Billion years

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"Gya" redirects here. For the village of Ladakh, India, see Gia, Leh.

A billion years (109 years) is a unit of time on the petasecond scale, more precisely equal to 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.[1] 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=3.15576×1016 s,
1 Gas=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.[2]

See also[edit]


  1. ^ Yarus, Michael (2010). Life from an RNA World: The Ancestor Within. Boston, Massachusetts: Harvard University Press. p. 38. ISBN 0-674-05075-4. 
  2. ^ Selsis; Kasting, J. F.; Levrard, B.; Paillet, J.; Ribas, I.; Delfosse, X.; et al. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics. 476 (3): 1373–1387. arXiv:0710.5294free to read. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091.