This document is part of the Martian Time Boneyard. It was originally located at
Author: Andrew Bridges
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In a Martian Minute: Telling Time on the Red Planet
By Andrew Bridges
Chief Pasadena Correspondent
posted: 04:34 pm ET
02 December 1999


When the Mars Polar Lander sets down at 12:14 p.m. PT on Friday, it will kick off what NASA hopes will be a mission lasting 60 to 90 days.

But how is one to count them? For all intents and purposes, there are no "days" on Mars.

On Earth, a day is a strictly defined period of time equal to 24 hours. But on Mars, the time it takes the planet to revolve once on its axis is slightly longer: 24 hours and 37 minutes.

For planning purposes on Mars Polar Lander and past missions like Pathfinder, engineers dub the Martian equivalent of a day a "sol" -- Latin for "sun" -- in reference to the planets period of rotation.

Following that logic, Friday -- Day 1 of the mission -- will be Sol 0. (Unlike most of the world, NASA starts things off at 0, not one, including its rockets at launch.)

So how is the sol broken down?

With that, the can of Martian worms opens.

"None of it is worked out, there is no word for a Martian hour or minute. The whole concept of Martian time will have to be decided," said Jim Bell, a Cornell University astronomy professor who is part of a team sending a version of the worlds oldest timepiece -- a sundial -- to Mars on a mission slated for 2001.

On Earth, time in the form of days, months and years is governed by the rotation of the planet, the revolution of the moon around it and its own revolution around the sun.

On Mars, days, as we have seen, are 37 minutes longer. It has two moons, Phobos and Deimos, that whip around it every 1.3 and 0.3 days, respectively, making for very frequent months. And to make the full, year-long trek around the sun, Mars takes 687 Earth days -- or more properly, 669 sols.

So whats a Martian month, much less a week?

Puzzling over Mars time has been a constant occupation for a select audience for more than a century -- an Earth century, that is.

Mars enthusiast Percival Lowell simply stretched the Earth calendar to fit the longer Mars year. Author Edgar Rice Burroughs divvied the sol into 10 "zodes." Fellow novelist Arthur C. Clarke made his Martian months as long as 60 sols.

Today, promoters of various Martian calendars tend to make their case on the Internet, where dozens of calendar variations -- some with 22-month years, others with sols partioned into tens -- can be found.

"There doesnt appear to be any consensus yet, there are more schemes for Mars calendars than you can shake a stick at," said Michael Allison, a space scientist with the Goddard Institute for Space Studies in New York, who has has chimed in with his own method of keeping Mars time.

So whos right?

Until the International Astronomical Union weighs in, probably nobody. But in the meantime, that hasnt kept anyone from scheming.

Robert Zubrin, president of the Mars Society, claims "natural time units" are the best solution: divide the sol into 24 hours, each hour into 60 minutes and each minute into 60 seconds. End discussion.

Put Zubrins Martian clock next to an Earth clock, and the astute observer will notice that for every one second the former ticks off, the latter measures 1.025 seconds.

"The time system is already implicitly in use," Zubrin said.

But mean time -- the stuff of wrist watches, Earthly or Martian -- and solar time -- dictated by where the sun is in the sky -- can also be at odds.

On Earth, mean and solar times differ very little throughout the year. If a clock says it is noon, the sun will be highest over head within plus or minus 16 minutes of that moment. On Mars, because of its highly eccentric orbit, mean and solar times can be off by as much as 51 minutes.

That led Allison, the Goddard scientist, to devise a means of calculating the solar time on Mars.

After all, a spacecraft dependant on solar power cares more if the sun is directly overhead than it does if the clock claims it is actually 11:08 a.m. or 12:40 p.m.

Or in the case of Allison, who hoped to use data from the ill-fated Mars Climate Orbiter to map the planets atmosphere through the seasons, what the sun is doing at any particular moment has huge implications.

"It is an issue on Mars because the atmosphere is so hypersensitive: When the sun comes up, the thermometer comes screaming upward and conversely when it comes down," Allison said. "I thought to do this mapping carefully, I should work out the position of the sun in the sky."

His colleague, Robert Schmunck, then took Allison's algorithms and worked them into an actual solar map, which shows the sunlit portion of Mars and, with the click of a mouse, the local mean and solar times for any spot on the planet.


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