   Among Catholics, Christoph Clavius (1537–1612) was the leading astronomer in the sixteenth century. A Jesuit himself, he incorporated astronomy into the Jesuit curriculum and was the principal scholar behind the creation of the Gregorian calendar. Like the Wittenberg astronomers, Clavius adopted Copernican mathematical models when he felt them superior, but he believed that Ptolemy’s cosmology – both his ordering of the planets and his use of the equant – was correct.