Christoph Clavius (1537–1612)
The Jesuit mathematician and astronomer Christopher Clavius was born in Bamberg, Germany in 1537. Clavius joined the Jesuit order as a young man, and studied at Coimbra. Clavius was one of the major architects of the Gregorian calendar reform of 1582 and remained professor of mathematics at the Jesuit's Collegio Romano until his death on February 6, 1612.
In 1570 Clavius wrote what was to become one of the most influential textbooks on astronomy of its day, in the form of a commentary on the so-called Spheres of Sacrobosco (see "Clavius' Spheres" below). In the Catholic world, this was the textbook for three generations of astronomers, including Galileo Galilei, and most particularly for Jesuit astronomers throughout the world. In later editions of his book, Clavius had a favorable opinion of the Copernican system as a mathematical model, but to the end of his life he rejected its physical reality. Until Galileo's condemnation in 1633, this was also the official position of the Roman ecclesiastic authorities.
Clavius was chief astronomer at the Jesuit's Collegio Romano at the time of Galileo's first telescopic discoveries, and was still quite active despite his advancing age. He had a favorable view of the physical reality of most of these telescopic observations. His doing so effectively silenced the first wave of opposition to Galileo. He did not agree with all of Galileo's interpretations however, and in particular remained staunchly opposed to the notion of mountains on the Moon.
“Clavius' Spheres,” along with his influential commentary on “Euclid's Elements,” was regularly reprinted for many decades after his death in 1612. The book is basically a presentation of Ptolemaic astronomy, although in later editions the "Tychonian" and "Copernican" planetary systems are also discussed.
Lattis, J. 1994, Between Copernicus and Galileo: Christopher Clavius and the collapse of Ptolemaic Astronomy (Chicago: The University of ChicagoPress).