How Long Is A Year?


What is a year? How long is a year? What is a tropical (solar) year? What is a light year? Information on year.

yearYear; a unit of time intended to equal the period of revolution of the earth around the sun. Prior to 4000 b.c. the Egyptians established the length of the year as 365 days by counting the days that elapsed between two successive heliacal risings of the star Sirius; that is, two successive appearances of Sirius just before sunrise, each occurring after a period of invisibility caused by its nearness to the sun. Astronomers distinguish several kinds of year, of which three—the tropical, sidereal, and anomalistic—are most common.

The tropical year, also called solar year or equinoctial year, is the year to which the Gregorian calendar is adjusted. It is the interval between two successive returns of the sun to the vernal equinox and is approximately equal to 365.24220 mean solar days, or 365 days 5 hours 48 minutes 46 seconds. Since 1956 the tropical year has been the interval in terms of which the second, the fundamental unit of time in the several systems of units employed in physics and engineering, is defined. In that year the International Committee on Weights and Measures redefined the second as 1/31,556,925.9747 of the tropical year for Dec. 31, 1899 (called January 0, 1900, in astronomical literature) at 12 hours ephemeris time. It was necessary to use a specific date because the length of the year is not precisely constant: the tropical year is slowly decreasing in length, while the sidereal and anomalistic years are slowly increasing. The rate of change, however, is less than 1 second per century.

The sidereal year is the average period of revolution of the earth with respect to the fixed stars; its duration is 365.25636 mean solar days. The light-year, a unit used to express stellar distances, is the distance light travels in 1 sidereal year; it is equal to 5.879 x 1012 miles. The anomalistic year is the average interval between successive closest approaches of the earth to the sun; its length is 365.25964 mean solar days.

The calendar year for the Gregorian calendar consists of 365 days (366 in leap years) and is divided into 12 months. It begins on January 1 and ends on December 31. Since the length of the tropical year is nearly 365% days, it is necessary once every 4 years (with certain exceptions; see Leap Year ) to add a day to the calendar year. This day, which falls on February 29, is known as leap day.

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