Date: Mon, 9 Feb 1998
To: cruse@usfca.edu
From: Robert Alan Wolf
Subject: leap years
For the old calendar, the Julian, in use until recent centuries, the
rule was simply "years divisible by 4, and only such years, are leap years."
However, this made the average length of the calendar-year slightly longer
than the solar-year (I'm not sure if the latter term is the right one to use
here, but you know what I mean); in other words, under the Julian system,
too many years were being designated as leap years. And by the late 16th
century, the calendar was reading 10 days behind what it should have; e.g.,
maybe the calendar was reading March 4 when the actual position of the earth
in its orbit about the sun would have called for March 14.
In papal countries, after skipping 10 days in October 1582 to get
back on track (i.e., the day after Oct. 4, 1582 was decreed to be Oct. 15,
as I recollect), the Gregorian calendar thinned out the leap years a bit, in
order to produce a shorter calendar year on average, one that kept up with
the earth's movement about the sun. (Protestant countries adopted the new
calendar only somewhat later, by the way. Apparently a lot of people grumbled
about losing 10 days' pay and such.)
Oct. 15 seems like a day we should perhaps celebrate as Gregorian
Calendar Day, being the birthday of this wonderful calendar. After all, it
works so well that we never stop to appreciate our good fortune in having so
accurate a calendar. Its rules are
1) years divisible by 400 ARE leap years (so, for example,
2000 will indeed be a leap year),
2) years divisible by 100 but not by 400 are NOT leap years
(so, for example, 1700, 1800, and 1900 were NOT leap years, NOR will 2100 be
a leap year),
3) years divisible by 4 but not by 100 ARE leap years
(e.g., 1988, 1992, 1996),
4) years not divisible by 4 are NOT leap years.
The only difference, then, between the Julian system and the Gregorian
system (besides the 10-day catchup) is that in the latter system, century
years like 1700, 1800, 1900 are NOT leap years. (In both calendars, the
extra day in a leap year is always February 29, by the way.
The Gregorian calendar keeps excellent time, but it could even
better (i.e., very, very excellent) if years like 4000, 8000, etc., were NOT
leap years. Therefore, it has been proposed that the rules be changed to
0) years divisible by 4000 are NOT leap years,
1') years divisible by 400 but not by 4000 ARE leap years
(so, for example, 2000 will still be a leap year),
2), 3), and 4) as above.
We have a long, long time to decide on this possible change. Whatever the
ultimate decision, let's hope we don't have a "year 4000 problem"!!!