Nature Bulletin No. 447-A March 4, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
This is a Leap Year with 366 days instead of the usual 365, and 29 days
in February. Julius Caesar was responsible for that. The Roman
calendar was patterned after those of the ancient Greeks and Egyptians
which, in turn, had been modeled on that of the early Sumerians in
Babylonia. The Sumerians had 12 lunar months in a year but every so
often, to make up for the difference between that year and the year of
the natural seasons, their astronomer-priests inserted an extra month.
The Roman year was too short. It had had only 365 days for so many
centuries that their calendar was badly out of step with the seasons and
something had to be done about it. The summer months were coming in
spring. Caesar's astronomers told him the reason: instead of being
exactly 365 days long, a year was 365 and one-quarter days in length.
Julius then solved the problem -- so he thought -- by establishing a leap
year of 366 days every fourth year. He put the extra day in February
because that was the last month on the old Roman calendar. Their new
year started on the first day of March.
The Roman astronomers did not realize that they were just a little bit off
in their calculations. We know now that twice a year, as the earth
travels around the sun, it arrives at a point where its axis is
perpendicular to a line drawn from the earth to the sun. That happens
each spring and fall. The length of time from when that happens in
spring, until it happens again the next spring, is called a "tropical" year.
It contains 365 days, 5 hours, 48 minutes, and 46 seconds -- 11 minutes
and 14 seconds less than 365-1/4 days. The seasons are always in step
with the tropical year and our present calendar is based upon it.
About 1500 years after Julius Caesar's reform of the calendar, that
excess of 11 minutes and 14 seconds had added up to about 12 days.
The Julian calendar had had 12 leap years too many! From observations
of the sun, it was obvious that the calendar was out of step, so Pope
Gregory XIII corrected it again. Under our Gregorian calendar, every
fourth year is a leap year except those century years which are not
divisible by 400. In other words, there was no leap year in 1900, 1800
nor 1700 but there will be one in 2000 A. D. Even this is not quite
accurate and so, in the year 4000, it will be necessary to drop another
day and there will be no leap year.
The Pope directed that 10 days be dropped from the calendar and that
the day after October 4, 1582, should be October 15; also that each new
year should begin on January 1. Until then, some countries began it on
December 25 and some, as in England, on March 25. The Gregorian or
New Style calendar was not adopted in England until 1752 and almost
caused a revolution because it was necessary to drop 11 days. The
people held great meetings and processions demanding: "Give us back
our eleven days !" According to the Julian or Old Style calendar then in
effect, George Washington was born on February 11, 1731, which
corresponds to February 22, 1732, on our "New Style" calendar.
Many people have urged that our calendar be revised to have a year
with 13 months of exactly 4 weeks each, and an extra day at the end --
or 2 days in Leap Years. The Aztecs had a year with 18 months, each
having 20 days, and 5 extra days -- considered very unlucky -- between
years. Every 52 years they corrected their calendar and began a new
"bundle" or cycle.
Reading one of their complicated but very accurate calendar "wheels",
carved on stone, is like trying to read a gas meter.
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Update: June 2012