Moon and Spica
The Moon and the star Spica stage quite an encounter tonight. At their closest, during early evening, they'll be just a couple of degrees apart, with Spica to the Moon's left or upper left. They'll remain together as they wheel across the south during the night.
If you were standing on the Moon and looking back toward Earth, you wouldn't see Spica at all -- it's below the horizon for the lunar hemisphere that faces our way. It won't rise for about another week. When Spica does rise, it'll remain in view for two weeks, rotating across the sky at a leisurely pace. And when it sets, it'll remain hidden for two weeks more.
That's because the Moon rotates much more slowly than Earth does, so it takes four weeks to complete one turn on its axis with respect to the stars.
It takes a couple of days longer to complete its cycle of phases and have the Sun return to the same position in the sky, though. During that time, Earth and the Moon move a good distance in their mutual orbit around the Sun. So each time the Sun sets, a slightly different slice of the night sky is climbing into view.
Of course, on the airless Moon, you wouldn't have to wait for nightfall to see the stars -- they're in view all the time. But you would have to shield your eyes against the glare of sunlight reflecting off the lunar surface. Only after your eyes adapt to the darkness could you see the stars -- shining without twinkling in the lunar sky.
Script by Damond Benningfield, Copyright 2008
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