Figures and Tables

1.0 The Darian Calendar for Mars

1.1 Introduction

1.2 Years

1.2.1 An Extended Intercalation Scheme

1.3 Months and Seasons

1.4 Weeks

1.4.1 The Martiana Calendar

1.5 The Telescopic Epoch

1.6 Darian-Gregorian Calendar Displays

1.7 Children and Collateral Relatives

2.0 The Calendars of Jupiter

2.1 Introduction

2.2 Circads

2.3 Years

2.4 Weeks

2.5 Months

2.6 Intercalation

2.7 Calibration

2.8 Variations on a Martian Theme

3.0 The Darian Calendar for Titan

3.1 Overview of the Darian Calendar System

3.2 Astronomical Cycles on Titan

3.3 Circads and Weeks

3.4 Months and Years

3.5 Intercalation

3.6 Calibration

4.0 Conclusion

5.0 References

Appendix 1: Intercalation Precision on Mars

Appendix 2: Perturbations of Mars

Appendix 3: Martian Daylight Time


Copyright 1986-2005 by Thomas Gangale


In the twentieth century, the Gregorian calendar has achieved a distinction that is unique in human history. While several other calendars continue to be practiced in various parts of the world in order to maintain religious and cultural traditions, for civil functions the Gregorian calendar has become the universal calendar for humankind. However, with the coming colonization of the Solar System in the next century, the Gregorian calendar's monopoly on civil timekeeping will come to an end. On some of the new worlds it will be reasonable to ignore the local astronomical cycles as being useless for regulating human biological and social rhythms, and on such planets the use of Universal time and the Gregorian calendar can be maintained. But on Mars the need for a new convention of timekeeping is compelling, and there is a body of literature by dozens of authors that addresses this issue.

In addition to being based on the two natural cycles of Mars--its periods of rotation and revolution--the Darian calendar incorporates two social units of time--the week and the month. The Darian month of 28 sols is approximately the same duration as the human menstrual cycle, a natural unit of time.

The Darian calendar for Mars combines the advantages of the two major proposals for calendar reform here on Earth in the 20th century, the World calendar and International Fixed calendar:

  • Every common year is the same, and every leap year is the same (World calendar and International Fixed calendar).
  • To within one sol, each year divides evenly into halves, thirds, quarters, sixths, and twelfths, while simultaneously into an integral number of months (World calendar). Additionally, it divides into eighths. This is an important consideration, since an eighth of a Martian year approximates a quarter of an Earth year. As we all know, corporations issue financial reports on a quarterly basis, and one can predict that the emerging Martian economy will closely follow terrestrial financial practices in order to attract investment capital.
  • The quarters are equal in common years and contain an integral number of months (World calendar). Each has exactly 167 sols, 24 weeks or 6 months. The quarters are identical in form with the week at the end of each quarter truncated to six sols.
  • Each month begins on the first sol of the week (International Fixed calendar).
  • The leap sol occurs at the end of the year (International Fixed calendar).

Far more than any other Martian calendar proposal, the Darian calendar has been both unconsciously repeated and consciously imitated by numerous authors, plus it has been cited by still other authors. Additionally, most of the structural features of the Darian calendar have consistently scored high in the Martian Time Survey. The following list shows the preferences for features of the Darian calendar over the nearest competing option:

  • A 7-day week (6 to 1).
  • 24 equal-duration months, nominally 28 days each (4 to 1).
  • A 668-day non-leap year and a 669-day leap year (7 to 1).
  • A leap-year pattern of odd-numbered years plus decennial years (6 to 1).
  • A leap day added at the end of the year (7 to 1).
  • A perpetual calendar with an integral number of weeks per month (2 to 1).
  • Begin using a Martian calendar now rather than wait for a future event (1.7 to 1).
  • Increment the numerical year on the Martian cycle (14 to 1).
  • Begin the calendar year on the northern hemispheric vernal equinox (4 to 1).

Moreover, the Darian calendar is not restricted to Mars. The Darian system is an integrated timekeeping system for six worlds, not an isolated system for just one planet.

For the Galilean moons of Jupiter as well, it is possible to develop a new system that both takes into account the local astronomical cycles and satisfies human needs. Two key conditions make this possible. First of all, the local solar days of the four Galileans divide into new units of time -- the circads -- that approximate the terrestrial diurnal cycle and that differ from each other by only twenty minutes. Secondly, for the inner three Galileans, the number of circads per revolution around Jupiter form a geometric relationship enabling the construction of an eight-circad week which has definite physical meaning on these three worlds. The fact that these calendars can be further organized into four-week months and conveniently reconciled with the annual cycles of either Earth or Mars is an added bonus.

Of course, there is a price to be paid in trading the Gregorian calendar for a new chronometric system that is based on the astronomical cycles of the Jovian System. Although this new timekeeping system emphasizes commonality to the maximum extent, no system of multiple calendars can possibly have as much commonality as the case in which a single calendar -- the Gregorian calendar -- is imposed on all worlds. But any one calendar can only have physical meaning on a single world, and on all other worlds it will be an arbitrary thing. On Mars the Gregorian calendar is clearly unacceptable and must be replaced by a new system. The setting of this precedent increases the plausibility of a new timekeeping system for the Galileans as well.

It turns out that a timekeeping system for the Galileans based on the Darian calendar is more convenient in terms of reconciling the annual Martian cycle and the cycles of the Galileans, in that a division of the Martian year by 24 months is easily achieved. On the other hand, an attempt to adapt the Gregorian calendar to the Galileans resulted in calendars containing 13 months. Not only is this a serious deviation from the 12-month Gregorian calendar itself, the division of the year by a prime number is unattractive as well. Furthermore, the Darian calendar for Mars can also be adapted for Saturn's largest moon Titan.

However, the most immediate prospect for off-world human habitation, aside from the Moon, is Mars. Some have cautioned against the adoption of a Martian calendar any time soon, preferring instead to leave this decision to the Martians. Well, we are the Martians, aren't we? If a future generation of Martians determines that it is in their interest to modify a Martian calendar that was developed and adopted on Earth, or to adopt an entirely new calendar, they will certainly feel free to do so. Meanwhile, the decision is ours now. We know enough about the astronomical cycles of Mars and the societal requirements for a calendar to make a pretty good stab at it. Furthermore, I assert that the necessity to move forward far outweighs any arguments to hold back.

The enterprise of sending human expeditions to Mars and of permanently settling that planet will obviously require broad political support among many nations, sustained over several decades. I believe that the early institution of a Martian calendar will serve a significant political and social purpose as a symbol of the human commitment to establish a permanent presence on that new world in the coming decades. Mars will become more of a human place in the public imagination as familiar human references are adapted for that planet. The realization will become more widespread that the concept of colonies on Mars is transitioning from the realm of science fiction to that of imminent accomplishment. Although much engineering development remains to be done before the first human landing can be achieved, the process of humanizing Mars and laying the foundation for a new culture can and should begin now. The early promulgation of a human-oriented Martian calendar can be a symbol of a spreading awareness that human beings will not be going to Mars merely as visitors, but that we are going there with every intention of staying, putting down our roots, and flourishing on that new world.