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Leap Year

Written for the KidsKnowit Network by: Meredith Tennant

This year, 2012, is a leap year.  What does that mean?

Leap year happens every four years

Every four years an extra day is added to the calendar, making
the length of the year 366 days, instead of the normal 365.

Why on earth does that need to happen?

The calendar is supposed to match the solar year, in other words, the length of time it takes for Earth to orbit the Sun once. But things aren’t quite that simple. It actually takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to complete its orbit (about 365 1/4 days). Those extra hours gradually add up so that after four years the calendar is out of step by about one day. Adding a day every four years allows the calendar to match up to the solar year again.

However,because the solar year isn’t exactly 365 days, even adding a leap day every four years means that the calendar is still out of step by 11 minutes and 14 seconds each year. Over the course of 400 years this would add up to three extra days. In order to solve this problem it was decided to leave out the leap year three times every 400 years. So, the new rule was, a century year (1600, 1700, 1800, etc.) would only be a leap year if it was evenly divisible by 400. This means that the year 2000 was a leap year, but 2100 will not be.

Phew! So, who figured all this out?

The Egyptians were the first civilization to add a leap day to the calendar

The Egyptians were the first people to think of adding a leap day to the calendar every four years. Later, the Romans copied the idea. In 1582 Pope Gregory XIII reformed the Julian calendar (introduced by Julius Caesar in 45 BCE). By Pope Gregory’s time the calendar had drifted 14 days off track. He neatly solved this by lopping ten days off the calendar, telling everyone that the day after October 4th was going to be October 15th. Bad luck for people with birthdays during that time!

The early Roman calendar, way before Julius Caesar’s time, began the year with March. It consisted of ten months, each lasting about 30 days, ending with December. They didn’t seem to have counted the winter months. It is thought that two extra months, January and February, were added sometime around 715-673 BCE. This would have made February the end of the year, which might explain why a leap day was added to that month. Later it was decided to start the year with January, as that month contained a festival dedicated to Janus, the god of gates (and, later, all beginnings).

Here is an example of an early Roman calendar.You can see that January and February are included on this calendar.

Did You Know?

With the current system of adding leap days, it will be 3,300 years before the calendar is again off by a day.

Other nations have different calendars and different methods of keeping the calendar in line with the solar year. A day, or in some cases a month, gets added every few years, according to the organization of the particular calendar. The Chinese, for example, add a month about every three years, whereas in Islamic Hijri calendar a day is added 11 times during a 30-year cycle.

It’s pretty confusing, but just remember that for our calendar:

Thirty days hath September,
April, June and November;
All the rest have thirty-one
Save February, she alone
Hath eight days and a score
Til leap year gives her one day more.

You can see the days go from the 4th to the 15th
An example of the Gregorian calendar when ten days were removed in 1582.

Did You Know?

There is a tradition that women are allowed to propose marriage to men on leap days. One day in the 5th century, St. Bridget complained to St.Patrick about the unfairness of the system which only allowed men to propose, so he decided to let women do the asking once every four years!