rwl@uvacs.UUCP (11/12/87)
The Julian calendar system and the Julian date are two distinct time measures. The former was the calendar reform instituted in 46 B.C. (of our Gregorian calendar) under the reign of Julius Caesar which fixed the calendar year to 365 days with an intercalary day added every four years to make the average year 365.25 days long. In 1583, French scholar Joseph Justus Scaliger introduced a system for use in comparing historical dates. Named in honor of his father, Julius Caesar Scaliger, the Julian period was based on a cycle of 7980 years -- long enough to encompass all of history as far as Scaliger was concerned. The number of 7980 was arrived at from the combination of three cycles: the 19-year Metonic Cycle (which roughly interrelates the lunar and solar cycles), a 28-year ``solar cycle'' (the period after which the days of the seven-day week repeat on the same dates [given a leap day every four years]), and the 15-year Indiction cycle (apparently a civil census period of Egypt in the third century A.D.). Scaliger calculated that these cycles coincided in the year 4317 B.C. so he started his system of reckoning on January 1, 4317 B.C. As arbitrary as the system is, it caught on in astronomy where dates can be given as an ordinal number (and fraction) of days from 0h UT January 0, 4317 B.C. The original intent of expressing dates as a Julian year plus the ordinal position of the day within the year fell into disuse. -- | Ray Lubinsky Department of Computer Science, University of Virginia | | UUCP: ...!uunet!virginia!uvacs!rwl | | CSNET: rwl@cs.virginia.edu | | BITNET: rwl8y@virginia |
wyatt@cfa.UUCP (11/12/87)
in article <2078@uvacs.CS.VIRGINIA.EDU>, rwl@uvacs.CS.VIRGINIA.EDU (Ray Lubinsky) says: > Keywords: calendar systems > > The Julian calendar system and the Julian date are two distinct time measures. [...] [correct info, ommitted for brevity.] > > Scaliger calculated that these cycles coincided in the year 4317 B.C. so he > started his system of reckoning on January 1, 4317 B.C. As arbitrary as the ^^^^ typo! (4713) > system is, it caught on in astronomy where dates can be given as an ordinal > number (and fraction) of days from 0h UT January 0, 4317 B.C. [...] Except that the zero point is Jan 1, 4713 B.C. at NOON (GMT). This 12-hour offset is the source for the majority of JDN calculation errors. Before 1925, astronomers reckoned mean solar time from 12:00, hence the relic. -- Bill UUCP: {seismo|ihnp4}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: 17410::wyatt (this will change, sometime)