Time and time zones on Mars (draft text)

Paul Eggert eggert at twinsun.com
Mon Jan 19 01:09:05 UTC 2004

Wednesday's L.A. Times article by Jia-Rui Chong about JPL workers'
wristwatches keeping Martian time prompted me to do a little bit of
research about time zones on Mars.  I'll eventually propose a patch to
the "Theory" file, but thought I'd first submit the following rough
draft on this mailing list for comment.

----- Time and time zones on Mars -----

Some people have adjusted their work schedules to fit Mars time.
Dozens of special Mars watches were built for Jet Propulsion
Laboratory workers who kept Mars time during the Mars Exploration
Rovers mission (2004).  These timepieces look like normal Seikos and
Citizens but use Mars seconds rather than terrestrial seconds.

A Mars solar day is called a "sol" and has a mean period equal to
about 24 hours 39 minutes 35.244 seconds in terrestrial time.  It is
divided into a conventional 24-hour clock, so each Mars second equals
about 1.02749125 terrestrial seconds.

The prime meridian of Mars goes through the center of the crater
Airy-0, named in honor of the British astronomer who built the
Greenwich telescope that defines Earth's prime meridian.  Mean solar
time on the Mars prime meridian is called Mars Coordinated Time (MTC).

Each landed mission on Mars has adopted a different reference for
solar time keeping, so there is no real standard for Mars time zones.
For example, the Mars Exploration Rovers mission defined two time
zones "Local Solar Time A" and "Local Solar Time B" for its two
missions, each zone designed so that its time equals local true solar
time at approximately the middle of the nominal mission.  Such a "time
zone" is not particularly suited for any application other than the
mission itself.

Many calendars have been proposed for Mars, but none have achieved
wide acceptance.  Astronomers often use Mars Sol Date (MSD) which is a
sequential count of Mars solar days elapsed since about 1873-12-29
12:00 GMT.

The tz database does not currently support Mars time, but it is
documented here in the hopes that support will be added eventually.


Michael Allison and Robert Schmunk,
"Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
<http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-01-12).

Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
(2004-01-14), pp A1, A20-A21.

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