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Probably the most reliable of these estimates was produced by the British geologist Charles Lyell, who estimated that 240 million years have passed since the appearance of the first animals with shells.
Today scientists know his estimate was too young; we know that this occurred about 530 million years ago.
Thomson's calculations, however, were soon shown to be flawed when radioactivity was discovered in 1896.
Radioactivity is the tendency of certain atoms to decay into lighter atoms, emitting energy in the process.
Each dark band represents a winter; by counting rings it is possible to find the age of the tree (Figure 11.22).
Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.In 1892, William Thomson (later known as Lord Kelvin) calculated the age of Earth in a systematic fashion (Figure 11.24).