Julian dates

Mark Wieder mwieder at ahsoftware.net
Fri Apr 16 03:07:36 CDT 2004


Thursday, April 15, 2004, 1:33:56 PM, you wrote:

DS> Uh, I mean URLs that give me confidence that there really is such thing
DS> as Julian day.  I guess I can look those up myself; I have some info on
DS> Julian day as astronomers use it.

Like this? From the Time FAQ for sci.astro:

What's a Julian date?

It's the number of days since noon 4713 BC January 1. What's so
special about this date?

Joseph Justus Scaliger (1540--1609) was a noted Italian-French
philologist and historian who was interested in chronology and
reconciling the dates in historical documents. Before the western
civil calendar was adopted by most countries, each little city or
principality reckoned dates in its own fashion, using descriptions
like "the 5th year of the Great Poo-bah Magnaminus." Scaliger wanted
to make sense out of these disparate references so he invented his own
era and reckoned dates by counting days. He started with 4713 BC
January 1 because that was when solar cycle of 28 years (when the days
of the week and the days of the month in the Julian calendar coincide
again), the Metonic cycle of 19 years (because 19 solar years are
roughly equal to 235 lunar months) and the Roman indiction of 15 years
(decreed by the Emperor Constantine) all coincide. There was no
recorded history as old as 4713 BC known in Scaliger's day, so it had
the advantage of avoiding negative dates. Joseph Justus's father was
Julius Caesar Scaliger, which might be why he called it the Julian
Cycle. Astronomers adopted the Julian Cycle to avoid having to
remember "30 days hath September ...."

For reference, noon on 1995 October 9 was Julian day 2450000.

Note that this has nothing to do with the Julian calendar:

The Julian calendar, instituted by Julius Caesar (who else?), has a
365-day ordinary year with a 366-day leap year every fourth year. This
gives a mean year length of 365.25 days, not a very large error.
However, the error builds up, and by the sixteenth century, reform was
considered desirable. A new calendar was established in most Roman
Catholic countries in 1582 under the authority of Pope Gregory XIII;
in that year, the date October 4 was followed by October 15 -- a
correction of 10 days. Most non-Catholic countries adopted this
"Gregorian" calendar somewhat later (Great Britain and the American
colonies in 1752), and by then the difference between Julian and
Gregorian dates was even greater than 10 days. (Russia didn't adopt
the Gregorian calendar until after the "October Revolution" -- which
took place in November under the new calendar!) Many of the calendar
changeovers elicited strong emotional reactions from the populations
involved; people objected to "losing ten (or more) days of our lives".

-Mark Wieder
 mwieder at ahsoftware.net

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