There’s a thin crescent Moon in the western sky this evening. It stands a little above Venus, the radiant “evening star.” Sunlight illuminates only about a tenth of the lunar hemisphere that faces our way — a bare sliver against the fading color of twilight.
As the nights pass by, though, that sliver will grow larger as the Sun casts its light on an increasing fraction of that hemisphere of the Moon. It’ll light up exactly half of the Moon just about the time the clock strikes midnight on New Year’s Eve — a point in the Moon’s cycle known as first quarter.
The Moon will continue to grow fatter over the following week, as it passes through its “gibbous” phase — when the Sun illuminates more than half of the visible lunar surface, but not quite all. That phase ends when the Moon is “full” on the night of January 8th. After that, the phases run in reverse, with darkness engulfing a larger slice of the lunar disk until the Moon is “new” on the night of the 22nd.
That whole cycle is the result of the Moon’s orbit around Earth. As it circles our planet, the viewing angle between Earth, Sun, and Moon is constantly changing. At full Moon, the Moon lines up opposite the Sun in our sky. And at new Moon, it lines up between Earth and the Sun. And when the Moon is an evening crescent, it’s just starting to pull away from the Sun.
So over the next few nights, watch the Moon as it gets fatter and sets later — a beautiful process that repeats month after month.
Script by Damond Benningfield, Copyright 2011