Guide to Mars
by Patrick Moore
edited by Thomas Gangale
from Chapter 2:
This may be the moment to introduce the Martian seasons, which are all-important in any consideration of the surface conditions. As a preliminary, I must say something about our own seasons, which are due in the main not to our changing distance from the Sun, but to the tilt of the Earth's axis, which is 23 1/2 degrees to the perpendicular. In Figure 6a, the Earth is shown in two positions. During northern summer, the northern hemisphere is tilted toward the Sun; the north pole is in continual sunlight, while the south pole has no daytime at all.
Six months later, the situation is reversed. The Earth is at perihelion during northern winter--only 91 1/2 million miles from the Sun, as against 94.1 million in late June--but the difference is not enough to have any marked effect, and in any case the greater amount of ocean in the southern part of the Earth has a stabilizing influence.
Not so with Mars, which has no oceans at all, and where the range between perihelion and aphelion is so much greater (Fig. 6b). At the present epoch, the axial tilt is virtually the same as ours (24 degrees), and, as with us, southern summer occurs near perihelion. This means that on Mars the southern summers are shorter but hotter than those in the northern hemisphere, while the southern winters are longer and colder.
It is one of Kepler's fundamental laws that a planet moves fastest when at its closest to the Sun; this is a fundamental traffic rule of the Solar System.
The effects of this situation are far-reaching, and I will refer to them over and over again during the course of this book. In short: the southern hemisphere of Mars has climates which are more extreme than those of the north, and the differences are quite marked. It is also worth noting that during close oppositions it is the south pole which is tilted in our direction, so that before the Space Age the southern hemisphere was the better-mapped of the two.
Though Mars has such a long 'year', its 'day' is much the same as ours--to be precise, a little more than half an hour longer: 24 hours 37, minutes 22.6 seconds, a period which has been measured very exactly from Earth. Since the Viking landings, a Martian day has become known as a 'sol', and this is the term which I propose to use from now on. Needless to say, a sol indicates a complete rotation--approximately 12 1/4 hours of daylight followed by 12 1/4 hours of darkness at the equator at the time of the equinoxes. The solar day (or, rather, solar sol) is 24 hours 39 minutes 35 seconds. This is the mean interval between successive transits of the Sun across the meridian as seen by an observer on Mars.
The near-equality between Earth day and Martian sol means that we can draw up a calendar which is not entirely unfamiliar. The lengths of the seasons, for instance, can be worked out easily enough:
Timekeeping is not going to be a real problem for future colonists (as it would be on Venus, where a rotation period or 'day' is actually longer than the revolution period or 'year', leading to a very peculiar state of affairs). A Martian clock will no doubt be divided into 24 hours, though each will be slightly longer than an Earth hour. So far as a calendar is concerned--well, there have already been plenty of suggestions. My own idea is to divide up a Martian year into 18 months, each of 37 sols. This makes 666 sols. We need 669, so an extra sol can be tacked on to Months 6, 12 and 18, giving them 38 sols instead of 37. This should work reasonably well, though there will have to be some kind of adjustment to allow for the fact that the revolution period of Mars is not exactly 687 days, but 686 days 23 hours 52 minutes 3I seconds according to Earth reckoning. The problem of naming our eighteen Martian months is not, I feel, something which need be considered yet awhile, though no doubt it will produce the usual tedious international squabbles when it eventually has to be tackled.