What is a Leap Year

Instead of the usual 365 days, the year 2008 will be 1 day longer. Why?

Because the year 2008 is a leap year.

In case you've ever wondered what leap year means, here's a quick overview:

What is a Leap Year

The calendar marks 365 days during the twelve months of the year. But every 4 years, the year is 366 days long. And that extra day is always added to the February. Why?

To understand what a leap year is, we have to look at how "a year" is defined:
A year is completed, when the Earth has rotated around the Sun. That takes exactly 365 days, 5 hours, 48 minutes, and 46.5 seconds. That means the solar year is slightly longer than our 365 days calendar.

If we wouldn't have the leap year, these extra hours would accumulate over the centuries. In order to keep our seasons in the months where they belong, 1 day is added to our calendars every 4 years. Otherwise, the day would come when it snows in September.

Who started the concept of Leap Year

Ancient Egyptians were the first to notice that the solar year wasn't exactly 365 days long. Using the shadow of an obelisk when the sun was at its highest point in the sky, they documented the shadow at different lengths for each day of the year. After a while, they noticed that the shadow did not return to the same spot each 365 days.

They concluded that they needed an extra one-quarter day. Thus the leap year was created. The Egyptians added one extra day to their calendars, but did that only every four years.

So, the Egyptians were definitely the forerunners when it comes to the leap year, but the Romans were the first to pick February 29 as leap day.

The Romans and the Leap Year

It was around 46 B.C., when Julius Caesar adopted the idea of the Egyptians. Julius Caesar invented the "Julian" calendar, adding February 29 to every year which could be evenly divisible by four.

However, Julius Caesar made a mistake. Julius Caesar assumed that the year was 365.25 days long, a calculation that was false by 11 minutes and 14 seconds. His advisers, aware of the problem, deemed it to be too insignificant to have any noticeable effect. But the time added up, and by the 16^th century, the official calendar was a mess. Spring equinox--when the day and night are equal in length--was 10 days later than the real spring equinox. Why? The "Julian" calendar added too many leap days.

The Pope and the Leap Year

Pope Gregory XIII modified the "Julius" calendar. The Pope instituted a calendar reform in 1582 which resulted that the spring equinox was back on March 21, in sync with the solar year.

Pope Gregory knew by then that Caesar's "Julius" calendar was wrong by about 3 days every 4 centuries. The Pope, with the help of astronomers, came up with the solution that only century years evenly dividable by 400 should be leap years, thus creating the calendar we know as the "Gregorian" calendar.

While the "Gregorian" calendar is not perfect, it only errs every 3300 years by 1 day. And in the year 2008, we still use Pope Gregory's "Gregorian" calendar.

What are the rules for Leap Years

Simply put, the calendar year is 365 days long. Except when the year is divisible by 4, then an extra day is added to February. If the year is an end-of-century year, then it is only a leap year if exactly divisible by 400. (For example, the year 2000 was a leap year, although not the year 1900, because its not divisible by 400).

When was the first Leap Year

Italy, Poland, Portugal and Spain adopted the "Gregorian" calendar in 1582. But the first leap year for the USA was in 1752, when the "Gregorian" calendar was finally accepted.

By the way, the next leap year is in 2012.

Source:
National Maritime Museum, http://www.nmm.ac.uk/server/show/conWebDoc.349
http://inkido.indiana.edu/a100/celestialsphere14.html
http://www.wsu.edu/DrUniverse/leapyear.html
http://en.wikipedia.org/wiki/Leap_year