If I may expand a bit, as I recall, the physicists scoffed for what they thought were good reasons. 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. Simple, no? Aided, no doubt, by the fact that Lord Kelvin was one of the pre-eminent physicists of the day, even Darwin conceded that this was a huge flaw in his theory, since he estimated that at least ten times that many years would have been necessary for certain geological features to have formed.
Alas, Lord Kelvin will have to be remembered as the discoverer of the Second Law of Thermodynamics - which is also popular in these parts - and not as the man who drove a stake through the heart of Darwinian evolution. Although his calculations were undoubtedly correct, his assumption about what powered the sun was completely wrong. Lord Kelvin simply had no idea about the process of nuclear fusion, being sixty years too early for Eddington, and eighty years too early for Bethe ;)
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.