The Martian calendar

Feb 2018

There are beginning to be serious proposals for sending people to Mars. An important aspect of life for a colonist on Mars, which I sure some will have thought about already, is the measurement of time. Trying to remain synchronised with terrestrial clocks and calendars would prove at least disconcerting or probably very disturbing psychologically.

The rotation of Mars is such that a solar day lasts for 37.4 minutes longer than a day on Earth. So if a terrestrial clock is used the daily rhythms will be progressively out of step by more than half an hour per day. I think this would be extremely hard to cope with. Dividing the day into 24 hours is something to which we are accustomed and that should therefore still be the case but Martian clocks should run slower than those on Earth (by a factor of 0.9746997). To reiterate: for human comfort the Martian day will still comprise 24 hours but each will be slightly longer than an hour on Earth.

It takes Mars 686.971 Earth days to orbit once around the Sun. That equates to 669.590 Martian days. There are seasons on Mars similar to those on Earth because the planet's rotation axis is tilted by 25 degrees to the plane of its orbit (very similar to Earth's tilt of 23.5 degrees). Therefore it will make sense for a resident of Mars to measure years in units of 669.590 days. Our 12 months will not be much use but it may be worth noting that 24 months of 28 days would come to 672 days, just 2.5 days longer than the Martian year. (It is easy to see that months of other lengths around 30 days leave much larger remainders at the year end.) So 23 months of 28 days each and one of 25 or 26 days in alternate years would keep the calendar almost in step with the seasons. Unlike our leap year every 4 years, alternate years would have one day's difference in length. There would also be a further day's adjustment after many years just like the 4-century correction in our Gregorian calendar.

It remains to come up with names for the 24 months. 12 months of 56 days each, trying to keep the same names as we have now would also be confusing. A similar consideration has arisen before, for example during the French Revolution, but this time I think there is much more justification.

Most months having 28 days is good because a week of 7 days can still be used and it means that a given numbered date in each month will fall on the same weekday throughout a year, just changing for the next year (assuming that the leap month is at the end of the year, unlike our February).

There will need to be time zones on Mars, to make noon occur about halfway between sunrise and sunset at any given location. There is already an equivalent of the Greenwich meridian on Mars which astronomers defined long ago for mapping the surface, though its position is perhaps rather arbitrary. Tables of the position of the "Central Meridian" of Mars for any given Earth time are available in current almanacs.

Note that similar considerations do not apply for a colony on the Moon because it does not have the confusingly similar yet different day length. A day on the Moon lasts for about 2 weeks. So on the Moon we can usefully remain synchronised with the existing UTC and Gregorian Calendar on Earth.