Leap Year

Leap Year, calendar year in which an extra day is added at the end of February, giving a total of 366 days instead of the usual 365. Leap years are necessary because the orbital period of the Earth around the Sun (on which the length of the year is based) is not an exact number of days—its actual length is 365.242 days. The rule for adding days to the Gregorian calendar (the one currently in use in most of the world) is that every fourth year is a leap year, but century years are not leap years unless they are divisible by 400. Hence, the year 2000 is a leap year but 1900 was not and 2100 will not be. This arrangement ensures that the calendar remains in step with the seasons.