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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 aerospace engineer and political scientist Thomas Gangale in 1985 and named by him after his son Darius.
Year length and intercalation
The basic time periods from which the calendar is constructed are the Martian 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 than 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 than non-leap years) are years that are either odd (not evenly divisible by 2) or else are evenly divisible by 10.
Calendar Months and Weeks
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. If a month has 27 sols, this causes the final sol of the week to be omitted. This is partly for tidiness. It can also be rationalised as making the average length of the Martian week close to the average length of the Terrestrial week, although it must be remembered that 28 Earth days is roughly equal to 27+1/4 Martian sols and not 27+5/6 Martian sols.
The sols of the week are Sol Solis, Sol Lunae, Sol Martius, Sol Mercurii, Sol Jovis, Sol Veneris, Sol Saturni.
The odd-numbered months are named after the Latin names for the constellations of the zodiac starting with Sagittarius and the even-numbered months are the Sanskrit equivalents for the same constellations. At any time, the Sun is generally found within the constellation corresponding to the name of the current month, whether Latin or Sanskrit.
Other Darian calendars
In 1998, Gangale adapted the Darian calendar for use on the four Galilean moons of Jupiter discovered by Galileo in 1610.