Leap Year 101
Why and when we
have leap years
by Borgna Brunner
2008 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 365¼ 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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