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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Steamboat Springs — Have you ever wondered why the month of February has only 28 days most years, but occasionally has 29 days, as it did this year? This whole leap year thing started in the days of the Roman Empire under the rule of Julius Caesar. Astronomers, even that long ago, realized there were not a whole number of days in a year. In fact, there are about 365 and 1/4 days in a year. Since it wouldn't make any sense to have the last day of the year be only one-fourth of a day long, Julius Caesar decreed in 44 B.C. that we would let the one-quarter days accumulate and then add in a whole day, a leap day, every fourth year, sticking it at the end of February.

This calendar reform by Julius Caesar assumed the year was exactly 365.25 days long, but it isn't. It's really 365.2422 days long, about 11 minutes and 14 seconds shorter than assumed by Julius Caesar. So, using Julius Caesar's method, we were adding in too many leap days throughout the years. By the 1500s, all those 11 minutes and 14 seconds had added up to a full 10 days on the calendar, causing the vernal equinox, or first day of spring, to shift from March 21 to March 11. If that error continued, we'd eventually be celebrating the first day of spring in December!

In 1582, Pope Gregory XIII tried to rectify this situation by reforming the Julian leap year system. First, he threw 10 days out of the calendar (Oct. 5, 1582, became Oct. 15, 1582!) to bring the date of the vernal equinox back to March 21. He then declared that any year evenly divisible by four would remain a leap year, unless it was a century year such as 1900 or 2000. A century year must be evenly divisible by 400, not four, in order to be a leap year. So, the century year 1600 was a leap year, but the century years 1700, 1800 and 1900 were not. The century year 2000 was again a leap year. Using Pope Gregory's method, we leave out three leap days every four centuries and keep the calendar in sync with the seasons.

The Gregorian reform assumes that a year is 365.2425 days long, very close to the actual length of 365.2422 days. It will take 3,300 years for this small discrepancy to add up to a full day, so we needn't worry about it for a long, long time.

Why did the extra leap day get added to February instead of some other month? The original Roman calendar only had ten months in it: Martius, Aprilus, Maius, Iunius, Quintilus (the "fifth month"), Sextilus (the "sixth month"), Septembris (the "seventh month"), Octobris (the "eighth month"), Novembris (the "ninth month"), and Decembris (the "tenth month"). Januarius and Februarius were added later, and Februarius was only given 28 days, making it the shortest month and the last month of the calendar year. It seemed logical to tack the extra leap day onto the end of the calendar year, giving us an occasional Feb. 29. Eventually, the first day of the new year was shifted from Martius 1 backward to Januarius 1. Quintilus was renamed July to honor Julius Caesar, and Sextilus was renamed August to honor Augustus Caesar, and we are left with our current calendar where Decembris (the "tenth month") is actually the twelfth month. Weird.

I wonder when all of the people born on Feb. 29 celebrate their birthday in non-leap years?

Oh, and don't forget to set your clocks ahead (spring forward) one hour at 2 a.m. March 9 when we go back on daylight-saving time, six weeks earlier than usual. (sigh)

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day" Web site, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His "Celestial News" article appears weekly in the Steamboat Pilot & Today. His "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out his Web site at www.jwestlake.com.