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| Martian Timekeeping
Timekeeping has traditionally been an integral part of astronomy and has
grown quite complex along with the rest of life. A full introduction to
timekeeping and calendars as they are related to astronomy is given in the
Explanatory Supplement to the Astronomical Almanac. There are many time
standards, including atomic time kept by a large array of clocks which is
thought to be the most accurate standard. Universal time, governed by the
rotation of the Earth, is what most of us think of as the true measure of
time. Being closely related to the time indicated by a sundial, ephemeris
time is governed by the orbital motion of objects in the solar system, but
due to relative effects, there are many possible frames for ephemeris time.
In seeking a martian time standard and calendar system, we should be driven
by the practical needs of those who will, in the not too distant future, be
working and living in the Martian environment. It will no doubt be some
time before an extremely accurate local standard is available there.
Calendar constructs based on terrestrial analogy are useful since they can
be easily understood by all and they may incorporate psychologically valid
ideas (e.g., circadian rhythms). Consequently, the same terms may be used
to refer to somewhat similar constructs in the martian and terrestrial
contexts. To avoid confusion, we will adopt the practice of referring to
the martian analogy of a terrestrial quantity by using the prefix areo-
(from the Greek Ares for Mars). For example, the martian equivalent of a
day has long been referred to as a sol (a more precise definition is given
below). If Martians were to make use of an interval of 7 sols,
corresponding to a terrestrial week of 7 days, we would refer to this unit
of time as an areoweek.
The martian sidereal rotation period, with respect to the fixed stars, is
24 hours, 37 minutes, 22.66 seconds or 1.026 days. Its sidereal orbital
period is 686.98 days. Thus, Mars rotates 669.6 times on its axis during
each orbit around the sun. The orbital and rotational motions are in the
same sense; there are only 668.6 sunrises and sunsets for an observer on
Mars during the course of the orbit, so the Martian analog of a year
consists of 668.6 sols. Each sol, the analog of a day, is 686.98/668.6 =
1.0275 Earth days (24 hours, 39 minutes, 35 seconds).
The eccentricity of the Martian orbit is great enough (.0934) that the
planet's angular speed varies greatly over its orbit. The insolation rate
(i.e. the amount of sunlight falling on Mars) also varies measurably. The
planet has very pronounced seasonal changes which should be incorporated
into a calendar system.
The traditional challenge of calendar making has been to take
disproportionate periods and to find an approximate that allows humans to
keep track of celestial events. In the terrestrial case, the lunar period
is generally also a part of the calendar, in addition to the day and solar
year, though the association has been largely lost in modern times. On
Mars, there is no appropriate lunar period to include, but months may be
retained because of the strong association of different terrestrial months
with the seasons.
Astronomers measure the Martian seasons in terms of the solar longitude or
Ls, a measure of the apparent motion of the sun through the sky. At the
vernal equinox, Ls =0°; at summer solstice, Ls =90°; at autumnal equinox,
Ls=180°; and at winter solstice, Ls =270°. The association between
"months" and the seasons can be retained if each month corresponds to 30°
of Ls. The analogous construct in terrestrial timekeeping is the motion of
the sun through the signs of the zodiac.
Download the full Martian Timekeeping report for more
The Mars Calendar was developed by Geoffrey Briggs and Howard Houben.