Retrograde (e.g., Mercury Retrograde)

Every planet orbits the Sun in the same direction — none of them actually runs backward. The apparent backward motion happens when Earth overtakes a slower outer planet (like a train that appears to fall behind when viewed from a faster train passing it) or when an inner planet like Mercury overtakes Earth on the inside track. Mercury is affected most often, because of its fast orbit: roughly three to four times a year, for about three weeks each time.

In the ephemerides, retrograde motion shows up as a planet's ecliptic longitude decreasing from day to day; in charts it's marked with an "R" or "Rx." Astrological tradition reads retrograde phases as times of repetition and review — Mercury retrograde, for instance, in communication, contracts and technology. Pop culture has since turned it into a scapegoat for every mishap.

Soberly considered, "Mercury retrograde" is neither an astronomical event happening to the planet itself nor a demonstrable cause of mishaps. Used astrologically, it makes the most sense read as an invitation to care and review — not an excuse.