The Julian calendar estimated the length of the natural solar year
(the time it takes for the earth to make one orbit of the sun) to be
365 days and 6 hours. Every fourth year the extra six hours were
collected and added as an extra day to the year, creating a leap year
of 366 days.
But the calendar's underlying estimate was off by 11 minutes and 14
seconds. It was longer than the natural solar year. The extra minutes
accumulated to one whole day. By 325 AD, the Spring Equinox was
arriving on March 21st on the Julian Calendar - instead of March 25.
The First Ecumenical Council met in Nicea in 325 and determined that
the date to celebrate Pascha was on the first Sunday, after the first
full moon, after the Spring Equinox on March 21st. In other words, it
enshrined the Julian calendar's aberration.
Thus, by 1582, the Spring Equinox was arriving on March 11.
Half-hearted measures by Popes Paul III and Pius V failed to restore
the essential correspondence between the calendar and the seasons.
Pope Gregory XIII decided - in his tenth year in office - to drop 3
leap years every 400 years by specifying that any year whose number
ended with 00 must also be evenly divisible by 400 in order to have a
29-day February.
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