Many scholars trace the birth of modern science back to Galileo Galilei (1564-1642), who used instruments to observe nature and experiments to understand it. Like Copernicus, he began training for a career in medicine, but later switched to a subject more to his liking, mathematics. Galileo long accepted Copernicus’ idea that Earth and the other planets orbited the sun, but he was the first able to prove it based on his observations with a telescope.

       Many people think Galileo invented the telescope, but that’s not true. Spectacle makers in Europe had probably discovered how to make distant objects appear closer well before Galileo. The first telescope to arouse interest, however, was made in 1608 by the Dutch optician Hans Lippershey. When Galileo heard of it, he quickly made his own and turned it on the heavens.

       Within a few months he had discovered four moons orbiting Jupiter—destroying the Greek idea that Earth was the center of all motion—and the phases of Venus— overturning Ptolemy’s concept that the Sun and planets all orbited Earth. He wrote of his sensational discoveries in Italian rather than academic Latin so the general public could read about them. His observations improved our knowledge of the universe we live in and helped turn science into an experimental endeavor. For his efforts, and their devastating effect on the religious dogma of the time, he was forced to recant his findings before the Inquisition and spent the last decade of his life under house arrest.