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From: dave@sun.physics.heriot-watt.ac.UK (Dave Hutchings)
Newsgroups: comp.sys.atari.st
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Message-ID: <18581.8908101512@s.phy.hw.ac.uk>
Date: 10 Aug 89 15:12:57 GMT
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In article 368 Roger Critchlow writes,
>>
>> The Julian day number is the number of days since 4004 BC or when ever it was
>> that someone calculated that God created the earth.
> 
>The Julian day number counts from Greenwich Mean Noon, January 1, 4713 BC.
>Given the Julian day number (JD) the formula ((JD + 1.5) remainder 7) gives
>the day of the week counting Sunday as 0.  So Julian day number 0 was a
>Monday.  Hmmm.

All will now be revealed!

In 1583 a French scholar Joseph Scalinger devised a calender system which 
counts the days in order to match up the chronology of historical events of 
different civilisations. Scalinger desired to make day 1 in his system have 
some calendrical signifigance and therefore chose day 1 to be

1) Sunday January 1st          (once every 7 years)
2) a leap year                 (once every 4 years)
3) a new moon                  (once every 19 years = 235 lunar months)
4) a Roman indication (census) (once every 15 years)

As all these numbers are mutually prime this means this "Julian-cycle" is 
7x4x19x15=7980 years long. The last such date, Scalinger calculated was 
January 1 4713 B.C. (note 4713 B.C. is a leap year though not divisible by 4 
as there is no year 0 between 1 B.C. and 1 A.D.). This date suited 
his purposes as recorded history came after this date.

In 1582, Pope Gregory XIII recognised the year is not 365.25 days long but 
365.2422 days and decreed that ten days which had been accumulated over a 
thousand years be dropped thus the day after 4 October 1582 was 
15 October 1582. This "Gregorian" calender took a while to be recognised 
in Protestant countries but it was eventually universally accepted. Thus 
Scalinger's day 1 no longer has its significance in our modern calender 
system, but this doesn't matter once it is chosen - one simply counts the 
days without regard to calender rules.

P.S. in 1650, the Archbishop James Ussher of the Anglical Church calculated 
the creation of the earth took place at 8pm on October 22, 4004 B.C. which 
is where the other date came from. He didn't specify if this was G.M.T. but 
maybe he assumed a flat earth.

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 David C. Hutchings            JANET    -   dave@uk.ac.hw.phy.s
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