Monday, February 20, 2012
Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.
Find more columns by Westlake here.
Have you ever wondered why February has only 28 days most years but occasionally has 29 days, as it does this year?
This whole leap year thing started back in the days of the Roman Empire under the reign of Julius Caesar. Even that long ago, astronomers realized that there weren’t a whole number of days in a year. The Earth rotates 365 and one-quarter times in a year. Because it wouldn’t make any sense to have the last day of the year be only six hours long, Julius Caesar decreed in 44 B.C. that we would let the quarter-days days accumulate and then add in one whole day — a leap day — every fourth year. This would take up the slack between our calendar and Earth’s actual orbit around the sun.
This calendar reform by Caesar assumed that 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 Caesar assumed. So, using 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 fix this problem by reforming the Julian leap year calendar. First, he cut 10 days out of the calendar and declared that Oct. 5 was actually Oct. 15. This brought 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, like 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 attached to February instead of some other month? The original Roman calendar had only 10 months: 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 “10th month”). Januarius and Februarius were added several centuries later and Februarius was only given 28 days, making it the shortest 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 month of the new year was shifted from Martius backward to Januarius. 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 “10th month”) is actually our 12th month.
Did you know that in leap years, Halloween always occurs on the same day of the week as Leap Year Day? Both fall on Wednesdays this year.
Children born on Leap Year Day are sometimes called “leaplings.” Because their actual birthday rolls around only once every four years, leaplings usually celebrate their birthdays on March 1 in common years.
Oh, and don’t forget to set your clocks ahead (spring forward) one hour at 2 a.m. March 11 when we go back on daylight savings time.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. Check out Jimmy’s astrophotography website at www.jwestlake.com.