age of the earth

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The time since the earth emerged as a planet of the sun, estimated by dating techniques to be about 4.6 × 109 years. The oldest known rocks on earth are estimated by radioactive dating to be about 3.5 × 109 years old. The earth is older than this because of the long time it took to cool. An estimate for the cooling time is included in the estimate for the age of the earth.

Subjects: Physics.

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