the earliest known calculator, built in 1623 by the German astronomer and mathematician Wilhelm Schickard. He described it in a letter to his friend the astronomer Johannes Kepler (Kepler, Johannes), and in 1624 he wrote again to explain that a machine that he had commissioned to be built for Kepler was, apparently along with the prototype, destroyed in a fire. He called it a Calculating Clock, which modern engineers have been able to reproduce from details in his letters. Even general knowledge of the clock had been temporarily lost when Schickard and his entire family perished during the Thirty Years' War.