A
leap year (or
intercalary year) is a year containing one or more extra days (or, in the case of
lunisolar calendars, an extra month) in order to keep the
calendar year synchronised with the
astronomical or
seasonal year. For example, in the
Gregorian calendar, February has 29 days in a leap year instead of the usual 28 - and consequently, the year lasts 366 days instead of the common 365. Because seasons and astronomical events do not repeat at an exact number of full days, a calendar that had the same number of days in each year would, over time, drift with respect to the event it was supposed to track. By occasionally inserting (or
intercalating) an additional day or month into the year, the drift can be corrected. A year that is
not a leap year is called a
common year.
In the Gregorian calendar, the current standard calendar in most of the world, most years whose division by 4 equals an integer are leap years. In each leap year, the month of February has 29 days instead of 28. Adding an extra day to the calendar every four years compensates for the fact that a solar year is almost 6 hours longer than 365 days.
However, some exceptions to this rule are required since the duration of a solar year is slightly less than 365.25 days. Years that are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years.[1][2] For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not in the 1582 papal bull (the 1750 British Calendar Act ignores 1700). Similarly, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 will not be leap years, but 2400 and 2800 will be. By this rule, the average number of days per year will be 365 + 1/4 - 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds.
The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon (i.e. a full moon) that falls on or after March 21) remains correct with respect to the vernal equinox.[3] The vernal equinox year is about 365.242374 days long (and increasing), whereas the average year length of the Gregorian calendar is 365.2425.
