Bya

For other uses, see BYA (disambiguation).

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 U.S.,^[1] not the long-scale 10¹² "billion" of some European usage. Billion by this convention (10⁹) is often called a "thousand million" in the UK and a "milliard" in some other countries.^[2] For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Ga (Giga-annum), however, bya remains in more widespread use.^[1] In 1974, the UK switched from the long scale to the short scale.^[1]

Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.

See also

• Chronology
• Myr
• Before Present

References

1. Gale, Joseph (2009). Astrobiology of Earth: The Emergence. Oxford: Oxford University Press, p. 21.
2. Chartrand, Mark (2004). Satellite Communications for the Nonspecialist. New York: SPIE Press, p. 73.