This document is part of the Martian Time Boneyard. It was originally located at http://www.marsengineering.com/Calendar/Months.asp.
Author: Shaun Moss

Months

Lengths

The system of month lengths we have devised is very easy to use and understand. There are 23 months of 28 sols each, and one slightly shorter month with 24 sols, or 25 in leap mirs. Having 24 months makes it easy to divide the mir into 2, 3, 4, 6, 8, or 12 parts.

A month length of 28 sols is ideal. If we define a Martian week as having 7 sols (see the Week Lengths section), 28 sols divides conveniently into 4 weeks (or two fortnights). A Martian month is therefore also very close to the duration of a Lunar month (28.74 sols) and not much shorter than a Gregorian month (29.62 sols on average). Considering future interplanetary commerce between Mars, Luna, and Earth, it will be useful for a month to mean approximately the same thing on each world. Months of this length are also more harmonious with the female menstrual cycle.

The table further down this page illustrates this arrangement of month lengths.

Names

This system uses constellation names for month names, as do several other Mars calendars. The Darian Calendar, for example, uses the names of the well-known constellations of the zodiac - odd months are assigned the common Latin names, and the even months are assigned equivalent Sanskrit names. We use the same names for odd-numbered months, but names for other constellations for the even-numbered months.

The resulting solution was worked out by Francois Guesdon, John Darcy, and Shaun Moss within the Mars Time Group. Part of the motivation for this system is to provide a mechanism to teach the constellations to children. To determine the 24 constellations, we started with the 13 that lie on the ecliptic, namely, the 12 well-known ones and Ophiuchus. The remaining 11 constellations were selected according to the following criteria:

• Easily recognisable constellations were preferred, in consideration of the design goal of using the calendar to teach constellations to children.
• Constellations with one word in their names were preferred, as those with two words do not make convenient month names. This excluded bright constellations such as Canis Major and Canis Minor.
• Constellations closer to the equator were preferred, as these are visible to the maximum number of people. This excluded bright constellations such as Crux and Carina.

The names are sorted by ecliptic longitude of the centre of the constellation. The reason for starting with Sagittarius is because the mir begins at the northern vernal equinox, just after the Sun has entered Sagittarius. This is simply meant to be a convenient way to choose a starting month - the Sun does not pass through each of the listed constellations, only the ones on the ecliptic. Furthermore, because of precession of the equinoxes, the Sun will not always be in Sagittarius at the vernal equinox.

Note how Ophiuchus could be thought of as the Martian February, being slightly shorter and having an extra sol in leap mirs.