This was in response to someone who asked, “How can we really know the age of the Earth?” during a discussion of Archbishop James Ussher estimated it to be 5654 years old in the year 1650. Today (2019), Ussher's estimate puts the age of the Earth at 6000 years.
It took more than a century to arrive at the accurate age of the Earth. Darwin knew that it was far older than 4000 years based on his knowledge of natural selection and evolution of life. He never quoted numbers, as far as I can tell, but described it as “great expanses of time”.
Scientists in the 1800s (Lord Kelvin and others) estimated Earth's age by determining how long it would take a molten Earth to cool to its present temperature. They put it at between 20 and 400 million years. That's quite a range, but at least they knew it was millions of years older that Ussher's estimate.
In the 1890s, radioactivity and radioactive elements were discovered by Marie Curie and others. Radiometric dating was then used to determine the age of things, such as rocks. In 1911, a rock was determined to be more than one billion years old. In 1956, rocks from Diablo Canyon in Arizona were found to be about 4.5 billion years old.
Today, the state of the art in dating techniques ensures a better than 1 percent accuracy. So, the Earth is 4.54 billion years old—precisely.
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