António Cidadão

This is an animation that shows the Moon through one lunar cycle, revealing all of its phases from New Moon to New Moon. The phases of the Moon are caused by the relative positions of the Sun, Earth, and Moon. The Moon orbits the Earth, on average, every 27 days (27 days, 7 hours, 43 minutes, to be precise). Because it simply reflects sunlight, the amount of illuminated surface we see depends on where the Moon is in its orbit.

Motions of the Moon

The phases of the Moon change depending on where the Moon is in its orbit around the Earth. If the Moon is in the direction of the Sun, then we call it a New Moon. In this case, the entire back side of the Moon is illuminated, leaving the side we see in darkness. About seven days later, the Moon is in First Quarter phase; the illuminated portion we see is a quarter of the entire Moon (the other illuminated quarter is on the back side of the Moon). In another seven days the Moon will be opposite the Sun, rising when the Sun sets and setting when the Sun rises. This is a Full Moon, when the entire disk as seen from Earth is illuminated. After Full Moon, the Moon is waning, the illuminated portion grows smaller and smaller. Seven days after Full is Third Quarter, and then finally another New Moon, when the Moon rises approximately when the Sun rises, is in the sky all day, and sets when the Sun sets.

Ellen Cohen