Clavius (or Klau), Christoph (1537–1612)

German mathematician and astronomer. A leading Jesuit and professor of mathematics at the College of Rome, he was frequently consulted by the Vatican on controversial scientific matters. The best-known case was his defence of the Copernican system against the theories of Galileo. His books on mathematics were among the most widely used mathematics textbooks in the 17th century.

Between 1588 and 1603 he wrote no fewer than five separate works defending the calendar reforms of Pope Gregory XIII in 1582. He was again called upon in 1611 to advise the Vatican authorities upon the reliability and significance of Galileo's telescopic observations of the Moon. While responding sympathetically to Galileo's work, he advised, nonetheless, that the observations did not constitute a convincing proof of the Copernican system. He argued that the lunar mountains described by Galileo – which indicated that the celestial system was not unchanging – were covered with a smooth but transparent crystalline surface.

His widely used textbooks were Epitome arithmeticae (1583) and Algebra (1608). He also published a major treatise on gnomonics in 1581, and in 1612 he wrote a treatise on the construction and use of sundials.