The Millennium Mars Calendar

By James M. Graham and Kandis Elliott
Department of Botany, University of Wisconsin-Madison, Madison, Wisconsin 53706

presented at
the Founding Convention of the Mars Society, August 13-16 1998,
University of Colorado, Boulder, Colorado, USA

reprinted by permission

For a long time Mars calendars were exclusively the domain of science fiction writers, going back at least as far as Edgar Rice Burroughs and his John Carter of Mars series. These sci-fi calendars were just a detail in the narrative. Many of them did not correctly present such basic facts about Mars as Mars' rotation period of 24h 39m 35.238s or its orbital period of 668.59906 Martian days, or sols, in a Martian year. Since a sol on Mars is about 0.66h longer than a day on Earth, the orbital period of Mars expressed in Earth days is 687 days. This is the value found in astronomy texts for Mars' orbital period, which some writers have incorrectly used as the length of the Mars year, even though it has no significance there.

An early attempt at a working calendar for Mars appeared in the NASA document Mars Scientific Model (Michaux and Newburn 1972). The authors composed a list of Mars days, but did not define weeks or months. In 1986 Thomas Gangale published a working Mars calendar of 24 months named for the 12 zodiacal constellations, whose names are given first in Latin and then in Sanskrit; these months have 27 or 28 sols. The first day of each year is the spring equinox in the northern hemisphere. The calendar's year 0 occurs in the year when the Viking 1 lander touched down, which was July 20, 1976, on Earth.

No significantly new calendar proposals were offered until the 1990s. Since the basic facts of Mars cannot change, proposals for calendars differ mainly in the way they define months or name days.

In 1993 Robert Zubrin described a calendar for Mars in Ad Astra magazine. His calendar has 12 months named for the 12 zodiacal signs beginning with Gemini. Gemini 1 is spring equinox in the northern hemisphere. Each month represents 30 of arc of the Martian orbit around the Sun. If the same principle were applied to the Earth calendar, each month would have 30.4 days, which would translate into either 30 or 31 days per month. That pattern of days would occur because the orbit of Earth is nearly circular. However, the orbit of Mars is shaped like an ellipse; thus Mars takes different numbers of days to traverse each 30 of arc around its orbit, resulting in a different number of sols (from 46 to 66) for each month.

One final proposed calendar is that of William Becker (1994). This calendar is unusual in having 19 months, named for a mixture of Egyptian, Greek and Roman gods and goddesses. In general each month has 5 weeks of 7 sols each, for 35 sols per month. The first day of each month is always Monday, and the last is Sunday. Since 19 times 35 equals 665 sols, this calendar is either 3 or 4 sols shorter than the observed Martian year of 668 or 669 sols. To correct this discrepancy, Becker added what he calls "intercalary" days to his calendar, which are not part of the regular week. Intercalary days occur on the first month Ra, the fifth Selene, tenth Europa and 15th Muse. The intercalary day in Ra is New Year's sol and is called Mars Sol. That in Selene is Phobos Sol, in Europa Horus Sol (which is the Martian leap year sol), and that in Muse is Deimos Sol. Horus Sol is used to make the Martian year 669 sols long on leap years.

The Millennium Mars Calendar (MMC) divides the Martian year into 20 months in 4 groups of 5 months each (Table 1). The months are named for Greek gods and goddesses. Within each group of 5 months, the first three have 33 sols each and the last two have 34. That makes 668 sols per year; to account for leap years we add one sol to Zeus for 35 sols about every other year. The use of Greek gods and goddesses allows us to avoid the controversy that would arise if we used the names of real persons, and also represses the old inequity in our Earth calendar, which has only the month of June named after a female. Our calendar actually has 10 goddesses and 9 gods. This arrangement came about because many Greek gods have associations we did not want in a calendar. Would you really want a month named after Hades, the lord of the underworld?

Table 1
Eos 33 Athena 33 Artemis 33 Hestia 33
Hermes 33 Poseidon 33 Gaea 33 Hephaestus 33
Eros 33 Helius 33 Selene 33 Asclepias 33
Aphrodite 34 Demeter 34 Theia 34 Hera 34
Apollo 34 Persephone 34 Boreas 34 Zeus 34 (35)

Mars' orbit is an ellipse, resulting in seasons of different lengths. Specifically, northern spring and summer are longer than fall and winter. As Table 2 shows, in the MMC northern spring runs from Eos 1 to Athena 26 for 193 sols. Northern summer extends from Athena 26 to Gaea 4 for 178 sols. Fall is 143 sols long from Gaea 4 to Hestia 13, and winter lasts 154 sols from Hestia 13 to Zeus 34 (or 35 in leap years).

Table 2
Northern Hemisphere Southern Hemisphere Dates on Mars Mars Days
Spring Fall Eos 1 to Athena 26 193
Summer Winter Athena 26 to Gaea 4 178
Fall Spring Gaea 4 to Hestia 13 143
Winter Summer Hestia 13 to Zeus 34 154 (155)
668 (699)

Every calendar has to have a beginning. The MMC begins in the year of the landing of Viking 1 on July 20, 1976. The Martian year in which this landing occurred is called year 0, and it starts on the northern spring equinox at Eos 1, 0. Eos 1, 0, corresponds to December 26, 1975. The published MMC will begin on December 20, 1999 (which is Hestia 1, 12), and runs through to February 1, 2002 (Ascelpius 33, 13). The calendar thus covers the turn of the millennium on Earth (Graham and Elliot 1998).

Since there is no real relation between a Tuesday on Earth and any particular day on Mars, we have renamed the days of the week in the MMC after the Sun and first six planets: Solday, Mercuryday, Venusday, Earthday, Marsday, Jupiterday, and Saturnday. Some old Earth expressions like TGIF may become TGIJ on Mars! The published MMC is two calendars in one. Pale orange denotes the Mars sols or days. The blue bar below the Mars sols shows the corresponding Earth days and months together with some significant dates in space exploration.

The MMC is intended to be both entertaining and educational while promoting the exploration of Mars. The calendar also proposes a goal for all of humanity for the coming millennium, specifically the exploration, colonization and terraforming of the planet Mars. We purposely designed the final calendar page illustration with a globe of a fully terraformed Mars around 3000 AD, with the ghostly image of present day Mars behind it, to represent our hope that a living Mars will one day be possible. The MMC is one of many possible forms for a Mars calendar. Future colonists will no doubt adopt a calendar which best suits their needs in their new environment.


Becker, W. H., "Perpetual 19 month Mars calendar", A letter to Dr. Chris McKay at NASA Ames Laboratory, dated 19 October (1994).

Gangale, T., "Martian Standard Time", JBIS, 39, 282-288 (1986).

Graham, J. M. and K. Elliot, Millennium Mars Calendar, The Institute of Implied Science, P.O. Box 45926, Madison, WI 53744 (1998).

Michaux, C. M. and R. L. Newborn, "Mars Scientific Model", Jet Propulsion Laboratory Document 606-1, (1972).

Zubrin, R., "A Calendar for Mars", Ad Astra, Nov.-Dec., 25-27 (1993).