yohannparis / frcal

Interactive French Republican Calendar (a.k.a. French Revolutionary Calendar), using the historic equinox method for leap years.

French Republican Calendar

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 Revolutionary 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, 8, 12 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.