THIERRY LEGAULT / EURELIOS / SCIENCE PHOTO LIBRARY THIERRY LEGAULT / EURELIOS / SCIENCE PHOTO LIBRARY
Phases of the Moon. North is at bottom. As the Moon orbits the Earth, the amount of sunlight seen on its visible hemisphere varies in a regular 28-day cycle. This montage of images shows, from left: a waxing crescent Moon (3 days old); the Moon's first quarter (7 days old); a Full Moon (14 days old); the Moon's last quarter (21 days old); and a waning crescent Moon (26 days old). As the cycle progresses, the motion of the Moon in its orbit around the Earth moves its visible hemisphere into and then out of the sunlight. The lunar day dawns on a waxing Moon, and then the lunar night falls on a waning Moon. Photographed in Earth's southern hemisphere.
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