Up on Mars, time does not work the same way it does here on Earth.

On the face of it, this probably seems like a pretty counterintuitive remark. For one thing, time flows at a constant rate everywhere, doesn’t it? And anyway, you measure it the same way no matter where you are, don’t you?

Answers: No, and no.

First, although it seems to contradict your own experience, time does not flow — although even Isaac Newton thought it did. But 200 years after him, Einstein’s theory of relativity, which has been verified again and again as an accurate description of physical reality, showed that time is not a flowing entity at all, but it is a dimension bound up with the three spatial dimensions — space-time. For some reason no scientists have figured out, we experience this fourth dimension as a flow — time. So there is no “constant rate” of time on Mars any more than there is on Earth.

Second (and possibly more comprehensibly), time on Mars cannot be measured the same way we measure it on Earth because the motions our hours, days and years are based on are different there.

On Earth, one year is measured as the time it takes for the planet to make one complete orbit around the sun. For practical purposes, we round this off to 365 days of 24 hours each. It’s not quite an exact measure, though. Earth’s rotation rate — the time it takes a point on Earth’s surface to make one complete circuit with respect to the fixed stars — is actually just short of 24 hours, about 23.9345 hours. This is known as the sidereal rotation period (sidereal means “with respect to the stars”). Meanwhile, Earth’s orbital period around the sun is about 365.256 days. The small discrepancy between the actual period and the 365-day period accumulates, and so we insert a leap day every four years (with longer-term adjustments even to this) to keep our months falling in their proper seasons.


One year on Mars, however, is not one Earth year. We mark Mars completing one full orbit every 686.98 Earth days, about 43 days short of two full Earth years. So Martian calendars need to measure months (we’ll see what that means in a moment) on a different basis from Earth months.

The next complication is that Mars’ day is longer than Earth’s. Where Earth rotates once in just about 24 hours, Mars rotates once every 24 hours, 39 minutes, and 35.244 seconds of Earth time. So if astronauts were to use Earth-based watches on Mars, they’d be running almost 40 minutes fast. At this rate, it takes only about 10 Martian days for the Earth watch to be 6.5 hours ahead of Mars time, according to a calculation by amateur astronomer George Lardas. In about 37 days, a Thursday morning on Earth is corresponding to Wednesday morning on Mars.

If astronauts ever get to Mars, one way to deal with the confusion between mission control’s timekeeping and the astronauts’ timekeeping will be for the astronauts to use watches calibrated to 24 Martian hours — in which one Martian hour is 1.02749 times longer than one hour on Earth.

Lardas notes that Kim Stanley Robinson in his “Red Mars” science fiction saga has the colonists setting their watches to Earth time but keeping a 39-minute pause every night to maintain sidereal accuracy. Robinson calls the pause the “Martian time slip.” Hmm. In Philip K. Dick’s 1964 novel “Martian Time-Slip,” the slip refers to a difference in the experience of the rate at which autistic people experience time. Time really is a conundrum of the psyche.

Anyway, another problem in Martian timekeeping is that, because its orbital axis is slightly tipped (similar to Earth’s tip), Mars also has seasons. So the months in a Martian calendar of its 687-Earth-day year (669 Mars days) need to correspond reliably to winter, spring, summer and fall, the same as our Earth calendars do. Obviously, our 12 months are not going to fit into the 669-day year. The other discrepancy is that because Mars’ orbit is more elliptical than the Earth’s, distance-from-the-sun factors make the effects of the sun’s warmth much more pronounced on Mars. So while the lengths of Earth’s seasons differ by about three or four days, Mars’ seasons differ in length by from 15 to 50 Martian days.

Related to this is the complication that where our Earth months are based from deep antiquity on the roughly four-week cycle of the moon’s phases (it’s about one week each from new moon to first quarter, first quarter to full, full to third quarter, and third quarter back to new), no such corresponding cycle exists on Mars. Mars’ larger moon, Phobos, orbits every eight hours, instead of every four weeks as Earth’s moon does. The other moon, Deimos, is half the size of Phobos, has a roughly 30-hour orbit, and is not much brighter in the sky than Venus is to us on Earth, Lardas points out.

So if people get to Mars, clocks and calendars will have to be devised that correspond to Martian time without confusing mission control back in Earth time. To keep track of Martian dates for the activities of the rovers and other robots, astronomers at present refer to Martian sols (24 hour, 39 minute days) in Martian years calculated from a zero date of April 11, 1955. In Mars’ northern hemisphere, it is currently the summer of Martian year 33.

And by the way, Einstein’s relativity theory tells us there are going to be peculiarities in the time calculations between Mars and Earth no matter what we do because even when the two planets are relatively close, as they are this (Earth) spring, it takes 4.5 minutes for light to travel from one place to the other, slightly skewing clock measurements — not to mention the psyches of the people trying to make sense out of them.

Dana Wilde lives in Troy. His book on the stars and planets is “Nebulae: A Backyard Cosmography,” available online in print and e-book editions. You can contact him at [email protected] Backyard Naturalist appears the second and fourth Thursdays each month.

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