Radiocarbon year
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Raw radiocarbon dates are usually reported in years "before present" (BP). This is the number of radiocarbon years before 1950, based on the assumption that the level of carbon-14 in the atmosphere has always been at the 1950 level. Because this is untrue raw radiocarbon years do not correspond exactly to calendar years. Calibrated dates take into account this factor.
Willard F. Libby was awarded the Nobel Prize in Chemistry in 1960 for his work in developing radiocarbon dating. He measured the half-life of carbon-14 to be 5568±30 years. The accepted value today is 5730±40 years. To keep radiocarbon years reported today consistent with radiocarbon years reported in the past, the Libby half-life is still used. Thus on average, 5568 radiocarbon years equal 5730 calendar years; or the average radiocarbon year is about 1.03 calendar years. Calibration curves used to correct radiocarbon dates usually correct for this as well as for changing levels of carbon-14 in the atmosphere.
See radiocarbon dating for more information. de:Radiocarbon-Jahr