Today is the birthday of Robert Strutt, an English physicist born in 1875 who discovered Earth's ozone layer. In 1916, Strutt and his colleague Alfred Fowler confirmed the existence of ozone in the atmosphere and estimated its upper limit. But Strutt didn't just have his head in the clouds--he also discovered a way to date rocks by their concentration of radioactive helium. The key was alpha radiation, the natural decay process in which an element disintegrates into a lighter nucleus and an alpha particle (or helium nucleus). In 1904, Strutt published one of the first books on radioactivity, "The Becquerel Rays and the Properties of Radium."
[Source: Emily McMurray, Ed., Notable Twentieth Century Scientists (Gale Research Inc., ITP, 1995).]