Each year's length varied as the pontiffs, who were the men in charge of making up the numbers and lengths of months, changed things constantly. They added and subtracted days, and sometimes, even months to try to keep the dates in line with the solar and lunar schedule.

This new Julian calendar included the concept of leap years. This was a concept that was brilliant and ahead of its time. With the Julian calendar, leap years occurred every four years. Having an extra day once every 4 years helped to prevent years from becoming too short. Still, though this concept was sound, it was not perfect.

The Gregorian calendar

In 1582, Pope Gregory XIII instituted the Gregorian calendar. After many centuries had passed, Easter Sunday had been arriving closer to January. Having leap years occur every 4 years actually overcorrected the problem of the years being too short.

A mechanism to correct this problem was needed. Easter Sunday depends on full moon cycles and the vernal equinox. Because of this, Easter's date had moved up too far to maintain Easter as a springtime event.

The way the Gregorian calendar differs from the Julian is that it has 3 fewer leap years for every 400 hundred years. How does this happen? Finding out how the Gregorian calendar makes this adjustment requires knowing how leap years are determined with both the Julian and Gregorian calendars.

How Leap Years are Calculated

With the Julian, every forth year is a leap year. It is as simple as that. If you divide the year by 4 and the answer is a whole number, it is a leap year. So, 4 AD was a leap year. 8 AD was a leap year. So was 300. 1900 would have been a leap year also, if the Julian calendar had still been used in 1900.

With the Gregorian calendar, if the year is evenly divisible by 4 it is a leap year unless it is divisible by 100. If it is evenly divisible by 100, it is not a leap year unless it is divisible by 400. For example, 1896, since it is evenly divisible by 4, was a leap year. 1900, though it is divisible by 4, it is also divisible by 100 and so no adjustment was made to February in 1900.

1996, being divisible by 4 was a leap year. 2000 was also a leap year because, though it was divisible by 100, it was also divisible by 400. Another way of looking at it is that there was no February 29 in 1700, 1800 or 1900, even though these numbers are evenly divisible by 4. However, there was a February 29 in 1600 and 2000.

The World's Calendar Since 1753

Though Pope Gregory XIII made the Gregorian calendar official in 1582, not all countries were subservient to the pope. Great Britain was one of these countries. The colonies, which are now part of the United States were, of course, owned by Great Britain at that time so, the Gregorian calendar was not recognized in The Colonies in 1582. Instead, it was adapted in 1753. This means any dates noted before 1753 in the U.S. and many other parts of the world are questionable.

We still use the Gregorian calendar today and we probably will use it for a long time to come because it has made the necessary correction to the Julian concept. Still, even though it is very accurate, the Gregorian calendar is off by 3 minutes and 39 seconds each decade. This means we will gain a day over the next 40 centuries.