fredag 16 februari 2018

Calendar confusions

I have taken down my Christmas lights at last, yesterday being the Feast of the Presentation, 2nd February, which marks the end of the Christmas season. Except that yesterday was 15th February. It was 2nd February on the Julian calendar, which is now 13 days behind the Gregorian calendar in secular use.

Muslims use a lunar calendar with 12 months, but the months are  28 or 29 days long. The year is shorter than the real year, and feasts like Ramadan are 11 or 12 days earlier each year. At the moment, Ramadan is in the middle of the summer, which would be tough on those near the Arctic circle if the rules were not relaxed.

The Jewish calendar is also a lunar calendar but extra months are added according to a 19 year cycle of leap years. The extra month, called Adar Sheni, the Second Adar, is in the spring, and is inserted on the 3, 6, 8, 11, 14, 17, and 19th years of the cycle. In practice the Jewish calendar is more complicated than that. Passover is on 15th Nisan, the month after Adar. In leap years, Nisan is after Adar Sheni, which brings Passover into late April if there is a leap year. The earliest date for Passover is 26th March.

The Jewish calendar keeps reasonably good time, with a drift of one day every 231 years, about 4 days per millennium. It will be several millennia before Passover is so late in the year as to be a problem. There is no need for anyone alive today to worry about it.

The Julian calendar is worse, with a leap year every four years and a drift of 12 days per millennium. It came about like this. A solar year is reckoned as 365 and a quarter days; after four years, an extra day is added to the year. Each year is 11 minutes too long, a discrepancy which builds up over time. This drift had been noticed by the early middle ages but it took a long time to devise a way of fixing the problem.

The solution adopted was to make an adjustment every hundred years, by not having a leap year unless the year was divisible by 400. Thus 1600 and 2000 were leap years but 1700, 1800, and 1900 were not, and 2100 will not be. The calendar was first introduced in 1582 and its use spread gradually. By the time it was adopted in Britain, in 1752, 11 days had to be skipped; 2th September 1752 was followed by 14th September 1752. This change is the reason why the British tax year ends on 5th April; it is the old Lady Day, 25th March.

Thus the Gregorian calendar has 238 leap years per millennium compared to the 250 leap years of the Julian calendar. The separation of 12 days per thousand years is not desperate but becomes appreciable with the passing of the centuries. 

The trouble with the Gregorian calender, however, is that, every few years, 2005 and 2008, for example, Easter can be one month before the Jewish Passover. This matters from a theological perspective because the Last Supper was on 14th Nisan, the night before the Jewish Seder on the 15th. The earliest possible date for Easter is 22th March is but there will not be another until 2353; about once a century, Easter is on 23th March but it is not unusual for Easter to be a month before Passover.

The Julian calendar still used by some of the Orthodox churches avoids Easter from coming before Passover, but is drifting to the point where it is sometimes, and increasingly, in May; if the calendar is still in use, it will be on 10th May in 2268. This is of course nothing that any of us alive today needs to bother ourselves about, but at some time it would be a good idea to skip a fortnight and bring the Julian calendar back into line if the churches want to carry on using it. There is a lot to be said for not adopting the Gregorian calendar with its breaking of the link to the Jewish Passover.

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