The Moon is Falling!

The Moon puts on quite a show for us this week. Day to day we can watch it move away from Mars, through the brilliant stars of winter, past Saturn, and on to Leo. Wednesday: midway between Aldebaran in Taurus and Pollux in Gemini. Thursday: in the midst of Gemini. Friday: between Pollux and Saturn.

The stars and the planets move, too, but the Moon is so close that we can see it move from day to day. The Moon is a quarter of a million miles from the Earth, so its monthly path is a hundred fifty million miles long. To complete this journey, it must race through the sky at two thousand miles an hour.

There's more to the story. If the Moon were simply racing along at two thousand miles an hour it would continue to move in a straight line - like a car skidding on smooth ice - and would soon be lost to sight. But just like an apple falling from a tree, the Moon also falls. That's what Newton realized when he saw the apple fall: the moon is falling just like the apple. But since it's so far away, it falls just fifteern feet an hour.

It's this balance between the Moon's own motion and its falling to the Earth that gives it a stable orbit. Even though it's falling fifteen feet every hour, it never falls down, but hangs over us, a quarter of a million miles up.

(02/08/06)