Yearsimple:year A year is the term for any period of time that is derived from the period of the orbit of the Earth (or indeed any planet) around its 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:
- tropical year: the period for the Earth to complete one revolution with respect to the framework provided by 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 framework moves slowly backwards along the ecliptic with respect to the fixed stars; as a consequence, the Earth completes this year before it completes a full orbit as measured in a fixed reference frame. Therefore a tropical year is shorter than the sidereal year: its duration (averaged over all tropical ecliptic points) was:
- anomalistic year: the period 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:
- 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:
- 365.259635864 days (at the epoch J2000).
- eclipse or ecliptic year: the period for the Sun (as seen from the Earth) to complete one revolution with respect to a node of the Moon's 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 eclipse year. Hence the are two eclipse seasons every eclipse year. The average duration of the eclipse year is:
- similar to the eclipse year, there is the period for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moon's orbit. This has not received much attention in astronomical literature. This period is associated with the apparent size of the Full Moon, and also with the varying duration of the synodic month. The duration of one fumocy is:
Calendars usually try to match some tropical year, because the seasons and their cardinal points are determined by this sort of year. 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 March equinox tropical year (365.2424 days), almost every 4th year gets 366 days: this is called a leap year. The most important current exception is the Islamic calendar, a lunar calendar without leap years, in which holidays move through the seasons.
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°. 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
The distinction from one planet to another is made through use of a preceding adjective, such as: "the Martian year."