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:
tropical year: the period 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 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:
- the perihelion, where the Earth is closest to the Sun (around 2 January), and
- 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:
ecliptic year: the period for the Earth to complete one revolution with respect to a node of the Moons orbit (the points where the Moon's orbit intersects the ecliptic). This period is associated with eclipses: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a Month every half ecliptic year. The average duration of the eclipse year is:
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.
Besselian year: this is a tropical year that starts when the mean Sun reaches the ecliptic longitude of 280 deg. This is always on or close to the 1st day of January. It is named after the 19th-century astronomer and mathematician Friedrich Bessel. An approximate formula to compute the current time in Besselian years from the Julian day is:
B = 2000 + (JD - 2451544.53)/365.242189