samedi, décembre 27, 2003
The number of days since noon on January 1, -4712, i.e., January 1, 4713 BC (Seidelmann 1992). It was proposed by J. J. Scaliger in 1583, so the name for this system derived from Julius Scaliger, not Julius Caesar. Scaliger defined Day One was as a day when three calendrical cycles converged. The first cycle was the 28 year period over which the Julian calendar repeats days of the week (the so-called solar number). After 28 years, all the dates fall on the same days of the week, so one need only buy 28 calendars. (Note that since the Gregorian calendar was adopted the calendar now takes 400 years to repeat.) The second was the 19 year golden number cycle over which phases of the moon almost land on the same dates of the year. The third cycle was the 15 year ancient Roman tax cycle of Emperor Constantine (the so-called indiction). Scaliger picked January 1, 4713 BC on the Julian calendar as Day One. The three cycles coincide every 7980 years (Tøndering).
>>>Gregorian
Portions of this entry contributed by Alejandra Mercado
The calendar currently in worldwide use for secular purposes based on a cycle of 400 years comprising 146,097 days, giving a year of average length 365.2425 days. The Gregorian calendar is a modification of the Julian calendar in which leap years are omitted in years divisible by 100 but not divisible by 400. By this rule, the year 1900 was not a leap year (1900 is divisible by 100 and not divisible by 400), but the year 2000 will be a leap year (2000 is divisible by 400). The total number of days in 400 years is therefore given by
400 X 365 + 100 - 3 = 146, 097
This also gives an exact number of weeks per 400-year cycle.
The Gregorian calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for the Earth to complete one orbit around the Sun.
>>>Julian Date
The number of days since noon on January 1, -4712, i.e., January 1, 4713 BC (Seidelmann 1992). It was proposed by J. J. Scaliger in 1583, so the name for this system derived from Julius Scaliger, not Julius Caesar. Scaliger defined Day One was as a day when three calendrical cycles converged. The first cycle was the 28 year period over which the Julian calendar repeats days of the week (the so-called solar number). After 28 years, all the dates fall on the same days of the week, so one need only buy 28 calendars. (Note that since the Gregorian calendar was adopted the calendar now takes 400 years to repeat.) The second was the 19 year golden number cycle over which phases of the moon almost land on the same dates of the year. The third cycle was the 15 year ancient Roman tax cycle of Emperor Constantine (the so-called indiction). Scaliger picked January 1, 4713 BC on the Julian calendar as Day One. The three cycles coincide every 7980 years (Tøndering).
>>>Gregorian
Portions of this entry contributed by Alejandra Mercado
The calendar currently in worldwide use for secular purposes based on a cycle of 400 years comprising 146,097 days, giving a year of average length 365.2425 days. The Gregorian calendar is a modification of the Julian calendar in which leap years are omitted in years divisible by 100 but not divisible by 400. By this rule, the year 1900 was not a leap year (1900 is divisible by 100 and not divisible by 400), but the year 2000 will be a leap year (2000 is divisible by 400). The total number of days in 400 years is therefore given by
400 X 365 + 100 - 3 = 146, 097
This also gives an exact number of weeks per 400-year cycle.
The Gregorian calendar was constructed to give a close approximation to the tropical year, which is the actual length of time it takes for the Earth to complete one orbit around the Sun.
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