StonehengeIn 1952, Prof. Willard F. Libby determined the age of Stonehenge on Salisbury Plain, England, at about 1848 BC (+/- 275 years) through analysis of the carbon-14 radioisotope in charcoal remains excavated there there. Update of C-14 ceases when plants or animals die, and the proportion in the organic remains steadily declines through radioactive decay. Since the half-life of C-14 is about 5,600 years, measurement of the remaining proportion in dead organic matter, indicates the age of that sample. Astronomer Sir Joseph Norman Lockyer had previously calculated that on Midsummer Day, 1680 BC, the sun rose directly over a special marking notch that can still be seen on the Heel Stone. Libby's measurements support that estimate.« |
