The Rule:
According to the Gregorian calendar, which is the civilcalendar in usetoday, years evenly divisible by 4 are leap years, with the exception ofcenturial years that are not evenly divisible by 400. Therefore, the years1700, 1800, 1900 and 2100 are not leap years, but 1600, 2000, and 2400are leap years.
Background:
The Gregorian calendar year is intended to be of the same length asthe cycle of the seasons. However, the cycle of the seasons, technicallyknown as the tropical year, is approximately 365.2422 days. Sincea calendar year consists of an integral number of whole days, a calendar yearcannot exactly match the tropical year. If the calendar year always consistedof 365 days, it would be short of the tropical year by about 0.2422 days everyyear. Over a century, the calendar andthe seasons would depart by about 24 days, so that the beginning of Springin the northern hemisphere would shift from March 20 to April 13.
To synchronize the calendar and tropical years, leap days are periodicallyadded to the calendar, forming leap years. If a leap day is added everyfourth year, the average length of the calendar year is 365.25 days. Thiswas the basis of the Julian calendar, introduced by Julius Caesar in46 B.C. In this case the calendar year is longer than the tropical year byabout 0.0078 days. Over a century this difference accumulates to a little overthree quarters of a day. From the time of Julius Caesar to the sixteenthcentury A.D., the beginning of Spring shifted from March 23 to March 11.
When Pope Gregory XIII instituted the Gregorian calendar in 1582, thecalendar was shifted to make the beginning of Spring fall on March 21 anda new system of leap days was introduced. Instead of intercalating a leapday every fourth year, 97 leap days would be introduced every 400 years,according to the rule given above. Thus, the average Gregorian calendaryear is 365.2425 days in length. This agrees to within a half a minute ofthe length of the tropical year. It will take about 3300 years before theGregorian calendar is as much as one day out of step with the seasons.
