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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 United States,Gale, Joseph (2009). Astrobiology of Earth: The Emergence. Oxford: Oxford University Press, p. 21. not the long-scale 1012 "billion" of some usage. Billion by this convention (109) is often called a "thousand million" in the UK and a "milliard" in some other countries.Chartrand, Mark (2004). Satellite Communications for the Nonspecialist. New York: SPIE Press, p. 73. For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Giga-annum (Giga-annum), however, bya remains in more widespread use. In 1974, the UK switched from the long scale to the short scale.
Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Giga-annum or Gya has a Capitalization first letter instead.
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