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Animation showing the positions of the Earth and Moon during a lunar month, seen from above. The Moon's phase is seen in the inset at upper left. At the start, the Moon is at a right angle to the Earth-Sun line, and from Earth is seen as a half Moon, or last quarter, as only half of the sunlit side is visible. The Earth orbits anticlockwise around the Sun and the Moon orbits anticlockwise around the Earth. The Moon completes one full orbit of Earth in 27.32 days. This is known as a sidereal month. However, as the Earth has moved around the Sun in this time, the Moon is no longer at a right angle with respect to the Earth and Sun, and therefore has not reached the last quarter again, instead remaining slightly fuller, which is called a waning gibbous Moon. It takes another 2.21 days for the Moon to reach the alignment for a last quarter phase again. This longer period of the Moon's phases takes 29.53 days, and is called a synodic month. These periods are shown in graph form in clip K006 7161.
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