Why every 4 years have February 29th?

Question

why must every 4 years have february the 29th?

Answer 1

it is because earth takes 365 days n 6hrs for each revolution...

so.. 4 times 6 hrs becomes a day..

so 29 th feb comes in every four years..

Answer 2

The vernal equinox is time when the sun is directly above the Earth's equator, appearantly moving from the southern to the northern hemisphere.

The mean time between two successive vernal equinoxes is called a tropical year, and it is about 365.2422 days long.

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.

In the Gregorian calendar, which is the calendar used by most modern countries, the following rules decide 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.