|
|
|
|
|
|
| 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. (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.) |