Explanation
+What is this?
+The French Republican + calendar was a calendar created and implemented during the French Revolution.
+It was also frequently known as the French Republican Calendar, but this was a misnomer: year 1 + of the calendar started on 22 September 1792, the day after the + abolition of the + monarchy and the founding of the French + First Republic.
+How does it work?
+A year consists of 12 months of 30 days each, divided into three décades of 10 days + each, followed by 5 complementary days (6 in leap years).
+The year starts on the day of the autumnal equinox at the Paris Observatory (longitude 2°20′14.03″ E). A + leap year follow directly from this definition: a year is a leap year when the next autumnal equinox + happens 366 days later instead of the normal 365. By this definition, the year will never drift + with respect to the seasons.
+The 12 months are: Vendémiaire, Brumaire, Frimaire, Nivôse, Pluviôse, + Ventôse, Germinal, Floréal, Prairial, Messidor, Thermidor, + Fructidor.
+The complementary days are: la Fête de la Vertu, la Fête du Génie, la Fête du + Travail, la Fête de l'Opinion, la Fête des Récompenses, and la Fête de la + Révolution (leap years only).
+What's so special about this version?
+Most versions of the calendar floating around doesn't use the original definition above.
+Most versions uses the so-called Romme method for leap years, using the same leap year rules as + the Gregorian calendar, i.e. every year divisible by four, except century years not divisible by 400. + This method might make sense, except years 3, 7, and 11 were leap years under the original rules and + were observed as such in real life, but the Romme method instead makes years 4, 6, 11 leap + years instead.
+This version uses the original rules. The JPL's + DE440 and DE441 ephemerides were used to calculate the exact timings of the autumnal equinoxes + between the Gregorian years 13201 BCE and 17191 CE (corresponding to the French Republican years -14991 + to 15399). The times were then converted to UT1+00:09:21, the exact local time at the Paris Observatory. + UT1 was chosen to keep track of the Earth's rotation without having to worry about the issues posed by + leap seconds in UTC. Note that due to the uncertainty over + ΔT — the difference between UT1 and + Terrestrial Time (TT) used in the ephemerides — it is theoretically possible for there to be + inaccuracies when the equinox occurs very close to midnight.
+What are those numbers below the Gregorian date?
+The five (or more) numbers separated by dots is the corresponding + Mesoamerican Long Count + calendar date. This is commonly known as the “Mayan calendar.” This calendar is not + available for dates before August 11, 3114 BCE (25 Thermidor -4905).
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