bya or b.y.a. is an

abbreviation An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters or words taken from the full version of the word or phrase; for example, the word ''abbrevia ...

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 An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...

is often used in the sciences of astronomy, geology, and paleontology. The "billion" in bya is the 10⁹ "billion" of the

short scale The long and short scales are two of several naming systems for integer power of 10, powers of ten which use some of the same terms for different magnitudes. For whole numbers smaller than 1,000,000,000 (109), such as one thousand or one m ...

of the U.S.,Gale, Joseph (2009). ''Astrobiology of Earth: The Emergence''. Oxford: Oxford University Press, p
21.
/ref> 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.Chartrand, Mark (2004). ''Satellite Communications for the Nonspecialist''. New York: SPIE Press, p
73.
/ref> For this reason, there is potential for some confusion, and some scientists prefer the unit

Gya A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to seconds (or simply 1,000,000,000 years). It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or ...

, while others prefer Ga (Giga-annum), however bya remains in more widespread use. In 1974, the UK switched from the long scale to the short scale. Related units are

mya Mya may refer to: Brands and product names * Mya (program), an intelligent personal assistant created by Motorola * Mya (TV channel), an Italian Television channel * Midwest Young Artists, a comprehensive youth music program Codes * Burmese ...

("million years ago), and byr ("billion years"). These are traditionally written in

lowercase Letter case is the distinction between the Letter (alphabet), letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller lowercase (or more formally ''minuscule'') in the written representation of certain lang ...

. Ga or Gya has a

capitalized Capitalization (American English) or capitalisation (British English) is writing a word with its first letter as a capital letter (uppercase letter) and the remaining letters in lower case, in writing systems with a case distinction. The term a ...

first letter instead.

References

Units of time Chronology Metrology {{time-stub

See also

References