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Moon and Spica

November 28, 2013

The Moon bears down on the star Spica at dawn tomorrow — its second encounter with 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 lifetimes for the motions of any star to become apparent. 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 that background, so it returns to each of the bright stars along its path about every four weeks. That’s not 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 rhythm.

Look for the Moon and Spica beginning a couple of hours before sunrise. Spica stands just slightly below the Moon. The Moon will pass by Spica during the daylight hours here in the U.S., but it’ll return to the star again in four weeks — just like clockwork.


Script by Damond Benningfield, Copyright 2010, 2013

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