Units Summary Mars' History
the optimal timekeeping system for Mars
developed by the Mars Time Group in 2001
As the calendar begins each mir at the Northern Vernal Equinox, the precise mir length used as a basis for the calendar is the average duration between two consecutive Northern Vernal Equinoxes. This is currently 668.5906 sols (see What is a "Year" (on Earth or Mars)? by Michael Allison) - however, current data indicates that this is increasing by about 3x10-7 sols per mir.
A useful Mars calendar must describe mirs that are whole numbers of sols in duration. On Earth, the Gregorian calendar describes a combinination of regular years of 365 days and leap years of 366 days, which results in an average calendar length equal to the Earth's average tropical year: 365.242199 days. We can easily use a similar system for Mars, by defining a pattern of short mirs with 668 sols and leap mirs with 669 sols.
The most accurate system of determing leap mirs is that described in the Darian Calendar. Because the vernal equinox mir appears to be gradually increasing in length, a system is used which results in an average calendar mir length of 668.5910 sols. Althouth this is slightly longer than the current vernal equinox mir, if the rate of increase in the duration of the vernal equinox mir is constant (something which is not yet known with certainty), then 668.5910 sols will be the average vernal equinox mir length over the next 2667 mirs.
Therefore, this system for leap mir determination can be used for 2667 mirs with a high degree of accuracy, according to current data. It may need fine-tuning as more accurate data about Mars' orbit becomes available, however, it provides us with a usable and accurate system to begin with. To properly compensate for the fraction of 0.591 sols, the calendar specifies 591 leap mirs per 1000 mirs.
To put it simply, a mir is a leap mir if it's odd or divisible by 10, unless it's divisible by 100 but not 1000. Or, said another way, a mir is a leap mir if it ends in 0, 1, 3, 5, 7, or 9, unless it ends with 100, 200, 300, 400, 500, 600, 700, 800, or 900.
Where should the extra sol in a leap mir be inserted?
Many Mars calendars, including this one, place the leap sol at the end of the mir, which has the advantage that sols identified by counting from the beginning of the mir (e.g. the 200th sol) always fall on the same date.
From what point in time should we start counting mirs?
We need to define the zero point, the point from where we begin counting mirs. There are several important episodes in Mars' history that could be suitable.
We decided to choose 1609, an extraordinarily important year for astronomy, as the epoch at which to start counting mirs. This is the year that the German astronomer Johannes Kepler published his first two laws of planetary motion in Astronomia Nova* (known in English as "New Astronomy - Commentary on the Motions of Mars", or simply "New Astronomy"). Significantly, it was through Kepler's study of Mars' ecliptic orbit that he discovered these laws, thus proving and augmenting Copernicus' heliocentric model of the solar system.
It isn't known for certain who first suggested counting mirs from 1609, although the idea appears on the Martian Time website. It was suggested again by John Darcy in the Mars Time Group, and after careful consideration, the logic of this choice became clear.
1609 was also the year the telescope was first used in astronomy, by the famous Italian astronomer Galileo Galilei. By choosing to begin counting mirs from 1609, this means that all telescopic observations of Mars, and all Mars missions, occur in positive mirs. Furthermore, 1609 could make a logical starting point for calendars for any other bodies in the solar system (particularly the Galilean moons), as they, too, could have their complete history of telescopic observation and exploration expressed in positive numbers. It may be advantageous in the future to have a common starting point for all the various calendars in the solar system.
There is a Martian northern vernal equinox that occurs in 1609, on March 11, which makes an ideal starting point for mir 000 (see Mars' History).
* Or more fully, "Astronomia Nova a t t sue Physica Coelestis, tradita Commentariis de Motibus Stellae Martis. Ex Observationibus G. V. Tychonis Brahe"
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