The Astrolabe: a Mechanism for Reading the Stars

Abstract. The astrolabe is one of the most interesting astronomical instruments of the past, a mixture of science and art that represents the projection of the celestial sphere on the level of the equator. In essence, it responds to the practical need to represent the armillary sphere on a plane, in order to obtain a more comfortable instrument to handle and above all portable. The stars, and the position assumed by the Sun during the year, are shown on a disk that could be rotated and positioned to represent the sky in a given moment. All this was possible thanks to the stereographic projection. Chapter deriving from the report, by invitation, presented at the conference: International Federation for the Promotion of Mechanism and Machine Science, HMM2018 in Beijing, China, organized by Permanent Commission for the History of Mechanism and Machine Science, IFToMM, Institute for the History of Natural Sciences ( IHNS), Chinese Academy of Sciences, 26-28 September 2018.