This document is part of the Martian Time Boneyard. It was originally located at
Author: William J. Clancey

Living On Mars Time

By William J. Clancey

I lived in Pasadena, CA for two weeks in February 2004, observing how geologists and engineers controlled the Mars Exploration Rovers. There were two active rovers near the equator on opposite sides of Mars, at places called Gusev Crater (the rover we called "Spirit") and at Meridiani Planum ("Opportunity").

I was working with the Opportunity team. At the time (a possible pun you will soon realize), the Opportunity team was living their lives as if they were actually at Meridiani. So for us, sunrise occurred when the sun rose and shined on the Opportunity rover at Meridiani. And because Mars turns on its axis somewhat more slowly than the Earth, a day (more specifically a "Mars sol") lasts somewhat more than 24 hours. Roughly, a Mars sol is 24 hours and 40 minutes long.

Okay, the theory seems simple, what's it like in practice?

Twenty-six sols after Opportunity landed at Meridiani, M26 for short, I glanced at our fancy computer clock, which was displayed on a kind of moveable touch-screen board that you can write on with computer pens. I saw the time: 19:41 LST-B. In human-speak that means 7:41 PM on mission B, otherwise known as Opportunity (which meant us). LST means Local Sol Time, which means the time at this place on Mars, Meridiani. In short, if you were living at Opportunity's landing site, you would look at your watch and say, "It's 7:41 PM."

Just below the LST-B time, I could read the Los Angeles time: 19:53 PST, which of course is 7:53 PM.

Now, as we know on Earth, places in different time zones are never at the same time. So the time in Paris can never be the same time as LA. But the Earth and Mars are turning independently, and one turns more slowly than the other. So if you are LST-B time zone on Mars, every few weeks your clock will be identical to a clock in LA. But that will just last for a moment, and then your clocks will get out of synch again. The question is, what does that look like? Which clock is falling behind? The one at Meridiani or the one in LA?

At 19:41 LST-B I could easily see that those of us living at Meridiani on Mars were now 12 minutes behind LA (19:53 PST). The LA clock was ahead of us.

Three hours before, at the start of the Science Assessment meeting I was observing, we had been only 7 minutes apart. So our clock, the one we were using on Mars, was falling behind LA's clock. How could this be? Mars has more minutes a day, how could our clock be slower?

This question reminded me of an article I had recently read on JPL's web site, profiling the geologist I knew from Haughton, Jim Rice (Feb 8, "A day in the life of a Martian Scientist"). The author wrote: "They next day, they'd come in at 9:40 a.m., and the next day at 10:20 a.m., and so on. They end up running multiple laps around Earth's 24-hour schedule throughout the mission." She made it sound like the Mars people are living a much more frantic life.

But if the Mars clock is losing time (in three hours I saw it fall behind by 5 minutes more), how could it "run laps" around Earth's 24-hour schedule? Clearly, the opposite is true, Earthlings are running laps around people living on the Mars! If after 3 hours we had lost 5 minutes, then on every sol those of us living on Mars would lose 40 minutes. After a day, Earth would be ahead of us by 40 minutes, and over the course of the mission, Earthlings will run laps around the Mars 24-hour 40 min schedule. On Earth, they will say 90 days have passed, while on Mars only 88.5 sols will have gone by. That makes sense, Mars sols are longer than Earth days, so relative to Earth, there will be fewer sols than days.

The trick is that the Mars clock has 24 hours just like Earth, but each second is longer! 24 hours x 60 minutes x 60 seconds = 1 Martian sol, but a sol will take place in the time Earthlings say is 24 hours and 40 mins (roughly). Time goes more slowly on Mars. Martians are the tortoises, and Earthlings the hares.

February 24, 2004
Pasadena, CA (pretending to be at Merdiani Planum, Mars)


Postscript. Reread my story and you'll see the original misconception -- Mars does not have more minutes per day. Rather a Mars minute takes longer to occur than an Earth minute. A sol has the same number of minutes as a day (24 x 60). A Mars minute has the same number of seconds as an Earth minute (60). But each second on Mars last slightly longer than 1 second on Earth.

PPS. For advanced credit: How many days are there in a Martian year? How many sols is that?

PPPS. Isn't a "second" a fundamental physics metric, just like a meter, a gram and a degree centigrade? Is there no fundamental unit of time? Is this a reflection of relativity? Nevertheless, did scientists make a mistake in deciding that "a second" will have different meanings, depending on what planet you live on? If you know the answers to these questions, send me an email!

Copyright © 2004 William J. Clancey. All Rights Reserved.

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