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In article <4842BF0172%rav21@cam.ac.uk> Richard van der Hoff wrote:
> In message <na.cda3dc4842.a70220nhawkes@NO.SPAM.co.uk> Nigel Hawkes           wrote:

>> A 'Julian date' is the interval, in days and fractions of a day,
>> since Greenwich noon on 1 January 4713 BC.
Fractions of a day expressed in decimals. Quoting from Norton's 
Star Atlas:

Jan 1, 1926, 09:00 (9 a.m.), astronomical time, is J.D. 2,424,516.875,
i.e. Dec 31 1925, 21 hours of the Julian Period.

> What happened in 4713 BC for it to be such a milestone?
Julian Period, chronological system now used chiefly by astronomers
and based on the consecutive numbering of days elapsed from 
Jan 1, 4713 BCE. Not to be confused with the Julian calendar, the 
Julian period was devised by the scholar Joseph Justus Scaliger in 
1582 and named by him for his father, Julius Caesar Scaliger.
 
Josephus Scaliger proposed a period of 7,980 years of numbered days 
to be used in determining time elapsed between various historical 
events otherwise recorded only in different chronologies, eras, or
calendars. The length of 7,980 years was chosen as the product of 
28 times 19 times 15; these, respectively, are the numbers of years
in the so-called solar cycle of the Julian calendar in which dates
recur on the same days of the week; the lunar or metonic cycle, after
which the phases of the moon recur on a particular day in the solar 
year, or year of the seasons; and the cycle of indiction, originally
a schedule of periodic taxes or government requisitions in ancient
Rome. The epoch, or starting point, of 4713 BCE was chosen as the 
nearest past year in which the three cycles began together.
- Enc.Brit.
* Never be afraid to mislabel a product - Ferengi rule 239 of acquisition.

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