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Darian calendar

The Darian Calendar is a system of time keeping designed to serve the needs of any possible future human settlers on the planet Mars. It was created by Thomas Gangale in 1985 and named by him after his son Darius.

The basic time periods from which the calendar is constructed are the Martian solar day (sometimes called a sol) and the Martian vernal equinox year, which is slightly different from the tropical year. The sol is 39 minutes 35.244 seconds longer that the Terrestrial solar day and the Martian vernal equinox year is 668.5907 sols in length. The basic intercalation formula therefore allocates six 669-sol years and four 668-sol years to each Martian decade. The former (still called leap years even though they are more common that non-leap years) are years that are either odd (not evenly divisible by 2) or else are evenly divisible by 10. The year is divided into 24 months. The first 5 months in each quarter have 28 sols. The final month has only 27 sols unless it is the final month of a leap year when it contains the leap sol as its final sol.

The calendar maintains a seven sol week, but the week is restarted from its first sol at the start of each month (ie the final sol of the week is omitted at the end of each 27-sol month). This is partly for tidiness, but can be rationalised as making the average length of the Martian week close the average length of the Terrestrial week.

The Martian year is treated as beginning near the equinox marking spring in the northern hemisphere of the planet. Mars currently has an axial inclination similar to that of the Earth, so the Martian seasons are perceptible, though the greater eccentricity of Mars' orbit about the Sun compared with that of the Earth means that their significance is strongly amplified in one hemisphere and masked in the other. The most sophisticated calculations of the Darian calendar extend to the point of making allowance for the slight increase in the length of the Martian vernal equinox year over several thousand years. These prescribe a more complicated intercalation formula (for details see the link cited below).

Certain details of the Darian calendar have been the subject of dispute. The names of the 24 months (provisionally chosen as the Latin names of constellations of the zodiac and their Sanskrit equivalents in alternation) and the 7 sols of the week (provisionally named after the Sun, Moon and the 5 brightest planets as seen from Mars - including the Earth) were less contentious than the selection of the Martian epoch. Originally this was chosen as late 1975 in recognition of the American Viking program as the first fully successful soft landing mission to Mars. This was recognised as being excessively parochial, and also resulted in the many telescopic observations of Mars over the past 400 years being relegated to negative dates. The currently favoured epoch, first suggested by Peter Kokh, is in 1609 in recognition of Johannes Kepler's use of Tycho Brahe's observations of Mars to elucidate the laws of planetary motion, and also Galileo Galilei's first observations of Mars with a telescope.

In 1998, Gangale adapted the Darian calendar for use on the four Galilean moons of Jupiter discovered by Galileo in 1610: Io, Europa, Ganymede, and Callisto. In 2003, he created a variant of the calendar for Titan.

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