**Christopher Clavius** was a German Jesuit astronomer who helped Pope Gregory XIII to introduce what is now called the Gregorian calendar.

- Several guesses such as 'Clau' or 'Klau', or even 'Schlüssel' which means 'key' so might have led to him taking the Latin 'Clavius' which also means 'key', have been made but none have ever been substantiated with any evidence.
- Mathematics had always been a topic which interested Clavius, and he excelled in the mathematical courses which he took as part of a general degree.
- Although Clavius gives the year as 1559, he has made an error since we know that the only possible eclipse that fits his description occurred on 21 August 1560.
- This event was important for Clavius since it convinced him to devote his life to mathematical and astronomical study.
- In fact, except for a period in Naples around 1596 and a visit to Spain in 1597, Clavius was to remain Professor of Mathematics at the Collegio Romano for the rest of his life.
- Another suggestion is that the 'certain narrow circle' seen by Clavius might be the inner corona.
- It is perhaps for his work on the reform of the calendar that Clavius is most widely known.
- Clavius proposed that Wednesday, 4 October 1582 (Julian) should be followed by Thursday, 15 October, 1582 (Gregorian).
- Viète did not like Clavius's calendar and the people of Frankfurt rioted against the Pope and mathematicians who, they believed, had conspired together to rob them of 11 days.
- Clavius wrote Novi calendarii romani apologia Ⓣ(An defence of the new Roman calendar) (1595) which justified the new calendar reforms defending them against these attacks.
- Although Clavius produced little original mathematics of his own, he did more than any other German scholar of the 16th Century to promote a knowledge of mathematics.
- Clavius also produced a number of instruments, perhaps the most interesting being an instrument to measure fractions of angles.
- Perhaps the best summary of his work would be to look briefly at the five volumes Christophori Clavii e Scoietate Jesu opera mathematica, quinque tomis distributa Ⓣ(Five volumes of the mathematical works of Christopher Clavius of the Society of Jesus) which was produced near the end of his life and contains his collected works.
- Finally let us look at Clavius's reaction to the astronomical discoveries made by Galileo.
- When Clavius published a book he would always send a copy to his friend Galileo.
- When Galileo published Sidereus Nuncius in 1610, Clavius was an old man and it must have been extremely difficult to grasp these new discoveries both from a scientific and religious point of view.
- Hence Clavius's plea to astronomers to come up with a theory to 'save these phenomena'.

Born 25 March 1538, Bamberg (now in Germany). Died 2 February 1612, Rome (now in Italy).

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Astronomy, Origin Germany, Physics, Special Numbers And Numerals

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive