An Explanation of Zubrin's Mars Calendar

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 L_S=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 L_S=70⁰, Mars is at aphelion (furthest point from the Sun); while at L_S=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	Begins 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

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