Leap Year: Invented by Caesar; Perfected by a Pope

As I slog into work on Monday mornings and try to repair my mood with a cup of Geophysical Institute coffee, I often wish in vain for an extra day. I'd fill it with sleep and other activities I failed to accomplish during the weekend.

Because 1996 is a leap year, my wish will be granted on Feb. 29, thanks to Julius Caesar, who established the leap year as part of his Julian calendar in 45 B.C.

Caesar's astronomer, Sosigenes, developed the Julian calendar based on the fact that it takes the earth 365 days, 5 hours, 48 minutes and 46 seconds to revolve around the sun. This time was abbreviated to 365 1/4 days, and a calendar year was defined as 365 days, with one "leap day" added every four years to compensate for the lost quarter day.

But a nagging, although slow-developing, problem arose: 5 hours, 48 minutes and 46 seconds isn't exactly a quarter day. In 730 A.D., the Venerable Bede, a mathematically skilled Anglo-Saxon monk, pointed out that the 365 1/4-day Julian year was 11 minutes, 14 seconds too long. But since the error added up so slowly (the calendar fell out of sync with the sun by only a day every 128 years) no one did anything about it.

As time ticked on, people began noticing the flaws of the Julian calendar. In 1582, Pope Gregory XII noticed that the spring equinox---when day and night are of equal length---fell upon March 11 instead of March 21.

The pope fixed the problem by erasing 10 days, declaring that the day following Oct. 4, 1582 would be known as Oct. 15, 1583. To make the calendar more sun-accurate than Caesar's, Pope Gregory XII pulled out his abacus and calculated the following leap year restrictions (I'd advise a swig of coffee here): If the first year of a century is divisible by 400, it is a leap year; if it's not, then that year isn't a leap year.

The pope's mathematical modification knocked out leap years in 1800 and 1900, but in 2000 we'll have another one, because 2000 divides evenly into 400. Unless it's a centennial year, you can determine a leap year without looking at a calendar by dividing it by four. It if divides evenly, with no reaminder, it's a leap year. If you're lazy, like me, you can also flip through a calendar in search of February 29.

Pope Gregory XII's calendar, known as the Gregorian calendar, now hangs on most of our walls. It wasn't as popular when it was first developed, though.

Although most Roman Catholic countries adopted it at once because it recalibrated the beginning of spring and restored Easter to its proper time, Protestant countries didn't make the change for 200 years. England resisted the switch until 1752, and the loss of 11 days caused by the date adjustment spurred riots in the streets.

Russia didn't accept the Gregorian calendar until 1918, which means that when the U.S. purchased Alaska in 1867, 11 days were lost in the transition from the Julian calendar.

Leap years are also significant for Alaskans because summer solstice occurs about 18 hours earlier in a leap year. Since we have an extra day this year, summer solstice---when the earth tilts Alaska nearest the sun---happens at 5:24 p.m. on June 20th instead of on the traditional June 21.

I'll try to keep that sunny thought in mind next Monday morning.