LARRY LANDOLFI / SCIENCE PHOTO LIBRARY LARRY LANDOLFI / SCIENCE PHOTO LIBRARY
Image 16 of 27. Waning (decreasing) gibbous Moon 16 days into its 28-day cycle. The lunar phases arise as the Moons orbit of the Earth shows the Earth-facing side moving into and out of the light of the Sun. This Earth-facing side of the Moon is locked in place by the gravitational influence of the Earth. This results in a lunar day that equals the length of the lunar Earth orbit. On a waning Moon the day-night line (terminator) marks a lunar sunset. The Moon is gibbous when it is more than half full.
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