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Mathematician and cosmographer, b. at Perugia, Italy, 1537; d. at Alatri, 19 Oct., 1586. As a boy he learned the rudiments of painting and architecture from his father and aunt, but mathematics and science were his favorite studies. He received the Dominican habit 7 March, 1555, changing his baptismal name Pellegrino to Ignazio. After completing his philosophy and theology he gave some time to preaching, but soon devoted himself zealously to mathematics, astronomy, and geography. About 1567 he was invited to Florence by Cosmo I, Duke of Tuscany, who wished to avail himself of his services in reviving mathematical and astronomical studies in his newly acquired dominion. About the same time Pope Sixtus V, who belonged to the Order of Preachers, is said to have commissioned him to furnish plans for the construction of a Dominican church and convent at Bosco. During his stay in Florence Danti taught mathematics with much success and may be said to have prepared the way for Galileo and his contemporaries. He resided at the convent of Sta Maria Novella, and designed the first gnomon on the facade of its church in 1572. He was chosen to direct the building of a canal which was to place Florence in communication with both the Mediterranean and the Adriatic. Cosmo did not live to carry out his project and shortly after his death (1574) Danti became professor of mathematics at the University of Bologna. While occupying this chair he spent some time in his native city, at the invitation of the governor, where he prepared maps of the Perugian republic.
On account of his mathematical attainments Gregory XIII invited him to Rome, appointed him pontifical mathematician and made him a member of the commission for the reform of the calendar. He also placed him in charge of the painters whom he had summoned to the Vatican to continue the work so brilliantly begun by Raphael during the reign of Leo X and at the same time desired him to make a number of maps of ancient and modern Italy. When the pontiff commissioned the architect Fontana to repair the Claudian harbour it was Danti who furnished the necessary plans. While at Rome Danti published a translation of a portion of Euclid with annotations and wrote a life of the architect Vignola, preparing also notes for the latter's work on perspective. In recognition of his labours Gregory, in 1583, made him Bishop of Alatri in the Campagna. Danti showed himself a zealous pastor in his new office. He convoked a diocesan synod, corrected many abuses, and showed great solicitude for the poor. Shortly before his death Sixtus V summoned him to Rome to assist in the erection of the grand obelisk in the piazza of the Vatican. Besides the works already mentioned, Danti was the author of "Trattato del'uso e della fabbrica dell'astrolabo con la giunta del planifero del Raja"; "Le Scienze matematiche ridotte in tavole", also a revised and annotated edition of "La Sfera di Messer G. Sacrobosco tradotta da Pier Vincenzio Danti".