Astronomy:Billion years
A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to 3.16×1016 seconds (or simply 1,000,000,000 years).
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 to 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).
- 1 Gas = 1×109 y
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 Gy as abbreviation for the 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.[2]
See also
References
- ↑ Yarus, Michael (2010). Life from an RNA World: The Ancestor Within. Boston, Massachusetts: Harvard University Press. p. 38. ISBN 978-0-674-05075-4. https://books.google.com/books?id=-YLBMmJE1WwC&pg=PA38.
- ↑ Selsis, Franck; Kasting, James F.; Levrard, Benjamin; Paillet, Jimmy; Ribas, Ignasi; Delfosse, Xavier (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics 476 (3): 1373–1387. doi:10.1051/0004-6361:20078091. Bibcode: 2007A&A...476.1373S. https://hal.archives-ouvertes.fr/hal-00182743.
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