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<p><b>New page</b></p><div>{{short description|10^9 years; geological unit of time}}<br />
[[File:Nature Timespiral.png|thumb|right|upright=1.2|[[Astronomy:Chronology of the universe|History of the universe]] timelines usually use the Gya or "billion years ago" scale.]]<br />
A '''billion years''' or '''giga-annum''' (10<sup>9</sup> [[Year|year]]s) is a unit of time on the petasecond scale, more precisely equal to {{val|3.16|e=16}} [[Second|second]]s (or simply 1,000,000,000 years).<br />
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It is sometimes abbreviated '''Gy''', '''Ga''' ("giga-annum"), '''Byr''' and variants. The abbreviations '''Gya''' or '''bya''' are for "billion years ago", i.e. billion years before present.<ref>{{cite book |first=Michael |last=Yarus |date=2010 |url=https://books.google.com/books?id=-YLBMmJE1WwC&pg=PA38 |title=Life from an RNA World: The Ancestor Within |publisher=Harvard University Press |location=Boston, Massachusetts |isbn=978-0-674-05075-4 |page=38}}</ref> <br />
The terms are used in [[Earth:Geology|geology]], [[Biology:Paleontology|paleontology]], [[Physics:Geophysics|geophysics]], [[Astronomy:Astronomy|astronomy]], and [[Astronomy:Physical cosmology|physical cosmology]].<br />
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The prefix [[Giga-|giga-]] is preferred to [[Billion|billion]]- to avoid confusion in the [[Long and short scales|long and short scales]] over the meaning of billion; the postfix annum may be further qualified for precision as a [[Astronomy:Sidereal year|sidereal year]] or [[Astronomy:Julian year|Julian year]]:<br />
:1&nbsp;Ga<sub>j</sub> = {{val|3.15576|e=16}}&nbsp;s,<br />
:1&nbsp;Ga<sub>s</sub> = {{val|3.15581|e=16}}&nbsp;s (epoch [[Astronomy:J2000.0|J2000.0]]).<br />
:1&nbsp;Ga<sub>s</sub> = {{val|1|e=9}}&nbsp;y<br />
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Byr was formerly used in English-language geology and astronomy as a unit of one billion [[Year|year]]s. Subsequently, the term gigaannum (Ga) has increased in usage, with Gy or Gyr still sometimes used in English-language works (at the risk of confusion with Gy as abbreviation for the [[Physics:Gray (unit)|gray]], a unit of radiation exposure). Astronomers use '''Gyr '''or''' Gy''' (or '''Ga''', where the '''a''' stands for ''annum'', Latin for "year") as the symbol for '''gigayear'''.<ref name="sel07">{{cite journal |doi=10.1051/0004-6361:20078091 |bibcode=2007A&A...476.1373S |title=Habitable planets around the star Gl 581? |journal=Astronomy and Astrophysics |volume=476 |issue=3 |year=2007 |pages=1373–1387 |last1=Selsis |first1=Franck |last2=Kasting |first2=James F. |last3=Levrard |first3=Benjamin |last4=Paillet |first4=Jimmy |last5=Ribas |first5=Ignasi |last6=Delfosse |first6=Xavier |arxiv=0710.5294 |s2cid=11492499 |ref=Selsis |url=https://hal.archives-ouvertes.fr/hal-00182743 }}</ref><br />
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==See also==<br />
* [[Astronomy:Kyr|Kyr]]<br />
* [[Astronomy:Myr|Myr]]<br />
* [[Physics:Aeon|Aeon]]<br />
* [[Orders of magnitude (time)]]<br />
* [[Year]]<br />
* [[Year]]<br />
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==References==<br />
{{Reflist}}<br />
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[[Category:Units of time]]<br />
[[Category:Geology]]<br />
[[Category:Units of measurement in astronomy]]<br />
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