Galileo Galilei first learnt about the recent invention of the telescope by a Dutchman in May 1609. His former student, Jacques Badover, sent him a letter from Paris reporting that a merchant was selling telescopes on the Pont Neuf. In short, the invention consisted of placing two lenses at the right distance with their optical axes in line so they would provide a magnified image of distant places and objects. Upon learning about this discovery, Galileo began experimenting with building his own telescope. By late August he had made a nine-power telescope that he showed to the Senate of Venice, displaying its extraordinary visual capacities from the campanile of San Marco. As a result of this demonstration, Galileo obtained a permanent contract, and a considerate increase in his salary, as a professor of mathematics at the University of Padua. In the Fall of 1609 Galileo continued to improve the lenses of his telescope, building a twenty-power one with which he began to observe the skies and the phases of the Moon, discovering previously unseen stars in the constellations and the Milky Way. On January 7, 1610, Galileo began a series of daily observations of Jupiter, discovering four new “fixed stars” or “planets,” that is, satellites, which he called the Medicean stars in honor of his Florentine patrons. Indeed, this breathtaking discovery compelled Galileo to publish all his astronomical observations in a short treatise called Sidereus Nuncius (The Starry Messenger).
On page 7 of the Sidereus Nuncius (The Starry Messenger) is a woodcut diagram whereby Galileo explains how this new telescope works. ABCD represents the tube and E the eye of the observer. Without glasses in the tube, the rays proceed to the object FG along the straight lines ECF and EDG. But with the glasses in the tube the rays proceed along the refracted lines ECH and EDI.
- Drake, Stillman. 2008. “Galilei, Galileo.” In Complete Dictionary of Scientific Biography, Vol. 5. (digital edition). Detroit, MI: Charles Scribner’s Sons: 237-249.
- Dupré, Sven, Albert Van Helden, Rob Van Gent, and Huib Zuidervaart, eds. 2010. The Origins of the Telescope. Amsterdam: KNAW Press.
- Reeves, Eileen and Albert Van Helden. 2007. “Verifying Galileo’s Discoveries: Telescope Making at the Collegio Romano.” In Der Meister und die Fernrohre, edited by Juergen Hamel and Inge Keil. Frankfurt am Main: H. Deutsch: 127-141.
- Willach, Rolf. 2008. The Long Route to the Invention of the Telescope. Philadelphia: American Philosophical Society.