Happy Leap Year Day!!!
Yeah, as the title suggests, Happy Leap Year Day...whoo hoo...another day...just another day :-
Here's some info for ya:
In the Gregorian calendar, which is the calendar used by most modern countries, the following rules decides which years are leap years:
1. Every year divisible by 4 is a leap year.
2. But every year divisible by 100 is NOT a leap year
3. 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?
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.)