You are here

Moon and Companions

January 15, 2012

The last-quarter Moon cozies up to a couple of bright companions at dawn tomorrow: the star Spica and the planet Saturn. They’re well up in the south, with Spica close to the upper right of the Moon, and Saturn a little farther to the upper left.

At last quarter, sunlight illuminates half of the lunar hemisphere that faces our way. But the “last-quarter” name refers not to the Moon’s appearance, but to its location in its month-long cycle of phases. At last quarter, the Moon is three-quarters of the way through that cycle.

You might expect a quarter Moon to shine half as brightly as a full Moon, but it doesn’t — it’s only about a tenth as bright.

That’s because the Moon shines by reflecting sunlight. Most of the reflected light aims back at the Sun. At full Moon, Earth is between the Sun and Moon, so we get a faceful of moonlight. But at a quarter Moon, we’re off to the side of that path. Some of the sunlight reflects in our direction, but most of it is going back toward the Sun.

You can see the same effect if you aim a flashlight at a mirror in a darkened room. If you stand squarely in front of the mirror, the reflected beam hits you right in the face, so the flashlight looks bright. But if you aim the beam at a shallow angle, so that the light reflects away from you, the flashlight looks much fainter.

So it’s all about geometry — the angles of the Sun, Moon, and Earth as the Moon moves through its never-ending cycle of phases.


Script by Damond Benningfield, Copyright 2011


Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.