Etching of Christophorus Clavius, (1537/38 - 1612), a German born in Bamberg, who joined the Jesuit order in 1555 and spent the rest of his life in Rome, where he died. He was most eminent as a mathematician, a subject on which he wrote several books, and in 1579 was charged by Pope Gregory XIII with recalibrating the Julian calendar, by then well out of step with astronomical measurement. This work resulted in the change of 1582 when Roman Catholic Europe adopted what became known as the Gregorian calendar. This was only adopted in England in 1752, by the removal of 11 days from the Julian sequence then still in use. This is one of a series of engraved portraits by Boulonois who is reported to have flourished about 1682.