by Thomas Gangale
originally published in the Journal Of The British Interplanetary Society, volume 39, pp. 282, June 1986
if you just want the facts and don't want to read this article, go to The Darian Calendar
or visit the Author's excellent Martian Time Web Page for even more on this subject
HTML conversion by Jim Kelley; all mistakes or omissions are mine
(NOTE: Gangale is very wordy, and I have shortened this article tremendously, though the meaning is intact)
1.0 Evolution Of The Existing System
There are many reasons for the system of time we use on Earth, but the most fundamental are the cycles of the sky. Of all the periods marked out by the motion of the celestial bodies, the most conspicuous, and the most intimately connected with the affairs of Humankind, are the Solar day, distinguished by the diurnal rotation of the Earth and the alternation of light and darkness, and the Solar year, which completes the cycle of the seasons. Another astronomical cycle is that of the Moon, the 2nd most obvious object in the sky. The Moon's appearance undergoes a pronounced cyclical change due to its orbit around the Earth, and this cycle is the basis for the month. In Lunar calendars the months are tied directly to the Lunar cycle, whereas in Solar calendars (such as the Gregorian Calendar) months are only approximations of the Moon's orbital period. The revolution of the Moon and the revolution and rotation of Earth are our natural divisions of time. All others, though of the most ancient and general use, are only arbitrary and conventional
Human psychology, sociology, politics, economics and religion have all influenced the way we mark the passage of time. That one month has only 28 days while the rest have 30 or 31 is blamed on the vanity of the Roman emperor Augustus, who stole a day from February so his month, August, would have 31. The 7-day week no doubt stems from the Biblical account of the Creation, but also satisfies economic and psychological needs; there are days of work and days of rest. The increasing complexity of civilization gave rise to the need for dividing the day into smaller and smaller units. The division of the day into 24 hours exists in some of the earliest historical records, and just why the number 24 was chosen is shrouded in antiquity. The hour was divided into 60 minutes because the numerical system of ancient Mesopotamia was based on 60 rather than 10. By extension, the second was then defined as 1/60 of a minute.
Convenience also dictated that there be no fractional days in a calendar month or calendar year. Despite the fact that the Lunar period is about 29.5 days and the Solar year about 365.25 days, no month or year begins in the middle of a day, nor does a year begin in the middle of a month. As travel and communication between distant parts of the Earth became quicker and more frequent, the system used by one culture came to be universally adopted due to economic or political influence or of precedent, so that the Gregorian Calendar is now predominant. Eventually the need arose for Universal Time, a.k.a. Greenwich Mean Time, a standardized time for the whole planet
While Greenwich Mean Time has become the standard time for all of Earth, other standard times coexist which are referenced to GMT by use of 24 standard time zones. Most nations use these standard zones, while a few offset their local time by some fraction of an hour. In some places there are seasonal adjustments to the local standard known as daylight time. It is these local times that the vast majority use, for it is local time and not GMT which is more relevant to their daily lives. Only a few professions, such as astronomers and aviators, use GMT on the job, but when they come home from work even they revert to local time
But on other planets, will local conditions favor the adoption of new local standards of time?
2.0 The Need For A New System
What planets are likely to have permanent populations in the 21st Century? The advance of technology can only be guessed, and likewise the interplay of various human motivations to reach beyond Earth. Venus will be ruled out due to its tremendous surface temperature and pressure, as will Jupiter and Saturn due to their very dense atmospheres and the doubt that they even have surfaces. Lastly, we disregard the worlds beyond Saturn on the assumption that flight times to such bodies will will continue to be so long that they will not be visited by humans for many decades. [NOTE: the GIT changes this -- Jim] Those left are Mercury, the Moon, Mars, the Galilean worlds of Callisto, Ganymede, Io and Europa and the Saturnian moon Titan.
With one exception, each of these is tidally locked to its primary. Because of its highly elliptical orbit, Mercury has evolved a unique synchronism. The innermost planet rotates exactly 3 times in the course of 2 local years; because of this, its Solar day is twice as long as its year! The Solar days of all these tidally-locked worlds are too long to be practical as days in the human sense. There, GMT will be as useful as any other system, so why invent a new one? [Because I wanted to -- Jim] Maybe on Io the 42.5-hour local day could be bisected to approximate the terrestrial day, and similarly Europa's could be divided by 4, Ganymede's by 7 and Callisto's and Titan's by 16 - or perhaps people on these worlds will feel GMT is good enough for them. There is only one planet here that isn't tidally locked, and that planet has a day nearly identical to Earth's
That planet is Mars
3.0 Telling Time On Mars
3.1 Keeping In Time With Earth
Since a Martian Solar day is longer than an Earhtly Solar day by roughly 2/3 of an hour, holding Mars to an Earth-referenced clock would mean a 3-day cycle in which one day would be 24 hours long and the other two 25 hours. After 3 days exactly 74 hours will have passed by standard time, while 3 Martian Solar days equal 73 hours, 58 minutes, 45.7 seconds. A further correction would be needed to account for this, so that every 48th cycle would have two 24-hour days and one 25-hour day. Thus 144 Martian official days would have 3,551 hours and would agree with as many Martian Solar days to within 36 seconds. Another correction is needed only once every 14,688 Martian days, or more than 41 Earth years. A system requiring days of different lengths would be bad enough, yet as awkward as this system has already become, the purpose of it, to keep in time with Earth, has yet to be addressed, and this is where its second great defect is exposed
As on Earth, Mars would have 24 time zones, each separated by an hour. These would be referenced to the Martian Prime Meridian, which in turn would be referenced to the Greenwich Meridian by some multiple of 60 minutes. On 25-hour days this reference would of course change by 60 minutes, so that any place on Mars would be synchronized with a different place on Earth every day or so. Also because of this, the Martian Date Line would move 15 deg. east from its position on the previous 25-hour day. The Martian Date Line would thus skip all the way around Mars about every 36 Earth days, or 35 Martian days, so that at any given location on Mars a date would be skipped over every 35 days or so to keep it in step with the date on Earth. After much hammering and bending we have forged a device that keeps in time with Earth after a fashion. It is hard to believe such a clumsy system would find many advocates on Mars.
Clearly, a new system is needed that is independent of Earth
3.2 Hours, Minutes and Seconds
It would be far more convenient to divide the Martian day into equal parts as is done on Earth. The problem is, how does one evenly divide a day consisting of 88,775.2 seconds? Dividing by 24 yields 3699.97 seconds to a Martian hour, and dividing further by 60 yiedls 61.6494 seconds to the Martian minute. Things are starting to look very messy. Let us try another approach. The Martian solar day is only 2.74913% longer than an Earthly day of 86,400 seconds. The simplest solution would be to define the Martian second as 1.0274913 Earth seconds. Thus Martian clocks would look just like earthly clocks but would run a bit slower. he Martians would have 60-second minutes, 60-minute hours, 24-hour days and 24 time zones referenced to a fixed Martian Date Line
The defining of a new unit of time, the Martian Second, suggests an even more radical approach: going decimal. After all, it can be argued that if we had it to do over again here on Earth, this is the way we would do it, since we now have a base 10 number system. In a Martian decimal time system the second would be defined as 1/100,000 of a Martian day, or 0.887752 Earth seconds, and larger units of 100 and 10,000 seconds would be analogous to minutes and hours. But would the mathematical simplicity of such a scheme outweigh the long tradition of 60, 60 and 24? It seems doubtful the Martians would want this when the adjustments of the Earthly system would be so small that they'd never feel the difference
Given that the Martian year is 686.9796 Earth days long and the Martian day 1.0274913 Earth days long, the Martian year thus has 668.599 Martian days. What can be done with a 668.599-day year? On Earth, because the year is about 365.25 days long, the normal calendar year is 365 days and every 4th year is 366 days. We achieve further accuracy by inserting corrections every 100, 400 and 4000 years. The same method can be applied to Mars to develop a practical and accurate calendar
In the calendar proposed for Mars, all even-numbered years have 668 days except those divisible by 10 and the first year of the Martian Era, or 0 ME. All other years are 669 days, so that in 10 calendar years there are 4 years of 668 days and 6 of 669 days, for a total of 6,686 days. In 10 Martian Solar years there are 6,685.99 days, the difference being 0.010 days. A further correction is thus needed every 1,000 years, so every year divisible by 1,000 has only 668 days
3.4 Months and Seasons
Phobos is only 24km across and orbits so closely that it rises and sets twice a day. Deimos, with an orbital period of little more than a day, is only half the size of Phobos and far enough away that it presents no disc to the naked eye. As the basis for natural divisions of time, they should be ignored. The Earthly month, however, is useful and familiar and can easily be tailored to Mars. Admittedly, the Martian month will be an artificial unit, but then, so are Earthly months.
In the proposed calendar, 669-day years have 21 28-day months and 3 27-day months. Years with 668 days have 20 28-day months and 4 27-day months. The short months are the 6th, 12th and 18th and, in short years, the 24th. The variable day is thus the last day of the year
In naming the Martian months the use of the zodiac constellations naturally comes to mind. The Sun appears to pass through these constellations as seen from Earth during the course of a year. Mars' orbit is inclined from Earth's by less than 2 deg., so the view from Mars is the same. There are only 12 constellations, however, so 2 name must be used for each. Here, the odd-numbered months use Latin names for their respective constellations, and the even-numbered months use Hindu and Sanskrit names for the same (see list). The intent is for the Sun to be in a given constellation for the 2 corresponding months of the Martian year
This would not work well on Earth. The Earth's axis does not always point in the same direction, but precesses on a ~26,000- year cycle. Thus the first day of the northern spring, the Point of Aries, is now in Pisces, and by the 26th Century will pass into Aquarius; thus the term "Age of Aquarius." Spring will not begin in Aries again until about 24,000 AD
Earth's relatively rapid precession is due mainly to the influence of the Moon and to a lesser extent that of the Sun. As Mars is much further from the Sun and has no large moon nearby, its rate of precession far less than Earth's. Mars' axis takes 176,000 Earth years to complete a circle, so it will be 15,000 Earth years or 8,000 Martian years before the Martian calendar loses alignment
In the proposed calendar the year begins at the vernal equinox, or the beginning of the northern spring. This occurs shortly after the Sun enters Sagittarius as seen from Mars, and may be called the first point of Sagittarius. The first month of the Martian year is thus Sagittarius, and the rest follow in order, as seen in the list of the months.
As the axes of Earth and Mars precess at their vastly different rates, eventually both planets will come to point in exactly the same direction. In the 91st Century AD (38th Century ME) the vernal equinoxes of both worlds will be about halfway between Scorpio and Sagittarius.
The present position of Mars' vernal equinox suggests 2 areographic names. In ancient times, when the Sun reached its maximum northern latitude of 23.5 deg. during the summer solstice in the constellation Cancer, that latitude became known as the Tropic of Cancer. Capricorn was then the constellation of the winter solstice, so 23.5 deg. South became known as the Tropic of Capricorn. On Mars the summer olstice occurs during Pisces, so the Martian tropic at ~25 deg. North should become the Tropic of Pisces. Likewise its southern counterpart should be the Tropic of Virgo, the constellation of Mars' winter solstice.
Due to the eccentricity of Mars' orbit, the lengths of its seasons are unequal. Mars' apehelion (furthest point from the Sun) occurs late in the northern spring. This makes spring the longest season, and as apehelion occurs only 49 days before the summer solstice, summer is the 2nd-longest season. Similarly perihelion (Mars' closest approach to the Sun) occurs 34 days before the winter solstice. Fall and winter are thus the shortest and 2nd-shortest seasons, respectively. Spring in the north lasts 194 days, so the first day of summer does not occur until the 28th of Pisces. Summer lasts 177 days, so although the fall equinox is in the constellation Gemini, it does not occur in the month of Gemini but on the 10th of Mithuna, the 2nd month under that constellation. After a fall of 142 days, winter begins on the 13th of Virgo and lasts 156 days.
Of course, just as on Earth, the seasons of the southern hemisphere are exactly the opposite of their northern complements, so that in the South the 1st of sagittarius marks the beginning of a 194-day autumn.
3.5 Days Of The Week
With a very minor adjustment, the 7-day week can be made to work better on Mars than it does on Earth, as there are exactly 4 weeks in a 28-day month. Since weeks on Mars will rarely match those of Earth, using the same names for days will be confusing; Monday on Mars may be Friday on Earth [depending on your location - let's not forget the Date Lines -- Jim]. To avoid this problem the Latin names for the days are used: Solis, Lunae, Martis, Mercurii, Jovis, Veneris and Saturn. As these are the antecedents of the day names in most European languages today, they are familiar and can be readily adopted, yet won't be confused with Earthly days.
[I don't think there'd be much of a problem -- Jim]
On Earth, these names were taken from objects in the sky that could be seen by the naked eye. Applying the same idea to Mars leads to 2 sunbstitutions. Martis or "Mars' Day" becomes Terrae, "Earth's Day." And, though tiny, from the surface of Mars Phobos is much more impressive than the Moon, so Lunae or "Moon's Day" becomes Phobotis, "Phobos' day."
Each month begins on Solis, so that regardless of the month a given day can only occur on 4 dates, 7 days apart. For example, Jovis will always fall on the 5th, 12th, 19th and 26th of any month. No one on Mars will ever have towonder which day of the week a given date falls on. This eliminates the sloppiness we've had to live with under the Gregorian Calendar [so do many of the new calendars proposed for Earth, such as the 13-Month Calendar -- Jim] However, this requires all 27-day months to end on Veneris, and that the Saturni right after, which would have been the 28th, be skipped over. Once again, the last week of a short month ends on Veneris, and the next day is Solis. This will happen at most only 4 times a year, and almost certainly the 27th of each short month will be a holiday by Martian custom so they will not be deprived of the usual 2-day weekend. [:)]
3.6 The First Year On Mars
The need to keep a Martian calendar began with the landing of Viking 1, when project scientists began expressing Martian Solar days as "sols." The day of the landing they designated "Sol 0" and the sols that followed were numbered consecutively [as in the Julian Dating System -- Jim]. With the landing of this first unmanned spacecraft humans began working on the surface of Mars, albeit by proxy, and thus it was that humans began working by Martian time.
This need to keep account of Martian Solar days will continue as further landings are made, so it seems best to establish a formal Martian calendar now rather than later. Given this, the Viking 1 landing, a human event by extrapolation, is the appropriate beginning for a Martian calendar. That Martian year, the Year 0 ME, began with the Martian vernal equinox on December 19, 1975 AD, when midnight on the Martian date Line occurred around 04:09 GMT. Viking 1 landed 208 sols later in Chryse Planitia
On Earth the date was Tuesday, July 20th, 1976
On Mars it was Saturni, Mina 7th, 0
3.7 The Interface With Earth
This system is intended purely for local application to local conditions. The differences stem from the fact that the Martian day and year are different in length than Earth's. GMT will remain the standard time wherever humans may travel an settle, but just as we need local time on Earth we shall on Mars as well. Of course, the Martians will never be completely free of the Earthly second, which will continue as the standard unit of time in science and engineering.
Also, they will still have to refer to the Gregorian Calendar occasionally, but it will not rule their daily lives. For instance, they will think of the anniversary of the Viking 1 landing as Mina 7th, not July 20th, and will observe it once every Martian year because, after all, it was a Martian event.
At the same time the Gregorian Calendar will be used for events that occurred on Earth. Christmas will always be on December 25th, and Independence Day on the 4th of July, so that on Mars they will be observed on different dates twice every year. This practice has precedence in the many days, such as Easter and Hannukah, which are celebrated on differing dates every year.
Early in the next century there will be permanent settlements on Mars, and the local environment will have considerable effect on the settlers' daily lives. This will never happen on the less hospitable worlds, where people will always have to live heavily protected from the extreme environments. These planets do not undergo any meaningful seasonal changes, so the concept of a local year will be largely irrelevant. Likewise the local day, which on many of these planets is weeks long, if not months. On such worlds Earthly timekeeping may as well remain in use.
On Mars, however, the colonists will find it more useful to express the order of events in their lives in local terms.
To the men and women who watch over the robot explorers of Mars, to the crews of the first expeditions, and tothe settlers who will follow them: should any of you choose to adopt the calendar I have proposed, I wish it to be known as the Darian Calendar, which I name for my son, Darius.
[DONE ... and done! -- Jim]
The Martian year is 668.599 Martian days ("sols") long
The Martian year is ~4 days short of 96 weeks long (7 x 96 = 672) so every year either 3 or 4 days have to be eliminated to make the days of the week fall on the same dates from year to year
One month, a completely arbitrary unit, is considered to be 4 weeks long
Each month contains 28 days, unless it is a short month
Short months have only 27 days. These are the 6th, 12th and 18th months of every year - also, in every even-numbered year (except those divisible by 10) the 24th month is short
Thus, every other year is a short year. It's easy to remember:
The Months Of The Martian Year
_______________________________________________ # Name Days # Name Days 1 Sagittarius 28 13 Gemini 28 2 Dhanasu 28 14 Mithuna 28 3 Capricorn 28 15 Cancer 28 4 Makara 28 16 Karkata 28 5 Aquarius 28 17 Leo 28 6 Kumba 27 18 Asleha 27 7 Pisces 28 19 Virgo 28 8 Mina 28 20 Kanya 28 9 Aries 28 21 Libra 28 10 Mesha 28 22 Tula 28 11 Taurus 28 23 Scorpio 28 12 Vrisha 27 24 Ali 27 or 28 _______________________________________________
The Seasons Of Mars
________________________________ spring equinox 1 Sag spring 194 days (29%) apehelion 6 Kum summer solstice 28 Pis summer 177 days (26%) fall equinox 10 Mit fall 142 days (22%) perihelion 6 Asl winter solstice 13 Vir winter 156 days (23%) ________________________________
[ percentages by Jim Kelley]
Darian and Gregorian Dates: Six Years In The Life Of Mars
______________________________________________________ MARS EARTH 01 Sag 00 spring equinox, 0 ME 19 Dec 75 07 Min 00 Viking 1 landing 20 Jul 76 23 Ari 00 Viking 2 landing 03 Sep 76 01 Sag 05 spring equinox, 5 ME 22 May 85 06 Kum 05 apehelion, 5 ME 18 Oct 85 28 Pis 05 summer solstice, 5 ME 07 Dec 85 07 Min 05 Viking 1 5th Anniversary 14 Dec 85 17 Min 05 Christmas, 1985 25 Dec 85 23 Ari 05 Viking 2 5th Anniversary 29 Jan 86 10 Mit 05 fall equinox, 5 ME 07 Jun 86 20 Can 05 Earth/Mars closest aproach 16 Jul 86 06 Asl 05 perihelion, 5 ME 26 Sep 86 13 Vir 05 winter solsrice, 5 ME 31 Oct 86 10 Lib 05 Christmas, 1986 25 Dec 86 01 Sag 06 spring equinox, 6 ME 09 Apr 87 ______________________________________________________