The Gregorian Calendar
In 1582, a refinement to the Julian calendar became effective. It became known as the Gregorian calendar after Pope Gregory XIII. The Julian calendar had a difference of 11 minutes and 15 seconds with the tropical (solar) year. This meant that by the time the Gregorian calendar became effective (1627 years after the Julian calendar became effective), a mismatch of about 12 days with the solar cycle had occurred. For this reason, there were 10 days that were removed in the transition from the Julian to the Gregorian calendar. The calendar moved from October 4 to October 15, 1582. The century years stopped being leap years except if divisible by 400. By doing this the the effective duration of the year in the Gregorian calendar became 365.2425 days, still not the same as the tropical year' duration of 365.24219. Another change was the creation of February 29th for the leap year, as opposed to the repetition of February 23rd as it happened in the Julian calendar.
