Basically, a leap year is a year containing one extra day. It supposedly is every year evenly divisible by 4, but it's more complex than that.
A tropical year, or solar year, is approxima
tely 6 hours more than 365 days. 6 hours is a quarter of a day, so one year is approximately 365.25 days long. Therefore adding one more day to the calendar every 4 years would make sense, making an average year 365.25 days long, and that's exactly what the Julian calendar does. But notice that a solar year is only approximately 6 hours more than 365 days; it's actually a bit shorter, about 365.24237
4 days long, and increasing. So to better approximate the length of the year, a better system was developed.
The Gregorian calendar is the most widely used one in the world. It was decreed by Pope Gregory XIII in 1582 to make sure Easter and the Vernal Equinox would fall at the same time. It modified the Julian calendar with a new rule. Leap years are years exactly divisible by 4, except years that are exactly divisible by 100, but centurial years exactly divisible by 400 are still leap years. What that means is that the
year 1900 was not a leap year, but 2000 was. While in the Julian calendar, both 1900 and 2000 were leap years. The Gregorian calendar results in a better average year: 365.2425 days long, which gives a more accurate approximation.
Update: Google has made a Google Doodle for the Leap Year. It shows a frog "leaping" instead of the l in Google.
