Soon enough, humans will be living on Mars, almost 40 million miles from their home planet. This will require some major work, like finding new ways to grow crops, build houses, and treat disease. Not the least on the list is settling on a whole new clock and calendar.

Thankfully, astronomers have already figured out some of the basics.

Time on Mars

Days, at least, are pretty similar on Mars. The name for one rotation of the Red Planet is called a sol. A sol is 24 hours, 39 minutes, and 35 seconds, whereas a day on Earth is 23 hours, 56 minutes, and 4 seconds. This will make it fairly easy for the first humans traveling to Mars to keep track of the time once they are there.

“The day length is almost exactly the same, it’s actually quite a coincidence,” Emily Lakdawalla, senior editor and planetary evangelist, for The Planetary Society, tells Inverse. “The first humans living on Mars will probably just use a 24-hour clock and not think too much about it.”

If using a 24-hour clock for a 24.7-hour day, of course, Martians would have to make some adjustments to maintain consistency.

One option would be to have special “leap days” that are extra long or short to correct for time slippage: Lakdawalla suggests a five-day workweek paired with one super-long weekend day.

Another option, already used by some, is a slow watch. NASA engineer Nagin Cox says she and colleagues started using a “Mars watch” back in 2004 when they were monitoring the Spirit and Opportunity rovers. For a while, they were also keeping to consistent Martian hours, which sometimes meant getting up in the middle of the night. “You really needed something on your wrist to tell you: What time is it on the Earth? What time is it on Mars?” Nagin said in a recent TED talk.

Nagin and others working with Mars informally developed new terms to talk about time over there. The current sol is tosol; there’s also yestersol and nextersol or solorrow.

Mars really is a red planet, as apparent in this true-color image.

Dates on Mars

Keeping track of the date on Mars might be trickier.

To start, a Martian year is nearly twice as long as an Earth year. It takes the Red Planet 669 sols — or 687 days — to orbit the Sun. If annual dates are to have any meaning on Mars, then, our sister planet will need to keep its own count.

As for months, Mars can’t go with a lunar cycle as Earth does since it’s got two moons: Phobos and Deimos. Still, Martians might just divide the year into a certain number of segments to help keep track of things, and various scientists and authors have proposed just how. Among them: Robert Heinlein described a 24-month version in his 1949 book Red Planet; Kurt Vonnegut had a 21-month version in his 1959 novel The Sirens of Titan.

Any attempt to track years on Mars will, of course, require us to pick a starting point. There are some suggestions here, with a few plans settling on Earth’s 1609, when the telescope was invented; or, for unclear reasons, on December 29, 1873 or April 11, 1955.

For current missions to Mars, at least, space organizations have kept things simple, counting the number of sols since the mission started. If there is a future colony on Mars, perhaps it will track sols — and Martian years — from the date the first colonists land.

As of June 14, 2017, Curiosity has been on Mars for 1,726 sols.

Seasons on Mars

Until humans make up their minds, there is really only one set-in-stone way to think about the time of year on Mars: seasons.

“The main reason we care about date-keeping on Earth is because of seasons,” Lakdawalla says. “We need to know what to wear for the weather. The same will go for Mars, it’s just that the seasons happen to last a lot longer.”

Mars has a similar axial tilt to Earth’s at 25 degrees, just a fraction away from the Earth’s 23.5 degrees, which means the planet has similar seasons to ours.

Since Mars has a much more elliptical orbit than Earth, however, its seasons vary greatly in length. Northern hemisphere spring clocks (or southern hemisphere autumn) is the longest season at 194 sols; northern hemisphere autumn is the shortest at 142.

The seasons also vary drastically depending on what hemisphere you reside on. On the northern hemisphere, the winter solstice happens to be the point at which the north is closest to the Sun (perihelion), this would create a very moderate winter. Likewise, in the summer, the north is farthest from the Sun (aphelion), so summer would be cool. But, in the southern hemisphere, where the winter comes near aphelion and summer comes near perihelion, the seasons are extreme.

Mars NASA time
Mars' orbit (red) is more elliptical than Earth's (blue).

The future

Whatever NASA and other Earth-based groups choose for Martian time, there’s a chance that future colonists will come up with their own system — with, say, a second system used for universal coordination.

“If there is one thing I’ve learned from science fiction, once humans leave Earth and go off into space they aren’t really going to care what happens on Earth anymore,” Lakdawalla says. “They’ll do what they want to do and say ‘hey if you want to fix it, then you come here.’ And we have to accept that.”

Manned Mars Missions are scheduled for the near future. NASA is hoping for a 2030s landing and the determined commercial space company, SpaceX, is shooting for 2020.

Photos via ESA Rosetta, Getty Images / NASA, Areong/Wikimedia Commons, Getty Images / ESA