MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Phases of the Moon, computer 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. The Moon is always half-illuminated by the Sun (out of frame to the right of this image). The phases arise because the observer sees more or less of the illuminated side as the angle of observation varies. The Moon rotates so that it always presents the same face to the Earth. It does not have a "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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