What is a leap year?
A leap year is a year with one extra day inserted into February, the leap year is 366 days with 29 days in February as opposed to the normal 28 days. (There are a few past exceptions to this)
Which years are leap years?
In the Gregorian calendar, which is the calendar used by most modern countries, the following rules decides which years are leap years:
Every year divisible by 4 is a leap year.
But every year divisible by 100 is NOT a leap year
Unless the year is also divisible by 400, then it is still a leap year.
This means that year 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years, while year 2000 and 2400 are leap years.
This actually means year 2000 is kind of special, as it is the first time the third rule is used in many parts of the world.
In the old Julian Calendar, there was only one rule: Every year divisible by 4 is a leap year. This calendar was used before the Gregorian calendar was adopted.
Why are leap years needed?
Short answer:
Leap years are needed so that the calendar is in alignment with the earth's motion around the sun.
Long answer:
The mean time between two successive vernal equinoxes is called a tropical year, and it is about 365.2422 days long. This means that it takes 365.2422 days for the earth to make one revolution around the sun (the time is takes to orbit the sun).
Using a calendar with 365 days would result in an error of 0.2422 days or almost 6 hours per year. After 100 years, this calendar would be more than 24 days ahead of the seasons (tropical year), which is not a desirable situation. It is desirable to align the calendar with the seasons, and make the difference as small as possible.
By adding leap years approximately every 4th year, this difference between the calendar and the seasons can be reduced significantly, and the calendar will follow the seasons much more closely than without leap years.
(One day is here used in the sense of "mean solar day", which is the mean time between two transits of the sun across the meridian of the observer.)
Is there a perfect calendar?
None of the calendars used today are perfect, they go wrong by seconds, minutes, hours or days every year. To make a calendar even better, new leap year rules have to be introduced, complicating the calculation of the calendar even more. The currently used Gregorian calendar may need some modification a few thousand years ahead. A tropical year is approximately 365.242199 days, but it varies from year to year, because of influence by the other planets.
Name of calendar |
Introduced |
Average year |
Approximate
error introduced |
Gregorian calendar |
AD 1582 |
365.2425 days |
27 secs (1 day every 3236 years) |
Julian calendar |
45 BC |
365.25 days |
11 mins (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 like the Julian Calendar (with every 4th year as a leap year) was first introduced by king Ptolemy III, Egypt in 238 BC.
In ancient times, it was very usual 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 2-3 years.
Note: Many other calendars have been and are still used throughout the world.
(With permission from http://www.timeanddate.com/date/leapyear.html ) |