Year
simple: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:
- 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 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:
- 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:
- 365.259635864 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.
The Gaussian year lasts 365.2568983 days, and is derived from the Gaussian gravitational constant that is expressed in units of the solar system.
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."
- See also calendar year, fiscal year, leap year, solar year, time