Our concept of time has been fundamentally shaped by the natural rhythms of Earth's interactions with the Sun and the Moon: the day, the month and the year. These are the natural units of time. All others - the second, minute, hour, week, quarter, decade, century, and millennium - are derived from these three. As the cultures of the world have become more integrated, a common system of timekeeping has been universally adopted for civil purposes. The 24-hour clock, with its further divisions into 60 minutes per hour and 60 seconds per minute, is used exclusively throughout the world. The Gregorian calendar has become the international standard for civil time, although a number of other calendars - such as the Julian, Chinese, Jewish, Islamic, and Hindu calendars - continue to be used for cultural and religious purposes.

So why Martian time? Since we have a standard for civil time on Earth, why not export it to Mars in order to maintain commonality? This straightforward idea overlooks the fact that the time standard we have adopted on Earth works for everyone only because we all live on Earth and are subject to all of the same natural cycles. The problem is that Mars has its own distinct natural cycles, and the impact of these cycles on human activities on Mars will be impossible to ignore. Our daily routine will be synchronized with the Martian day, not the Earth day, and it will be the annual passing of the Martian seasons, not those of Earth, that will have a significant effect on our activities.

How long is a day on Mars? Just about any astronomy book will tell you that the rotational period of Mars is 24.6229 hours, or 24 hours, 37 minutes. However, note that the same table also gives Earth's rotational period as 23 hours 56 minutes. But isn't Earth's day 24 hours long? What happened to the missing four minutes? The difference is that the 23 hours 56 minute figure is a sidereal day, i.e., Earth's rotation as measured from the point of view of a fixed reference angle. But as Earth turns once on its axis, it also moves along its orbit around the sun, and the direction from the Earth to the sun changes slightly. It takes Earth an extra four minutes to rotate through this additional angle, and so Earth's solar day, measured from the point of view of the sun, is 24 hours. The same principle applies to Mars. Although its sidereal day is 24 hours, 37 minutes, its solar day is 24 hours, 39 minutes, 35.244 seconds (88775.244 seconds).

Humans have already experienced the need to work according to a Martian daily schedule at times during the past quarter century. During the Viking missions in the 1970s, operations teams had to schedule tasks for the two landers based on the daylight hours at the two sites. A new term - sol - was coined for the Martian solar day. The sol on which each lander touched down was designated "Sol 0," and each successive sol was numbered consecutively. In order to express the local time at each site, the sol was divided into 24 Martian hours, which were in turn divided into 60 minutes per hour and 60 seconds per minute, just as on Earth. This same system was later used during the Mars Pathfinder mission in 1997. Exactly who developed this system? We wish we knew, for we would certainly like for him or her to be recognized.

Notice that the system of time used during the Viking surface operations, and which was later adopted for the Mars Pathfinder mission, made use of only one natural Martian cycle: the solar day. What about the Martian year? Viking Lander 2 operated for 1,280 sols (nearly two Martian years), and Viking Lander 1 lasted even longer - 2,244 sols, or more than three Martian years.

How long is a year on Mars? Just about any astronomy book will tell you that the orbital period of Mars is a bit less than 687 days. But this measurement is in 24-hour Earth days, not Martian sols, which are almost forty minutes longer. If you lived on Mars, you would count 668.5906 sols from one vernal equinox to the next.

On Earth, the vernal equinox (the beginning of spring) is used to define the beginning of the astronomical year. This occurs when the Sun is directly above the Earth's equator, and the daylight and night periods are exactly 12 hours each (the term equinox is derived from two Latin words and translates literally as "equal night"). In organizing data for Martian phenomena that are influenced by the annual cycle, scientists often use the Martian vernal equinox as the starting point of the Martian year. Data is then graphed on a time scale from 0 to 668.6 sols.

The question then arises, how does one refer to one specific Martian year versus another? How do we organize our description of annual phenomena? For the Viking era, this was easy. One could refer to Sol 207 of Viking Year 1, for instance, and compare conditions at a landing site then with the phenomena observed on Sol 207 of Viking Year 3, exactly two Martian years later. But the situation becomes more complicated when one wishes to compare data across two or more Mars missions. A researcher interested in global weather patterns might want to compare data from several orbiter missions obtained during a number of different Martian years. For instance, suppose one needed to refer to a data point on Sol 475 of the second Martian year of Viking Orbiter 1 operations and compare that to a data point on Sol 475 of first Martian year of Mars Global Surveyor operations. This is a rather cumbersome way of expressing what are essentially two Martian dates. To simplify expressing Martian dates, we need to agree on a standard epoch, that is, a starting date from which we all agree to count Martian years.

So far, we have discussed some of the technical requirements for measuring time on Mars. That is a quite narrow perspective, and what we currently have on Mars is a fairly rudimentary time system that serves the needs of a specific community of space scientists. Even so, one can see that as more spacecraft are sent to Mars and as more data accumulates, the need for a more comprehensive Martian timekeeping system grows.

Let us fast-forward to a time in which there is a human society on Mars, with people from all conceivable walks of life, not just scientists, engineers, and technicians, but accountants, artists, and athletes. What sort of timekeeping system will these Martians need? In asking that question, one needs to understand that developing a timekeeping system to serve the broad spectrum of humanity and all of its activities is really not a technical problem. The astronomy of developing a clock and a calendar is relatively straightforward. Once the space scientists determine the length of the sol and the Martian year, their part of the job is pretty much done. Almost anyone can do the math; it's only on the level of middle school algebra. However, the true scope of the problem goes far beyond that, because developing a comprehensive civil time system is mostly a human problem involving social necessity. One needs to understand how human societies organize themselves in the temporal dimension at various social levels. Also, since each human society has done it differently, each of us has a cultural bias when it comes to measuring time. Finally, since there is no obvious best way to organize time for a society, arriving at a universally accepted system of Martian timekeeping will involve developing a consensus via a social process.

Such a process has been underway for several years now, primarily due to the increased access to information and the increased ability to form issue-oriented communities on the Internet. Years ago, from time to time a person would consider the problem of Martian timekeeping and publish a paper in some (often obscure) journal, usually ignorant of the fact that others had published on the subject years before in some other obscure journal or in a book that had a limited printing run. Historically, the subject has not attracted wide attention. It may come as a surprise to you that the roots of Martian timekeeping go far back indeed. A couple of excellent Martian calendars were devised by astronomers in the 1930s and 1950s, and one of these gentlemen went so far as to have a working Martian clock built. Going back even further, the subject of Martian time was discussed in a science fiction novel published in 1880.

In the late 1990s, as the United States recommitted itself to a sustained program of Mars exploration, and the Mars Society was founded with the goal of furthering that exploration to eventually include human missions, bases, and settlements, more people became interested in Martian timekeeping. Nowadays, we find several new Martian timekeeping websites every year. While in the old days, most writers thought they were the first to look at the problem, that is becoming less and less the case. Today, most people who get interested in the subject go online and find at least some of the work that has already been published before they finalize their own ideas. So now, rather than working in isolation, there is an online discussion in progress. We are increasingly being influenced by each other's ideas, and it is in this environment that we can begin to form a consensus.

Consensus will probably develop in phases on an issue-by-issue basis. Some issues may need to be settled in the near future (perhaps the need for a standard epoch is one of these), while others can be deferred indefinitely (the names of the days of the week or the months of the year, for instance). Some issues may not need to be settled early, yet it is possible that there may develop such an overwhelming preference for specific solutions that consensus is achieved rapidly (the seven-day week, for example).

Another factor that will determine how we arrive at a consensus is the rise of various user communities, their specific needs for Martian time, and their influence on each other. The space science community was the first user community to emerge, but its requirements for Martian time are fairly basic. This community will probably be incremental in its approach to Martian time, adopting new standards only as the technical need arises. At the other end of the scale are enthusiast groups who look forward to a future human society thriving on Mars. For some of these groups, a fully-developed timekeeping system, complete with a calendar as well as a clock, is part of the characterization of an emerging Martian cultural identity. These comprehensive time systems may either be developed by the groups themselves or adopted from previously published sources. In any case, these systems will compete with each other by attracting either greater or fewer adherents over time. Some of these enthusiast groups have developed websites and even web-based Mars simulations that feature Martian timekeeping systems. One can speculate that the relative success of the competing systems in the environment of the World Wide Web will be a function of the popularity of their associated websites and not necessarily of the merits of the timekeeping systems themselves.

It may be useful to develop statistical tools to measure - and perhaps even to guide - the development of a consensus. To that end, we created the Martian Time Survey. The survey is currently in its second version. The results of the first version were reported at the Third International Convention of the Mars Society (Gangale & Dudley-Rowley, 2000). The survey records respondents' selection of options for specific aspects of Martian timekeeping, such as the number of primary divisions (hours) in a Martian solar day, or the choice of an epoch for counting Martian years. Thus the individual ideas that have been proposed over the years are deconstructed from their original comprehensive timekeeping packages and the associated cachet of individual authors or specific groups. Respondents can make selections on an issue-by-issue basis according to the merits of the proposed options, rather than choose between whole systems authored by a given person or touted by a specific group. Additionally, in the current version of the survey, respondents are encouraged to propose new options, and to furnish background information on each issue as well as pro and con arguments for each option as they desire. Thus not only can people respond to the survey, they can also influence the evolution of the survey.

Let's now turn to some specific issues.

Going back to the Viking project, the 24-60-60 system was not the only possible choice for a Martian clock. A number of writers on the subject of Martian time have considered clocks based on powers of ten. Technically speaking, there's no reason not to use such a system, and in fact the idea has a great deal of merit. The real issue is, once again, human. The 24-60-60 clock is deeply ingrained in human culture. On Earth, we agree to disagree on a number of other points when it comes to time. We variously prefer Friday or Saturday or Sunday as our sabbatical day, and we maintain a number of cultural calendars. But everyone tells the time of day by hours, minutes and seconds! So it was not unreasonable for the Viking program to stretch Earth's 24-60-60 clock to fit the longer Martian sol. In any case, the precedent for Mars has been set, and metric clock advocates have their work cut out for them.

More information on the topic The Martian Day: Structure

Given the number of cultural calendars that persist on Earth, despite the Gregorian calendar being the agreed civil standard, you can imagine the number of issues that we as a species bring to the problem of developing a Martian calendar.

Some people want to keep the seven-day week, some favor other numbers, and some want to abolish the week entirely.

More information on the topic The Martian Week: Structure

It is very much the same with the month. Some want to retain the 12-month year, while some think this makes for awfully long months (remember that the Martian year is nearly twice as long as Earth's) and would rather have twice as many in order to keep them to about the same number of days as Earthly months. Some want to keep the structure of the year simple and have months that are nearly equal in duration. Others, taking note of Mars' lopsided orbit, think that the months should span equal arcs of that orbit, even though this means that each month would contain a different number of sols (presenting a challenge Martian poets to come up with a mnemonic poem that would be truly memorable, if not very short). Then again, some want to abolish the month altogether.

More information on the topic The Martian Month: Structure

The season in which the calendar year should begin is another point of contention. Some favor the vernal equinox, others the winter solstice or another astronomically significant point, and still others still have picked arbitrary points in Mars' orbit to serve as the first sol of the year.

Picking a leap year system is far from simple either. The year lasts about 668.6 sols, whereas the length of each calendar year must be a whole number. There are leap year schemes that contain a pattern of 668-sol and 669-sol years. Other schemes include double leap years of 670 sols. Others ideas involve adding entire leap weeks or leap months.

Once one has decided on how many extra sols to have in leap years, when should those extra sols occur: at the beginning of the year, at the end of the year, or somewhere in between?

One idea for improving on the Gregorian calendar is to have each year begin on the same day of the week (Sunday, for instance). A number of ideas for reforming the Gregorian calendar that were proposed in the 19th and 20th centuries involved making it perpetual, that is, making each common year and each leap year begin on a Sunday and end on a Saturday. The fact is that the Gregorian calendar isn't really a single calendar but rather a set of 14 different calendars. A common year can begin on any of the seven days of the week, and so can a leap year. This is why you have to throw out your old calendar at the end of each year and buy a new one. Extending this idea of regularizing the calendar, each month could begin on the same day of the week. This would have the advantage of always placing the numbered day of the month on the same day of the week (one would know that the 16th always falls on a Tuesday, for example).

Assuming that we want to have a perpetual calendar on Mars, how do we implement it? If we keep the seven-day week, we have to come up with some deft manoeuver to get around the fact that neither 668 nor 669 are divisible by seven. Some solutions add one or more sols that fall outside the normal weekly rotation of seven sols, and these extra sols are counted as holidays. Another idea is to have an occasional six-sol week, eliminating a workday. Alternatively, one could have a number of sols at the end of the year that either comprises an irregular week or are outside the weekly rotation. Still another scheme is to have 665-sol years (divisible by seven) and add a seven-sol leap week as necessary.

More information on the topic The Martian Year: Structure

Finally, as we touched on earlier, there is the question of determining an epoch, a starting point for counting the Martian years. Ideas range from dates thousands of years in the past (such as the beginning of the Julian period), to much more recent events (such as the landing of Mars 3 or Viking 1), to waiting for the first human landing (which of course makes it impossible to count Martian years in the meantime).

More information on the topic The Martian Epoch

These are just the structural issues of characterizing a complete calendar system. We haven't even begun to discuss the problem of nomenclature. Should all of these Martian units of time - the second, minute, hour, week, month, and year - have new Martian names to distinguish them from Terran time measurements? What should we name the sols of the week and the months of the year? Such issues do not lend themselves to logical arguments, but are rather a matter of personal taste. This class of issues will probably be the last to be resolved.

Further information:

As you can see, there's far more to Martian timekeeping that we can possibly cover in one short article. We invite you to visit the Martian Time website to learn more. Also, feel free to join the Martian Time Virtual Conference and share your ideas. Finally, the crucible of all the ideas that have been put forth is how they will be received by Martian society, and at present, you, dear readers, are Martian society, and so we hope you will take the time to register your preferences in the Martian Time Survey.

CopyrightA9 2002 by Thomas Gangale and Marilyn Dudley-Rowley.