The Moon does not produce any light of its own. Everything we see when we look at the Moon is sunlight being reflected off its surface. The Sun always lights up one half of the Moon, just as it always lights up one half of the Earth. The reason the Moon appears to change shape from night to night is not because the lit half changes — it is because our view of that lit half changes as the Moon travels around the Earth.

The Moon takes roughly a month to complete one orbit around the Earth. As it moves along this path, the angle between the Sun, the Moon, and the Earth is constantly changing. When the Moon is between the Earth and the Sun, its sunlit side faces away from us, and we see a dark New Moon. When the Earth sits between the Sun and the Moon, the full sunlit side faces us, and we see a Full Moon. The positions in between produce the familiar crescent and half shapes.

The Moon’s cycle is divided into recognizable phases. After the New Moon, a thin sliver appears and grows night by night — this growing stage is called waxing. The Moon passes through a crescent, then a half-lit shape known as the first quarter, then a bulging gibbous shape, until it reaches the Full Moon. After that, the lit portion shrinks, or wanes, back through gibbous, last quarter, and crescent stages until it returns to New Moon and the cycle begins again.

People sometimes assume the Moon’s phases are caused by the Earth’s shadow falling on it, but that is not the case. The Earth’s shadow only touches the Moon during a lunar eclipse, which is a rare event that happens a few times a year at most. The ordinary monthly phases have nothing to do with shadows — they are simply the result of geometry, the changing viewing angle between three moving bodies in space as the Moon circles the Earth.

This article was written using information from Wikipedia.