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First-Quarter Moon

April 11, 2011

The first-quarter Moon sails across the southwestern quadrant of the sky this evening. It lines up with the bright "twins" of Gemini, which are to the Moon's upper right as darkness falls.

The Moon actually reaches "first quarter" at 7:05 Central Time this morning, so it'll be a few hours past that marker by the time it rises in early afternoon, and about a half-day past it by nightfall. Still, it'll be hard to tell much of a difference -- sunlight will illuminate just a smidge more than half of the lunar hemisphere that faces our way.

That appearance makes the "first-quarter" title a bit confusing. If we see half of the Moon, shouldn't it be a half Moon instead of a quarter one?

If you're going by appearance alone, then the answer is absolutely -- that description works perfectly well. But "first quarter" refers not to the Moon's appearance, but to its position in its month-long cycle of phases. And today, it's one-quarter of the way through that cycle. It'll be half-way through the cycle on the 17th. To continue the confusion, though, that half-way point is the full Moon -- the entire disk is bathed in sunshine.

After that comes last quarter, followed on May 3rd by new Moon, as the Moon crosses the line between Earth and the Sun. That marks the end of one cycle of phases and the beginning of another, which will bring the next first-quarter Moon just 29 and a half days from now.

Tomorrow: a "spacey" date.

Script by Damond Benningfield, Copyright 2011


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