"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.
We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].
Radiocarbon is not stable; over time radiocarbon atoms decay into nitrogen atoms.
From then on, the ratio of radiocarbon to stable carbon will decrease, because the unstable radiocarbon atoms will slowly decay. From this measurement the age in radiocarbon years is calculated. Modern radiocarbon dates are calibrated using long tree-ring chronologies.Animals (and humans) get their carbon atoms primarily from what they eat (i.e., plants).Thus the ratio of radiocarbon to stable carbon in living animal tissue is also virtually the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.Plants obtain all their carbon atoms from the atmosphere.Thus, the ratio of radiocarbon to stable carbon in a living plant is the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.