The planet that walks backwards

The ancient Greeks called the planets planētes, "wanderers", because they drift slowly against the fixed pattern of the stars. Watch Mars over a few weeks and it creeps eastward through the zodiac, night after night. But roughly every 26 months it slows down, stops, and for a couple of months slides back west. Then it stops again and carries on as if nothing happened.

This is called retrograde motion, and you can catch the next one yourself: Mars turns backwards on 10 January 2027, reaches opposition (directly opposite the Sun, and at its brightest) on 19 February, and resumes its normal eastward drift on 1 April, all in front of the constellation Leo.

Mars never actually reverses. What reverses is your line of sight. Drag the Earth below, or press play, and watch.

It's an overtaking illusion

You already know this effect from the motorway. Overtake a slower lorry and, for a few seconds, it seems to slide backwards against the distant hills, even though it's still driving forward. The hills play the role of the stars.

Earth is on the inside lane. It circles the Sun in 365 days at about 30 km/s, while Mars, one and a half times farther out, takes 687 days at about 24 km/s. So every 780 days or so Earth catches up with Mars and passes it. For most of that cycle, the direction from Earth to Mars swings steadily eastward. But in the weeks around the overtake, when the two planets are on the same side of the Sun, Earth's own sideways speed dominates, and the sight line swings the other way. Mars appears to back up against the stars.

That's why retrograde always happens around opposition, when Mars rises at sunset and sits opposite the Sun in the sky. It's also when Mars is closest and brightest, so the planet's strangest behaviour always happens when it's easiest to see.

Why a loop, not a straight line

If every orbit lay in exactly the same flat plane, Mars would just slide back and forth along one line. But Mars's orbit is tilted by about 1.85° relative to Earth's. Depending on where in its orbit the overtake happens, Mars is a little above or below the plane, so the backward stretch becomes a loop, a Z or an S. Switch planets in the simulation, or let Mars run for a few cycles, and you'll see the shape change from one retrograde to the next.

Wheels within wheels

Earth-centred astronomy had to explain this with machinery. In the system Claudius Ptolemy laid out in the Almagest around 150 CE, each planet rode on a small circle, an epicycle, whose centre moved on a larger circle around the Earth. When the planet was on the inner part of its little circle, its backward spin outran the forward motion of the big one, and it appeared to reverse. The scheme predicted planetary positions well enough to be used for about fourteen centuries.

But it had a suspicious coincidence built in. For Mars, Jupiter and Saturn, the epicycles had to turn in lockstep with the Sun, or the loops would not line up with opposition. In 1543 Nicolaus Copernicus published De revolutionibus, putting the Sun at the centre. Retrograde loops now fell out for free: they are simply what you see from a moving platform that laps its neighbours. The lockstep with the Sun stopped being a coincidence, because the "Sun's motion" was really Earth's.

Farther planets, longer reversals

Every planet goes retrograde as seen from Earth. For the outer ones, Earth laps them more often the slower they are, because they barely move while we go round:

The farther the planet, the longer it spends going backwards, but the smaller its loop: distant Saturn's backward step is tiny, because Earth's orbit looks small from out there. Try it in the simulation and compare the size of the loops. Mercury and Venus, on the inside lanes, do the overtaking themselves, which is why Mercury goes retrograde about three times a year for roughly three weeks each time.

So when Mars stalls in Leo next January and drifts backwards for eleven weeks, nothing strange is happening out there. You're watching the Earth take the inside line.