Leap Years Begin
Julius Caesar introduced leap years during the time he ruled in Rome (48 - 44 BC) to keep the calendar in step with the seasons. By adding one extra day to every 4 years, the average length of a year becomes 365.25 days. However, the time for the Earth to rotate around the sun is slightly less than this at 365.24219 days. This small difference equals about 11 minutes per year.
The Calendar Is Fixed
Julius Caesarís fix worked well for a while, but by the year 1582 the calendar was 11 days ahead of the seasons. The Catholic Church became worried about this because Easter was slowly occurring earlier each year, moving from March into February. Pope Gregory asked for the calendar to be fixed again. It was found that the error in the calendar gave us three extra days every 400 years. So, it was decided that every 400 years, 3 leaps years would be removed and treated as normal years of 365 days. An easy way to remember this was to make a rule that:
All years ending in "00" would not be leap years unless they were divisible by 400.
So the year 1600 was a leap year, then 1700, 1800 and 1900 were not. The year 2000 was a leap year, then 2100, 2200, and 2300 will not be. And so the Gregorian calendar was born!
Pope Gregory Makes Days Vanish
Even with this correction, Pope Gregory still had a calendar that was 11 days off, so he decided that October 4, 1582 would be followed by October 15, 1582, skipping 11 days to put the calendar in line with the seasons.
This loss of 11 days was adopted immediately in Catholic countries, but it didnít occur in England until 1752. The Russian people didnít switch to the Gregorian calendar until 1917.
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