We've all known for the longest time that the reason the Sun, Moon and stars appear to move across the sky is that the Earth is turning. Yet we still say that the Sun rises, the Moon sets, the stars move from east to west during the night.
How dizzying to think that sunset is really this: the mountains in the west, those mountains that seem so solid, actually move upwards like a great elevator to cover the Sun. And in the east, the mountains and fields lower to reveal ever new stars, and at daybreak, once again, the Sun shines forth.
The Earth has not always turned once every twenty-four hours, as it does now. When it was first formed, the day was only five or six hours long. But for billions of years, the tides, caused by the Moon, have been crashing against the shore and rubbing against the ocean bottom. Over the ages, this friction has slowed the Earth.
As the Earth slows, the Moon moves away from us. Together, the Moon and the Earth act like a twirling skater. Usually, a skater draws her arms in to spin faster and throws them out to slow down. If the Moon were somehow drawn away from the Earth, the Earth would slow down. As the Earth slows down, the Moon is "thrown out," and gets ever further away. This was confirmed by actual measurements from the Earth. The Apollo astronauts placed a set of large mirrors on the Moon. The beam of a laser on the Earth was aimed at these mirrors, and reflected
Careful measurements showed that the Moon is now receding from the Earth several inches each year. The Moon was actually about ten times closer when it was first formed, about four and a half billion years ago.
A Moon ten times closer would have appeared ten times larger. Of course, there was no life on Earth then, but what a sight that would have been. And in the future? As the Moon continues to recede it will appear ever smaller in the sky. But now it looks just right!
Keep looking up!
"The Sky Tonight" is written by Dr. Errol Pomerance. He runs SKYSHOWS OF VERMONT, dedicated to bringing complete planetarium shows directly to schools throughout Vermont. Visit the SKYSHOWS Web site for previous columns and show descriptions: www.sover.net/~skyshows Have an astronomical question? E-mail Dr. Pomerance at skyshows@sover.net.
"The Sky Tonight" is written by Dr. Errol Pomerance. He runs SKYSHOWS OF VERMONT, dedicated to bringing complete planetarium shows directly to schools throughout Vermont. Visit the SKYSHOWS Web site for previous columns and show descriptions: www.sover.net/~skyshows.
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