sourcery (87455) writes "At noon of 21 September 2006 begins Julian Day Number 2,454,000. A day and a half later, the Autumnal Equinox will occur (2006-09-23T04:03 Universal Time.)
The Hebrew New Year starts at sundown on 22 September (a.k.a Rosh Hoshana.) Note that the first month of the Hebrew year is traditionally known as the seventh month, not the first month.
Julian Day Numbers are an integer count of days since a specific epoch date. Julian Day Zero begins at Noon on 14 November 4714 BC, according to the proleptic Gregorian Calendar — or at Noon on 1 January 4713 BC, according to the proleptic Julian Calendar. Julian Day Zero was a Monday.
A Julian Date is a count of days, including any fractional part of the day, since -4713-11-24T12:00:00+0000 (24 Nov -4713 12:00:00 Universal Time, using Astronomical year numbering, where the year prior to the year 0001 is the year 0000, and not the year 1 BC, as would be traditional.)
It is common, but nevertheless technically incorrect, to refer to an ordinal date (Year-DayOfYear) as a "Julian Date."
Using a Julian Day Number to specify a date, or a Julian Date to specify a precise point in time, is useful for two reasons:
1) It permits dates to specified without reference to any particular calendrical system; and
2) It vastly simplifies astronomical calculations, and other computations where the amount of time between two dates needs to be computed.
The Julian Day system of specifying dates was invented by the astronomer Joseph Scaliger in 1583 (the year after the Gregorian Calendar Reform was put into effect in the Catholic countries of Europe.)
The epoch day of the Julian Day system was chosen to be the most recent day on which three calendrical cycles all were at their respective zero points. The three cycles are the 15-year Indiction Cycle (important in Roman tax law,) the 19-year Metonic Cycle (important for obtaining approximate synchronization of lunar and solar calendars,) and the 28-year Solar Cycle (all possible patterns of Julian Calendar dates and days-of-the-week recur once every 28 years.)
The "Julian" in "Julian Day" refers to Scaliger's father, and not to either Julius Caesar or to the Julian Calendar."