The Julian Period isn't associated with the Julian calendar, and must not be confused with one another. The Julian Period was created by the French scholar Josef Justus Scaliger (1540-1609). The name Julian Period is probably after the Julian calendar, although it is possible Scaliger named it after his father Julius Caesar Scaliger.
Scaliger's goal was to create a method of determining year with each year being positive, so as not to worry about AD and BC. The period starts at January 1, 4713 BC, and last for 7980 years, where it will repeat on January 1, 3268.
Why January 1, 4713 BC and 7980 years. Because on January 1, 4713 BC, the Indiction, the Golden Number, and the Solar Number all become 1, and this won't repeat for 15 * 19 * 28 (7980) years.
Scientist often use the Julian period for giving a number to each day. 12:00 PM UTC, January 1, 4713 BC for 24 hours marks Julian Day 0, each day that follows adds another number. Therefore, at 12:00 PM UTC, January 1, 2000, the Julian Day would become 2,451,545.
Calculating the Julian Day of 12:00 PM UTC, January 1, 2000 is simple:
- From 4713 BC to 2000 AD, there are 6712 years.
- The year in the Julian calendar is 365.25 days, so 6712 * 365.25 is 2,451,558 days.
- For the Gregorian calendars, subtract 13 days to convert from Julian to Gregorian, so you get 2,451,545
Often the Julian Day is used in fractions to determine the time of day as well.
The Julian Date Number starts at 12:00 PM UTC on the given date.
(The divisions are integers, with the remainders discarded. )
For a date in the Gregorian calendar:
Julian Date Number = day + (306*M+5)/10 +Y*365 + Y/4 - Y/100 + Y/400 + 1721119
For a date in the Julian calendar:
Julian Date Number = day + (306*M+5)/10 +Y*365 + Y/4 + 1721119
This works fine for AD dates. For BC dates, you must convert BC year to a negative year (10 BC = -9), and make sure you round down instead of up (-9/4 = -3, not -2).
Sometimes a Modifies Julian Day number is used in replace of the standard Julian number. In the MJD, 2,400,000.5 days are subtracted to make the number more manageable, and the numbers change at midnight rather than noon. MJD 0 falls on November 17, 1858 of the Gregorian calendar.