From Wikipedia, the free encyclopedia
Annum is a form of the Latin noun annus meaning year, from which words such as annual and annuity are derived. Annum is
the accusative singular
of the 2nd declension
masculine noun
annus (nominative singular: this is the reference
form of the word), anni (genitive singular and
nominative plural). Thus it is synonymous with year.
In astronomy it is defined as 365.25 days (that is, the average
length of a year in the Julian calendar) of 86,400 SI seconds each.[1]
Although there is no universally accepted symbol for the year, NIST
SP811[2] and ISO
80000-3:2006[3] suggest
the symbol a (in the International System of
Units a is also the symbol for the unit of
area called the "are", but context
is usually enough to disambiguate). In English, the abbreviation
yr is still used informally but is deprecated in
most scientific usage.[4][5][6]
The Unified Code for Units of
Measure[7]
disambiguates the varying symbologies of ISO 1000, ISO 2955 and
ANSI X3.50 [1] by using
- ar for are (unit),
and:
- at = a_t = 365.24219
days for the mean tropical year
- aj = a_j = 365.25 days for the mean Julian year
- ag = a_g = 365.2425
days for the mean Gregorian year
- a = 1 aj year
(without further qualifier)
Prefix
multipliers
- per annum means "yearly". (The Latin
preposition per takes the accusative case.)
- kiloannum, usual symbol ka,
is a unit of time equal
to one thousand years.
- megannum (more correctly megannus,
sometimes megaannum - and similarly for the other
prefixes), 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 into the past or
future. 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; the alternative but deprecated "mya" unit includes "ago" explicitly.). In
astronomical applications, the year used is the Julian year of precisely 365.25
days. In geology and palentology, the year is not so precise and
varies depending on the author.
- gigannum, 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.
- terannum, 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. It is the same
order of magnitude as the expected life span of a small red dwarf star.
- petannum, 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[8].
- exannum, 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.
Deprecated unit symbols
- bya - Formerly
used for Ga (ago)
- byr - Formerly
used for Ga (either elapsed or ago)
- gya - Formerly
used for Ga (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 unit symbols are now deprecated in modern geophysics[4][5],
and their use is controversial in modern geology[9]. Except
for kyr they do not use accepted SI prefixes. Further, the suffixes
ya and yr are not accepted SI units for time. However
ya would be the symbol for the yoctoannum unit of time. 1
ya would be 10-24 a which would be about of 3.15 x
10-17 s.
See also
References
Wikis
Search
