The julian date changes at noon, a time when astronomers were usually asleep.
The Julian *Day* changes at noon. "Julian date" is an ambiguous term
that can refer to "Julian Day including time fraction" or "date in the
Julian calendar" or "day of the year". It's best avoided.
More specifically, the Julian Day is always counted in Universal Time,
so it changes at noon UT. (Astronomers use different varieties of UT
for different purposes, with corresponding varieies of JD, but for
civil use it uses the civil time, UTC.)
Noon vs midnight, btw, is the reason the term "GMT" is deprecated,
since "0:00 GMT" can mean either noon or midnight UTC depending on the
context date.
The Gregorian calendar was instituted in 1582. For those who followed
Pope Gregory's edict, Thursday, October 4, 1582 was followed by
Friday, October 15, 1582 - a 10-day jump. England and her overseas
colonies didn't get around to adopting the new calendar until almost
200 years later. By then, the year 1700 (a leap year in the Julian
but not Gregorian calendar) had added another day's difference, so
when the switch was made there was an 11-day jump: Wednesday,
September 2, 1752 was followed by Thursday, September 14. This is the
date switch used by the UNIX cal(1) program, so if you open a Terminal
window and type "cal 9 1752" you can see the jump.
Since then, the years 1800 and 1900 have added another two days to the
difference, so the Julian calendar is currently 13 days behind the
Gregorian, where it will stay until 2100.
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