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        Science | Astronomy Terms

Year, a unit of time. It is based on the time it takes the earth to make one revolution around the sun. The tropical, or solar, year, also called year of the seasons, is the length of time between two successive spring equinoxes. Its length is 365 days, 5 hours, 48 minutes, 46 seconds, or slightly less than 365 1/4 days. The calendar year is based on the tropical year but avoids fractional days. Extra days are periodically added to the calendar year so that its average length will equal the actual length of the tropical year. These adjustments must be made in order to have the spring and fall equinoxes occur in the same months each year.

The calendar year normally has 365 days. Each fourth year, however, usually has 366 days. (The extra day is February 29.) The 366-day years, called leap years, are those divisible by 4, such as 1992, 1996, and 2004. except for century years (1800, 1900, etc.). which are leap years only if they are divisible by 400, such as 1600 and 2000. The four-year adjustment makes the calendar year about 11 minutes longer than the tropical year. The difference amounts to about three days for every 400 years; it is for this reason that only every fourth century year is a leap year.

When a new year should begin has been determined by agricultural, religious, and political events. Pope Gregory XIII in 1582 chose January 1 to be the first day of the year, replacing March 25. (March 25Annunciation Dayhad been chosen early in the Christian Era because of its religious significance.) Gregory's system was quickly adopted in Catholic countries but was resisted in Protestant ones: England and the American colonies continued to use March 25 until 1752.

Other Kinds of Years

A fiscal year is a 12-month period used by government bodies and many businesses for budgetary and accounting purposes. It may be the same as the calendar year but often is not; the United States government, for example, uses a fiscal year running from July 1 to June 30. The academic, or school, year commonly begins in September and lasts for nine months. The church year, also called ecclesiastical or liturgical year, is established according to various religious observances. In most Christian churches it begins with the first Sunday in Advent.

Two additional kinds of years are based on natural cycles and are of interest mainly to astronomers:

The Sidereal Year

is the length of time taken by the earth to make one revolution about the sun. It lasts 365 days, 6 hours, 9 minutes, and 9.5 seconds, and is gradually becoming longer. The tropical year is shorter than the sidereal because the axis of the earth is very slowly wobbling like a top. The result is that equinoxes occur about 20 minutes sooner than they would if there were no wobble.

The Anomalistic Year

is the time between two successive passages of the earth through the point in its orbit that is closest to the sun. It is 4 minutes, 44 seconds longer than the sidereal year and is also gradually becoming longer.