Leap Year Explained

Today we celebrate the Leap Year, the extra day at the end of February that comes around once every four years.


For some people the leap year has a special significance. This year, for instance, there have been initiatives to encourage people to put that extra day to use helping the planet.

But where does the idea of a leap year come from, and why exactly do we have one?

The second question is a bit easier to answer than the first. Many of you might recall from your school days that a year is not exactly 365 days. It actually takes the planet 365.242 days to complete a revolution around the sun. After four years, those extra .242 days add up to a whole extra day.

Even then it’s not exactly perfect. Obviously, .242 times four is not 1. We get around this problem by skipping a leap year for three out of four century years. So the year 2000 was a leap year, but the years 2100, 2200 and 2300 will not be.

Like many advancements in civilization, the concept of a leap year came from the Egyptians. The Egyptians first began using a calendar with a leap year during the Ptolemaic dynasty (300-30 B.C.E.) Many people credit Julius Caesar for the use of the leap year in his Julian calendar, but he almost certainly took the ideas from the Egyptians. One legend says that he took the idea from his lover Cleopatra.

When Caesar returned to Rome and implemented the calendar the leap year became popularized. It would remain unchanged until the 16th century A.D.

In 1582 Pope Gregory XIII introduced a new, but very similar, calendar. As we mentioned earlier, you actually add slightly too much time to the year by adding a day every four years. This results in an unnecessary extra day being added to the calendar once every 128 years. This annoyed Gregory. He believed that this extra day ever century and a quarter had resulted in Christian holidays being celebrated on the wrong days.

So Gregory went ahead and did something about it by inventing the Gregorian calendar, which we still use today. The Gregorian calendar introduced the concept of skipping three of four century years as a leap year, and so keeps the balance a bit better than the Julian calendar.

So there you are. Now that you know the science and history behind your extra day, what are you going to do on it?

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