| LEAP YEAR
from Bill Petro, friendly neighborhood historian
The Leap Day, February 29, depicts a day that occurs only once every
four years, every Leap Year or intercalary year when an extra day is
inserted. But not every fourth year, if that year ends in "00" like
1900, then it is not a Leap Year. Except if that year ending in 00 is
also divisible by 400 then it is a Leap Year. Unless it is a Tuesday
and it is dark. OK, I made up that last rule. So, years like 2008 are
Leap Years, being divisible by 4. 1900 is not a Leap Year as it ends
in 00. The year 2000, you remember, the famous Y2K, when computers
programmers only obeyed the first two rules and assumed that it
wasn't a Leap Year so that all the computers failed and the world
came to and end? That was a Leap Year, as it was divisible by 4, and
though it ended in 00, it was divisible by 400 (indeed, it's
divisible five times, if you're still with me.)
How did we get into this calculatory conundrum? It has to do with a
cumulative rounding error in trying to reconcile the Julian calendar
with the tropical or astronomical calendar. The Julian calendar,
established by Julius Caesar in 46 B.C. lasted from 45 B.C. until
A.D. 1582. and stipulated that they year should be 365 days for 3
years in a row, with every 4th year having 366 days. This meant that
an average year was 365.25 days. But according to the tropical
calendar, the year has 365.24219 days.
This tropical (or seasonal) calendar recognizes that the year is
marked by two successive passages of the Sun through the vernal
equinox (equal nights). You and I know that the Sun does not pass
through the Earth's sky, but rather the Earth orbits around the Sun
-- or at least you probably realized it since the Sun came up this
morning -- but it's easier to explain this by considering this
apparent motion of the Sun in our sky. And of course, this is just
the easy explanation.
So where does this cumulative rounding error come in? Back in
A.D. 730, an Anglo-Saxon monk named the Venerable Bede recognized
that the Julian year was 11 minutes and 14 seconds too long, which
would produce an error of about one day every 128 years. But there
were a lot of other things going on then, and the Venerable Bede
didn't have a blog, so nothing was done about it for 800 years.
In A.D. 1582 this accumulated error was estimated at 10 days, and
Pope Gregory XIII decreed that the day following Oct. 4 would be
Oct. 15, pretty handy if you had a library book due during that
time. This Gregorian calendar was adopted throughout much of the
Catholic world, but not everywhere. Uncivilized parts of the British
Empire, like America, made the change in 1752 when 2 September was
followed by 14 September and New Year's Day was changed from 25 March
to 1 January.
Ultimately, to make future adjustments for the error, which amounts
to about three days every 400 years, it was decided that years ending
in "00" would be normal years rather than Leap Years, with the
exception of those divisible by 400. Unless it's a Tuesday and dark. |
