La penna geometrica di Giambattista Suardi ispirata al Sistema tolemaico

Giambattista Suardi's “Penna Geometrica” was an instrument based on wires and gears and could be used to draw a variety of curves generated by combining different rotary motions based on the combination of three parameters: angular velocity of the wheels, ratio of their radii or number of teeth, and possible directions of the motions. Suardi described the device in his famous treatise of 1752 “Nuovi istromenti per la descrizione di diverse curve antiche e moderne”. He was inspired to the Ptolemaic motion and indeed, due to mechanical limitations, Suardi’s Geometric pen only allowed to make approximately a few orbits of the planets. This paper describes some of the complex mechanical devices that Suardi used to draw curves, the connections with the Ptolemaic theory as well as connections between Suardi's machine, the spirograph and some later instruments. We show how in reality Suardi's machine did not allow to trace the known orbits of the planets, due to its physical limitations