Annular Eclipse

StarDate: January 25, 2009

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You probably can't tell with your eyes alone, but the Moon doesn't always look the same size in our sky. That's because the distance to the Moon varies by about 30,000 miles. When the Moon is closest, it looks a little bigger than when it's farthest.

Even though the change isn't much, it comes into play tonight -- or tomorrow, for folks in the eastern hemisphere. There's a solar eclipse, as the Moon passes directly between Earth and the Sun. Because the Moon's a little farther from us than average, though, it's not quite big enough to cover the entire Sun. Instead, a bright ring of fire will encircle the Moon -- an event known as an annular eclipse.

The path of the eclipse stretches from the South Atlantic, past the southern tip of Africa, across the Indian Ocean, and into the islands of Southeast Asia.

Solar eclipses occur when the Moon's path across our sky intersects the Sun's path at new Moon. The Moon's orbit is tilted with respect to the Sun, so most months the Moon passes a little above or below the Sun. But at least twice a year, the paths intersect.

Right now, the Moon and Sun are almost exactly the same apparent size in the sky. But the Moon is moving away from Earth by about an inch and a half a year. As the Moon moves away, it looks a little smaller with each passing millennium. Eventually, it'll move so far away that it won't be big enough to cover the Sun -- and total solar eclipses will fade into history.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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