The abbreviation Byr means "billion years" (109 or 1,000,000,000 years). A closely related abbreviation is bya "billion years ago".
Byr was formerly used in English-language geology and astronomy as a unit of one billion years. Today, the term gigaannum (Ga) is also used, but Gy or Gyr are 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. Specifically, one Gyr is equal to 109 years.
Because a billion means 1000 million in some countries but can mean a million million in others its use is deprecated in favour of giga- which unambiguously means 109 and annum which for precision may be qualified by (sidereal), (Julian), etc.
- Before Present
- Gigaannum or Ga
- mya (unit)
- Orders of magnitude (time)#Years
- Symbols y and yr
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