Phases of the Moon

The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth, on average, in 27 days 7 hours 43 minutes. In the case of February it is possible to have a moon with NO moon. When a month has two moons, the second moon is called a, blue moon.

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 moons 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. Because the horns of the moon at the ends of the crescent are always facing away from the setting or rising sun, they always point upward in the sky.