
The Gregorian calendar, like the , is a with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a being added to February in the . The months and length of months in the Gregorian calendar are the same as for the Julian calendar. The only difference is that the Gregorian calendar omits a leap day in three centurial years every 400 years and leaves the leap day unchanged. [pdf]
The Gregorian calendar, used in Europe and in a very large part of the world, takes its name from Pope Gregory XIII who set it up in 1582. This calendar is a correction to the previous calendar, the Julian calendar introduced by Julius Caesar in 46 BC. The starting point of Year 1 is an approximate date of the birth of Jesus.
By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days).
It proposes two major corrections. The first is an 11-day jump in the calendar: the day after 4 October 1582 will be 15 October 1582, and 10 days are therefore removed from the calendar. The second is a new way of calculating leap years. In the Gregorian calendar, the tropical year was approximated to 365.2425 days.
The Gregorian calendar reduces the number of intercalary days to 97 in 400 years, as opposed to 100 intercalary days in 400 Julian years. The change is small but profound. It brings the mean length of the calendar year into much closer agreement with the tropical year, providing, a mean cal endar year of 365.2425 days.
There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception.
The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long. [ 82 ] This approximation has an error of about one day per 3,030 years [ s ] with respect to the current value of the mean tropical year.
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