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LEAP YEAR

As February 29, 2008 approaches, some people may notice something unusual
about the date. Last year, there was no February 29th. In fact, there has not
been a February 29th since 2004. Why does February 29th occur only once in
four years? To understand this confusing arrangement, it is necessary to
understand the calendar that is currently used in the United States.
Nearly all modern societies use some kind of calendar to decide on the dates
and times of everything, from religious holidays to business meetings. The kind
of calendar used determines what makes up a week, a month, or a year. Some
societies use lunar calendars, which are based on the rotation of the moon
around Earth, and others use solar calendars, which are based on the rotation
of Earth around the sun. The United States, like much of the rest of the world,
uses a solar calendar.
The solar calendar used by most of the world today is known as the Gregorian
calendar. Named after Pope Gregory XIII, who introduced it in 1582, this
modern calendar is the end result of hundreds of years of fine-tuning. It was
developed from the Julian calendar, which was created in 46 BC by Julius
Caesar.
The Julian calendar was also a solar calendar, based on the time it takes for
Earth to travel one complete loop around the sun. In Julius Caesar’s time,
although astronomers believed that the sun revolved around Earth, they still
managed to make fairly accurate measurements of the length of a complete
cycle. A solar year, they calculated, was about 365.25 days long.
Julius Caesar, deciding that it would be difficult to add ¼ of a day onto each
year, ordered one extra day to be added every four years to the month of
February, creating what would be called “leap” years. This calendar was used
by the western world for over a thousand years.
Unfortunately, there was a problem with the Julian calendar. When Julius
Caesar ordered a leap year every four years, he was putting in too many extra
days. A solar year is not 365.25 days long, but in fact 365.24219 days. The
difference between the actual length of the solar year and the approximated
value is very small, but over several hundred years it began to add up. By the
1500s, spring holidays were starting to happen in the summer.
In 1582, Pope Gregory XIII came up with an idea of how to fix the problem. He
realized that there were too many leap years in the Julian calendar, so he
reorganized it into a more complicated system. In the new calendar, there is
still a leap year almost every four years, but there is also a new rule. If the year
ends in 00, it is only a leap year if the year’s number can be divided by 400. For
this reason, 1800 and 1900 were not leap years, but 2000 was.
The new calendar was so accurate that over the next several hundred years, it
was adopted by almost every country in the world. Over 400 years after its
introduction, the Gregorian calendar is currently in use world-wide.

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