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bya or b.y.a. is an abbreviation for "billion years ago". It is commonly used as a unit of time to denote length of time before the present in 10⁹ years. This initialism is often used in the sciences of astronomy, geology, and paleontology.

The "billion" in bya is the 10⁹ "billion" of the short scale of the U.S.,^[1] not the long-scale 10¹² "billion" of some European usage. Billion by this convention (10⁹) is often called a "thousand million" in the UK and a "milliard" in some other countries.^[2] For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Ga (Giga-annum), however, bya remains in more widespread use.^[1] In 1974, the UK switched from the long scale to the short scale.^[1]

Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.

References

^ ^a ^b ^c Gale, Joseph (2009). Astrobiology of Earth: The Emergence. Oxford: Oxford University Press, p. 21.
^ Chartrand, Mark (2004). Satellite Communications for the Nonspecialist. New York: SPIE Press, p. 73.

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[Gale2009-1] Gale, Joseph (2009). Astrobiology of Earth: The Emergence. Oxford: Oxford University Press, p. 21.

[2] Chartrand, Mark (2004). Satellite Communications for the Nonspecialist. New York: SPIE Press, p. 73.

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+{{Short description|Abbreviation for "billion years ago"}}
 {{Other uses|BYA (disambiguation)}}
 {{lowercase}}
-'''bya''' or '''b.y.a.''' is an [[abbreviation]] for "'''[[1000000000 (number)|billion]] years ago'''". It is commonly used as a [in 10<SUP>9</SUP> years. This [[Acronym#Nomenclature|initialism]] is often used in the sciences of [[astronomy]], [[geology]], and [[paleontology]].
+'''bya''' or '''b.y.a.''' is an [[abbreviation]] for "'''[[1000000000 (number)|billion]] years ago'''". It is commonly used as a [[unit of time]] to denote length of time before the present in 10<SUP>9</SUP> years. This [[Acronym#Nomenclature|initialism]] is often used in the sciences of [[astronomy]], [[geology]], and [[paleontology]].
-The "billion" in bya is the 10<SUP>9</SUP> "billion" of the [[Long and short scales|short scale]] of the [[United States|U.S.]],<ref name="Gale2009">Gale, Joseph (2009). ''Astrobiology of Earth: The Emergence''. Oxford: Oxford University Press, p. [https://books.google.com/books?id=64zn0nxDVUYC&pg=PA21 21.]</ref> not the long-scale 10<SUP>12</SUP> "billion" of some [[Europe]]an usage. Billion by this convention (10<SUP>9</SUP>) is often called a "thousand million" in the UK and a "milliard" in some other countries.<ref>Chartrand, Mark (2004). ''Satellite Communications for the Nonspecialist''. New York: SPIE Press, p. [https://books.google.com/books?id=MM0d2cMUWbEC&pg=PA73 73.]</ref> For this reason, there is potential for some confusion, and some scientists prefer the unit [[Gya]], while others prefer [[Giga-annum|Ga]] (Giga-annum), however bya remains in more widespread use.<ref name="Gale2009"/> In 1974, the UK switched from the long scale to the short scale.<ref name="Gale2009"/>
+The "billion" in bya is the 10<SUP>9</SUP> "billion" of the [[Long and short scales|short scale]] of the [[United States|U.S.]],<ref name="Gale2009">Gale, Joseph (2009). ''Astrobiology of Earth: The Emergence''. Oxford: Oxford University Press, p. [https://books.google.com/books?id=64zn0nxDVUYC&pg=PA21 21.]</ref> not the long-scale 10<SUP>12</SUP> "billion" of some [[Europe]]an usage. Billion by this convention (10<SUP>9</SUP>) is often called a "thousand million" in the UK and a "milliard" in some other countries.<ref>Chartrand, Mark (2004). ''Satellite Communications for the Nonspecialist''. New York: SPIE Press, p. [https://books.google.com/books?id=MM0d2cMUWbEC&pg=PA73 73.]</ref> For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer [[Giga-annum|Ga]] (Giga-annum), however, bya remains in more widespread use.<ref name="Gale2009"/> In 1974, the UK switched from the long scale to the short scale.<ref name="Gale2009"/>
-Related units are [[Mya (unit)|mya]] ("million years ago), and '''byr''' ("billion years"). These are traditionally written in [[lowercase]].  [[Giga-annum|Ga]] or Gya has a [[capitalization|capitalized]] first letter instead.
+Related units are [[Mya (unit)|mya]] ("million years ago"), and '''byr''' ("billion years"). These are traditionally written in [[lowercase]].  [[Giga-annum|Ga]] or Gya has a [[capitalization|capitalized]] first letter instead.
 ==See also==