How long is a day on Mars?

Two ways to time a spinning planet

Planetary scientists keep two clocks. The sidereal day is one full rotation measured against the distant stars: for Mars, 24 hours, 37 minutes, 22 seconds. The solar day is what a person standing on the surface actually experiences — noon to noon — and it runs slightly longer, at 24 hours, 39 minutes, 35 seconds. The gap exists because Mars is also traveling along its orbit while it spins, so the planet has to rotate a couple of extra minutes' worth to bring the Sun back overhead. Earth has the same quirk (our rotation takes 23 hours 56 minutes; our solar day is 24 hours). When anyone asks how long a day on Mars is, the solar day is the honest answer: 24.66 hours.

It is one of the friendlier coincidences in the solar system. Of all the worlds we might try to live on, the nearest candidate happens to spin at almost exactly the rhythm our bodies already keep.

What is a sol?

A sol is one Martian solar day. NASA adopted the word — from the Latin for sun — during the Viking lander missions in 1976, because mission planning gets dangerous when "day" might mean either planet's. Every surface mission since has counted its own time in sols, starting from landing: Perseverance touched down on Sol 0, and the count simply climbs from there. Rover teams even coined "yestersol," which is exactly what it sounds like and remains the best word the space program has produced.

Living on Mars time (and the watches it took)

Because a sol runs 39.6 minutes longer than an Earth day, Mars time drifts against ours — every day, Martian noon arrives about 40 minutes later on your Earth clock. A rover runs on solar power and does its best work in Martian daylight, so when Spirit and Opportunity landed in 2004, hundreds of engineers and scientists at NASA's Jet Propulsion Laboratory did the only logical thing: they abandoned Earth time entirely and lived on sols.

It meant reporting to work 40 minutes later each day, forever sliding around the clock — breakfast at midnight one week, at dawn the next. Team members blacked out their bedroom windows and wore two watches. A watchmaker in Montrose, California named Garo Anserlian hand-built mechanical Mars watches for the teams, tuned to lose 39 minutes a day so they'd keep perfect sol time. The chronic fog of living 2.75% out of sync with your own planet earned a name, too: sol lag, jet lag's meaner cousin, except the jet never lands. Most teams held out for about 90 sols before returning to Earth schedules; the Curiosity team repeated the whole ritual in 2012.

Day length across the solar system

Mars is the outlier for being so normal. Elsewhere, "a day" gets strange fast — here's each planet's rotation period, in hours:

The year: 687 days, 668.6 sols, seasons that won't quit

Mars orbits farther from the Sun and moves slower, so its year lasts 687 Earth days — which, in local units, works out to 668.6 sols. That's just shy of two Earth years, and it means every Martian season stretches nearly twice as long as the Earth version. They're lopsided, too: Mars's orbit is noticeably elliptical, so northern spring lasts about 194 sols while northern winter gets only about 154. If you dislike February, Mars offers five consecutive months of it. (Kids tend to find the year math delightful — a ten-year-old is barely five in Mars years, which is exactly the sort of thing Rusty the Rover puts in his letters, dated in sols, naturally.)

Quick answers

More instruments: your age on Mars · your weight on Mars · Mars facts for kids