Distinct from his star catalogue, this entry documents the attribution — held by historians such as G.J. Toomer, John North, and David King — of the invention or adaptation of the planispheric astrolabe to Hipparchus of Nicaea. The attribution lacks direct contemporary evidence: the earliest supporting testimony is a letter of Synesius of Cyrene, centuries after Hipparchus, and historian Emilie Savage-Smith has explicitly noted there is no convincing evidence Hipparchus or Ptolemy knew the planispheric astrolabe.