Leap years are needed to keep our calendar in alignment with the earth's revolutions around the sun. The vernal equinox is the time when the sun is directly above the Earth's equator, moving from the southern to the northern hemisphere. The mean time between two successive vernal equinoxes is called a tropical year–also known as a solar year–and is about 365.2422 days long. Using a calendar with 365 days every year would result in a loss of 0.2422 days, or almost six hours per year. After 100 years, this calendar would be more than 24 days ahead of the season (tropical year), which is not desirable or accurate. It is desirable to align the calendar with the seasons and to make any difference as insignificant as possible. By adding a leap year approximately every fourth year, the difference between the calendar and the seasons can be reduced significantly, and the calendar will align with the seasons much more accurately. (courtesy of timeanddate.com)
