Julian Day numbering was invented in 1583 by the French scholar, Joseph Justus Scaliger. He constructed the Julian Period, an interval of 7980 years, based on three cycles of years: the 28-year solar cycle (the time taken before the days of the week next align with the Julian year), the 19-year lunar cycle (when the phase of the moon aligns with the days), and the 15-year Roman indiction cycles (used for taxation, census and other legal purposes which continued to be used in to the Middle ages). Assuming all dates are reckoned in the Julian calendar, for which it was named, the Julian period began at 12 noon, 1st. January 4713 BC and will end end at 12 noon, 1st. January 3268 AD, when all three cycles will once again co-incide. Days are numbered consecutively from zero within the Julian Period, without any subdivisions in to months or years.

Here are some Julian days for some interesting dates:

Gregorian Date Julian Day MJD Noon 1752-09-14 2361222 Noon 1858-11-16 2400000 Midnight 1858-11-17 2400000.5 0 Noon 1858-11-17 2400001 0.5 Midnight 1900-01-01 2415020.5 15020 Noon 1996-09-03 2450330 50329.5 Midnight 2000-01-01 2451544.5 51544 Midnight 2100-01-01 2488069.5 88069 Midnight 2132-08-31 2499999.5 99999 Midnight 2132-09-01 2500000.5 100000

Although the Julian Day is very useful for astronomical purposes, it does have some drawbacks:

- it begins at noon, rather than at midnight as is civil convention. This offset of 0.5 day makes it awkward to talk about calendar days as single Julian day numbers.
- it is rather long, with all the dates in the current and next centuries beginning with the decimal digits``24''.

For several years since his triumph over Pompey in 705 AUC (49 BC), Julius Caesar had been in increasingly complete command of Rome, regarded as semi-divine, and Emperor in all but name. Caesar was also a gifted astronomer, and well aware of the difficulties the existing calendar caused. He decreed that 708 AUC (46 BC) should be lengthened to 445 days in order that his new calendar be inaugurated in step with the constellations on 1st. January, 709 AUC (45 BC). His new calendar had a year of 365 days plus a leap year every fourth year with 366 days. This made the average length of the year 365.25 days, a good approximation to the real length of 365.24219 days, and the Julian calendar continued almost unchanged for many centuries.

Julius Caesar's nephew and adopted heir, Gaius Octavian Augustus (691--767 AUC, 63 BC--14 AD) became first Roman Emperor in 727 AUC (27 BC) and renamed the month Quinctilis to Augustus and stole a day from February to give his month 31 days. The base date for the calendar was moved from the founding of Rome to various dates of local importance, and it was not until the 6th. century AD when the scholar Dionysius Exiguus made a study of Easter days that the Anno Domini epoch we now use came in to being.

This page was created on MJD 50329. It was last updated MJD 50376.9.

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