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From: tth@dhruva.caltech.edu (Thomas Hamilton)
Subject: Re: Why AD Latin, BC English? - summary & follow-up
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Date: Fri, 14 Oct 1994 20:54:24 GMT
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In article <37m6d7$qrq@nntp1.u.washington.edu> aardvark@ial4.jsc.nasa.gov (Michael W. Martin) writes:
>In Article <mccombtmCxMJ0J.9s2@netcom.com>, velde@jhunix.hcf.jhu.edu
>(Francois Velde) wrote:
>
>>No, you'd need a Nova explosion, or Halley's comet (renamed, of course,
>>to remove that DWEM's name); making sure the event was observable by
>>the whole planet.
>
>    Actually this 'renumbering' to a 'culturally neutral' system has already
>been done by astronomers. It is called the Julian Date. (Julian being the
>father of the inventor, not one of the Caesars). It was designed to provide
>a stable time standard over more or less recorded human history. 
>
The Julian date system is based on a "great year" which is equal to 
7980 ordinary years.  The number 7980 is the product of the 28 year period
with which a day of the week falls on the same date in the Julian calender,
the 19 year Metonic period with which the phases of the moon approximately
repeat, and the 15 year indiction cycle which was used for taxation purposes
by the Roman Empire.  The Julian date system is therefore not "culturally
neutral".  In fact, it is quite weird.  It was introduced in 1583, during a 
period of when calendar reform was much discussed.  Indeed the year before
Scaliger introduced the nutty Julian Period, the church had adopted
the Gregorian calender.  I don't know if anyone besides astronomers ever
took the Julian Period seriously.  I suspect JD caught on because astronomers
needed to record dates without taking sides in the Julian/Gregorian
controversy which was part of the 17th century wars of religion.

The use of Julian Date is by no means universal amoung today's astronomers, many of
whom prefer the Modified Julian Date, which is offset by 12 hours.  
In astronomy, the whole subject is a bit of an embarassment, especially
when students ask why we care about the Roman tax cycle.
