Annum
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 seconds, 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 abbreviation yr is frequently used as well.
- Per annum means "yearly".
- Kilo-annum, usual symbol ka, is a unit of time equal to one thousand years.
- <div id="Mega-annum">Mega-annum, usual symbol Ma, is a unit of time equal to one million (10<sup>6</sup>) 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. </div>
- Giga-annum, usual symbol Ga, is a unit of time equal to one billion (10<sup>9</sup>) 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.
- Exa-annum, usual symbol Ea, is a unit of time equal to (10<sup>18</sup>) 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.
See also
- Before Present
- bya
- byr
- Geologic time scale
- Giga-annum or Ga
- kyr
- mya
- myr
- tya (sometimes spelled kya)
- Orders of magnitude (time)
References
Citations
- [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)