A couple years ago I came into possession of a 6″ telescope mirror that my grandfather had made from a pyrex blank in his barn in the late 1950s. Last fall (2012) while teaching an undergraduate seminar entitled “Outer Space from Plato to Newton,” I was reading (for pleasure) R. Descartes’ Dioptrique – (On Optics). The treatise begins with a panegyric to the telescope, an instrument which, as Descartes enthused, was requiring a dramatic reconsideration of the structure of the cosmos. Descartes began writing the Dioptrique in the late 1620s. What is so remarkable is that Galileo had published his first work based on telescopic observations only as early as 1610. In this work – Siderius Nuncius – Galileo suggested, among other things, that the surface of the moon was not smooth but irregular, and that four satellites (what he called, after his patrons, the Medician stars; and what we call, after him, the Galilean moons) orbited Jupiter. These were hugely important observations and deductions because, if correct, they contradicted the laws of Aristotelian physics, which required that celestial bodies be perfectly spherical and that the universe have only one point around which objects could orbit, located the center of the earth. The scientific world was to be much rattled by the implications for generations to come, and Descartes – scarcely a decade after Siderius Nuncius – was celebrating the new challenges the new tool, the telescope, opened up.