Year

A year is the term for any period of time that is derived from the period of the orbit of the Earth around the Sun. In astronomy, several types of year are defined:

[sidereal year]?: the actual period for the Earth to complete one revolution of its orbit, as measured in a fixed frame of reference (such as the fixed stars, Latin sidus). Its duration is on average 365.25... days. {ToDo?: I'll fill this in later -- Tompeters} . The actual duration varies from year to year because the motion of the Earth is influenced by the gravity of the Moon and other planets.

tropical year: the time it takes for the Earth to complete one revolution with respect to the equinox. This point is on the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). Because of the precession, this points moves slowly backwards along the ecliptic; as a a consequence, the Earth reaches this point before it completes a full orbit as measured in a fixed reference frame. Therefore the tropical year is shorter than the sideral year: its average duration is 365.24218967 days (365d 5h 48m 45s) (at the epoch J2000? = 1 Jan. 2000 12h TDT?).

[anomalistic year]?: the time it takes for the Earth to complete one revolution with respect to its apsides?. The orbit of the Earth is elliptical; the extreme points, called apsides?, are:

1. the perihelion, where the Earth is closest to the Sun (around 2 January), and
2. the aphelion, where the Earth is furthest from the Sun (around 2 July).

Because of gravitational disturbances by the other planets, the shape and orientation of the orbit are not fixed, and the apsides slowly move with respect to a fixed frame of reference. Therefore the anomalistic year is slightly longer than the sidereal year: on average, 365.25... days. {ToDo?: I'll fill this in later -- Tompeters}

Calendars usually try to match the tropical year, because the seasons are determined by this period. For practical reasons, a calendar year consists of an integer number of days. In the calendar currently in use in western societies, the Gregorian Calendar, most years have 365 days. In order to keep synchronized with the actual tropical year, almost every 4th year gets 366 days: this is called a leap year.

Julian year: 365.25 days, the average length of the year in the Julian Calendar.