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A billion years (109 years) is a unit of time on the petasecond scale, more precisely equal to 7016316000000000000♠3.16×1016 seconds.
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. The terms are used in geology, paleontology, geophysics, astronomy and physical cosmology.
The prefix giga- is preferred over billion- to avoid confusion in the long and short scales over the meaning of billion; the postfix annum may be further qualified for precision as a sidereal year or Julian year:
1 Gaj=7016315576000000000♠3.15576×1016 s, 1 Gas=7016315580999999999♠3.15581×1016 s (epoch J2000.0).Byr was formerly used in English-language geology and astronomy as a unit of one billion years. 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 the gray). Astronomers use Gyr or Gy as an abbreviation for gigayear.