What this really is
No planet drives backwards. What happens is that the Earth and the planet run in different lanes at different speeds, and when one overtakes the other, the one alongside seems to slip back for a while against the background of stars — exactly like the car you overtake on the motorway. With Mercury it happens three times a year because it is the fastest of them all: it laps us often. The phenomenon is real and can be calculated to the day; what is tradition, and not astronomy, is what it is said to mean.