Leap Year 101
Why and when we
have leap years
by Borgna Brunner
2012 is a leap year,
which means that it has 366 days instead of the usual 365 days that an
ordinary year has. An extra day is added in a leap year—February 29 —which is
called an intercalary day or a leap day.
Why is a Leap Year Necessary?
Leap years are added to the
calendar to keep it working properly. The 365 days of the annual calendar
are meant to match up with the solar year. A solar year is the time it takes
the Earth to complete its orbit around the Sun — about one year. But the
actual time it takes for the Earth to travel around the Sun is in fact a
little longer than that—about 365 ¼ days (365 days, 5 hours, 48 minutes, and
46 seconds, to be precise). So the calendar and the solar year don't
completely match—the calendar year is a touch shorter than the solar
year.
It may not seem like much of a difference, but after a few
years those extra quarter days in the solar year begin to add up. After four
years, for example, the four extra quarter days would make the calendar fall
behind the solar year by about a day. Over the course of a century, the
difference between the solar year and the calendar year would become 25
days! Instead of summer beginning in June, for example, it wouldn't start
until nearly a month later, in July. As every kid looking forward to summer
vacation knows—calendar or no calendar—that's way too late! So every four
years a leap day is added to the calendar to allow it to catch up to the
solar year.
A Quick History Lesson
The Egyptians were the
first to come up with the idea of adding a leap day once every four years to
keep the calendar in sync with the solar year. Later, the Romans adopted
this solution for their calendar, and they became the first to designate
February 29 as the leap day.
But Wait! It's Not Quite that
Simple!
The math seems to work out beautifully when you add an extra
day to the calendar every four years to compensate for the extra quarter of
a day in the solar year. As we said earlier, however, the solar year is
just about 365 ¼ days long, but not exactly! The exact length of a
solar year is actually 11 minutes and 14 seconds less than 365 ¼ days. That
means that even if you add a leap day every four years, the calendar would
still overshoot the solar year by a little bit—11 minutes and 14 seconds per
year. These minutes and seconds really start to add up: after 128 years, the
calendar would gain an entire extra day. So, the leap year rule, "add a leap
year every four years" was a good rule, but not good enough!
Calendar Correction, Part II
To rectify the situation, the
creators of our calendar (the Gregorian
calendar, introduced in 1582) decided to omit leap years three times
every four hundred years. This would shorten the calendar every so often and
rid it of the annual excess of 11 minutes and 14 seconds. So in addition to
the rule that a leap year occurs every four years, a new rule was added: a
century year is not a leap year unless it is evenly divisible by 400. This
rule manages to eliminate three leap years every few hundred years.
It's Smooth Sailing for the Next 3,300 Years
This ingenious
correction worked beautifully in bringing the calendar and the solar year in
harmony, pretty much eliminating those pesky extra 11 minutes and 14
seconds. Now the calendar year and the solar year are just about a half a
minute off. At that rate, it takes 3,300 years for the calendar year and
solar year to diverge by a day.
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