Cornelius Gemma

He was a professor of medicine at the Catholic University of Leuven, and shared in his father's efforts to restore ancient Ptolemaic practice to astrology, drawing on the Tetrabiblos.

As an astronomer, Gemma is significant for his observations of a lunar eclipse in 1569 and of the 1572 supernova appearing in Cassiopeia, which he recorded on 9 November, two days before Tycho Brahe, calling it a "New Venus.

[10] Gemma attempted to formulate a universal philosophy that brought together inferiors and celestials, nature, soul and intellect, numbers, ideas and external objects.

This "cyclognomic art" is an arrangement of seven concentric circles, starting from the outermost: A profusion of charts, celestial diagrams, and spherical triangles is characteristic of Gemma's ars cyclognomica, as is the use of three as a mystic number.

Disciplines are grouped under three faculties or spheres: Gemma's two volumes De naturae divinis characterismis (1575), on divine marks or features in nature, included tales of medical marvels.

An example may be found online in the Compendium Maleficiarum of Francesco Maria Guazzo: a 15-year-old girl was reported to excrete a live eel and to vomit a prodigious stream of hairs, skin fragments, stones and bones.

Gemma was one of the few astronomers — most famously Tycho Brahe, but also Helisaeus Roeslin, William IV, Landgrave of Hesse-Kassel and Michael Mästlin — who identified the comet as superlunary.