An Irish Bishop named James Ussher, who lived from 1581 to 1656, compiled research from a literal reading of the Bible and concluded that Earth was a little over 6,000 years old. He based his assertion on chronologies, genealogies, and the biblical account of Earth's creation given in the first eleven chapters of Genesis, in which Earth is said to have been created by God in six days. Ussher said that Earth must have been created in October of the year 4004 B. C. His biblical chronology of Earth's history was widely accepted in the Judeo-Christian Western world for nearly a century and is still accepted by some today who believe a literal interpretation of the Bible’s creation account. (Other believers of the Bible accept it as scripture but assert that the word "day" should not be interpreted as the 24 hours we define it as now, but instead as a creative period of time in which the Earth was formed in stages, with each stage lasting an unspecified amount of time.)

James Hutton, who lived from 1726 to 1797, and who you learned about in the previous section, presented the law of uniformitarianism, which is that the laws of nature do not change with time and that the rates at which things happen on Earth today are the same as they were in the past. Hutton observed, for example, that sediments could pile up to make huge rock cliffs and mountains, but that they must have done so in history at the same slow rate at which they do today, meaning that the Earth formed by gradual processes over a very long time. Rock layers as thick as those in the Grand Canyon could only have formed in a span of millions of years if sedimentation took place in the past at the same rate at which it does today. From this observation, Hutton believed the Earth to be millions of years old, not just a few thousand years old, as Bishop Ussher believed. James Hutton is often called the father of modern geology.

In the early 1830s, the Scottish geologist Charles Lyell published the Principles of Geology, a scientific text that supported the work of Hutton and advanced the idea of uniformitarianism. The subtitle to Lyell's book was "An attempt to explain the former changes of the Earth's surface by reference to causes now in operation." In other words, with uniformitarianism, Hutton and Lyell proposed that the past was the key to understanding the present. From his field studies, Lyell proposed that Earth was shaped entirely by slow-moving forces still in operation today, acting over a very long period of time. Like Hutton, Lyell asserted that Earth was millions of years old.

After the work of Hutton and Lyell, numerous scientists attempted to assign an age to Earth. Early attempts included estimates based on how long it would take for the oceans to become salty (about 100 million years) or how long it would take to accumulate the known thicknesses of fossil-bearing sedimentary rock layers (about 500 million years). The most accepted view of Earth’s age came from Lord Kelvin, a British physicist and engineer who lived from 1824 to 1907. Lord Kelvin estimated Earth’s age by calculating how long it would take the planet to cool from its originally molten state. He estimated the Earth to be 100 million years old.

A major breakthrough in assigning an age to Earth came in 1896 when Henri Becquerel discovered radioactivity. Becquerel was a French physicist who lived from 1852 to 1908. Later, the British physicist Lord Rutherford became the first to suggest that principles of radioactive decay could be used to date Earth and geologic events.

In 1956, American geologist Clair Patterson used uranium-lead dating of the Canyon Diablo meteorite, shown here, to calculate that it was 4.6 billion years old. The meteorite is a fragment of an asteroid that hit modern-day Arizona about 500,000 years ago. It came from our solar system, and thus is close in age to Earth because it would have formed at the same time that other bodies in our solar system formed. By dating it as 4.6 billion years old, we know that Earth is likewise about 4.6 billion years old. It is difficult to assign an age to Earth by dating materials formed on Earth because Earth materials are constantly cycled by the tectonic forces and rock cycle. But meteorites are considered a closed system, and the Canyon Diablo meteorite is assumed to have not undergone any chemical changes since its ancient formation.