This week saw, not just a once-in-a-lifetime event, but a once-in-four-centuries event: the first February 29 to fall in a year ending in 00 since 1600.
The three basic elements of the calendar are the day, month and year. The day is the time it takes the Earth to rotate once: 24 hours. The month approximates the time it takes the moon to change from new moon to full moon to new moon again: 29.53 days. The year is the time it takes the Earth to complete one orbit around the sun: 365.2422 days.
The first calendars were purely lunar: a new month was said to have begun whenever religious authorities saw a crescent moon. Unfortunately, 12 lunar months is only 354.36 days, almost 11 days shorter than a year. Dates in a lunar calendar, therefore, rapidly move backward through the seasons (as they still do in the Islamic calendar), so that a date in summer eventually ends up in winter, and vice versa. This makes it difficult to use the calendar to plant crops or schedule seasonal religious celebrations.
Early societies corrected their lunar calendars by throwing in an extra month or a few extra days whenever it seemed necessary. For example, the Egyptians held a five-day feast at the end of each year (not such a bad idea, if you ask me). There were literally hundreds of variations of these “lunar-solar” calendars; the Jewish calendar is one.
The Roman Empire originally used a lunar-solar calendar, but it ended up in a hopeless mess because the people charged with keeping the calendar started fiddling with it to lengthen their terms of office or hasten or delay elections. (Calendars change; politicians do not.)
Julius Caesar, in consultation with the Egyptian astronomer Sosigenes of Alexandria, put an end to the confusion with the “Julian” calendar. It divided the solar year into 12 months of slightly varying lengths, to ensure that the year had 365 days. The year 46 BC was given 445 days to get the new calendar off on the right foot.
The Julian calendar set the start of the new year as January 1; before that, it was in March, which made September, October, November and December the seventh, eighth, ninth, and 10th months–hence their names. February had 29 days most years, but every fourth year an extra day, February 30, was added to compensate for the fact that the year is actually just a little longer than 365 days.
February 30, however, never rolled around. While creating the calendar, Julius named a month after himself–July. Two years later he was dead. The new Caesar, Augustus, decided to name a month after himself, too. The month he chose was one day shorter than July–but not for long. To keep up with Julius, Augustus stole a day from February and added it to August.
A leap year every four years would fix everything if the year were exactly 365 days and six hours long–but it’s 11 minutes short. After 1600 years, that discrepancy had caused the Julian calendar to drift almost two weeks out of phase, so that religious festivals were again starting to slip into the wrong seasons, especially Easter, whose timing is connected to the Jewish lunar calendar.
This concerned Pope Gregory XIII. Like Julius Caesar, he went to an astronomer, Christopher Clavius, for a solution. The “Gregorian” calendar fixed the Julian calendar by making years ending in 00 leap years only when they are divisible by 400. Not having a leap year in most years ending in 00 slightly shortens most centuries, correcting for the extra time added by ordinary leap years, but it shortens them a bit too much; having a leap year in years divisible by 400 corrects for that over-correction.
The new calendar was installed in 1582 in Catholic countries, and the accumulated error of the Julian calendar was eliminated by decreeing that Wednesday, October 4, 1582 (Julian) was followed by Thursday, October 15 (Gregorian).
This year’s once-in-four-centuries leap day caused a few minor problems as computers around the world failed to recognize February 29, 2000, as a valid date. In Japan, for instance, some weather monitoring stations reported double-digit rainfall even though no rain fell, and in New Zealand, merchants had trouble verifying banking transactions. Most computers, however, were reprogrammed to recognize February 29, 2000, at the same time they were being recognized to recognize 2000.
But please remember, computer programmers, there won’t be a February 29 in 2100. You might want to leave a few sticky notes for your descendants.