In space, a “sol” is one full day on Mars. It’s the time it takes for Mars to rotate once relative to the Sun, and it lasts 24 hours, 39 minutes, and 35 seconds in Earth time. NASA uses “sol” instead of “day” so there’s no confusion between Mars time and Earth time when running missions on the red planet.
How Long Is a Sol?
A sol is only about 40 minutes longer than an Earth day, which makes Mars surprisingly convenient compared to other planets in the solar system. Jupiter’s day is under 10 hours. Venus takes 243 Earth days to complete one rotation. Mars sits close enough to Earth’s rhythm that mission teams can almost keep a normal schedule, just slightly stretched.
The precise average length is 24 hours, 39 minutes, and 35.244 seconds. That “average” matters because, like Earth, Mars doesn’t rotate at a perfectly uniform speed. Its orbit is more elliptical than ours, so the apparent length of a solar day shifts slightly throughout the Martian year. The figure NASA uses is the mean solar day, smoothing out those variations.
Mars also has a sidereal day, which measures rotation against the distant stars rather than the Sun. That’s slightly shorter: 24 hours, 37 minutes, and 22.663 seconds. The difference exists for the same reason Earth’s sidereal and solar days differ. As Mars orbits the Sun, it has to rotate a tiny bit extra each day for the Sun to return to the same position in the sky. For practical purposes on the surface, the solar day (the sol) is what matters, because it tracks sunrise and sunset.
Why NASA Needs a Separate Word
When you’re operating a rover millions of miles away, clear communication is everything. Saying “day 200 of the mission” could mean Earth day 200 or Mars day 200, and those fall on completely different calendar dates. The word “sol” eliminates that ambiguity instantly. Everyone at mission control knows that “Sol 198” means the 198th Martian day since landing, not the 198th Earth day.
The term comes from the Latin word for sun, which is fitting since a sol measures one full cycle of sunlight on Mars. NASA adopted the term during early Mars surface missions and has used it consistently ever since. Every lander and rover, from the Viking missions in the 1970s through Perseverance today, logs its timeline in sols. When Perseverance took its famous selfie over a rock nicknamed “Rochette” on September 10, 2021, NASA logged it as Sol 198 of the mission.
Converting Sols to Earth Days
The math is straightforward. One sol equals roughly 1.027 Earth days. So if a rover has been on Mars for 1,000 sols, that’s about 1,027 Earth days, or just over 2 years and 10 months. To convert any sol count to Earth time, multiply by 1.027.
A full Martian year is about 668.6 sols, which works out to roughly 687 Earth days, or nearly 23 Earth months. That means a Mars year is almost twice as long as ours, but the individual days are nearly the same length. This creates interesting calendar math: Mars packs fewer sunrises into each orbit around the Sun, not because its days are longer (they barely are), but because its orbit is so much wider.
What Living on Sol Time Feels Like
During active surface missions, some NASA team members actually shift their daily schedules to Mars time. This means their “workday” drifts about 40 minutes later each Earth day. After about three weeks, someone living on sol time who started waking at 7 a.m. would be waking at 7 p.m. instead. After roughly 36 Earth days, the cycle wraps all the way around and resets.
Mission teams have described this as a perpetual jet lag. Some cover their windows with blackout curtains and wear watches set to Mars time. The experience is disorienting but manageable in short stretches, and it’s typically only required during the early, critical phases of a mission when real-time coordination with the rover matters most. Once a mission settles into routine operations, most team members switch back to Earth schedules and simply queue up commands for the rover to execute during its next sol.
Does “Sol” Apply to Other Planets?
Technically, “sol” is specific to Mars. Other planets have their own day lengths, but none of them have active surface missions that require daily timekeeping the way Mars does. If humans ever land on Titan or another body with a day length close to Earth’s, a similar naming convention would likely follow. For now, though, “sol” belongs to Mars. You’ll sometimes see it used loosely in science fiction to mean any alien day, but in real space science, it refers to exactly one thing: the Martian solar day.