A Calendar for Mars

Note: This is a reprint of an article published in Ad Astra, the magazine of the National Space Society (Ad Astra, Nov/Dec93, vol. 5 Issue 6, p25). This material is reproduced with the permission of the author, Dr. Robert Zubrin.

Mars needs a calendar - using Earth dates just won't do. If I tell you it is February 1, you know that it is freezing in Minneapolis and high summer in Sydney, but what does it tell you about conditions on Mars?

The idea of a Martian calendar and timekeeping system is not original, and many have been designed in the past. Edgar Rice Burroughs developed an intricate calendar system for his Barsoomian civilization. But the problem with Burrough's, and all the successor Martian time systems that I have seen, is that they are not physical - they bear no relation to the Martian seasons or to the timekeeping needs of Mars surface navigation. Furthermore, the authors of such systems have neglected to provide a method of converting Martian dates to terrestrial ones, and vice-versa.

Now devising a Martian calendar is a bit trickier than it might seem at first. Mars has a year consisting of 669 Martian days, or "sols", each of which consists of 24 hours and 39.6 minutes of terrestrial time. Most previous calendars have simply divided this up into convenient subdivisions, such as 12 months of 55 to 56 sols each. Time-keeping systems have usually preserved terrestrial clock units, with an interstitial partial hour thrown in at some time of the day, or else employed some totally novel, usually decimal-based clock.

Such equipartitioned months don't work for Mars because Mars orbit is elliptical, which causes it's seasons to be of unequal length. And a clock that uses unequal hours would be a nightmare for those attempting navigation or astronomy from the Martian surface, while a decimal or other novelty would probably be disorienting and in any case would a correspondingly complete overhaul of the system of surface geographical co-ordinates in order to make it's use convenient.

The practical answer to the clock problem is simple - just divide up the Martian day into 24 Martian hours, each composed of 60 Martian minutes, each of which is composed of 60 Martian seconds. The conversion facto between Martian days, hours, minutes, and seconds and their terrestrial equivalents would thus be 1.0275 across the board. A time of day on Mars, say 9a.m, would have exactly the same physical significance with regard to the orientation of the planet towards the Sun as it does on Earth. All the equations of celestial navigation would also remain valid, although stellar latitude measurements would have to be taken with respect to the Martian pole star, which is located at 21.18 hours right ascension, 52.89 north declination (about halfway between Deneb and Alpha Cephei).

Such a clock solves all the practical problems associated with daily time keeping on Mars. Unfortunately, such a clock would annoy Physicists who regard the terrestrial second as the sacrosanct fundamental unit of physical time. They shouldn't worry - Martian crystallographers and others who require a high degree of precision in quoting their measurements of frequencies and whatnot can still quote their measurements in terms of terrestrial seconds. The Standard International system of physical units can remain intact. However, for purposes of operating on Mars, the terrestrial second is no more a useful unit of time-keeping than the terrestrial day, and must yield to its Martian counterpart.

The calendar is a more interesting problem. In order to predict the seasons, a calendar must divide the planet's orbit not into equal divisions of days, but into equal angles of travel around the Sun. If we want months to be useful units and choose to retain the terrestrial definition of a month as a twelfth of a year, then a month really is 30 degrees of travel around the Sun.

So, we have our months, but what should we name them? Using terrestrial month names could be confusing, and a totally new system would be completely arbitrary. However, there is a set of names available which has long been universally known to humanity and which has real physical significance not only to Mars, but to any planet in our solar system - the signs of the zodiac.

All the constellations of the zodiac lie in the plane of the planets. Ancient astrologers, having a geocentric (Earth - centered) point of view, named the months for whatever zodiacal constellation the Sun appeared to be located in as viewed from Earth. An interplanetary culture, though, must adopt a heliocentric (Sun - centered) point of view. Therefore, I have chosen to name the Martian months for whatever constellation mars would be found in as seen from the Sun.

For Martian colonists the sign of the month would be seen high in the sky during the midnight hours of a given month. It is currently the custom among planetary scientists to start a planet's year with the vernal equinox (the beginning of spring in the northern hemisphere, March 21 on Earth),and so, consistent with that custom, the Martian year begins with the month of Gemini and ends with Taurus.

Now for a complete system of dating, it is necessary not only to know he month of a year, but also what year it is in some absolute sense - you need some year "I". For this year the corresponding terrestrial year is 1961. It happens that the first day of the year, 1 January 1961, coincides with the first day of the Martian year, 1 Gemini. In addition, 1961 was the latest year prior to the commencement of Mars exploration where this held true. The notation I've adopted for this calendar places the day of the month first, followed by the name of the month followed by the year in Roman numerals. Thus the calendar starts with 1 Gemini I.

While such a calendar may be interesting, it is still difficult for residents of Earth to use, unless a means can be provided to convert Earth dates to mars dates. I have invented such a device, which I call an "areogator." You can use it to find the month (and therefore season) on Mars during any chosen month on Earth, or vice - versa; the relative positions and angles of Earth and Mars about the Sun; as well as where in the sky Mars will be as seen from Earth, or vice-versa, at any given time in the past or future.

It's my firm belief that we now posses the technology that could allow a human landing on Mars within 10 years of the time a decision is made to launch the program. It's now 1993, and it's worth noting that 2003 would be a particularly good year to launch a first Mars expedition, since a low energy trajectory is possible that year that would get astronauts to Mars in just 126 days. Launch would take place June 22, arrival would be October 26. On arrival at Mars the date will be 46 Pisces XXIII, and the dust storm season will be drawing to a close. Shortly after capture into orbit, the last of the clouds will lift, revealing the surface of Mars in fine southern weather, and a landing will be called for. It'll be about time.

Month	Sols	Begins on Sol	Notes
Gemini	61	1	Gemini 1 - vernal equinox
Cancer	65	62
Leo	66	127	Leo 24 - Mars at aphelion
Virgo	65	193	Virgo 1 - summer solstice
Libra	60	258
Scorpius	54	318
Sagittarius	50	372	Sagittarius 1 - autumnal equinox
Capricorn	47	422	Dust storm season begins
Aquarius	46	469	Aquarius 16 - Mars at perihelion
Pisces	48	515	Pisces 1 - winter solstice
Aries	51	563	Dust storm season ends
Taurus	56	614	Taurus 56 - Martian New Year's Eve

Calculating a Martian date is not too difficult. The trickiest part is figuring out what day it is on Earth. It's not enough to say, "It's 23 November 1993, dummy." You need to know, instead, how far along in the year you are. Is November 23 the 314th day of the year, 336th, 202nd, what? You could add up the days from January through October and add 23. You could simply pull out a calendar and count the days. Or you could fudge it a bit and do the following: multiply the number of full months that have passed by 30.4, (the average number of days in an Earthly month) and then add the number of days that have passed in the month in question. In the case of November 23 that would be 10 months (January through October) times 30.4, or 304. Add 23 and you have 327. To figure out how far along you are in the year, simply divide the number of days by 365 - in this case you would divide 327 by 365 and end up with 0.896 - you're 896/1000ths of the way through the year. This may seem a bit esoteric, but it necessary for calculating Martian dates.

To calculate the Martian date for an Earth date, you use the following equation: Mars date = (8/15) * (Earth date - 1961) +1."Earth date" in this equation is composed of the year, plus that bit of esoteric information, how far along you are in the year. So, for purposes of translating to a Martian date, 23 November 1993 becomes 1993 + 0.896, or 1993.896. Now your ready to undertake some math, to wit: (8/15) * (1993.896 - 1961) +1. And the answer is ... 18.544. What does that tell you? Think of it as the Martian equivalent of the Earth date you used in the equation - the integer portion gives you the year, the remainder how far along you are in the year. Ergo. 183544 indicates that you're 544/1000ths through the Martian year 18.

To determine the Martian month and sol, simply work backwards from the fact that you're 544/1000ths of the way through a Martian year. Multiplying 669 (the number of sols in a Martian year) by .544results in sol 364. From the table above, you can see that sol 364 resides in the Martian month of Scorpius, which begins on sol 318. Subtracting 318 from 364 gives you sol 46. Since Martian months (like Earth Months) begin on sol 1 (it's November 1, not November 0), add 1 to the sol number: 46 + 1 = 47. The date? 47 Scorpius XVIII.

Occasion	Earth Date	Mars Date
Calendar begins	1 January 1961	1 Gemini I
Mariner 4 flyby	15 July 1965	25 Libra III
Mariner 6 flyby	31 July 1969	16 Sagittarius V
Mariner 7 flyby	5 August 1969	20 Sagittarius V
Mariner 9 in orbit	14 November 1971	20 Pisces VI
Mars 2 and 3 land	2 December 1971	38 Pisces VI
Viking 1 arrives n orbit	19 June 1976	41 Leo IX
Viking 1 lands	20 July 1976	6 Virgo IX
Viking 2 lands	3 September 1976	49 Virgo IX
Mars Observer disappears	21 August 1993	16 Libra XVIII

You don't need a sixth sense to determine where Mars is at any given time - just six cents, if you use the areogator.

Lets say you want to know the position of Mars during a given year, 1993 for example. Place a penny, representing Mars, on the diamond on Mars' orbit (red) labeled "93", and a nickel, representing Earth, on the diamond on Earth's orbit (blue) at the beginning of January. These were the comparative positions of Earth and Mars around 1 January 1993. Note how close they are together, and that Mars was on the opposite side of the Earth from the Sun. Such an arrangement is called an "opposition," and as a result, Mars was bright and high in the sky around midnight during January 1993. You can also see that it was mid-Gemini, or early spring in the northern hemisphere on Mars at that time. To move forward in time, just move Mars (the penny) forward one diamond and Earth (the nickel) one diamond as well. You can see that by mid-February, Earth has passed Mars, which is now in the month of Cancer. Move the Earth forward four more diamonds to mid-August, the time Mars Observer was due at Mars, and Mars forward four diamonds. You can see that when Mars Observer vanished, it was the middle of the month of Libra, or mid-Summer in the Northern hemisphere on Mars. Counting forward, you can see that it would take four more diamonds before Mars enters the month of Capricorn, the beginning of the dust storm season, and this corresponds to February 1994. You can also see that about that time Mars and Earth will be on opposite sides of the Sun from each other.

If you want to plan a mission for the year 2001, just repeat the procedure , placing your Mars marker at the 01 diamond (in Virgo), and your Earth marker at January 1, and start move them forward in tandem. I've included markers for all years between 1993 and 2007. If you want too know the relative positions of Earth and Mars for years before or after those indicated, just add or subtract any multiple of 15 to the numbers given on the markings (i.e. 1975 is the same as 1990, which is the same as 2005, 2020, 2035, etc.)

If you want to know what constellation to find Mars in at night, lay a straight edge between Mars and Earth and then visualize a parallel line extending from the sun in the same direction. Thus, during February 1993, Mars was in the Month of Cancer, but a line parallel to Earth-Mars drawn through the Sun at that time would go through Gemini, and this is the constellation Mars was seen in by observers on Earth at that time. At the same time, any observers located on Mars would have seen Earth in Sagittarius.

The purpose of the Mars Society will be to further the goal of the exploration and settlement of the Red Planet. This will be done by:

Starting small, with hitchhiker payloads on government funded missions, we intend to use the credibility that such activity will engender to mobilize larger resources that will enable stand-alone private robotic missions and ultimately human exploration.

A Private Initiative to Send Humans to Mars

Mars is the new frontier. The only world aside from Earth in our solar system with sufficient resources to support life and, someday, a new branch of human civilization, the Red Planet poses the central challenge faced by our era of history.

Humanity: Do you have what it takes to leave your cradle Earth to become a multi-planet species?

It is now clear that millions of people around the world are ready and willing to answer this challenge in the affirmative. On July 4, 1997, when NASA's Pathfinder probe landed on Mars, there were 100 million hits on the mission web site. That is more than the number of people who vote in the United States in a presidential election! It is more than the total number of Americans actively for or against abortion, gun control, nationalized health care, or a balanced budget, combined. Since the landing there have 700 million more hits. Even assuming a significant repeat rate, the number is simply phenomenal.

Yet despite this massive demonstration of support for Mars exploration, the politicians in charge and their consultants and astrologers, appear deaf to the call, providing virtually no government funding to prepare the way for human Mars exploration. This has caused tremendous public frustration, exemplified for me by the fact that since the publication of my book The Case for Mars, in October 1996, I have received over 3000 letters or e-mails from people asking me if there was any way we could initiate a human Mars exploration privately.

I initially thought it impossible. However the public response to the Pathfinder mission has convinced me otherwise. After all 700 million hits! At $1 each that would be enough to fund four Pathfinder missions. At $10 each, that would be $7 billion, probably enough to fund a human Mars mission if done privately. But how does one get the credibility required to mobilize the funding that such broad based support represents?

The Mars Society

There is one way that can be done; by forming an organization that initiates a program of activity, which in a series of escalating steps, proves to all concerned that a real private Mars exploration effort is underway. This is the method that Jacques Cousteau employed to raise resources for a series of increasingly ambitious undersea exploration ventures. It is to carry out such a program that the Mars Society is being formed, with the Founding Convention to take place in Boulder Colorado August 13-16 of this year.

Unless we are unexpectedly lucky with fundraising, we will have to start small, perhaps with a hitchhiker payload or instrument flying on a NASA or ESA Mars probe. This could probably be managed at a cost of about $5 million. Once that is done, however, it should elevate the public profile of the Society enough to allow the raising of funds on the order of $100 million, sufficient to finance a complete private robotic exploration mission. My favorite candidate for such a mission would be a Mars aerial photography mission carried out with balloons, although there are many exciting alternative options, including long distance ground rovers, orbiters with ground penetrating radar to search for subsurface water, and others. If a private organization were to be successful in implementing any mission of this type, the public excitement generated all over the world could well be sufficient to allow billions of dollars to be raised, enough to fund human Mars exploration either by the Society acting alone, or perhaps on a cost-sharing basis with NASA or other government space agencies.

The Society's planned private Mars exploration effort and NASA's are not exclusive. On the contrary, because every government effort pursuing Mars exploration or technology development assists the cause and reduces the funding burden of the private initiative, the Society will also engage in broad outreach to instill the vision of pioneering Mars into the public and mobilize support for ever for aggressive government-funded robotic and human Mars exploration efforts.

A preliminary steering committee for the Society has been formed, which includes Mike Griffin, former NASA Associate Administrator for exploration, myself, NASA scientists Christopher McKay and Carol Stoker, science fiction author Kim Stanley Robinson, and many others. The Founding Convention of the Mars Society will be a historic step, one that may be remembered for centuries to come on the new world it opens. But history is not a spectator sport. We need your talents and energy if the initiative is to succeed.

Please send me mail if you have any suggestions on how I might improve this page.