In the Gregorian calendar, the calendar used by most modern countries, the following three criteria determine which years will be leap years:
It is interesting to note that 2000 was somewhat special as it was the first instance when the third criterion was used in most parts of the world.
In the Julian calendar–introduced by Julius Caesar in 46 BC and patterned after the Roman calendar–there was only one rule: any year evenly divisible by four would be a leap year. This calendar was used before the Gregorian calendar was adopted.
Note: The illustration is not to scale.
The vernal equinox is the time when the sun is directly above the Earth's equator, moving from the southern to the northern hemisphere.
The mean time between two successive vernal equinoxes is called a tropical year–also known as a solar year–and is about 365.2422 days long.
Using a calendar with 365 days every year would result in a loss of 0.2422 days, or almost six hours per year. After 100 years, this calendar would be more than 24 days ahead of the season (tropical year), which is not desirable or accurate. It is desirable to align the calendar with the seasons and to make any difference as insignificant as possible.
By adding a leap year approximately every fourth year, the difference between the calendar and the seasons can be reduced significantly, and the calendar will align with the seasons much more accurately.
(The term "day" is used to mean "solar day"–which is the mean time between two transits of the sun across the meridian of the observer.)
No calendars used today are perfect; they are off by seconds, minutes, hours or days every year. To make a calendar more accurate, new leap year rules have to be introduced to the Gregorian calendar, complicating the calculation of the calendar even more. It will, however, need some modifications in a few thousand years. As for the tropical year, it is approximately 365.242199 days, but varies from year to year because of the influence of other planets.
|Name of calendar||When introduced||Average year||Approximate error introduced|
|Gregorian calendar||AD 1582||365.2425 days||27 seconds (1 day every 3,236 years)|
|Julian calendar||45 BC||365.25 days||11 minutes (1 day every 128 years)|
|365-day calendar||-||365 days||6 hours (1 day every 4 years)|
|Lunar calendar||ancient||12-13 moon-months||variable|
A calendar similar to the Julian calendar, with every fourth year earmarked as a leap year, was first introduced by King Ptolemy III of Egypt in 238 BC.
In ancient times, it was customary to have lunar (moon) calendars, with 12 and/or 13 months every year. To align the calendar with the seasons, the 13th month was inserted as a "leap month" every two or three years. Many countries, especially in Asia still use such calendars. Read more about Leap Year in Other Calendars.
Note: Many other calendars have been and still are used throughout the world.
The Julian calendar introduced too many leap days, thus increasing the number of days between the vernal equinox of March 21, its scheduled date as noted in AD 325 during the Council of Nicaea. The introduction of the Gregorian calendar allowed for realignment with the equinox; however, a number of days had to be dropped when the change was made. Click on any one of the year links below for a better explanation of the calendars and the days that were dropped in order to make the switch to the Gregorian calendar.