Christopher Clavius S. J. (1537 - 1612) was a German mathematician and astronomer. Renowned as a teacher and writer of textbooks, Clavius was particularly active in the reform of the Gregorian calendar. This is the title page of his collected works, *Opera Mathematica*,* *(1612), five volumes. The first volume contains his commentaries on Euclid and the “Spheric” of Theodosius. The second volume contains his works on algebra and geometry and the third volume contains a commentary on Sarobosco’s “Sphaera” An account of the construction of sundials is given in the fourth volume, while the last volume contains information on calendar reform.

The English translation of this page is as follows:

Christopher Clavius of Bamberg and the Society of Jesus Mathematical Works in five volumes, now once more corrected by the author and revised in many places. Dedicated to the Most Reverend and Illustrious Prince and Lord, Dom John Geoffrey, Bishop of Bamberg at Mainz printed by Reinhard Eltz, at the expense of Anthony Heirat, with leave and permission of His Majesty the Holy Emperor. In the year of Our Lord1612. God has given me the knowledge of the progress of the years and of the constellations of the stars. Wisdom 7,19.

Index of problem demonstrations from volume 1 of Clavius’ *Opera.*