[<1/1,000,000th%]

Namely, Lord Kelvin had rather carefully calculated that the sun could be no older than 30 million years, based on the assumption that what powered the sun was the gravitational energy of the matter that had fallen together to form it in the first place - you add up how much gravitational energy a mass the size of the sun would have, and divide by the rate at which the sun is radiating away energy.

He also made a calculation about the age of the earth based on heat flow and current temperatures, but radioactivity was unknown at the time. The extra heating due to the radioactive decay is why his age calculation was wrong.

[VadeRetro]

He also made a calculation about the age of the earth based on heat flow and current temperatures, but radioactivity was unknown at the time. The extra heating due to the radioactive decay is why his age calculation was wrong.

Incredibly, you sometimes see Kelvin quote-mined with no mention of the source of his errors.

[Physicist]

He also made a calculation about the age of the earth based on heat flow and current temperatures, but radioactivity was unknown at the time. The extra heating due to the radioactive decay is why his age calculation was wrong.

I remember reading that what cemented everyone's opinion is that both calculations give just about the same answer. I can't find the reference, though.