year

For when a year begins, see New Year's.

Various intervals of time all roughly based on the time it takes the Earth to make one journey around the sun. They can be conveniently divided into four types: astronomical years, calendar years, metaphorical years and economic years.

Astronomical years

The lengths in days (= 86,400 seconds as defined in SI) for the year 1990 of the various years are:

Type of year	days = 86,400 SI seconds	days	hours	minutes	seconds
Tropical year	365.2421897	365	5	48	45.19
Sidereal year	365.25636	365	6	9	10
Anomalistic year	365.25964	365	6	13	53
Eclipse year	346.62005	365	14	52	52
Gaussian year	365.25690	365	6	9	56

Tropical year

The average interval between instants when the sun passes through the vernal equinox, which is an instant when it appears to be both in the plane of the Earth's orbit and in the plane of the Earth's equator. In the terminology of astronomers, the time required for the longitude of the sun to increase 360 degrees. = 365.24220 mean solar days. Symbol, a_trop.

The tropical year is the interval between one spring (or autumn) equinox and the next. It is the interval at which seasons repeat and is the basis of all solar calendars. The name comes from the Tropics of Cancer and Capricorn. The tropical year is not the length of the year in the tropics; it has nothing to do with Somerset Maugham, sunny beaches or banana plantations.

Sidereal year

The average period of revolution of Earth with respect to a fixed direction in space. See sidereal time. Symbol, a_sid.

Anomalistic year

The orbit of the Earth is not a circle, but an ellipse with the Sun at one focus. Consequently, during half of its orbit Earth keeps getting closer to the sun, and in the other half it is moving away. The anomalistic year is the interval between the instant when Earth's orbit brings it closest to the sun (perihelion) and the next such instant. Symbol, a_anom.

Astronomical year

The period of time in which the right ascension of the mean sun increases by 360 degrees. Symbol, a_astr. Also known as the annus fictus, or Bessel's year.

Eclipse year

lunar node to lunar node

Calendar years

A period of time consisting of an arbitrary number of whole days. In some calendars it also consists of a whole number of lunations, though the latter number must be obtained by rounding off. In most calendars today the year is 365 days, with a 366-day year roughly every four years. See calendar. However, years as short as 260 days have been used, and in some calendars some years are longer than 366 days. Perhaps the longest calendar year ever was the one Julius Caesar decreed for 46 bc: 445 days, to bring the equinoxes back to their traditional dates in the Roman calendar.

Name of Year	Days	Lunations	Average Value
Name of Year	Days	Lunations	days (decimal)	days	hours	minutes	seconds
Solar calendars
Julian year	365 or 366	NA	365.25	365	6	0	0
Gregorian year	365 or 366	NA	365.2425	365	5
Lunar calendars
Islamic	354 or 355	12	354.36707
Lunisolar Calendars
Hebrew	353, 354, 355, 383, 384 or 385	12 or 13

For the various calendar years, see particular calendars in the index.

Metaphorical Years

Great Platonic Year

An interval of time at the end of which all of Nature reverts to the state in which it was at the interval's beginning; often specifically, the planets complete their orbits together (Plato, Timaeus 39). The Greeks called it the αποκαταστασις (apokatastasis).

This idea of the Eternal Return is found in many cultures, even among modern scientists. Henri Poincaré showed that in an unlimited amount of time a limited system, in the absence of gravity, will return to its initial state.¹

1. H. Poincaré.
Comptes Rendus de l'Académie des Sciences (Paris) vol. 108, page 550 (1889).

Mircea Eliade.
The Myth of the Eternal Return.
Willard R. Trask, translator.
New York: Pantheon Books, [c1954]

Economic years

Fiscal year

The calendar year is not convenient for many economic activities; the year's end falls in the middle of the biggest retail sales season, for example. U.S. tax law generally permits taxpayers to set their own fiscal year.

Crop marketing year

A year that runs from harvest to harvest. It varies by agricultural commodity; for soybeans, for example, the crop year begins on 1 September and ends on 31 August. The term is used in futures trading.