Brief Explanation of the Moon Phases

Thursday, June 16, 2011

The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth in 27.3 days, or 27 days 7 hours 43 minutes, on average. This measurement is relative to the stars and is called the sidereal period or orbital period. However, because of the earth's motion around the sun, a complete moon cycle (New Moon to New Moon) appears to earthbound observers to take a couple of days longer: 29.5305882 days to be exact. This number is called the synodic period or "lunation", and is relative to the sun.

The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes through the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon. The edge of the shadow (the terminator) is always curved, being an oblique view of a circle, giving the moon its familiar crescent shape.