Moon and Spica 
The Moon passes the star Spica tonight -- the second time it's passed by the bright star this month. It's a reminder that the night sky is a giant celestial clock, ticking out the rhythms of the cosmos. And it also makes it easy to understand why ancient cultures saw the sky as a series of spheres revolving around our own planet.
One of these spheres contained the "fixed" stars -- like Spica. Today, of course, we know that the stars aren't fixed at all. Like our own Sun, they race around the center of the galaxy. But they're all so far away that it takes many lifetimes for the motions of any star to become apparent to the unaided eye. Spica, for example, will take tens of thousands of years to move a distance equal to the width of the full Moon.
Each of the bodies that moved against the background of stars had its own sphere, including the Moon. It takes about 27 and a half days for the Moon to complete one circle through the stars, so it returns to each of the bright stars along its path in just under four weeks. That motion isn't the result of the celestial spheres, though -- it's the Moon's orbit around Earth. Even so, the Moon moves across the sky with its own regular rhythm.
Look for the Moon and Spica climbing into view by mid-evening. Spica is the bright blue-white star just to the left of the Moon as they rise -- a star that the Moon will visit again in four weeks -- just like clockwork.
Script by Damond Benningfield, Copyright 2010