The Gregorian Calendar

~~ Paul V. Hartman ~~

       Ancient people based their calendars on the cycle of the Moon, which they put at 30 days. Since the actual rotation interval is 29.53 days on average, such calendars were obliged to go out of sync, the usual remedy being to insert a "month" of a certain number of days into the year somewhere. The early Egyptians may have been the first to employ a solar year of 365 days because of those lunar inaccuracies, using 12 months of 30 days with 5 days tacked on as a short month at the end. The early Romans employed a calendar of 10 months of 30 days: Mars, April, May, June, Quintilis (5th), Sextilis (6th), September (7th), October (8th), November (9th), December (10th). In 700 BC, January and February were added at the beginning, yielding a year of 360 days. Had they been added at the end, we would not now have months named for numbers, such as December ("dece" = 10), all being "off" by two places.

When Julius Caesar inaugurated the "Julian Calendar" in 45 BC, it was already known that the time of a complete year - a full journey of the Earth around the Sun (although thought to be a full journey of the Sun around the Earth) was a fraction more than 365 days. The existing calendar was about 3 months off in 45 BC, with Winter occuring in September. Caesar called upon Sosigenes, a Greco-Egyptian scholar/astronomer, to make suggestions. He calculated a transit of 365.25 days and recommended adding a day in every year which was divisible by 4. (Inaccurate, but it is highly unlikely that anyone could have done better, at that time, or offered a better solution.)

Thus there were three years with 365 days and a fourth with 366 (the "Leap Year"), the extra day being plugged into February just before the 25th of the month. To straighten out the seasons immediately, Caesar declared that 46 BC would have 445 days, which the Romans called "the year of confusion." He also took a day from February to give "his" month, the month of his birth - formerly Quintilis (5th month) but now renamed "Julius" an extra day: 31. His nephew, Octavius, who would become Caesar Augustus, did the same, renaming Sextilis "Augustus", stealing another day from February, leaving it with 28. Thus "tweaked", the Julian Calendar would serve Western Civilization for more than 16 centuries.

365.25 was pretty close, but no cigar. The actual trip took 365.24232 days, a discrepancy of 11 minutes and 4 seconds. By the year 1582, the calendar was off by 11 days, and the reason that this was important to the Catholic Church and Pope Gregory XIII in 1582 was that if the calendar were not corrected, Easter Sunday would eventually be celebrated in Summer.

Easter Sunday is a movable feast, and is determined by a formula based on the vernal equinox (vernal="spring", equinox="equal day and night") which traditionally was March 21. In order to immediately correct the 16 century drift, it was elected to remove 10 days from the year in 1582, and to do so, the new "Gregorian Calendar" would officially begin the day after October 4th, a Thursday, as October 15, a Friday. October happened to have, in the 16th Century, the fewest "official events", and so the choice of October was least disruptive to the church and the choice of a Friday/Friday substitution least disruptive to business. The extra day every fourth year was to appear at the end of February.

Of course, a calendar for all of Western Civilization is of little use if all countries do not adopt it, so to insure that each did, the Pope declared that anyone not accepting the new calendar would be excommunicated.

However, the Protestant Reformation was in full flower, and the debate on calendar reform was religious as well as academic. On the academic side, the debate had to do with actual transit time, and whether the Gregorian plan of curbing equinoctial drift by deleting Leap Years from 3 out of 4 centurial years (1700, 1800, and 1900 were the first three selected) would be sufficiently accurate. It would not, but the weight of argument eventually settled on what was simplest and most practical.

Religious differences made several states retain the Julian calendar. Germany did not adopt the Gregorian until 1700, Great Britain until 1752, Russia until 1918.

No calendar can be accurate if the transit time of the earth includes a fractional day, as it does, so periodic corrections are necessary. In addition, the transit time of the Earth is slowing, it being 365.24232 in 45 BC but 365.24298 now. (Put another way, because the transit of the Earth around the Sun is slowing, the transit time is getting longer, thus the larger number.)

Using the current calendar arrangement, a one-day correction will not be required until the year 4317 AD. Thus this calendar represents a very practical compromise between scientific accuracy and desired simplicity.

That's the whole story in one page, and all you need to know.

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