows the same face to the Earth.

By the way, in the same way the Moon has slowed the Earth, though not by so much. When the Earth was first formed, it rotated once in nine and a half hours. A very short day indeed.

(01/23/08)