Frequently Asked Questions

Would an Earth calendar work on Mars? What would be the problems of working with an Earth calendar?

For two reasons, an Earth calendar will not work on Mars. First of all, a Martian sol is nearly forty minutes longer than an Earth day. This may not seem like much, but over the course of a month this difference accumulates to nearly an entire day. So if one tried to use an Earth calendar on Mars, the months that contain 31 days on Earth would generally have only 30 sols on Mars, and most of our 30-day months would be only 29 sols long on Mars. In actual practice this adjustment would be much more complicated. For instance, to maintain the same relationship between the date of the month and the day of the week, you would lose a day of the week at the end of a month that had to be shortened by a day. Tuesday might be followed by Thursday, or even worse, Saturday might be followed by Monday, and you’d only get a one-sol weekend.

An Earth calendar is designed to mark the passage of the seasons on Earth. When it is July, we know that it is summer in Earth’s northern hemisphere, but what season is it on Mars? An Earth calendar just can’t tell you that.

Is there a generally accepted standard Martian calendar?

No. Several dozen calendars have been proposed for Mars since 1880, but there has been no pressing need to adopt a standard. However, as the prospect of sending humans to Mars and establishing permanent bases draws nearer, the need for a generally accepted standard Martian calendar becomes more immediate.

What factors would need to be considered to develop a Martian calendar?

Calendars must take into account the astronomical cycles of a planet: its day and its year. On Earth we have the additional complication of the Moon’s orbital period. Lunisolar calendars, such as the Hebrew, Hindu, and Chinese calendars, take all three of these cycles into consideration. Solar calendars, such as the Gregorian calendar, are based on the day and the year alone, and months are arbitrary divisions of the year.

Calendars must also serve to mark human cycles that are unrelated to astronomical cycles. A conspicuous example is the social cycle of the seven-day week. Also, in solar calendars, the month is arguably a human social cycle since it does not track the phases of the Moon.

Another consideration in designing a calendar is accounting for the fractional number of days in a year. Earth’s year is 365.24219 days long, and so calendar years contain either 365 or 366 days according to a specific intercalation formula. Such a formula would need to be developed to account for the 668.5921-sol Martian year.

What advantages would a Martian calendar have over an Earth calendar?

Since a Martian calendar is based on the length of a Martian year, it will keep time with the Martian seasons. An Earth calendar cannot. Furthermore, there are several deficiencies in the Gregorian calendar. For instance, each year and each month begins on a different day of the week, and on the whims of Roman autocrats, days were removed from February and given to July and August. There have been various plans to correct these defects over the past several centuries, but social inertia has doomed attempts to reform the calendar. On Mars we have the opportunity to create a calendar that has a much more rational structure than the Gregorian calendar.

Would weeks contain seven sols or some other number?

Theoretically, a week could contain any number of sols, within reason. One calendar has six-sol weeks, and a few have none at all, However, most calendars have seven-sol weeks, and given the history of calendars on Earth, this may be wise. The French Revolutionary calendar, which contained ten-day weeks, and two Soviet calendar, one having a five-day week and the other a four-day week, were each abandoned after few years for lack of acceptance.

Does a Martian calendar need months, or other units between day and year?

There is no astronomical cycle on Mars that corresponds to our idea of a month. The moons of Mars are tiny in comparison to Earth’s Moon, and they are much closer to Mars than the Moon is to the Earth. Phobos orbits Mars three times per sol, and Deimos circles Mars in a little more than a sol. For this reason, a few Martian calendars have no months, but most include the month as a social cycle, as with the week. In human terms, it is useful to divide the year into smaller units.

If you kept the same number of Earth months, what would you have to do?

Since the Martian year is nearly twice as long as the Earth year, months on Mars would need to be nearly twice as long as Earth months.

If you gave the calendar more months, how would you determine the number of months?

In order to have months of nearly the same length as they are on Earth, you would need to have 23 months on Mars. But 23 is a prime number. A much better choice might be to have a number of months that is divisible by many other numbers, so that trimesters, quarters, or even eights of a Martian year would contain an integral number of months.

How do the lopsided seasons on Mars affect choices for dividing the year into months?

Many Martian calendars divide the year into nearly equal lengths of time (to within a sol). A few instead divide the orbit of Mars around the sun into equal arcs, and since Mars travels at different speeds at different points in its elliptical orbit, the months on such calendars vary in their duration by about 50 percent. Although each season contains the same number of months on such calendars, each month has a different number of days from every other month.

What are some ideas for naming months?

If we kept the same number of months on Mars as on Earth, it would be natural to keep the same names as well. Even some of the calendars that have more than twelve months keep the familiar Roman names and add to them. An advantage to keeping the same names is that we would automatically know that Martian April occurs in the spring, just as it does on Earth. Of course, April (or any other month) would hardly ever occur on Earth and Mars simultaneously, and it would be confusing if someone on Earth said to a Martian settler, "I'll see you in July." Whose July? Earth's or Mars'? Some 24-month calendars use the Roman names twice, one after another, with some way of distinguishing between the two. The following list summarizes the various naming schemes that have been used:

• Roman
• Roman + others
• Roman x 2
• Constellations (and stars) of the zodiac
• Bodies of the solar system
• Mythology
• Numerical prefixes + seasons
• Greek alphabet
• Famous people
• Fictional Martian language

A number of calendars have been proposed for Earth which reconcile the relationship between weeks and months so that the year always begins on the same day of the week. What about perpetual calendars for Mars?

Dividing the Martian year by seven results in 95 weeks pus three or four sols left over. Several perpetual calendars have been proposed for Mars. As with the various perpetual Earth calendars, some of these make use of an occasional sol which does not count as part of the week. Other perpetual Martian calendars contain three or four six-sol weeks to arrive at a total of 96 weeks.

What are some considerations for setting an epoch (time to start the count of the years) for a Martian calendar?

Several epochs have been proposed:

• Beginning of the Julian period (1 Jan 4713 B.C.)
• Martian vernal equinox prior to 1st millennium (2 Aug 1 B.C.)
• Beginning of the Common Era (1 Jan 1 A.D.)
• Most recent Martian vernal equinox occuring on Earth's January 1st (1 Jan 1707)
• Mars 3 landing (6 Oct 1971)
• Martian vernal equinox prior to the Viking 1 landing (18 Dec 1975)
• Viking 1 landing (20 Jul 1976)
• Martian winter solstice prior to 3rd millennium (24 Dec 1999)
• Landing of the first human mission

Having an epoch too far in the past introduces accumumated errors due to the slight variability of the rotational periods of Earth and Mars. Setting an epoch to some future event precludes using any Martian calendar until that event occurs.