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SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Phases of the Moon, illustration. As the Moon orbits the Earth, the area that is illuminated appears (from the Earth's surface) to change in size, producing the phases shown on the outer circle. The Moon is always half-illuminated by the Sun. The phases arise because the observer sees more or less of the illuminated side as the angle of observation varies. At left is the full Moon, and at right is the new Moon, when the side facing the Earth is in shadow. The Moon rotates so that it always presents the same face to the Earth. It does not have a permanently "dark side" because it is rotating with respect to the Sun. The mean length of the cycle of phases (a lunar or synodic month) is 29.53 days.
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