Mercury
3x per year / 3 weeks
Astrological Atlas · Apparent movement
No planet ever really turns back. Retrogradation is an optical illusion: due to different orbital speeds, from Earth some planets seem, for certain periods, to move backward against the background of the zodiac.
Frequency
3x per year / 3 weeks
every 18 months / 40 days
every 2 years / 2-3 months
many months per year
A worked example
A simplified, top-down map of the solar system. The Sun holds the centre; Earth travels the tight inner orbit, Jupiter the broad outer one. Earth laps the Sun once a year, Jupiter roughly once every twelve, so our planet keeps catching up to and overtaking the slower giant.
Each thin line is a line of sight. Extended from Earth past Jupiter, it reaches the fixed ring of signs and marks where we actually see the planet: its real position projected onto the zodiac. The dashed line does the same for the Sun.
Follow that projected point and the loop appears. The trail is drawn in one colour while Jupiter moves forward through the signs, and in another during the weeks it appears to slide backward, exactly as a slower car seems to drift back the instant you overtake it.
In charts this phase is marked with the abbreviation Rx. Symbolically, the planet's energy turns introspective and reflective: a time to review, rethink, reconsider.
Astrology is not a mechanical science. It is an interpretative art: aspects are its words, but meaning always arises from the whole sentence.
Astrological Atlas / note to the reader