Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.
By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.
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.
As is known from studies of 400 million year old fossil corals, Earth years were 400 days in duration in distant times because the Earth rotated faster than it does today.
According to calculations based on the fundamental law of rotational motion dynamics involving the moment of inertia of a body, the radius of the ancient Earth 400 million years ago was 553.379 km less than it is today.
Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology.
At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet.
The resulting collision sent debris into orbit that eventually became the Moon.
James Hutton, a physician-farmer and one of the founders of the science of geology, wrote in 1788, “The result, therefore, of our present inquiry is, that we find no vestige of a beginning, — no prospect of an end.” Although this may now sound like an overstatement, it nicely expresses the tremendous intellectual leap required when geologic time was finally and forever severed from the artificial limits imposed by the length of the human lifetime.
By the mid- to late 1800s, geologists, physicists, and chemists were searching for ways to quantify the age of the Earth.