Christopher Clavius

Germany
1538-1612

This German Jesuit mathematician and astronomer modified the proposal of the modern Gregorian calendar after the death of its primary author, Aloysius Lilius. Clavius would later write defences and an explanation of the reformed calendar, including an emphatic acknowledgement of Lilio’s work. In his last years he was the most respected astronomer in Europe. His textbooks were used to teach astronomy for over fifty years in and out of Europe.

Clavius joined the Jesuit order in 1555. He attended the University of Coimbra in Portugal.  Here, his success in mathematics, and his observation of the total solar eclipse of 1560 turned astronomy into his life’s work. He was ordained in 1564, while still pursuing theological studies, and became a full member in 1575. He began teaching math at the college 1564 and was on the faculty of the Collegio Romano until his death in 1612. Within the Jesuit order, Clavius was almost solely responsible for the adoption of a rigorous mathematics curriculum in an age where math was often ridiculed by philosophers as well as fellow Jesuits like Benito Pereira. In logic, Clavius’ Law (inferring of the truth of a proposition from the inconsistency of its negation) is named after him.
Because of his prodigious output of mathematical works, he was called “the Euclid of the sixteenth century.” Through his teaching and textbooks, and also through several mathematical curricula drafted by him, Clavius shaped mathematical education in the Jesuit order all over the world. In 1579 he was assigned to compute the basis for a reformed calendar that would stop the slow process in which the Church’s holidays were drifting relative to the seasons of the year. Using the Prussian Tables of Erasmus Reinhold and building on the work of Aloysius Lilius, he proposed a calendar reform that was adopted in 1582 in Catholic countries by order of Pope Gregory XIII and is now the calendar used worldwide. If you like the modern calendar, thank the Catholic Church.

Galileo was familiar with Clavius’s books, and he visited the famous man during his first trip to Rome in 1587. After that they corresponded from time to time about mathematical problems, and Clavius sent Galileo copies of his books as they appeared. In this last edition of his Sphere, Clavius mentioned the telescopic discoveries of Galileo briefly as follows:  “This instrument shows many more stars in the firmament than can be seen in any way without it, especially in the Pleiades, around the nebulas of Cancer and Orion, in the Milky Way, and other places . . . and when the Moon is a crescent or half full, it appears so remarkably fractured and rough that I cannot marvel enough that there is such unevenness in the lunar body. Consult the reliable little book by Galileo Galilei, printed at Venice in 1610 and called Sidereus Nuncius, which describes various observations of the stars first made by him.”

He used the decimal point in the goniometric tables (the art of constructing all possible sun-dials;) of his astrolabium in 1593, one of the first who used it this way in the West. As an astronomer Clavius held strictly to the geocentric model of the solar system, in which all the heavens rotate about the Earth. Though he opposed the heliocentric model of Copernicus, he recognized problems with the Ptolemaic model. He was treated with great respect by Galileo, who visited him in 1611 and discussed the new observations being made with the telescope; Clavius had by that time accepted the new discoveries as genuine, though he retained doubts about the reality of the mountains on the Moon. Clavius states among other things a method of dividing a measuring scale into subdivisions of any desired smallness, which is far more complete than that given by Nonius and must be considered the precursor of the measuring instrument named after Vernier, to which perhaps the name Clavius ought accordingly to be given. The chief merit of Clavius, however, lies in the profound exposition and masterly defence of the Gregorian calendar reform, the execution and ultimate success of which are due chiefly to him.  A large crater on the Moon is named for him.