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Why does Venus show phases when viewed from Earth?
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Venus
Venus photographed in ultraviolet light by the Pioneer Venus Orbiter (Pioneer 12) spacecraft, February 26, 1979. Although Venus's cloud cover is nearly featureless in visible light, ultraviolet imaging reveals distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). Added color in the image emulates Venus's yellow-white appearance to the eye.
Venus exhibits a full cycle of phases when viewed from Earth because its orbit lies within Earth’s orbit around the Sun.12
As both planets revolve around the Sun, the amount of the illuminated portion of Venus visible to an observer on Earth changes, similar to the way the Moon displays different phases. Because Venus is closer to the Sun than Earth, it can appear in various states of illumination, ranging from a crescent to a full disk, depending on its relative position to the Sun and Earth.12
The observation of these phases by Galileo in 1610 provided critical, direct observational evidence supporting the heliocentric model of the solar system, as it was inconsistent with the prevailing geocentric view of the time.12