Hipparchus and the AstrolabeHipparchus (who flourished during the second half of the second century BC) was a careful astronomical observer, who operated in a systematic fashion. He may have been responsible for the invention (or adaptation) of several astronomical instruments. Some historians of astronomy (including G.J.Toomer, John North and David King) believe that Hipparchus may have been the inventor of the planispheric astrolabe, used to tell the time at night from stellar positions. The earliest surviving description of the planispheric astrolabe is to be found in the writings of John Philoponus, who lived during the sixth century AD, long after Hipparchus. However, the mathematical theory which serves as the foundation for the stereographic projection used in the planispheric astrolabe was provided in the second century AD, by Ptolemy, in his Planispherium. Given Ptolemy's acknowledged debt to his predecessor, it is entirely possible that he was here, too, building on the work of Hipparchus. (It should be noted that the 'armillary astrolabe' or star-taker made of rings or bracelets described by Ptolemy in the Almagest Book 5, chapter 1, was a completely different sort of instrument.)
John North points to Synesius (about AD 400) as a source of information relevant to the claim that Hipparchus invented the planispheric astrolabe (but without providing a quotation or citation). North speculates that Hipparchus may have used an instrument 'of the astrolabe type' to produce the calculations of the risings and settings of stars provided in his writings, including the Commentary on the Phenomena of Aratus and Eudoxus (his only extant work). North's suggestion seems entirely reasonable.