The Origin of Leap Year

Leap Year seems to be such a strange thing. Why do we have to add an extra day to the month of February every four years? On the surface, this practice seems to make no sense, and it causes some confusion when dealing with computers, voice mails, PBXs and faxes that have time and date stamping.

There is a valid reason for the calendar and clocks to be adjusted periodically. It is also interesting to note that it originated over 2000 years ago during the time of Julius Caesar, leader of the Roman Empire. The calendar used at that time was not accurate at all. It was based on calculations that were not correct and sometimes not even close. There were conflicts about which calendar to use since not everyone in the world had the same time measuring tools. No one really knew the exact length of a day and therefore could not know the exact length of a year. No standards existed, information was inaccurate, and almost everyone believed the world was flat. To temporarily get things back in sync for the Julian Calendar, ninety days were added to the year 46 BC.

Caesar wanted some method devised to keep this from happening again. The astronomer Sosigenes was requested, by Caesar, to investigate the possibility of a solution. Sosigenes worked on the calculations and came up with the length of a year as being 365.25 days. This is very good work considering the times and the fact that he did not have benefit of computers or satellites, but just his mind, his observations and some papyrus.

His solution was to add an extra day to the last month of every fourth year. This would keep the calendar in synchronization with the seasons and address the problem with a simple solution that was easy to implement. The last month of the year during the Roman Empire period was February.

Over 1600 years later, the problem needed to be addressed again since the calendar was now ahead by more than ten days. The astronomer Christopher Clavius now had the benefit of some past history. Using Sosigenes' calculations and comparing the effects over time, he knew that the original figure of 365.25 days in a year had to be too long. For the calendar to be out of sync again meant that about 11 minutes was being added to each year, and it was time for another adjustment. Clavius recalculated to address the immediate problem and the future effect. This adjustment would still result in a calendar that would be off 3.12 days every 400 years. He gave his results and a solution to the Pope.

Pope Gregory XIII took action in the year 1582 by cutting 10 days off the month of October and devising the Gregorian Calendar, the one we still use today. The last day of the Julian Calendar was Thursday, October 4th, 1582, followed by Friday, October 15th, 1582.

Clavius' solution was to make no Centennial Year a leap year unless it was divisible by 400. Since 1600 was coming up, it was noted that it would be a leap year, whereas 1700, 1800 and 1900 would not. The year 2000 will be a Centennial Leap Year. He also realized that this solution slightly overcorrected the calendar. Therefore, any year that is divisible by 4,000 would be called a Common Year and would not be a leap year. This has the effect of bringing the calendar back in line. There will not be a Common Centennial Year until the year 4000.

Today we estimate that a year is 365.242199 days long and a day is 24.0159254137 hours long. We still tweak it very now and then when a more accurate assessment is made with better measuring instruments. However, the calendar system that was put into place so long ago will not have to be altered anymore, as long as we accept months and days as described by this calendar.

I wonder what Sosigenes and Clavius would think about computers and databases today that only address a two digit year field and have no programming provisions for leap year calculations.....

CAN A CENTURY YEAR BE A LEAP YEAR?
A year is a leap year if it is divisible by four, but century years are not leap years unless they are divisible by 400. So, the years 1700, 1800, and 1900 were not leap years, but the year 2000, as of course you now know, was a leap year. Why do we have leap years? The actual length of a year (the rotation of Earth around the Sun) is 365.2422 days. If we didn't have leap years, the seasons would shift about a quarter of a day every year, and after 100 years the seasons would be off by 25 days. The extra leap day adjusts this drift

EXACTLY HOW LONG IS ONE YEAR?
365 days, 5 hours, 48 minutes, and 46 seconds.

What year was missing ten days in October?
In 1582 Pope Gregory decreed that October's dates would skip from the fourth to the fifteenth, dropping ten days. The reason for this seemingly strange act had to do with the calendar system that was in use at the time.

Unlike our current system, the old Julian calendar had a leap year every four years without exception. Because a year is really a fraction shorter than 365.25 days, tiny errors began to accumulate.  By the time of Pope Gregory's decree, the calendar was adjusted by ten days compared to Earth's solar year.

When he issued his decree, Gregory also fixed the leap year rule, so that leap years do not occur on century years (divisible by 100), unless the year is also divisible by 400. There is one other exception: years divisible by 4000 are not leap years. For example, 1900 was not a leap year, but 2000 is one.

WHY DOES FEBRUARY HAVE ONLY 28 DAYS?
January and February both date from about the time of Rome's founding. They were added to a calendar that had been divided into ten month-like periods whose lengths varied from 20 to 35 or more days. A winter season was not included, so those period lengths are believed to have been intended to reflect growth stages of crops and cattle. When introduced, January was given 29 days and put at the beginning of the calendar year. February was given 23 days and put at the end. Then, for an undetermined period shortly after Rome's founding, months were said to have begun when a new moon was first sighted. At some later time, month lengths were separated from lunations and again became fixed. At that time, February's original length was extended by five days which gave it a total of 28.

WHY IS OUR LEAP DAY IN FEBRUARY, AND NOT AT THE END OF THE YEAR?
Romans always reconciled differences between calendar and solar year lengths during the "Month of Purification." Whenever and however Roman calendars were modified to correspond to year length, it was always done after the 23rd day of February, traditionally the last day of the year. Even in our time, leap year is observed with a 29-day February. To purists, "leap day" is February 24, not the 29th.