Galileo Galilei s earthshaking book, Sidereus Nuncius (The starry messenger), was a definitive moment in the Renaissance s departure from ancient cosmology and its assumptions. Entering the debate between the astronomies of Ptolemy and Copernicus, Sidereus Nuncius provided rich and detailed evidence for Copernican heliocentrism. That evidence came from a telescope that Galileo modified and improved for his lunar observations and with which, most significantly, he also discovered four moons circling Jupiter. Sidereus Nuncius contains an introductory passage about the telescope, a section on Galileo s lunar observations, a description of how the planets and the fixed stars appeared through his telescope, a daily log of sightings of Jupiter and its satellites, and a brief conclusion in which Galileo contended that his discoveries answered some of the objections to Copernicus theory, promising that the reader could expect more news from the heavens soon. The age of the telescope began modestly with a patent application in 1608 for a three powered spyglass, filed with the Dutch Republic by a spectacle maker from Middleburg, Hans Lipperhey. News of the device traveled quickly to other parts of Europe, and when Galileo heard about it in the spring of 1609, he built his own instrument, a three powered spyglass with a convex objective lens and a concave ocular lens that he bought in a spectacle maker s shop. By the end of August, he presented an eight powered telescope of his own devising to the Venetian senate. By November, Galileo had fashioned a twenty powered telescope, and with it he undertook to observe the Moon, discovering that its surface was rugged and mountainous rather than perfect, as would befit a heavenly body according to classical cosmology. Galileo began writing up his lunar research in January 1610. Commentary by Albert Van Helden, searchable English translation and Latin live text.