Tuesday, February 28, 2012

When Is A Leap Year Not A Leap Year?

A February 29th in 2012 may be proof that its a bona fide leap year, but are there supposed "leap years" that are disqualified from getting their own February 29ths?

By: Ringo Bones

Believe it or not, the year 2000 might be the only century year within most of existing folks lifetimes that passed muster to necessitate a February 29th. On the other hand, century years - those ending in double zeroes - ordinarily do not deserve a February 29th, as in the century years of 1700, 1800 and 1900 didn't have this additional day. But once every four centuries, the double-0 year is a leap year, and the year 2000 was one of them.

The complexity of adding leap years is due to the fact that humans like to work with nice whole numbers while Mother Nature never obliges to such a "square" convention. Measured against astronomical observations, a typical Earth year lasts 365.2422 days - i.e 365 days, 5 hours, 48 minutes and 46 seconds. If the Earth year were 365.25 days long - or about 11 minutes longer when measured against background stars - rules for adding a February 29th would have been much simpler and we would add one leap day every fourth year in order to eliminate the fraction. But the planet Earth circles our Sun a tad faster than that, so we occasionally have to skip the once-every-four-year February 29th.

Bypassing three leap years every 400 years may be confusing, but it gets the job done. This extra "tweak" - introduced by Pope Gregory XIII in 1582 - keeps our calendar accurate to one day every 3,300 years. Years divisible by 4,000 are not leap years, which fixes that little glitch. Although critics of Pope Gregory's method say this tweak needs further refinement and necessitates the addition of a February 30th in the year 3000 to keep the Gregorian Calendar accurate beyond that date.

If it weren't for this elaborate system of leap years, each January 1 would begin at a slightly different point in the Earth's orbit around our Sun. That might not seem like such a big deal, but eventually the seasons would start at odd times. Christmas would come in August - or summer in the Northern Hemisphere - if this went on long enough. Our previous calendar - i.e. the Julian Calendar - in which all century years were leap years, was good enough for Julius Caesar, but its annual 11-minute error eventually did add up. By the 1500s, equinoxes and solstices took place a full 10 days too early.

In 1582, Pope Gregory XIII boldly returned the seasons to their intended schedules by simply eliminating 10 days from the calendar. So in that year, October 4 was immediately followed by October 15. Pope Gregory also decreed at that time that all century years divisible by 400 would be leap years.


  1. Heard about this adding a February 30th to the month of February in the year 3000 to make the Gregorian Calendar still accurate after that date back in 1998. Will it work?

  2. I do find the idea of a once-every-3,00-year-February 30 cool, but that's a problem best left to future generations.
    To people over 30 - like me - this February 29 business can be disconcerting as time goes on. I once remember a time when the first day of spring fell on March 21, yet long since then the date slid to March 20 due to scores of additional February 29ths. The Gregorian Calendar tend to allow equinoxes to drift behind as each century progresses, only to push them ahead a bit too much by the omitted leap years at most end-of-century years.