The Sol: Mars's Version of a Day

A day on Mars isn't quite the same as a day on Earth — but it's surprisingly close. One Martian day, called a sol, lasts 24 hours, 39 minutes, and 35.244 seconds. That's just about 40 minutes longer than an Earth day, a difference that sounds minor until you're scheduling a rover's activities or trying to keep a future colonist's sleep schedule in sync with local sunrise.

The sol is defined by Mars's rotation period relative to the Sun — what astronomers call a solar day. Mars's sidereal rotation period (relative to distant stars) is slightly shorter, at 24 hours, 37 minutes, and 22 seconds, but for practical purposes — sunrise to sunrise — the sol is the number that matters.

Why 40 Minutes Matters More Than You'd Think

NASA mission controllers learned this lesson the hard way with the Mars Pathfinder mission in 1997 and have lived it with every rover since. When the Curiosity rover landed in Gale Crater in August 2012, scientists and engineers at NASA's Jet Propulsion Laboratory began working on Mars time. That means their workday shifted by roughly 40 minutes each Earth day — some days starting at 3 a.m., others at noon — to stay aligned with operations on the Martian surface.

Mission teams have used specially designed Mars clocks and even worn watches that display Martian time. It sounds like a quirky detail, but drift accumulates fast: after just 36 sols, a Mars-synced schedule is already a full Earth day out of phase with normal life. Families of rover operators have joked — and sometimes complained — about the lifestyle it creates.

The Martian Year: Long, Cold, and Elliptical

While a Martian sol is close to Earth's 24-hour day, a Martian year is a different story. Mars takes 686.97 Earth days — about 1.88 Earth years — to complete one orbit around the Sun. In Martian terms, that's 668.6 sols per year.

Mars's orbit is also notably more elliptical than Earth's, meaning the distance between Mars and the Sun varies significantly. At its closest approach to the Sun (perihelion), Mars is about 206.6 million kilometers away; at its farthest (aphelion), it's about 249.2 million kilometers. This 20 percent variation drives dramatic seasonal differences — southern hemisphere summers on Mars are shorter but more intense, while southern winters are long and frigid.

Keeping a Martian Calendar

Scientists use a system called the Mars Sol Date (MSD) and divide the Martian year into 24 seasons using a parameter called solar longitude (Ls). Ls = 0° marks the northern spring equinox, while Ls = 180° marks the northern autumn equinox. This system allows researchers to compare dust storm seasons, polar ice behavior, and atmospheric pressure across different Martian years.

Mars years are numbered starting from April 11, 1955 — chosen because it was close to a northern spring equinox and falls within the era of telescopic observation. Curiosity landed during Mars Year 31. Perseverance, which touched down on February 18, 2021, has been operating into Mars Year 36.

Proposed Colonist Calendars

Several Mars calendar systems have been proposed for future human settlers, including:

  • The Darian Calendar, developed by aerospace engineer Thomas Gangale in 1986, which divides the Martian year into 24 months across 668 or 669 sols
  • Hybrid systems that retain Earth-like week structures (7-sol weeks) to ease psychological adaptation
  • Decimal-based systems designed for scientific simplicity

No standard has been adopted, and that's largely fine for now — there are no humans on Mars yet. But mission planners take the question seriously, because timekeeping shapes everything from work schedules to agricultural cycles.

What This Means for Future Settlers

For a human crew living on Mars, the extra 39 minutes per day is actually considered a gift by some researchers. Studies suggest the human circadian rhythm naturally runs slightly longer than 24 hours, meaning a 24-hour-39-minute sol might feel more natural than Earth time for some people. Whether that holds up over months of Martian living remains an open and genuinely fascinating question — one that future missions will help answer.