2022-02-12 21:57:13 -05:00
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# French Republican Calendar
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## What is this?
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The [French Republican calendar][0] was a calendar created and implemented
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during the French Revolution.
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It was also frequently known as the *French Republican Calendar*, but this
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was a misnomer: year 1 of the calendar started on 22 September 1792, the day
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after the [abolition of the monarchy][1] and the founding of the [French
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First Republic][2].
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## How does it work?
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A year consists of 12 months of 30 days each, divided into three *décades* of
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10 days each, followed by 5 complementary days (6 in leap years).
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The year starts on the day of the autumnal equinox at the Paris Observatory
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(longitude 2°20′14.03″ E). A leap year follow directly from this definition:
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a year is a leap year when the next autumnal equinox happens 366 days later
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instead of the normal 365. By this definition, the year will **never** drift
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with respect to the seasons.
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The 12 months are: *Vendémiaire*, *Brumaire*, *Frimaire*, *Nivôse*,
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*Pluviôse*, *Ventôse*, *Germinal*, *Floréal*, *Prairial*, *Messidor*,
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*Thermidor*, *Fructidor.*
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The complementary days are: *la Fête de la Vertu*, *la Fête du Génie*,
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*la Fête du Travail*, *la Fête de l'Opinion*, *la Fête des Récompenses,*
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and *la Fête de la Révolution* (leap years only).
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## What's so special about this version?
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Most versions of the calendar floating around doesn't use the original
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definition above.
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Most versions uses the so-called *Romme* method for leap years, using the
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same leap year rules as the Gregorian calendar, i.e. every year divisible
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by four, except century years not divisible by 400. This method might make
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sense, except years 3, 7, and 11 were leap years under the original rules
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and were observed as such in real life, but the *Romme* method instead makes
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2022-02-12 23:18:48 -05:00
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years 4, 8, 12 leap years instead.
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2022-02-12 21:57:13 -05:00
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This version uses the original rules. The [JPL's DE440 and DE441
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ephemerides][3] were used to calculate the exact timings of the autumnal
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equinoxes between the Gregorian years 13201 BCE and 17191 CE (corresponding
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to the French Republican years -14991 to 15399). The times were then converted
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to UT1+00:09:21, the exact local time at the Paris Observatory. UT1 was chosen
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to keep track of the Earth's rotation without having to worry about the issues
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posed by leap seconds in UTC. Note that due to the uncertainty over [ΔT][4] —
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the difference between UT1 and Terrestrial Time (TT) used in the ephemerides —
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it is theoretically possible for there to be inaccuracies when the equinox
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occurs very close to midnight.
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[0]: https://en.wikipedia.org/wiki/French_Republican_calendar
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[1]: https://en.wikipedia.org/wiki/Proclamation_of_the_abolition_of_the_monarchy
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[2]: https://en.wikipedia.org/wiki/French_First_Republic
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[3]: https://ssd.jpl.nasa.gov/planets/eph*export.html
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[4]: https://en.wikipedia.org/wiki/%CE%94T*(timekeeping)
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