[DB 1507 (40); OAB 25] This claim is known as "Kelvin's age of the Earth", and it is taught in introductory physics and geology classes as a classic example of a great calculation thatdidn't take everything into account. A hundred years ago, the eminent scientist Lord Kelvin assumed that the heat now escaping from the Earth must have come from gravitational contraction, the most powerful energy source known at the time. Calculating the gravitational energy available, and the cooling time, he derived a maximum age of the Earth: 24 million years. However, the powerful energy that could be released by radioactivity was not known until late in Kelvin's career. Calculations using the amount of radioactive materials known to exist in the Earth's crust show that the energy available is in good agreement with a cooling time of 4.6 billion years. Although Kelvin was in fact one of the great scientists of his time, he simply did not have all of the relevant information available to him in the late nineteenth century. Kelvin's calculation was shown one hundred years ago to be incomplete, giving a misleading answer, and it is rather disingenuous for young-Earth proponents to include it in their publications (as recently as 1995) as if it were still a valid argument.