The planets that turn back

How to read it

In astrology, retrograde motion is a recurring theme: it is much talked about, and interpretations are drawn from it. From the sky's point of view, however, no planet ever really turns back. Retrogradation is an effect of perspective: since the planets orbit at different speeds, seen from Earth a planet appears, for certain periods, to move backwards against the background of the zodiac.

While the animation plays, the three planets in the diagram — Mercury, Mars and Jupiter — each draw their own trail along their orbit: a thin line for direct motion, a thicker one for retrograde. The lines starting from Earth show the direction in which each planet appears in the sky, and the sign it is in.

Watch Mercury in particular: it is the planet closest to the Sun, hence the fastest, and it overtakes Earth several times in the course of a year — one retrogradation at every overtake. That is why "Mercury retrograde" is so frequent, and why during the animation you will see it move backwards several times. Here too the diagram is deliberately simplified — circular orbits, proportions not to scale — but speeds and positions are computed from true ephemerides.