The Night Sky: Phases of the Moon

Why does the Moon have phases?
The Moon has phases because it orbits around Earth, from our viewpoint this causes the portion of the Moon which we may see illuminated to change over time. While the Moon travels over 27.3 days to orbit Earth once, the lunar phase cycle (from new Moon to new Moon) is actually 29.5 days. The Moon spends the extra 2.2 days "catching up" while the Earth travels about 45 million miles along its voyage around the Sun; during this time the Moon completes one orbit around Earth.

At the "New Moon" phase, the Moon is positioned directly between the Earth and the Sun in the sky so that none of the side of the Moon facing Earth is illuminated by the Sun. At first quarter, the half-lit Moon is highest in the sky at sunset, then sets about six hours later. At full Moon, the Moon is behind Earth in space with respect to the Sun. As the Sun sets, the Moon rises with the side that faces Earth being fully exposed to sunlight.

Where And When Is The Moon?

Phase of the Moon	Best Time to View	Where to Look (Start and Motion)
New	Can Not Be Seen	Too Close to the Sun
Waxing Crescent	Soon After Sunset	Southwest towards the West
First Quarter	Dusk to Late Evening	South to West
Waxing Gibbous	Dusk to Midnight	Southeast to West
Full	All Night	East to West
Waning Gibbous	Late Evening to Dawn	East to Southwest
Last Quarter	After Midnight to Dawn	East to South
Waning Crescent	Shortly Before Dawn	East to Southeast