This week, Cherry Lewis of the University of Bristol presented a talk about the history of dating the Earth as part of the BA Festival of Science in York, England.Before so-called radiometric dating, Earth's age was anybody's guess.Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the "begats" since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus.
Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus.
Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine.
It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.giving an age for the Solar System and an upper limit for the age of Earth.It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling.This led him to estimate that Earth was about 75,000 years old.Meanwhile, Arthur Holmes (1890-1964) was finishing up a geology degree at the Imperial College of Science in London where he developed the technique of dating rocks using the uranium-lead method.