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Moon, Spica, and Saturn

May 31, 2012

The Moon snuggles especially close to a bright star tonight: Spica, the leading light of the constellation Virgo. As night falls, Spica stands just a couple of degrees above the Moon — the width of a finger held at arm’s length. And the planet Saturn, which is a little brighter than Spica, is just a few degrees farther along the same line, so it’s quite a cozy little trio.

The Moon is in its waxing gibbous phase right now. “Gibbous” means that sunlight illuminates more than half of the lunar hemisphere that faces our way, while “waxing” means the illuminated fraction is growing larger. That fraction will continue to increase until the Moon is full on Monday.

The word gibbous comes from a Latin word that means “hunched” or “humped.” It was applied to the Moon because the half of the disk that’s not in full sunlight looks like a hump attached to the side that is.

Since more than half of the Moon is lit up right now, you might expect it to be more than half as bright as a full Moon, but it’s not. The Moon shines by reflecting sunlight, and much of the light is reflected back toward the Sun. At full Moon, we’re between the Moon and Sun, so we get the full reflection. At other angles, though, less light is reflected our way from any given area of the lunar surface. When you figure it all up, the Moon is half as bright as a full Moon about two-and-a-half days before or after it’s full — which will happen late tomorrow.


Script by Damond Benningfield, Copyright 2012



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