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# Leap year

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The leap year is a method of keeping the calendar year in sync with the seasons. The Earth's seasons repeat once every [tropical year]? (the time it takes the Earth to complete an orbit around the Sun), which is about 365.2422 days long, so a 365-day year will cause the seasons to slowly drift. By occasionally adding an additional day (in the Gregorian Calendar this is February 29) to the year, making it 366 days long instead of the usual 365, this can be corrected.

There are some rules for leap year:

- The year has to be divisible by 4, like 1984.
- When the year is divisible by 100 the leap year will not happen, like 1900.
- Except if the year is divisible by 400 the leap year will usually happen, like 2000.

The logic behind the above rules are as follows:

By adding a day every four years, an average year is adjusted to 365.25 days. There is still descrepency from the actual 365.2422 days period. To make the average year more accurate, a leap year is cancelled in each century. That takes 0.01 days out to bring the average to 365.24 days. That is still not accurate enough. So the cancelled leap year returns every four centuries. That adds back 0.0025 days to bring the average to 365.2425 days. The adjusted average is still 0.0003 day ahead of the actual period. In other words, the Gregorian Calendar will still gain a day every 3333 years.

Is this 0.0003 day difference adjusted by the occasional [leap second]? maintained by some international time keeping organisation?

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Last edited August 7, 2001 7:04 pm (diff)
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