Calendars, a story of mistakes

As it was noted elsewhere, in the English speaking world, the monthly calendar for September, 1752 is unusually short with a gap of 11 days between September 2 and 14:

Su Mo Tu We Th Fr Sa
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

On September 2, 1752 England and its colonies finally adopted a calendar reform proclaimed by Pope Gregory XIII in October, 1582. The need for the reform stemmed from the error made by the Alexandrian astronomer Sosigenes when evaluating the duration of the solar year.

If ever asked: What's better, the Sun or the Moon? -
reply: The Moon. For the Sun only shines during daytime
when it's light anyway whereas the Moon shines at night.
But, on the other hand, the Sun is better in that it shines
and also warms whereas the Moon only shines; and that
during bright nights only.
Kozma Prutkov

The old style Julian calendar was established by Julius Caesar in 45 B.C. as a reform of the Roman republican calendar (reckoned from the founding of Rome). Caesar, advised by Sosigenes, made the new calendar solar, not lunar. The length of the solar year was estimated at 365.25 days which was off by about 11 minutes. The year was divided into 12 months, all of which had either 30 or 31 days except February, which contained 28 common (365-days) years and 29 days in every fourth (leap, 366-days) year. The calendar went into smooth operation in 8 A.D. (By another mistake, from 45 B.C. through 9 B.C. every third year was counted as leap to amend which leap years from 9 B.C. through 8 A.D. have been all canceled.) The current reckoning of years starting with the Christ's birth was not introduced until the 6th century. It's kind of surprising that leap years are divisible by 4.

By 1582, the cumulative effect of 11 minutes error has shifted the dates of the seasons by 13 days from Caesar's time. Pope Gregory XIII's reform reclaimed only 11 of the lost 13 days so that the date of the vernal equinox was restored to March 21, the date it had at the time of the Council of Nicaea, 325 A.D. With this reform we have fewer leap years too. In general, as before, every fourth year is leap except for those that are divisible by 100 but not by 400. So, for example, 1996 and 2000 are leap whereas 1900 and 2100 are not. The fact is, however, that, if all years divisible by 4,000 are denied their exceptional leap status, we would get even better conformity with actual time measurements. So far, no decision has been made to this effect. There is time yet.


  1. Encyclopædia Britannica

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