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Julian Day number JavaScript calculator

Jean Meeus, in his book Astronomical Algorithms, defines the Julian Day number as "a continuous count of days and fractions thereof from the beginning of the year -4712". Meeus includes a "zero year" in his book's calculations, since it's easier to program a computer to count 1 BC, 0, AD 1, etc. than to skip over zero as historians usually prefer. The beginning of a Julian Day starts at noon UTC (or GMT). And the starting point is based on a backward extension of the Julian Calendar.

In other words, Julian Day 0.0 corresponds to exactly 12 hours on January 1, 4712 BC under the Julian Calendar system when you include a zero year when extending the calendar deep into history. If for your purposes, you need to exclude a zero year, Julian Day 0.0 occurred on January 1, 4713 BC at noon. In either case, you should see above that more than 2.4 million days have since passed.

This is the astronomer's version of an absolute calendar. This is used because months and years aren't all of equal lengths. It often makes more sense to count the number of days between events and make things correspond to a common calendar later. One may also count Julian period years. 2002 is number 6715.

According to Collier's Encyclopedia, French chronologist Joseph Scaliger devised the Julian period and Julian Day number in 1582. He multiplied the 28-year solar cycle, the 19-year Metonic cycle, and the 15-year Roman Indiction cycle and determined that they began together on the same day every 7,980 years under the Julian Calendar in use at that time. The last time was in 4712 BC. The next time will be in AD 3267. Scaliger named the period for his father, Julius Caesar Scaliger. The Julian period has nothing to do with the Julian Calendar except that it follows the Julian Calendar's leap year rule (one leap year every four years without exception) when counting those 7,980 years.

If you want to look at the formulae Meeus published for the operations carried out above, look at the source code. You can copy the code if you want to. Meeus wrote the book for computer programmers, so I don't think I'm violating the copyright by putting the program here. On the other hand, he wrote the book before the Internet was publicly available so he may not have envisioned people distributing their programs like this. If you like calendars or astronomy and want to write your own programs, I recommend you get your own copy of the Meeus book.