Retrograde Motion of Mars

Watch Mars for a few months and it does something strange. Night after night it creeps eastward against the stars, then it slows down, stops, and drifts back west for about two and a half months. Then it stops again and carries on east as if nothing happened. This backward stretch is called retrograde motion, and it puzzled astronomers for thousands of years.

Mars is not actually reversing. The loop is an effect of where we are watching from: a planet that is itself moving, and moving faster than Mars.

How to read the simulation

The view looks down on the Solar System from above the north pole. Earth and Mars orbit the Sun counter-clockwise at their real periods (365 days and 687 days) and real relative distances. The solid line from Earth to Mars is our line of sight.

The ring of stars around Earth stands in for Earth’s sky, so it is centred on Earth and travels with it. Real stars are so far away that Earth’s motion doesn’t shift them at all, and keeping the ring centred on Earth preserves that: every direction from Earth points at the right place on the ring. The dashed line continues our line of sight past Mars to the stars, and where it lands, Mars leaves a trail. The trail is white while Mars moves east as normal (prograde) and turns red while it moves west (retrograde).

Why Mars seems to move backwards

Earth is closer to the Sun, so it moves faster on a shorter track. Every 26 months or so, Earth catches up with Mars and passes it on the inside, like a car overtaking a slower one on a curved highway.

Most of the time, the line from Earth to Mars swings eastward because Mars’s own motion carries it that way. But for a couple of months around the overtaking, Earth’s speed dominates. As we pull ahead, the line of sight to Mars swings backward, and Mars appears to slide west against the far-off stars. Once Earth rounds the curve and the geometry changes, the line swings east again and Mars resumes its usual direction.

The moment Earth sits exactly between the Sun and Mars is called opposition. It always falls in the middle of the retrograde stretch, and it is also when Mars is closest, brightest and up all night, which makes it the best time to observe it.

Why is it a loop and not a straight line?

If Earth and Mars orbited in exactly the same plane, Mars would just slide back and forth along a line. But Mars’s orbit is tilted by about 1.85 degrees relative to Earth’s. While we overtake it, Mars also drifts a little above or below the plane, so its path on the sky opens up into a loop, or sometimes a Z-shaped zigzag, depending on where in its tilted orbit the overtaking happens. In the simulation, that small north-south drift pushes the trail slightly outward or inward (exaggerated so you can see it), which is why the red section opens into a loop instead of retracing the white line.

How long does Mars retrograde last?

Mars spends roughly 70 to 80 days in retrograde, and it happens once every synodic period of about 780 days (about 2 years and 2 months). The exact length changes from one opposition to the next because Mars’s orbit is noticeably elliptical: when Earth overtakes Mars near its closest point to the Sun, the loop is larger and faster.

When is the next Mars retrograde?

The next one runs from about January 10 to April 1, 2027, with opposition on February 19, 2027. Mars will be in Leo, retreating westward through the evening sky. It is a distant opposition, because Mars will be near the far point of its orbit, so the planet will look smaller than during the close oppositions of 2003 or 2018, but its backward drift is easy to follow with the naked eye over a few weeks. After that, the next opposition is on March 25, 2029.

Do other planets go retrograde?

Yes, all of them do, as seen from Earth. Jupiter, Saturn, Uranus and Neptune go retrograde once a year, each time Earth overtakes them, and the farther out a planet is, the longer its retrograde lasts. Mercury and Venus are faster than us, so for them it is the other way around: they appear to go backwards when they overtake Earth on the inside. Mercury does this three or four times a year, which is why “Mercury retrograde” comes up so often.

A puzzle that shaped astronomy

Ancient astronomers who put Earth at the center of everything had to explain these loops somehow. In Ptolemy’s model, each planet moved on a small circle (an epicycle) that rode on a larger circle around Earth, so the planet would loop backwards at regular intervals. It worked reasonably well for predictions, but it needed more and more circles to keep up with observations.

In the 16th century Copernicus showed that if Earth and the planets all orbit the Sun, retrograde motion appears on its own, with no extra circles needed: it is simply what overtaking looks like. That simple explanation was one of the strongest arguments for the Sun-centered model, and it is exactly the geometry the simulation above shows.

See the planets’ relative speeds in the top-down Solar System simulation, learn more about the planet itself on the Mars page, compare its size with ours in Mars vs. Earth, or work out orbital periods with the Kepler’s third law calculator.