William Hamblen <william.hamblen@nashville.com>

It's the number of days since noon GMT 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reconciling the dates in historical documents.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, Julian day 2450000 began at noon on 1995 October 9.
Because Julian dates are so large, astronomers often make use of a
"modified Julian date"; MJD = JD - 2400000.5.  (Though, sometimes 
they're sloppy and subtract 2400000 instead.)