Cornelius (or Cornelio) Gemma (28 February 1535 – 12 October 1578) was a Flemish physician, astronomer and astrologer, and the oldest son of cartographer and instrument-maker Gemma Frisius. He was a professor of medicine at 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." With Brahe, he was one of the few astronomers to identify the Great Comet of 1577 as superlunary. Gemma is also credited with publishing the first scientific illustration of the aurora, in his 1575 book on the supernova.
Another milestone appears in his medical writings: in 1552, Gemma published the first illustration of a human tapeworm.
Gemma's two major works, De arte cyclognomica (Antwerp, 1569) and De naturae divinis characterismis (Antwerp, 1575), have been called "true 'hidden gems' in early modern intellectual history," bringing together such topics as medicine, astronomy, astrology, teratology, divination, eschatology, and encyclopaedism.
Gemma also has the distinction of being called "the first true orchid hobbyist, in the modern sense
"'Vere Gemmeum est?': Cornelio Gemma e la stella nuova del 1572"
"Cornelius Gemma and Universal Method"
"Cornelius Gemma et l'épidémie de 1574"
"Cornelius Gemma, Philosophie und Methode: Eine Analyse des ersten Buches der Ars cyclognomica"
"La notion de prodige selon Cornelius Gemma"