From Wikipedia, the free encyclopedia

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 10⁹ years (one billion on the short scale, one milliard on the long scale). 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.

• teraannum, symbol Ta, is a unit of time equal to 10¹² years (one trillion on the short scale, one billion on the long scale). It is an extremely long unit of time, about 70 times as long as the age of the universe. The expected life span of a small red dwarf star.

• petaannum, symbol Pa, is a unit of time equal to 10¹⁵ years (one quadrillion on the short scale, one billiard on the long scale). The half-life of the nuclear isomer tantalum-180m is about 1 Pa^[4].

• 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). The half-life of tungsten-180 is 1.8 Ea.

Ea is no a practical unit for use in cosmology, or most other branches of science, because the age of the universe is not considered to be more than 14.7 Ga, or 0.0000000147 Ea.

Contents 1 On hyphenation 2 Deprecated units 3 See also 4 References

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.

• 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.

It has been suggested that tya, kyr, mya (unit), myr, bya, byr be merged into this article or section. (Discuss)

• Before Present
• Geologic time scale
• Orders of magnitude (time)