The quest for a Martian calendar

In a few days, on the Dec. 21, the Earth will reach the point in its orbit where the northern end of its axis is leaning directly away from the Sun.

We will have the shortest day of the year. The Sun's rising and setting points will be at the southernmost extremes of its annual travels. This is the winter solstice. We also designate it the first day of winter.

This date has been important in a variety of religious and other ceremonies for many thousands of years, marking the point at which the days start getting longer again and we get the first tiny hints that spring will eventually return.

Our seasons are not due to changes in the Earth's distance from the Sun, they are due to the inclination of the Earth's axis, which is 23 degrees off vertical compared with the plane in which the planets orbit the Sun and points towards the Pole, or North, Star.

When the Northern Hemisphere is leaning towards the Sun, we have summer. When it is leaning away, we have winter. There are two occasions where the lean is sideways, neither towards nor away. These are the spring and autumn equinoxes. They provide the fixing points for our calendar, which helps us navigate our year.

Mars is the planet in the Solar System most like ours, and the one most likely to be colonized by us at some point in the near future. The axis of the Red Planet has an inclination of about 25 degrees, a little greater than ours, so we would expect that world to have definite seasons.

Back in the 19th century, during spring in the Martian northern hemisphere, astronomers saw a "wave of darkening" start in the northern polar regions and move slowly southward, towards the equator. This was interpreted as a movement of water from the poles, perhaps as rain or floods, greening up the vegetation as it went.

That idea collapsed when the spacecraft Mariner 4 flew past the planet on July 14 and 15 1967. The images it sent back showed no vegetation, just a cratered desert. However, the visual evidence of those seasonal changes was still there.

The cause of the "wave of darkening" is still a matter of discussion.

The Red Planet is currently populated by an increasing number of robot explorers and a variety of manned missions are being evaluated. All this interest has led to ideas for a Martian calendar, to tell those settlers when the seasons start and when to celebrate Thanksgiving and Christmas.

The Martian day is just over 24 hours, not much longer than ours and unlikely to be a problem for the colonists. A Martian day is called a “sol.” However, our calendar will not easily transplant to Mars.

The big role of a calendar is to lock the rhythm of our lives to the rhythm of nature around us. We will need to have a truly Martian calendar. The difference in day length is insignificant but the Martian year is almost twice as long as ours. The problem is that Mars orbits further from the Sun than we do and has further to travel during its year. That makes the Martian year—the time taken to complete a trip around the Sun—668.6 sols. Our year is 365.25 days, so like ours, the Martian calendar will have to include leap years and so on to keep it in step with the seasons.

One proposal is called the Darian calendar (it is described in detail in Wikipedia). Here is how it works.

The Martian year is divided into 24 months. The year starts with the spring equinox. The summer solstice and the beginning of summer happen in month six, the autumn equinox in month 12 and the winter solstice and the start of winter in month 18.

The major seasonal feature is the dust storm season, which begins around the autumn equinox and can persist until spring. It looks as though “brown Christmases" could be standard on Mars. That might sound rather way out there but there are probably people who are alive today, or maybe their children, who one day will be using the Martian calendar in their daily routine.

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• After sunset, Venus shines low in the southwest.

• Saturn lies in the south and Jupiter is rising in the northeast.

• Mars rises about three hours later.

• Mercury lies in the southeast just before dawn.

• The Moon will reach its last quarter on Dec. 22.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.