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A leap year (also known as an intercalary year or a bissextile year) is a year containing one additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year, drift over time with respect to the event that the year is supposed to track. By inserting (also called ''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. For example, in the Gregorian calendar, each leap year has 366 days instead of the usual 365, by extending February to 29 days rather than the common 28. Similarly, in the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. The name "leap year" comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in a leap year will advance two days (from March onwards) due to the extra day added at the end of February (thus "leaping over" one of the days in the week). For example, Christmas fell on Tuesday in 2001, Wednesday in 2002, and Thursday in 2003 but then "leapt" over Friday to fall on a Saturday in 2004. ==Gregorian calendar== In the Gregorian calendar, the current standard calendar in most of the world, ''most'' years that are multiples of 4 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 period of 365 days is shorter than a tropical year by almost 6 hours. This calendar was first used in 1582. Some exceptions to this basic rule are required since the duration of a tropical year is slightly ''less'' than 365.25 days. Over a period of 4 centuries, the accumulated error of adding a leap day ''every'' 4 years amounts to about 3 extra days. The Gregorian calendar therefore removes three leap days every 400 years, which is the length of its ''leap cycle''. This is done by removing February 29 in the three century years (multiples of 100) that cannot be exactly divided by 400. The years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are common years. By this rule, the average number of days per year is 365 + − + = 365.2425. The rule can be applied to years before the Gregorian reform (the proleptic Gregorian calendar). The rule is not directly applicable to years numbered with BC notation. Since 0 is divisible by 4, 100, and 400, the "year 0" (1 BC) is a leap year. Thus the rule can be applied to years before AD 1 if astronomical year numbering is used. 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 full moon that falls on or after March 21) remains close to the vernal equinox. The "Accuracy" section of the "Gregorian calendar" article discusses how well the Gregorian calendar achieves this design goal, and how well it approximates the tropical year. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Leap year」の詳細全文を読む スポンサード リンク
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