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'''Annum''' is a Latin noun meaning year. It is the accusative singular of the second declension masculine noun 'annus' (nominative), 'anni' (genitive) [1].
As a unit of time, it is defined as exactly 365.25 days (that is, the average length of a year in the Julian calendar) of 86,400 SI seconds each, representing the duration of one revolution of the Earth around the Sun. Although there is no universally accepted symbol for the year, NIST^[1] and ISO 31-1^[2] suggest the symbol 'a' (in the International System of Units 'a' is also the symbol for the are unit of area, but context is usually enough to disambiguate). In English, the deprecated abbreviation 'yr' is still frequently used.
The Unified Code for Units of Measure^[3] disambiguates the symbology by using
:''ar'' for are (unit), and:
:'a_t' = a_t = 365.24219 days for the mean tropical year
:'a_j' = a_j = 365.25 days for the mean Julian year
:'a_g' = a_g = 365.2425 days for the mean Gregorian year
:'a' = 1 a_j year (without further qualifier)

★ 'Per annum' means "yearly".

★ 'kiloannum', usual symbol 'ka,' is a unit of time equal to one thousand years.

★ 'megaannum', usual symbol 'Ma', is a unit of time equal to one million (10⁶) years. It is commonly used in scientific disciplines such as geology, paleontology, and celestial mechanics to signify very long time periods in the past. For example, the dinosaur species ''Tyrannosaurus rex'' was abundant approximately ''65 Ma'' (65 million years) ago (''ago'' may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; "mya" includes "ago" explicitly.). In astronomical applications, the year used is the Julian year of precisely 365.25 days.

★ 'gigaannum', usual symbol 'Ga', is a unit of time equal to one (short scale) billion (10⁹) years. It is commonly used in scientific disciplines such as cosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately ''4.57 Ga'' (4.57 billion years) ago. As is clear from the notation, ''billion'' as used here is on the short scale, also known as the American English preference.

★ 'exaannum', usual symbol 'Ea', is a unit of time equal to (10¹⁸) years (one quintillion on the short scale, one trillion on the long scale). It is an extremely long unit of time. The half-life of tungsten-180 is 1.8 Ea.

Contents

On hyphenation

Deprecated units

See also

References

On hyphenation

The use of the hyphen in SI is reserved for the infrequent practice of combining prefixes, e.g. ''deca-kiloannum''. It is not used in the fashion of ''kilo-annum''.

Deprecated units

★ 'bya' - Formerly used for Ga (ago)

★ 'byr' - Formerly used for Ga (either elapsed or ago)

★ 'mya' - Formerly used for Ma (ago)

★ 'myr' - Formerly used for Ma (either elapsed or ago)

★ 'tya' (sometimes spelled kya) - formerly used for ka (ago)

★ 'kyr' - Formerly used for ka (either elapsed or ago)
These units are deprecated because (except for ''kyr'') they do not use accepted SI prefixes and because the suffixes ''ya'' and ''yr'' are not accepted SI units.

See also

★ Before Present

★ Geologic time scale

★ Orders of magnitude (time)

References

1. National Institute of Standards and Technology (NIST) Special Publication 811, ''Guide for the Use of the International System of Units (SI)'', (1995)
2. International Organization for Standardization (ISO) 31-1, ''Quantities and units — Part 1: Space and time'', Geneva, Switzerland (1992)
3. Unified Code for Units of Measure