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Ceasar's assumption that
the year was 365.25 is inaccurate by 11 minutes and 14 seconds. This may not seem
like much, but it adds up to 3 days every four centuries. Ceasar's advisors were
aware of this problem, but believed it to be to insignificant so they ignored it.
However, by the 16th century the first day of spring was occuring on March 11. To alleviate this problem Pope Gregory XIII instituted a calender reform in 1582. he decreed that October 5, 1582 was really October 15, 1582 which placed the first day of spring back on March 21. He then modified Ceasars leap year system. Ceasar added February 29 to every year that is evenly divisible by four. However, we know that this system leads to an error of about 3 days every 4 centuries. Therefore, Pope Gregory XIII decreed that only the century years evenly divisible by 400 should be leap years. The Gregorian system is the one in use today. Its fundamental assumption is that the year is 365.2425 mean solar days long, which is very close to the true length of the tropical year. This is not error free, but unlike the Ceasar's system introduces and error of only 1 day every 3300 years. |
