Billion years

A billion years (10⁹ years) is a unit of time on the petasecond scale, more precisely equal to 3.16×10¹⁶ seconds.

History of the universe timelines usually use the Gya or "billion years ago" scale.

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 Ga_j=3.15576×10¹⁶ s,

1 Ga_s=3.15581×10¹⁶ s (epoch J2000.0).

1 Ga_s=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 as an abbreviation for gigayear.[2]

See also

• Kyr
• Myr
• Orders of magnitude (time)
• Year#Abbreviations yr and ya
• Year#SI prefix multipliers

References

1. Yarus, Michael (2010). Life from an RNA World: The Ancestor Within. Boston, Massachusetts: Harvard University Press. p. 38. ISBN 978-0-674-05075-4.
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:0710.5294. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091. S2CID 11492499.