Why do Catholic, Protestant and some Christian Orthodox Churches celebrate Christmas on 25 December, while some other Christian Orthodox Churches celebrate it on 7 January, 13 days later?
Everyone knows the answer – the majority of Christian Churches use the Gregorian calendar, while some Christian Orthodox Churches (including the Georgian, Russian and Ukrainian Christian Orthodox Churches) keep to the Julian calendar; and currently the Julian calendar is 13 days behind the Gregorian calendar. Also the Gregorian calendar is now almost universally used in secular life around the world.
The Julian calendar was introduced by Julius Caesar in 46 BCE. In it, leap years each fourth year were introduced, and it was a substantial improvement in comparison with previous calendars, which accumulated an error of 1 day every four years. However, it accumulated an error of 3 days in every 400 years. By 1582, when Pope Gregory XIII introduced a much more accurate calendar, this error was 10 days. Now it is 13 days, and it will become 14 days in the 22nd century, 15 days in the 23rd century, and so on.
The correction introduced in the Gregorian calendar was as follows:
- Years that are integer multiples of 100 (that is, divisible by 100 without remainder) are not leap years, even though they are also integer multiples of 4;
- Years that are integer multiple of 400 are leap years, even though they are also integer multiples of 100.
For instance, years 1700, 1800 and 1900 were not leap years, while years 1600 and 2000 were leap years. Years 2100, 2200 and 2300 will not be leap years, while 2400 will be.
The Gregorian calendar is still not absolutely correct, but it accumulates an error of one day in around 15 thousand years, and this can be corrected by introducing an additional day in a particular year in the far-away future.
