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Moon and Regulus

May 17, 2013

A bright star keeps company with the first-quarter Moon tonight. Regulus, the “heart” of Leo, the lion, stands a little above the Moon as night falls, and keeps that position as they slide down the southwestern sky later on.

The Moon reaches first quarter at 11:35 p.m. Central Time. At that moment, the Moon stands at a right angle to the line between Earth and the Sun. Sunlight illuminates exactly half of the lunar hemisphere that faces our way, so it looks as though someone sliced the Moon right down the middle.

Because of that, you might expect the quarter Moon to be half as bright as a full Moon, but it’s not. In fact, it’s not even close — the first-quarter Moon is only about one-tenth as bright as a full Moon.

That’s because the Moon isn’t a perfect mirror for reflecting sunlight. Instead, it’s most efficient at reflecting sunlight in the direction from which it came — back toward the Sun. At full Moon, Earth lines up between the Moon and Sun, so some of that bright glow hits us. At other phases, though, the Moon is off to the side of Earth, so we don’t get the full force of that reflection.

Over the next week, the illuminated portion of the Moon will grow fatter and brighter. It’ll be half as bright as a full Moon on the 22nd, when sunlight will illuminate all but a few percent of the lunar disk. The Moon will reach its peak a week from tonight, when it’ll be full — filling the night sky with brilliant light.


Script by Damond Benningfield, Copyright 2013

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