The abbreviation Myr, "million years", is a
unit of a quantity of (i.e. ) years, or 31.556926
teraseconds.
Usage
Myr (million years) is in common use in fields such as
Earth science and
cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
. Myr is also used with Mya (million years ago). Together they make a reference system, one to a quantity, the other to a particular place in a
year numbering system that is ''time before the present''.
Myr is deprecated in
geology, but in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galax ...
''Myr'' is standard. Where "myr" ''is'' seen in geology it is usually "Myr" (a unit of mega-years). In astronomy it is usually "Myr" (Million years).
Debate
In geology a debate remains open concerning the use of ''Myr'' (duration) plus ''Ma'' (million years ago) versus using only the term ''Ma''.
In either case the term ''
Ma'' is used in geology literature conforming to
ISO 31-1
ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to ''space and time''. It was superseded in 2006 by ISO 80000-3.
Definitions
Its definitions include:
Annex A
Annex A ...
(now
ISO 80000-3
ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ).
It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotech ...
) and NIST 811 recommended practices. Traditional style geology literature is written
The "ago" is implied, so that any such year number "X Ma" between 66 and 145 is "Cretaceous", for good reason. But the counter argument is that having ''myr'' for a duration and ''Mya'' for an age mixes unit systems, and tempts capitalization errors: "million" need not be capitalized, but "mega" must be; "ma" would technically imply a ''milliyear'' (a thousandth of a year, or 8 hours). On this side of the debate, one avoids ''myr'' and simply adds ''ago'' explicitly (or adds ''
BP''), as in
In this case, "79 Ma" means only a quantity of 79 million years, without the meaning of "79 million years ago".
See also
*
Byr
A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to seconds (or simply 1,000,000,000 years).
It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya o ...
*
Kyr
The abbreviation kyr means "thousand years".
kyr was formerly common in some English language works, especially in geology and astronomy, for the unit of 1,000 years or millennium. The "k" is the unit prefix for kilo- or thousand with the suffix ...
*
Megaannum
A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hour ...
(Ma)
*
Symbols y and yr
References
{{Portal bar, Earth science, Mathematics, Astronomy, Stars
Units of time
Units of measurement in astronomy
Geology