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Julian and Gregorian Calendars (view source)
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The previous calendar in Europe was the Julian Calendar, instituted in 46 BC and named after Julius Caesar. The old Julian Calendar assumed the earth went around the sun in exactly 365.25 days. For this calendar to follow the earth’s movement, this rule was used - every year that was divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. In actual fact, the earth travels around the sun in 365.2422 days, about 11 minutes shorter than the old Julian Calendar. This discrepancy accumulated about 3 days short every 4 centuries. By 1582, the calendar was 10 days early. | The previous calendar in Europe was the Julian Calendar, instituted in 46 BC and named after Julius Caesar. The old Julian Calendar assumed the earth went around the sun in exactly 365.25 days. For this calendar to follow the earth’s movement, this rule was used - every year that was divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. In actual fact, the earth travels around the sun in 365.2422 days, about 11 minutes shorter than the old Julian Calendar. This discrepancy accumulated about 3 days short every 4 centuries. By 1582, the calendar was 10 days early. | ||
The Catholic Church was very concerned because the celebration of Easter was figured from the spring equinox. And the spring equinox was now happening 10 days earlier | The Catholic Church was very concerned because the celebration of Easter was figured from the spring equinox. And the spring equinox was now happening 10 days earlier than it should. As a result, on 24 February 1582 Pope Gregory XIII issued a decree (a papal bull) instituting a new calendar. | ||
To return the spring equinox to 21 March, the new Gregorian Calendar chopped 10 days from the year. Also, a change already in progress was validated - the first day of the year was changed from the 25 March to 1 January. But most importantly, to keep consistent with the earth’s movement around the sun, a new rule was followed - every year that is exactly divisible by four is a leap year, except for centurial years that are not divisible by 400. This means that the year 2000, being exactly divisible by 400, was a leap year, but 2100, 2200, and 2300, although falling in the 4-year cycle, are not leap years because they are not evenly divisible by 400. | To return the spring equinox to 21 March, the new Gregorian Calendar chopped 10 days from the year. Also, a change already in progress was validated - the first day of the year was changed from the 25 March to 1 January. But most importantly, to keep consistent with the earth’s movement around the sun, a new rule was followed - every year that is exactly divisible by four is a leap year, except for centurial years that are not divisible by 400. This means that the year 2000, being exactly divisible by 400, was a leap year, but 2100, 2200, and 2300, although falling in the 4-year cycle, are not leap years because they are not evenly divisible by 400. | ||