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Ga is a common scientific abbreviation for Gigayears, 109 years, derived from the Latin Giga-annum. Note that the Latin accusative, annum, expresses an absolute age while English accusative, years, expresses a period of time. It is frequently used in fields such as astronomy and geology. Thus, the current best estimate for the age of the universe, from the Wilkinson Microwave Anisotropy Probe (spacecraft) data on the cosmic microwave background radiation, is 13.7 Ga = 13.7 Gy, and the age of the Earth, commonly taken as the time of the completion of accretion, is 4.56 Ga. In geology, it is often used to signify time before the present; for example, the time of the Moon-forming impact is 4.51 Ga and the age of the oldest rocks at the Earth’s surface is about 4.03 Ga.
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© 2011 Springer-Verlag Berlin Heidelberg
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Arndt, N. (2011). Ga. In: Gargaud, M., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-11274-4_611
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