Leap Year

A leap year is a calendar year made longer than a standard year by the addition of an extra day or month. Its purpose is to align the calendar with the actual cycle of the seasons and the movement of the moon, since the natural orbital period contains a fractional number of days.

In the Gregorian calendar, now used by most of the world, a standard year is 365 days. However, Earth's orbit around the sun takes approximately 365 days and a quarter. If that quarter is ignored, the calendar drifts from the seasons. For this reason, every fourth year an extra day is added—29 February—making that year 366 days.

The Gregorian calculation rule

A Gregorian year is a leap year if it is evenly divisible by 4. However, if it ends in a hundred (divisible by 100), it is a standard year, unless it is also divisible by 400. Therefore 1900 was not a leap year, while 2000 was. This rule reduces error from treating a quarter day as exact, since the actual orbit is slightly less than 365.25 days.

The Julian calendar and the transition

The Julian calendar, begun by Rome, added a day every four years without the hundred-year rule. This caused the calendar to drift progressively from the seasons. The Gregorian reform, introduced in the sixteenth century, added the hundred-year and four-hundred-year rules and adjusted dates to correct the accumulated drift. Many countries adopted this change at different times, making the dates of certain historical events appear under two different systems depending on source.

Lunar and lunisolar calendars

The Islamic Hijri calendar is lunar, consisting of 354 or 355 days; its leap year adds only one day counted within the months and is not tied to the solar seasons. Lunisolar calendars—such as the Hebrew calendar and some Asian calendars—add an extra month to certain years to align the lunar months with the solar year.