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Moon and Spica

May 14, 2011

The Moon pays a close call on an old friend tonight: Spica, the brightest star of the constellation Virgo. They're in the southeast at nightfall, with Spica just a few degrees to the left of the Moon. They arc across the south during the night and set around an hour before first light tomorrow.

The Moon passes by Spica once every four weeks or so. It usually skims a few degrees past the bright star, as it does tonight. But occasionally, the Moon crosses in front of Spica, briefly blocking it from view.

That's because Spica is quite close to the ecliptic -- the Sun's path across the sky. The Moon stays close to the ecliptic, too, although its path is tilted just slightly with respect to the ecliptic. So most months the Moon cruises right on by Spica, forming a pretty pairing with the star but not covering it up.

Every few years, though, the geometry is just right, and the Moon passes in front of Spica -- an event called an occultation. The last time that happened was in 2006. And it'll happen again next year -- several times, in fact, all during the second half of the year.

Unfortunately, though, none of the occultations will be visible from here in the United States. All will be limited to the far southern hemisphere. In fact, one of the best locations for viewing the occultations will be the south pole -- a chilly spot for an encounter between friends.


Script by Damond Benningfield, Copyright 2011


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