Jimmy Westlake: It’s leap year — time to take up the slack
February 23, 2016
Have you ever wondered why the month of February has only 28 days most years, but occasionally has 29 days, as it does this year? 2016 is a leap year, and it's time to take up the slack in the calendar.
Our use of the leap year system started in the days of the Roman Empire, during the reign of Julius Caesar. Even then, sky watchers realized there were not a whole number of days in a year.
The Earth spins around 365 times in one year, plus another one-quarter spin — an excess of six hours. It wouldn't make sense to have the last day of the year be only six hours long, so Julius Caesar decreed in 44 B.C. that we would let the one-quarter 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. Or, so Caesar thought.
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 Julius Caesar assumed.
So, using Julius Caesar's method, we were adding in too many leap days throughout the centuries. By the 1500s, all of those 11 minutes and 14 seconds added up to 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. Not good.
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In 1582, Pope Gregory XIII sought to fix this problem by reforming the Julian leap year system. First, he cut 10 days out of the calendar and declared that October 5 was actually October 15. This brought the date of the vernal equinox back to March 21, where it should be. He then declared 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, just enough to 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 time.
Why did the extra leap day get attached to February instead of another month?
The original Roman calendar contained 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 given only 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 backwards to Januarius. Quintilus was renamed July to honor Julius Caesar, and Sextilus was renamed August to honor Augustus Caesar, and thus we are left with our modern calendar, where Decembris, the 10th month, is actually our 12th month. Strange, but true.
Did you know that in leap years, Halloween always occurs on the same day of the week as Leap Year Day? That's also true. Both fall on Mondays this year.
Children born on Leap Year Day are sometimes called leaplings because their real birthday only happens once every four years, leaplings usually celebrate their birthdays on March 1 in non-leap years.
Put your extra day this year, Feb. 29, to good use.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. His "Celestial News" column appears weekly in Craig Daily Press and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website atProfessor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in Craig Daily Press and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Westlake’s astrophotography website atjwestlake.com.Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. His "Celestial News" column appears weekly in Craig Daily Press and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Check out Westlake's astrophotography website atjwestlake.com.