At 20:00 UT (Universal Time) Friday, August 20th, 2010, Mercury the wise communicator—and universal trickster—turns retrograde at 19°01' Virgo, in the sign of the Virgin, sending communications, travel, appointments, mail and the www into a general snarlup! The retro period begins some days before the actual turning point (as Mercury slows) and lasts for three weeks or so, until September 12, 2010, when the Winged Messenger reaches his direct station. At this time he halts and begins his return to direct motion through the zodiac.
Everything finally straightens out on September 27, as he passes the point where he first turned retrograde. Mercury normally turns retrograde three times a year, but this year he turns tail four times, which is unusual. The effects of each period differ, according to the sign in which it happens (see box for Retrograde Periods in 2010).
A planet is described as retrograde when it appears to be moving backwards through the zodiac. According to modern science, this traditional concept arises in the illusory planetary motion created by the orbital rotation of the earth with relation to other planets in our solar system. It's a bit like travelling on the road watching another car beside you: when the other car slows down, or you speed up, it looks as though the other car is moving backwards. Planets are never actually retrograde or stationary, they just seem that way due to this cosmic shadow-play. Click here for some neat graphics and more on the science of retrograde planetary motion.
Retrograde periods, although often problematic for us earthlings, are not particularly uncommon. Each planet retrogrades, except the Sun and Moon. Although a powerful astrological influence, Mercury is quite a small planet that travels at a relatively fast speed through the zodiac. Despite being the closest planet in our solar system to the Sun, Mercury is not always in the same sign as the Sun. Mercury turns retrograde this time in Virgo while the Sun is in Leo, but the Sun moves into Virgo on Aug. 23 (with Mercury still in Virgo). Mercury turns direct in Virgo on Sep. 12 with the Sun still in Virgo, but the Sun enters Libra at the equinox on Sep. 23, while Mercury remains in Virgo till Oct. 3.
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http://www.astrologycom.com/mercret.html