Julius Caesar took control over the chaotic Roman calendar, which was being exploited by politicians and others with the haphazard addition of days or months. It was a calendar horribly out-of-synch with the seasons of the earth, which are the result of the rotation of the earth around the sun. Caesar developed a new calendar of 364 1/4 days, closely approximating the length of the tropical year (the time it takes the earth to go around the sun from the beginning of spring to the beginning of spring). Caesar's calendar was normally 365 days long but included an extra day (a leap day) every four years to account for the extra one-quarter of a day. The intercalary (inserted into the calendar) day was added prior to February 25 each year.
Unfortunately, while Caesar's calendar was almost accurate, it wasn't quite accurate enough because the tropical year is not 365 days and 6 hours (365.25 days), but is approximately 365 days 5 hours 48 minutes, and 46 seconds (365.242199 days). Therefore, the calendar of Julius Caesar was 11 minutes and 14 seconds too slow. This added up to be a full day off every 128 years.
While it took from 46 BCE to 8 CE to get Caesar's calendar functioning properly (initially leap years were being celebrated every three years instead of every four), by the time of Pope Gregory XIII the one day every 128 years added up to a full ten days of error in the calendar. (Purely by luck did the Julian calendar happen to celebrate leap years on years divisible by four - during Caesar's time, the numbered years of today didn't exist).
A serious change needed to take place and Pope Gregory XIII decided to repair the calendar. Gregory was aided by astronomers in developing a calendar that would be more accurate than the Julian calendar. The solution they developed was almost perfect.
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