Timekeeping on Mars makes use of the 'extended second', which is 1.027491251041666... seconds long. This preserves a 24-hour 'sol'. Typically, many use the terms 'second', 'minute', 'hour', 'day', 'week', and 'month' interchangeably, though when accuracy is noted, standard seconds are assumed.
In Earth time, a sol averages 24 hours, 39 minutes and 35.24409 seconds long over the course of a tropical year, or 88775.24409 seconds. This creates some difficulty, since it is close enough to be conveniently considered a day yet not close enough for very accurate tracking.
A day on Mars is sometimes called a sol, primarily to differentiate it from the term 'day' on Earth. Because of the extended second, a Martian day still has 24 hours, each with sixty minutes, each with sixty seconds, even though a given second is slightly longer.
The 'weeksols' are named in order: Heliosol, Phobosol, Deimosol, Venusol, Terrasol, Jovesol and Lunasol, named after seven of the most brilliant planetary objects seen from the planet's surface (Saturn is omitted to avoid confusion with Saturday). As Phobos and Deimos were consumed in the voracious appetite of human industry, their memory will at least be aided by such naming.
Each month is divided into four weeks of seven sols, totaling 28 sols per month, every month.
A year normally has 24 months. Every eighth year, the last month is dropped from the calendar. Every 304 years, the drop is skipped and the year proceeds normally. They are named somewhat loosely after the Sanskrit ordinals. While originally specified to be much longer, linguistic drift had significantly shortened the names by the time they had come into common use.
- Admasa (from aadimamaasa - 'the first month')
- Dvityasa (from dvitiiyamaasa)
- Trityasa (from tritiiyamaasa)
- Caturthasa (from caturthamaasa)
- Pancamasa (from pancamamaasa)
- Sasthmasa (from sasthamaasa)
- Saptmasa (from saptamamaasa)
- Astmasa (from astamamaasa)
- Navmasa (from navamamaasa)
- Dashmasa (from dashamamaasa)
- Ekmasa (from ekaadashamaasa)
- Dvamasa (from dvaadashamaasa)
- Trayodesh (from trayodashamaasa)
- Caturdesh (from caturdashamaasa)
- Pancadesh (from pancadashamaasa)
- Sodesh (from sodashamaasa)
- Saptadesh (from saptadashamaasa)
- Astadesh (from astaadashamaasa)
- Navadesh (from navadashamaasa)
- Vimshasa (from vimshatitamamaasa)
- Ekavim (from ekavimshatitamamaasa)
- Dvavim (from dvaavimshatitamamaasa)
- Trayovim (from trayovimshatitamamaasa)
- Caturvim (from caturvimshatitamamaasa) - Skipped every 8 years, except for every 304th.
Shortly after the development of the Martian calendar, the Helios Calendar was designed and accepted. The instantaneous communication made available by tachyon transmission quickly eroded the popularity of this calendar. After the Purge, it is mostly used as an oddity, nostalgic types, and sometimes to throw others off in a conversation.
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