Clavius: leap year calculations still hold

Sunday 12 February 2012 marks the 400th anniversary of the death of Christopher Clavius, the Jesuit mathematician and astronomer, who was born in Bamberg, Franconia (now part of Bavaria), on 25 March 1538.

Little is known of his early life.?Even his original surname is a matter of conjecture, since Clavius was his adopted Latin name.

Clavius entered the Jesuit order in 1555, and a year later was sent to the Jesuit University of Coimbra in Portugal, where he excelled in mathematics. In 1564 he was called to the Collegium Romanium in Rome to teach the subject, a post he held almost uninterruptedly until his death.

He became the leading mathematician of the Jesuit order, and has been dubbed “the Euclid of the 16th century”.?He wrote a number of textbooks, including his version of Euclid’s ‘Elements’, as well as books on algebra, the astrolabe, practical arithmetic, geometry and astronomy.

After witnessing and describing the solar eclipse of 21 August 1560, Clavius decided to make astronomy his life’s work. He regularly corresponded with the astronomer Galileo Galilei, although he initially rejected Galileo’s modern ideas that contradicted the Ptolemic theory favoured by the church, in which the sun and planets revolve around the Earth. After Clavius’s death, the third largest crater on the moon was named after him.

However, it is perhaps for his work on the calendar reform of 1582, commissioned by Pope Gregory XIII, that Clavius is best known. The Julian calendar’s

leap-year rule created three leap years too many every 385 years. To correct this, Clavius proposed that Wednesday 4 October 1582 be followed by Thursday 15 October 1582, and that leap years follow the rule still in use today. So accurate were his calculations that it will take 3,500 years before an error of one day is reached.