Explanation
-What is this?
-The French Republican - calendar was a calendar created and implemented during the French Revolution.
-It is also frequently referred to as the French Revolutionary Calendar, but this is 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. Every three months represent a season, and the endings of the names reflect this - fact.
-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.
-For more details about how I calculated this calendar, please see - my blog - post on the topic. This is the fourth part of a series on time-keeping, and you are highly - encouraged to read the - first - three - parts - for a more complete understanding.
-What are those names above the Gregorian date?
-Those are the names of the days in the - rural version of the - calendar. This was intended to replace the Catholic Church's calendar of saints, as the French - Revolution wanted to reduce the influence of the church. Every day of the year has a unique name - associated with the rural economy and these names are supposed to correspond with the season.
-Every quintidi is named after an animal, every décadi is named after an agricultural - tool, and the remaining days are named after various plants or produce. The only exception is the winter - month of Nivôse, which has the remaining days named after minerals.
-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).
-What is decimal time?
-Decimal time is a time system used during the French Revolution that divided the day into 10 - hours, each with 100 minutes, which contained 100 seconds each.
-The result is 100,000 seconds in one day, compared to the 86,400 seconds with the normal 24-hour
- system. This makes it very easy to denote time as a decimal fraction of a day. For example, decimal time
- 5:67:72 (around 13:37:31) on January 1, 2000 can be represented as 2000-01-01.56772.
Also note that each decimal hour is 2.4 normal hours, each decimal minute is 1.44 normal minutes, and - each decimal second is 0.864 normal seconds.
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