Primum Mobile

In the Ptolemaic cosmological system, the primum mobile is the ninth heaven, devoid of stars, which rotates extremely rapidly around the Earth and the other planets, communicating its motion to the heavens below. The instrument 'of the Primum Mobile' was developed by the mathematician Petrus Apianus (or Pieter Bienewitz; Leisnig 1495 - Ingolstadt 1552). Apianus, in his Instrumentum sinuum sive primi mobilis (1534), imagined a device which allowed sines to be calculated. The instrument of the primum mobile, or instrument of sines, is nothing but an adaptation of the trigonometric grid already known to the Arabs and placed on the new quadrant by the medieval mathematicians. The name 'primum mobile' derives from the fact that astronomers referred to the ninth heaven to determine the true place of the stars.

The instrument is accompanied by sine tables for all of the minutes of the quadrant, divided into 100,000 parts. Astronomical calculations can be made which allow the position of the sun or a star to be determined. The instrument is derived from the quadrant and Oronce Fin? claimed its invention at around the same time.

Mara Miniati
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