An Explanation of Zubrin's Mars Calendar

Paul J. Thomas
Department of Physics and Astronomy
University of Wisconsin - Eau Claire
(715) 836-3615

Note: This page contains material from The Case for Mars, by Robert Zubrin with Richard Wagner (The Free Press, 1996). This material is reproduced with permission.

The Problem

A solar day (noon to noon) on Mars is called a Sol. It is quite similar in length to an Earth day: 24 hours and 39.6 minutes long. To date, the only sort of calendrical system used by humans on Mars is to number Sols for each spacecraft we have landed starting at Sol 1 for the landing day. See, for example, the Mars Pathfinder home page.

This is clearly unsustainable in the long term, particularly when we start to settle Mars permanently. We need to develop a calendar system for Mars similar to that of the Earth. In this system, there would be 669 Sols each year. We need to figure out a way to organize these into months and to number the years. The reason that this is non-trivial is (a) Mars's orbit around the Sun is quite elliptical, so the seasons are of grossly different lengths, and (b) what do we use as a year zero?

Determining the Months

Unlike Earth, we cannot base the length of a month on the motions of Mars's satellites. Phobos orbits Mars every 7 hours 39 minutes (a time shorter than a Sol!), while Deimos orbits Mars every 30 hours 7 minutes. In any case, they do not dominate Mars's night sky in the same way that that the Moon does Earth's.

Similarly, we cannot just map Earth months to Mars, as they will not fit into its year. The ellipticity of Mars's orbit, discovered by Johannes Kepler in 1619, is another serious problem, as shown below.

As is clear from the picture above, the various seasons of Mars have markedly different lengths. For example, winter in the Northern hemisphere is 154 days long, while winter in the Southern hemisphere is 178 days long. Scientists mark the first Sol of each year from the Northern spring equinox, and define this position as the line LS=00. The angles increase as Mars revolves around the Sun (in an anticlockwise direction as seen from a point North of the ecliptic plane, as in this diagram). As we can see, at LS=700, Mars is at aphelion (furthest point from the Sun); while at LS=2500, Mars is at perihelion (closest point to the Sun). Dust storms frequently occur around this time (and we have lost spacecraft in them), so this is a good date to have marked on a Mars calendar!

Robert Zubrin has extended this approach to develop a calendar for months. The basic idea of this is that the month corresponds to whatever constellation of the zodiac Mars would appear in as seen from the Sun. This marks the martian year into a set of 12 more-or-less equal months, as shown in the table below.

The Martian Year
Month Number
of Sols
on Sol #
Noteworthy Features
Gemini 61 1 Gemini 1, Vernal equinox
Cancer 65 62
Leo 66 127 Leo 24, Mars at Aphelion
Virgo 65 193 Virgo 1, Summer solstice
Libra 60 258
Scorpius 54 318
Sagittarius 50 372 Sagittarius 1, Autumnal equinox
Capricorn 47 422 Dust storm season begins
Aquarius 46 469 Aquarius 16, Mars at Perihelion
Pisces 48 515 Pisces 1, Winter solstice
Aries 51 563 Dust storm season ends
Taurus 56 614 Taurus 56, Martian New Year's Eve

This table of months is calculated on the main page, using a Javascript implementation.

Determining the Year

Determining the year is somewhat simpler. We just need to decide on an agreed on date for the beginning of our calendar. Zubrin noted that the beginning of the martian month of Gemini also corresponded to Mars's position around January 1 in the years 1946, 1961, 1976, 1991 and so on. The latest year that precedes all spacecraft sent to Mars is 1961. Zubrin therefore sets this year to I (he uses roman numerals).

It's worth noting that this perpetuates the annoying "lack of year zero" problem of the terrestrial calendar on to Mars.

Important events in the history of our exploration of Mars can be found in the table below.

Great Dates in Recent Martian History
Occasion Terrestrial Date Martian Date
Calendar begins January 1, 1961 Gemini 1, I
Mariner 4 flyby July 15, 1965 Libra 25, III
Mariner 6 flyby July 31, 1969 Sagittarius 16, V
Mariner 7 flyby August 5, 1969 Sagittarius 20, V
Mariner 9 arrival in orbit November 14, 1971 Pisces 20, VI
Mars 2 & 3 landings December 2, 1971 Pisces 38, VI
Viking 1 arrival in orbit June 19, 1976 Leo 41, IX
Viking 1 landing July 20, 1976 Virgo 6, IX
Viking 2 landing September 3, 1976 Virgo 49, IX
Mars Observer lost August 21, 1993 Libra 16, XVII
Mars Pathfinder lands July 4, 1997 Libra 26, XX
Mars Global Surveyor arrives September 11, 1997 Scorpius 33, XX