Year
From Wikipedia, the free encyclopedia.
A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, "Martian year".
Seasonal year
A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant.
Calendar year
A calendar year is the time between two dates with the same name in a calendar.
Calendars usually aim to predict the seasons, but because it is hard to determine the length of the seasonal year, they instead use an astronomical year as a surrogate for the seasonal year. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.
The Gregorian calendar aims to keep the vernal equinox on or close to March 21; hence it follows the vernal equinox tropical year.
No astronomical year has an integer number of days, so any calendar that follows an astronomical year must have a system of leap years.
A Julian year is exactly 365.25 days, the average length of the year in the Julian calendar.
Astronomical year
An astronomical year is the time between two recurrences of an astronomical event.
The sidereal year is the time 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:
- 365.256363051 days (at the epoch J2000 = 1 January 2000 12:00 TT).
A
tropical year is the time 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 of the equinoxes, 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. The exact length of a tropical year depends on the chosen starting point: for example the
vernal equinoctial year is the time between successive vernal equinoxes. The
mean tropical year (averaged over all tropical ecliptic points) is:
- 365.24218967 days (365d 5h 48m 45s) (at the epoch J2000).
The
anomalistic year is the time 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 the
perihelion, where the Earth is closest to the Sun (2 January in 2000), and the
aphelion, where the Earth is furthest from the Sun (2 July in 2000). 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 it is:
- 365.259635864 days (at the epoch J2000).
The
eclipse year or
ecliptic year is the time 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 there are
two eclipse seasons every eclipse year. The average duration of the eclipse year is:
- 346.620075883 days (at the epoch J2000).
The
full moon cycle or
fumocy is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the
perigee of the Moon's orbit. 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 full moon cycle is:
- 411.78443029 days (at the epoch J2000).
A
heliacal year is the interval between the heliacal risings of a star.
The Sothic year is the interval between the heliacal risings of the star Sirius.
The Gaussian year is the siderial year for an ideal planet governed by the Gaussian gravitational constant. Its length is:
- 365.2568983 days.
The
Besselian year is a
tropical year that starts when the mean
Sun reaches the ecliptic longitude of 280°. This is currently on or close to 1 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
Variation in the length of the year and the day
The exact length of a year changes over time. The main sources of this change are as follows:
- The precession of the equinoxes changes the position of astronomical events with respect to the apsides of the Earth's orbit. An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year.
- The gravitational influence of the Moon and planets changes the shape of the Earth's orbit.
Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month. This in turn depends on factors such as
continental rebound and
sea level rise.