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Space Venus :: Phases of Venus
Phases of Venus
Since its orbit takes it between the Earth and the Sun, as seen from the Earth, Venus exhibits visible phases as same as Earth's moon. Galileo Galilei was the first person to observe the phases of Venus in December 1610, an observation which supported Copernicus's then contentious heliocentric description of the solar system. He also noted changes in the size of Venus's visible diameter when it was in different phases, signifying that it was farther from Earth when it was full and nearer when it was a crescent. This inspection strongly supported the heliocentric model. Venus (and also Mercury) is not visible from Earth when it is full, since at that time it is at superior conjunction, rising and settings are along with with the Sun and so lost in the Sun's glare.
Venus is brightest when about 25% of its disk is illuminated; this usually occurs 37 days both before (in the evening sky) and after (in the morning sky), its inferior conjunction. Its greatest elongations happen approximately 70 days before and after inferior conjunction, at which time it is half full; between these two intervals Venus is actually visible in broad daylight, if the observer knows specifically where to look for it. The planet's period of retrograde motion is 20 days on either side of the inferior conjunction. Indeed, through a telescope Venus at greatest elongation appears less than half full due to Schröter's effect first noticed in 1793 and shown in 1996 as owing to its thick atmosphere.
Sometimes, Venus can be seen in both the morning (before sunrise) and evening (after sunset) on the same day . This circumstances arises when Venus is at its maximum separation from the ecliptic and concurrently at inferior conjunction; then one hemisphere (Northern or Southern) will be able to see it at both times. This occasion presented itself most recently for Northern Hemisphere observers within a few days on either side of March 29, 2001, and for those in the Southern Hemisphere, on and around August 19, 1999. These relevant events repeat themselves every eight years pursuant to the planet's synodic cycle.
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