Originally written by John Webb
The year is 365.2422 days long. To make this inconvenient number fit into calendars with whole numbers of days in a year, a system of Leap Years is needed.
Julius Caesar decreed a calendar with years of 365 days, with an extra day every four years. That was equivalent to using as the number of days in a year. This number was a little too large, so that more Leap Years were added than needed.
Over the years, the error built up to ten days, until in 1582 Pope Gregory XIII decreed that there should be a Leap Year in every year divisible by 4, except in years that were divisible by 100 but not by 400.
That amounted to using as the length of the year, which is a very good approximation to the true year of 365.2422 days. Today’s Western Calendar follows Pope Gregory’s rule.…