Leap Year

A leap year has 366 days, or one more day than an ordinary year. A leap year occurs in every year that can be divided evenly by four, except the year that mark the hundreds, such as 1500. The only century years that are leap years are those that can be evenly by 400, such as 1600 and 2000. Leap years were added to the calendar to make the calendar year nearly the same as the solar year, which is the time it takes for the sun to pass the vernal equinox twice. The extra day is added to the end of February and occurs as February 29 once every four years.

Year is the time the earth takes to make one complete revolution around the sun. There are two different kinds of years which are used by astronomers. The solar, equinoctial, or tropical year is the time between two passages of the sun through the vernal equinox, which occurs in March. This year is 365 days, 5 hours, 48 minutes and 46 seconds long. This year is used for all practical and astronomical purposes. It is the basis of our common or calendar year.

The sidereal year is made up of 365 days, 6 hours, 9 minutes and 9.5 seconds. This is the time it takes the earth to return to the same place in its orbit, with reference to the fixed stars. The sidereal year is longer than solar year because of the precession of the equinoxes. The sidereal year is seldom used except in the calculations of astronomers.

The calendar year is only 365 days long, and so we have to add an extra day every four years to correct the difference in time between the calendar year and the solar year. This fourth year is called leap year, and the extra day is February 29. Adding an extra day every fourth year makes the average calendar year 11 minutes, 14 seconds too long. So, the day is not added in the century years, except in those divisible by 400. The years 2009 will contain 366 days. Thus the difference between calendar and solar years will vary only one day over period of several thousand years.

The lunar year is made up of 12 lunar months. The ancient Greeks used this year. It contained 354 days. In most Christian nations the calendar year begins on January 1. During the Middle Ages, however, most European nations considered March 1 or March 25, Annunciation Day, as the first day of the calendar year. By 1600 nearly all civilized countries except England recognized January 1 as the first day of the year. England adopted the Gregorian calendar, which recognized January 1 as the beginning of the year, on September 14, 1752.

The church calendar, which is used in the Roman Catholic and in most Protestant churches, is regulated partly by the solar and partly by the lunar year. This causes a difference between the fixed feast day, which always fall on the same day every year, and movable feasts such as Easter, whose dates vary from year to year. The fixed feast days are determined by the solar year, and the movable feast days, by the lunar year.

In ancient Roman times, before the Julian calendar was adopted, the year began on March 1. The Jewish year begins near the time of the autumnal equinox, around the 22nd of September. The Islamic year, however, is based on the changing phases of the moon and lasts for 354 days. Therefore, the beginning of the Islamic year continually falls earlier in the seasons. Thirty Islamic years make up a cycle during which there are 11 leap years. These leap years occurs at irregular intervals.