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The age of the Earth is around 4.54 billion years (4.54 × 109 years ± 1%).[1][2][3] This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples. The Sun, in comparison, is about 4.57 billion years old, about 30 million years older.

Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.[4] The oldest such minerals analyzed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.[5][6][7] Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system cannot be much older than those rocks. Ca-Al-rich inclusions (inclusions rich in calcium and aluminium) – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,[8][9] 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 Ca-Al-rich inclusions and the meteorites. Because the exact accretion time of Earth is not yet known, and the predictions from different accretion models range from a few millions 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. The Acasta Gneiss of Northern Canada may be the oldest known exposed crustal rock.[10]

Studies of strata, the layering of rock and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.[11][12]

Abū Rayhān Bīrūnī (11th century CE) discovered the existence of shells and fossils in regions that were once sea floor, but were later uplifted to become dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly changing and proposed that the Earth had an age, but that its origin was too distant to measure.[13] The principle of superposition of strata was first proposed by Avicenna (11th century). He outlined the principle while discussing the origins of mountains in The Book of Healing in 1027.[14][15] Shen Kuo (11th century) also later recognized the concept of deep time.[16]

Nicolas Steno (17th century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata.[12] His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality").[17] In the 1790s, the British naturalist William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.[18] William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.[19]

The naturalist Mikhail Lomonosov, regarded as the founder of Russian science, suggested in the mid-18th century that Earth had been created separately from the rest of the universe, several hundred thousand years before. Lomonosov's ideas were mostly speculative, but in 1779, the French naturalist 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.

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