Christoph Clavius · 1612
Read the translation (PDF) · Editorial footnotes (PDF)
Tomus V is Clavius’s definitive account of the Gregorian calendar reform, which he principally engineered — the fullest primary exposition of the calendar the modern world still uses, written by the man most responsible for it.
The Explicatio Romani Calendarii expounds the reformed calendar in full: the correction of the solar year and the equinox, the epacts and golden numbers, the lunar cycle, and the reckoning of Easter, together with the great computus tables that put the scheme into daily practice. A compendium of the new method of restoring the calendar precedes the detail, and the perpetual Gregorian calendar follows it.
The volume then turns combative. The Apologia is Clavius’s two-book defence of the reform against Michael Maestlin’s attacks, and an appendix answers further critics — Joseph Scaliger, Georg Germanus, and François Viète. The dedications record what was at stake institutionally: the exposition is addressed to Pope Clement VIII, with a papal document ad perpetuam rei memoriam, while the volume as a whole is dedicated like its fellows to Prince Johann Gottfried.
The Gregorian calendar is the most widely adopted technical standard ever devised, and this is its primary source. What the volume preserves that later accounts lose is that the reform was fiercely contested — by Protestant astronomers who would not take a calendar from Rome, and by mathematicians of the first rank who disputed the method itself. Clavius answers them by argument rather than authority, which makes the Apologia a rare thing: a technical defence of a standard, written while its adoption was still in doubt, by the person who had to make the case.
The translated source is the Mainz printing of 1612, the year of Clavius’s death.