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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.
Scientists analyze these ice cores to determine how the climate has changed over time, as well as to measure concentrations of atmospheric gases.
The longest cores have helped to form a record of polar climate stretching hundreds of thousands of years back.
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.
Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.Radioactive materials in Earth's interior provide a steady source of heat.Calculations of Earth's age using radioactive decay showed that Earth is actually much older than Thomson calculated.While tree rings and other annual layers are useful for dating relatively recent events, they are not of much use on the vast scale of geologic time.During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques.