Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a
unit of time
A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as ...
equal to (i.e. years), or approximately 31.6
teraseconds.
Usage
Myr 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 cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...
. Myr is also used with Mya or Ma. Together they make a reference system, one to a quantity, the other to a particular point in a
year numbering system that is ''time before the present''.
Myr is deprecated in
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
, but in
astronomy
Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
''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 ''Mya'' (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 (now
ISO 80000-3) 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
*
Billion years ago
*
Kyr
*
Megaannum
A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more ...
(Ma)
*
Symbols y and yr
References
{{Portal bar, Earth science, Mathematics, Astronomy, Stars
Units of time
Units of measurement in astronomy
Geology
uk:Одиниці_вимірювання_часу#Мегарік_і_гігарік