Y

year

a period of time based on the revolution of the Earth around the Sun, or the period of the Sun’s apparent motion around the celestial sphere. The length of a given year depends on the choice of the reference point used to measure this motion.

year, anomalistic

the  period between successive passages of the Earth through perihelion. The anomalistic year is approximately 25 minutes longer than the tropical year.

year, Besselian

the period of one complete revolution in right ascension of the fictitious mean Sun, as defined by Newcomb. Its length is shorter than a tropical year by 0.148×T seconds, where T is centuries since 1900.0. The beginning of the Besselian year occurs when the fictitious mean Sun is at ecliptic longitude 280°. Now obsolete.

year, calendar

the period between two dates with the same name in a calendar, either 365 or 366 days. The Gregorian calendar, now universally used for civil purposes, is based on the tropical year.

year, eclipse

the period between successive passages of the Sun (as seen from the geocenter) through the same lunar node (one of two points where the Moon’s orbit intersects the ecliptic). It is approximately 346.62 days.

year, Julian

a period of 365.25 days. It served as the basis for the Julian caledar.

year, sidereal

the period of revolution of the Earth around the Sun in a fixed reference frame. It is the mean period of the Earth’s revolution with respect to the background stars. The sidereal year is approximately 20 minutes longer than the tropical year.

year, tropical

the period of revolution of the Earth around the Sun with respect to the dynamical equinox. The tropical year comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. It is approximately 365 days, 5 hours, 48 minutes, 45 seconds.