07 February 2012

Of Astrolabes and Other Things

PACHSmörgåsbord is always an enjoyable read - their audience is serious, engaged, and discerning. They have a terrific post on the history of astrolabes. Go check them out; a taste:-
The astrolabe was the most widely used scientific instrument in the middle ages. Nevertheless, its origins remain uncertain. The earliest surviving instruments date from medieval Islam. However, Greek and Syriac texts testify to a long theoretical and practical development that extends back to the second century BCE. The underlying mathematical principle of stereographic projection was described by Hipparchus of Nicaea (fl. 150 BCE). Less than two centuries later, Vitruvius (died post 27 CE) described a type of clock that depended on a similar stereographic projection. His suggestion that Eudoxus of Cnidos (ca. 408-355 BCE) or Apollonius of Perga (ca. 265-170 BCE) invented the rete or spider—the network of stars—almost certainly refers to the sundials he was discussing in the passage. Claudius Ptolemy (fl. 150 CE), the most famous astronomer from antiquity, wrote an extensive theoretical treatment of stereographic projection in his Planisphaerium, which included a short discussion of a horoscopic instrument. Although he described an instrument that resembles an astrolabe, including both a rete and the stereographic projection of a coordinate system, Ptolemy’s instrument does not seem to have included the apparatus needed to make direct observations and thus to measure the altitude of the sun or stars.