The loop in the sky
Why it seems to go backward
Mercury never actually reverses. Seen from above, both planets keep circling the Sun; it is the line of sight from the Earth that swings back when the faster Mercury overtakes us on the inside track.
Retrogrades of the year
What a retrograde is
Planets drift eastward through the zodiac, night after night. A few times a year Mercury seems to slow down, stand still, and slide back west for about three weeks before it turns forward again. That backward stretch is the retrograde; the moments it stands still are its stations.
It is an effect of perspective. Mercury goes around the Sun in 88 days, the Earth in 365, so about every 116 days Mercury catches up with us on the inside and passes between the Earth and the Sun. Around that pass, it moves across our line of sight in the opposite direction, like a car you overtake seems to slide backward against the hills. Venus does the same when it passes us, and the outer planets (Mars to Pluto) when the Earth overtakes them.
The shadow is the stretch of sky the planet crosses three times: forward before the retrograde (pre-shadow), backward during it, and forward again after it (post-shadow). Astrologers often count the shadow as part of the retrograde's influence.
In astrology, Mercury rules communication, travel, technology and contracts, so its retrograde is blamed for missed emails, delayed trains and crashing phones, and advised as a time to review and revise rather than to launch or sign. There is no known physical effect: Mercury is a small planet about 90 million km away when it is retrograde. Treat it as a reason to double-check, not as a verdict.
Positions are computed in your browser from the orbital elements of NASA JPL's Approximate Positions of the Planets, accurate to within hours for the stations between 1800 and 2050. Dates are shown in your time zone, and signs are those of the tropical zodiac.