Million years ago: Difference between revisions

{{Short description|AbbreviationUnit of time equal to 1,000,000 years}}
{{redirect|million years agoFor2|the [[Adele]] song|Million Years Ago (song)|1,000,000 BC|one million (disambiguation)}}

Myr (million years) is in common use in fields such as [[Earth science]] and [[cosmology]]. Myr is also used with 'mya' (millionMya yearsor ago)Ma. Together they make a reference system, one to a quantity, the other to a particular placepoint in a [[calendar era|year numbering system]] that is ''time before the present''.

Myr is deprecated in [[geology]], but in [[astronomy]] ''myrMyr'' 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).

In geology, a debate remains open concerning the use of ''Myr'' (duration) plus ''Ma'' (million years ago) versus using only the term ''Ma''.<ref>{{cite web|last=Mozley|first=Peter|title=Discussion of GSA Time Unit Conventions|url=https://www.geosociety.org/TimeUnits/|work=web page|publisher=[[Geological Society of America]]|archive-url=https://web.archive.org/web/20160303232640/https://www.geosociety.org/TimeUnits/|archive-date=2016-03-03}}</ref><ref name="Biever-war">{{cite journal |first=Celeste |last=Biever |title=Push to define year sparks time war |journal=[[New Scientist]] |volume=210 |issue=2810 |pages=10 |url=https://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html |date=April 27, 2011 |access-date=April 28, 2011|bibcode=2011NewSc.210R..10B |doi=10.1016/S0262-4079(11)60955-X }}</ref> In either case, the term ''[[Year#SI prefix multipliers|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: {{Quote|The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr.}}

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 ''[[Before Present|BP]]''), as in: {{Quote|The Cretaceous started 145 Ma ago and ended 66 Ma ago, lasting for 79 Ma.}}

* [[Billion years|Byr ago]]

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[[Category:AstronomyUnits of measurement in astronomy]]