Born Perugia, (Italy), April 1536
Died Alatri, (Lazio, Italy), 19 October 1586
Egnatio Danti was a master of observational instruments for astronomy and served on Pope Gregory XIII’s commission to reform the calendar.
The Rainaldi family had already become quite renowned in the field of humanities and mathematics when Carlo Pellegrino was born. Carlo’s grandfather, Pier Vincenzo, was a well-known man of letters and an expert in mechanics, and because of his cleverness his peers nicknamed him Dante or Danti. Pier Vincenzo’s brother Giovanni Battista thus decided to adopt the surname of Danti. Carlo Pellegrino received his early education from his father Giulio, who was particularly knowledgeable about instrumental techniques, and from his aunt Teodora, who had a reputation as a painter and scholar of astronomy, mathematics, and geometry.
At the age of 13, Danti entered the Dominican Order and changed his name to Egnatio. Several years later, fame of his knowledge reached the Medici court, where his brother Vincenzo was already working as a sculptor. Thus, in 1562 Cosimo I called him to Florence to paint maps of all the regions in the known world, based on Ptolemy ’s description, in the wardrobes of the room known as the Guardarobain the Palazzo Vecchio. Cosimo’s admiration for his cosmographer grew so much that in 1571 he asked the Dominican Order to allow Danti to live at the Medici palace.
Thanks to Cosimo’s benevolence, Danti obtained the mathematics chair at the University of Florence. Here, he began to study the inaccuracies of the Julian calendar then in use. To measure the exact duration of the year, Danti built an astronomical quadrant on the façade of the church of Santa Maria Novella with eight solar clocks and an equinoctial ring that we can still see today. Danti was thus able to observe the spring equinox in 1574, discovering that, in that year, it fell on 11 March of the Julian calendar. For the same purpose, in 1575 Danti started to construct a meridian line in the Santa Maria Novella, but he never completed it due to the death of Cosimo I. With Cosimo’s demise, he lost the protection he had enjoyed at the Medici court, and Cosimo’s son, Francesco I, banished Danti from Florence.
Consequently, Danti moved to Bologna, where in 1576 he was awarded the chair ad Mathematicamthat had replaced the chair of astronomy. This position entailed teaching Euclid’s Elements, John of Holywood ’s De Sphaera, and Ptolemy’s Almagestand Theorica Planetarum. Danti, who had already published the Italian translation of Proclus ’ De Sphaerain Florence in 1573, published his grandfather’s translation of Sacrobosco’s De Sphaerain 1579.
During his stay in Bologna, Danti constructed a large meridian line in the church of San Petronio in 1575, but no trace of it remains since the south section of the building was restored in the middle of the seventeenth century. Danti used the meridian line for further verification of the equinoctial day, in order to contribute to the necessary calendar reform. Danti described this meridian line in a rare loose sheet entitled Usus et tractatio gnomonis magni.
In Bologna, Danti also constructed a number of vertical anemoscopes, instruments he invented about 1570. The only extant one, which is partially preserved, is located in the cloister of the church of San Domenico and is described in his 1578 work Anemographia … In anemoscopium instrumentum ostensorem ventorum …. There he relates how he came up with the idea of taking the indications given by the weathervane, which turns on a horizontal plane, and placing them on a vertical plane.
In the summer of 1577 Danti returned to Perugia, where he built two anemoscopes and also began designing the topographical map of the city and the outlying area. This work was so successful that in 1578 he was appointed to carry out the topographical survey of the entire papal state, although this project did not keep him from continuing to teach in Bologna. This work yielded the now-rare Perusini Agrimap, printed in Rome in 1580, and the chorographic map of the Territorio di Orvieto, printed in 1583.
In 1580, Pope Gregory XIII called Danti to Rome as the pontifical cosmographer and mathematician, in order to reform the calendar. Danti became a member of the Reform Commission, chaired by Cardinal Guglielmo Sirleto. Danti created a meridian line on the floor of the loggia of the Torre dei Venti at the Vatican. The calendar was adjusted in 1582, and the reform was implemented in two parts. One proclaimed the new rules to follow for the future and the other set out the steps to be taken immediately to correct the errors of the past. In order to bring the vernal equinox back to 21 March, 10 days were subtracted from the year 1582, moving the calendar from Thursday, 4 October, to Friday, 15 October.
Danti was accepted as a member of the Accademia di San Lucain Rome in 1583 and in the same year he was named Bishop of Alatri in central Italy. In 1586, he participated in various engineering works, such as the restoration of the port of the Roman Emperor Claudius at Fiumicino, and the transfer of the Vatican obelisk to align it with Saint Peter’s Basilica. Working with the architect Giovanni Fontana, Danti’s specific task was to mark the solstices and equinoxes at the base, as well as the winds, thus treating the obelisk as if it were a giant gnomon. On his journey back to Alatri, Danti caught pneumonia in Valmontone and was brought back to his bishopric, where he died.
Danti was a passionate scholar of all kinds of instruments of observation. His most important works include those on the astrolabe (Dell’uso e della fabbrica dell’astrolabio), the anemograph (Anemographia), and the trigometer (Trattato del radio latino, instrumento giustissimo). In the second edition of his treatise on the astrolabe, Danti offered a highly detailed description of nine other astronomical instruments in use during the era.
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