An Easily Understood Calendar and Time System for Mars

Leon G. Heron

The clever mathematics on which this calendar and time system are based is not my doing. Credit for that must go to Thomas Gangale and others who have contributed articles posted on his website. I merely provide the following argument for a simple approach to the nomenclature of time-keeping on Mars. I have drawn names for some of the time periods from articles posted on Thomas Gangale’s website.


  • it will be Earthlings that colonise Mars, and
  • it will most likely be English-speaking Earthlings from the USA that lead the colonisation, and
  • Earthlings will communicate and trade with Martians

the time measurements used on Mars

  • must be easily understood by Earthlings (who might otherwise be unfamiliar with the intricacies of the orbit, seasons, and other natural cycles of Mars) and
  • should use easy-to-understand modifications of the English names for the equivalent terrestrial time periods (English is the most widespread language and is the major language of commerce and technology).

Thus, I propose that the time periods measured on Mars resemble their terrestrial counterparts in duration and nomenclature. The nomenclature, however, should easily differentiate the Martian time periods from their terrestrial counterparts. The following proposal is made after careful consideration of the history of terrestrial calendars and time-keeping methods and the historical origins of their nomenclature.

I propose that:

  1. The basic unit of time on Mars shall be the Martian solar day.
  2. The Martian solar day shall be called the ‘sol’
  3. The sol shall be divided 24:60:60
  4. The divisions shall be called mars-hour, mars-minute, and mars-second (note lower case m and hyphen for ease of reading and understanding). Abbreviations: m-hr, m-min, m-sec (note the hyphen prevents confusion of the m-sec with abbreviation for the millisecond – msec)
  5. The SI second (as measured by atomic clocks) shall be retained for scientific and technical applications
  6. Divisions of the mars-second shall be in decimal notation eg 1.3 mars-seconds (1.3 m-sec)
  7. Time shall be counted from crater Airy (Airy Mean Time, Airy Solar Time). Crater Airy marks the prime meridian and is named after George Bidwell Airy (b. 1801, d 1892 – Greenwich, England) the Pulmian Professor of Astronomy at Cambridge who chaired the Commission set up to construct Standard Weights and Measures.
  8. The week shall consist of 7 sols (the 7-day week is too ingrained into commerce, religion and ordinary life for it to be changed). If clear distinction is required between a Martian week and a terrestrial week, the name mars-week shall be used for the Martian week.
  9. The names of the weeksols shall be simple derivatives of the English weekday names: Monsol, Tuesol, Wednesol, Thursol, Frisol, Satursol, Sunsol. This provides for the simple transference of the work-related and religious significance of the weekdays. (I am aware of the duplication of meaning in the syllables of the word Sunsol, but so be it – ease of understanding is paramount)
  10. Each month shall consist of 27 or 28 sols (no change of name is required because Earthlings are familiar with months of 28, 29, 30 and 31 days from the Gregorian and other calendars). Clear distinction between a Martian month and a terrestrial month is not required because the word month is well known to be a ‘flexible’ term.
  11. The mars-year will consist of 24 months.
  12. The month names shall use names familiar to Earthlings. I propose that

  • Martian month names shall consist of a first part derived from the terrestrial equivalents plus a suffix that distinguishes the Martian month names from the terrestrial equivalents
  • Martian month names shall end in the suffix –mon so as to easily differentiate them from terrestrial months. That –mon is a derivative of month is obvious and easily understood.
  • the first part of the names of the Martian months shall be derived from either the names of the months of the Gregorian calendar or the names of the signs of the zodiac
  • the names of Martian months shall be arranged such that the months with names derived from the zodiac are intercalated between the months that have names derived from the months of the Gregorian calendar. For example, as all persons with even a passing interest in astrology know, the zodiac month of Aquarius (January 20 to February 18) lies ‘between’ January and February. Thus any Earthling with even a passing interest in astrology will know the order of the Martian months.

Proposed names and order of the Martian months: Janumon (28 sols), Aquarimon (28 sols), Februamon (27 sols, 28 sols in leap years), Piscemon (28 sols), Marsmon (28 sols), Ariesmon (28 sols), Aprilismon (28 sols), Taurusmon (28 sols), Mayamon (27 sols), Geminimon (28 sols), Junomon (28 sols), Cancermon (28 sols), Juliusmon (28 sols), Leomon (28 sols), Augustusmon (27 sols), Virgomon (28 sols), Septemon (28 sols), Libramon (28 sols), Octomon (28 sols), Scorpiomon (28 sols), Novemon (27 sols), Sagittarimon (28 sols), Decemon (28 sols), Capricormon (28 sols). Abbreviations easily distinguished from each other and the terrestrial equivalents: JANM, AQUM, FEBM, PISM, MARM, ARIM, APRM, TAUM, MAYM, GEMM, JUNM, CANM, JULM, LEOM, AUGM, VIRM, SEPM, LIBM, OCTM, SCOM, NOVM, SAGM, DECM, CAPM.

  1. The Martian year shall be called a mars-year
  2. Because a mars-year is almost double the duration of an Earth year, it will be difficult for Earthlings to automatically understand the age of Martians if the age of Martians is counted in mars-years (Martians will vote at 10 mars-years of age and retire at 32 mars-years). Therefore, I propose to introduce a new time measure equal to half a mars-year. This time measure shall be called a hemiannum (its duration is a few hours more than 21 days short of an Earth year). The hemiannum will be used as the unit for counting the ages of Martians. Martians will celebrate hemianniversaries. For example Martians will have two natal hemianniversaries or ‘birthsols’ per mars-year (or if you prefer to paraphrase Louis Carroll’s Mad Hatter, a Martian could, during one hemiannum, celebrate his/her ‘birthsol’ on the true sol of his/her birth and celebrate his/her ‘unbirthsol’ on the equivalent sol of the other hemiannum of the mars-year). Similarly Martians would celebrate their wedding hemianniversaries and perhaps get a pay rise on the hemianniversary of their employment. Note: the prefix hemi is used to ensure no confusion with the term semiannual (which is in terrestrial use). For those who wish to delve into astrology, birth signs could be assigned using the terrestrial date equivalents (eg the birth sign of Aries could be assigned to Martians with birthsols between Marsmon 21 to Aprilismon 19). However, I leave these decisions to the practitioners of astrology.
  3. Mars-years shall be counted using Arabic numerals.
  4. Ordinary mars-years shall have 668 sols and leap years shall have 669 sols. Because of the Gregorian calendar, we are all familiar with the concept of the leap year. Perpetual calendars proposed for Mars, keep ‘in step’ with the orbit of Mars by dropping a weeksol from the calendar from time to time to produce a 6-sol week. This is not likely to catch on because the 7-day week is inviolable for commerce, work and religious reasons. The concept of the leap year is already well accepted and should, therefore, be retained. For ease of understanding, the leap sol of a leap year shall be added to Februamon just as the leap day is added to February in the Gregorian calendar.
  5. M. D. Schmidt states that 1 mars-year = 668.5968606 sols. Thus, I have devised the following rules for leap years on Mars: Odd-numbered mars-years shall have 669 sols. Even-numbered mars-years shall have 668 sols except when the mars-year number ends in 0 when that mars-year shall have 669 sols except when dividing the mars-year by 100 yields a quotient that is an even integer other than an integer ending in zero – such mars-years shall have 668 sols. The mars-year, two mars-years after any mars-year that ends in 500 shall have 669 sols and every mars-year after a mars-years that when divided by 5000 yields a quotient that is an integer shall have 668 sols. These rules yield a calendar accurate to 10,000 mars-years.
  6. The first day of the mars-year shall be 1 Janumon (ie equivalent to the Gregorian calendar New Year’s Day) and the mars-year shall start at a point in Mars’s orbit so that the seasons on Mars occur (approximately) during the same months as they do on Earth. Thus, during Janumon the climate will be cold in the Northern Hemisphere of Mars and hot in the Southern Hemisphere in the same way that the climate is cold in the Northern Hemisphere of Earth and hot in the Southern Hemisphere during January. (Because the orbit of Mars is more eccentric than the orbit of Earth, it is not possible for Mars and Earth to have exact parity of the relationship of seasons to months). Thus, if we designate 1 Marsmon the date of the Martian northern vernal equinox (and my sources regarding the duration of the Martian seasons are correct and my calculations are correct), then because the Martian Northern Spring is 193 sols in duration, the Northern summer solstice will occur on 26 Junomon; and as the Northern Summer lasts 178 sols, the Martian autumnal equinox will occur on 9 Libramon; and as the Northern Autumn lasts 143 sols, the Northern Winter Solstice will occur on 13 Decemon; which leaves a Northern Winter of 154 sols (155 in a leap year).
  7. Mars-year zero shall be the mars-year on which Viking 1 landed on Mars. Mars-year 1 shall be the mars-year that began on Janumon 1 following the landing of Viking 1. Mars-years shall be designated AV (After Viking) and BV (Before Viking). The landing of Viking 1 is a significant date in the human history of the exploration of Mars. Few dates BV will have significance to the history of human exploration and colonisation of Mars.
  8. The mars-year shall be divided into 8 octants of three months each – four octants per hemiannum. Octants are roughly equivalent to the business quarters of the business years.
  9. Determination of dates of religious holy days and civil holidays shall be left to the relevant religious and civil authorities.