13 Feb Joel Achenbach

Early civilizations in the Near East built their calendars around the moon. Since a lunar month is about twenty-nine days, these ancient calendar months had either twenty-nine or thirty days. The Etyptians used a solar calendar, dividing the year into twelve months of thirty days each, with five or six extra days at the end of the year. The ancient Greeks, meanwhile, came up with a baffling system in which ten years of thirteen months each were interspersed among seventeen years of twelve months each. Come up with a rhyme for that, Plato.

In the year we would call 47 B.C., Julius Caesar threw out the moon-based Roman calendar and instituted what is, approximately, the calendar we use today, only more sensible. The first month, March, had thirty-one days. The second month, April, had thirty days. May had thirty-one. The number of days alternated between thirty and thirty-one. The final month, February, had thirty days only in the leap year; otherwise, twenty-nine.

Caesar decided that the calendar would go into effect on January 1 (“Ianuarius I”), which he decreed would be the day of the first new moon after the winter solstice. Bad move. New moons and solstices are in no way synchronous, so we now have a New Year’s Day with no astronomical significance, stuck in the dead of winter.

After Brutus and his ilk murdered Caesar, the Roman Senate decreed that the fifth month, Quintilis, would thereafter be called Julius. Then, in 8 B.C., during the reign of Augustus Caesar, the Roman Senate had another fit of obsequiousness and decreed that the sixth month, Sextilis, would become Augustus. This however, posed a potential embarassment: Julius had thirty-one days, and Augustus only thirty. So the Senate Stole the last day of the year, February 29, and affixed it to August. Politics.

The calendar still wasn’t quite right. The astronomical year is about eleven minutes and fourteen seconds short of 365 days. Over the centuries, this slight discrepancy caused the spring equinox, by which the date of Easter is determined, to drift inexorably toward February from its official date of March 21. By A.D. 1582 the real solar equinox was arriving on March 11. So Pope Gregory simply zapped ten days out of existence, and October 4, 1582 was followed by October 15. The Gregorian Adjustment included the proviso that, in the future, the leap year would not be observed at the ccentury mark, unless the year is divisible by 400. Thus, there was no February 29, 1900, but there will be a February 29, 2000.

The Gregorian calendar isn’t perfect. It’ll be a day off in three thousand years. Our advice: Start planning now.