Year

Year, name of various units of time, all depending on the revolution of the earth about the sun. The sidereal year (365.256636 mean solar days) is the average time the earth takes to complete one revolution measured with respect to a fixed direction in space. The tropical year (365.24220 mean solar days), the year measured by the changing seasons, is that in which the mean longitude of the sun moves through 360°. The anomalistic year (365.25964 mean solar days) is the average interval between successive terrestrial perihelions. The civil year is a period of variable duration, usually 365 or 366 days (leap year), depending on the type of calendar in use.