A leap of faith

Posted by Rich Talcott
on Friday, February 29, 2008

Astronomy“Slow down, you move too fast.” Paul Simon wrote those lyrics to open “The 59th Street Bridge Song,” from Simon and Garfunkel’s classic 1966 album Parsley, Sage, Rosemary & Thyme. As the title suggests, Simon was referring to what New Yorkers’ know better as the Queensboro Bridge. But he could just as easily have been feelin’ groovy about how our calendar runs faster than Earth’s revolution around the Sun.

Today — February 29 — marks the day we slow down the calendar so it can keep pace with Earth’s revolution. Without a leap day every 4 years, our calendar would soon get out of whack with Earth’s seasons. But the story is a little more complicated than that.

Our calendar is based on what astronomers call the tropical year: The time it takes for Earth to revolve around the Sun with respect to the vernal equinox, which keeps the calendar aligned with the seasons. The tropical year measures 365.242199 days long — not a number that allows a simple formula for leap years.

The first successful attempt to bring order out of chaos came from Julius Caesar, who in 46 B.C. instituted a calendar reform that decreed a leap year every fourth year. That gives a year averaging 365.25 days, just 11 minutes and 14 seconds longer than a tropical year. Not a big discrepancy, but those minutes add up.

By 1582, 13 days had accumulated and the vernal equinox (the first day of spring) had fallen back to March 11. If the trend continued, Easter eventually would fall during winter. To prevent further slippage, Pope Gregory XIII initiated a two-step calendar reform. First, to bring the vernal equinox back to where it was at the time of the Council of Nicea in the 4th century, the calendar dropped 10 days by proclaiming the day after October 4, 1582, to be October 15.

Second, the rule for leap years was modified so that years divisible evenly by 100 would not have an extra day unless they were also divisible by 400. Thus 1900 and 2100, for example, are not leap years, while 2000 was. The Gregorian calendar matches the tropical year to within a day about every 3,300 years. That should last a while. Still, some reformers have suggested a further modification — which would turn those years divisible by 4,000 into non-leap years — to make the calendar more precise.

Roman Catholic countries adopted the Gregorian reform immediately, but most Protestant countries and those under the Eastern church did not. England and the American colonies came on board in 1752, when September 2 was followed by the 14th. Rioting occurred in some areas as people demanded the lost time back (and some unscrupulous landlords demanded a full month’s rent for September). Russia stayed with the Julian calendar until the Bolshevik revolution in 1917, when it had to drop 13 days to rejoin the rest of the world.

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