A 'year' (from Old English ''
gēr'') 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, a "Martian year" is the time in which Mars completes its own orbit.
Calendar year
A '
calendar year' is the time between two dates with the same name in a
calendar.
Solar calendars usually aim to predict the four seasons, but because the length of individual seasonal years varies significantly, they instead use an astronomical year as a surrogate. 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 year. The average length of its year is 365.2425 days.
Among solar calendars in wide use today, the
Persian calendar is one of the most precise. Rather than being based on numerical rules, the Persian year begins on the day (for the time zone of
Tehran) on which the vernal equinox actually falls, as determined by precise astronomical computations.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of '
intercalation' such as '
leap years'.
In the
Julian calendar, the average length of a year was 365.25 days. This is still used as a convenient time unit in astronomy, see below.
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, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a
month from year to year.
Fiscal year
A '
fiscal year' is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulatory laws regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year.
Academic year
An '
academic year' refers to the annual period during which a student attends school or college.
The school year can be divided up in various ways, two of which are most common in North American educational systems. (1) Many schools divide the academic year into three roughly equal-length trimesters (called terms in the UK), more or less coinciding with autumn, winter, and spring. At some, a somewhat shortened summer session, not usually considered part of the regular academic year, is attended by students on a voluntary or elective basis. (2) Other schools break the year into two main semesters, a first (typically August through December) and a second (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a quarter. There may also be an elective summer session, and/or a short January session where one class may optionally be taken.
Astronomical years
Julian year
The '
Julian year', as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the
Latin ''annus'', ''annata'') used in various scientific contexts. The Julian century of 36,525 days and the Julian millennium of 365,250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. For the distance unit
light-year, by convention the Julian year is used in the computation.
Sidereal, tropical, and anomalistic years
:''The relations among these are considered more fully in
Precession (astronomy).''
Each of these three years can be loosely called an 'astronomical year'.
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 in
SI days of 86,400 SI seconds each is on average:
:365.256 363 051 days (365 d 6 h 9 min 9 s) (at the epoch
J2000.0 =
2000 January 1 12:00:00 TT).
The '
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). The exact length of a tropical year slightly depends on the chosen starting point: for example the 'vernal equinox year' is the time between successive vernal equinoxes. The 'mean tropical year' (averaged over all ecliptic points) is:
:365.242 189 67 days (365 d 5 h 48 min 45 s) (at the epoch J2000.0).
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 (
January 2 in
2000), and the
aphelion, where the Earth is farthest from the Sun (
July 2 in 2000). The average duration of the anomalistic year is:
:365.259 635 864 days (365 d 6 h 13 min 52 s) (at the epoch J2000.0).
Draconic year
The 'draconitic year', 'eclipse year' or 'ecliptic year' is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the same
lunar node (a point 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 of every half eclipse year. Hence there are ''two eclipse seasons'' every eclipse year. The average duration of the eclipse year is:
:346.620 075 883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0).
This term is sometimes erroneously used to designate the draconic or nodal period of
lunar precession, that is the time it takes for a complete revolution of the Moon's ascending node around the ecliptic: 18.612 815 932
Julian years (6798.331 019 days; at the epoch J2000.0).
Full moon cycle
The '
full moon cycle' 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:
:411.784 430 29 days (411 d 18 h 49 min 34 s) (at the epoch J2000.0).
Heliacal year
A 'heliacal year' is the interval between the
heliacal risings of a star. It differs from the
sidereal year for stars away from the
ecliptic due mainly to the
precession of the equinoxes (To visualise: the constellation
Crux which rose and set as seen from the
Mediterranean in
ancient Greek times, is never above the horizon in current times.)
Sothic year
The '
Sothic year' is the interval between heliacal risings of the star Sirius. Its duration is very close to the mean Julian year of 365.25 days.
Gaussian year
The '
Gaussian year' is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the
Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is:
:365.256 898 3 days (365 d 6 h 9 min 56 s).
Besselian year
The '
Besselian year' is a tropical year that starts when the fictitious mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to 1 January. It is named after the
19th century German astronomer and mathematician
Friedrich Bessel. An approximate formula to compute the current time in Besselian years from the
Julian day is:
:B = 2,000 + (JD - 2,451,544.53) /365.242189
Great year
The '
Great year', 'Platonic year', or 'Equinoctial cycle' corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years, and cannot be determined precisely as the precession speed is variable.
Galactic year
The '
Galactic year' is the time it takes Earth's
solar system to revolve once around the
galactic center. It comprises roughly 226 million Earth years.
Variation in the length of the year and the day
The exact length of an astronomical year changes over time. The main sources of this change are:
# The precession of the equinoxes changes the position of astronomical events with respect to the apsides of 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 motion of the Earth from a steady orbit around the Sun.
Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month; since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to change. Tidal drag in turn depends on factors such as
post-glacial rebound and
sea level rise.
It is also to be expected that changes in the effective mass of the sun, caused by
solar wind and radiation of energy generated by nuclear fusion, could affect the Earth's orbital period over a long time.
Summary of various kinds of year
★ 346.62 days - a
draconitic year in some septenary calendars
★ 353, 354 or 355 days — the lengths of common years in some
lunisolar calendars
★ 354.37 days/12 lunar months - the average length of a year in
lunar calendars
★ 365 days — a
common year in many
solar calendars
★ 365.24219 days — a mean tropical year near the year 2000
★ 365.2424 days — a vernal equinox year.
★ 365.2425 days — the average length of a year in the Gregorian calendar
★ 365.25 days — the average length of a year in the Julian calendar
★ 365.2564 days — a sidereal year
★ 366 days — a
leap year in many solar calendars
★ 383, 384 or 385 days — the lengths of leap years in some lunisolar calendars
★ 383.9 days/13 lunar months - a leap year in some lunisolar calendars
An average Gregorian year is 365.2425
days = 52.1775
weeks, 8,765.82
hours = 525,949.2
minutes = 31,556,952
seconds (mean solar, not SI).
A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.
A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.
The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks.
See also
numerical facts about the Gregorian calendar.
Numeration or designation of year numbers
A
Calendar era is used to designate a number to individual years. The most common era is nowadays called the
common era. It is used in many countries. However, other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
See also
★
Calendar
★
Calendar era
★
List of calendars
★
1 E7 s
★
Jera
★
annum - 10
0 years - base unit
★
Man-year
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