Born: 1578 in Brescia, Italy
Died: 9 April 1643 in Rome, Italy

Benedetto Castelli's parents were Annibale Castelli, a wealthy landowner born in 1553 to Ortensio Castelli, and Alda Tiberi. Benedetto was not the name that he was given when he was baptised, rather it was Antonio. He took the name Benedetto in 1595 when he entered the Benedictine order. Antonio was the eldest of his parents' seven children; his six younger brothers were Ortensio, Vespasiano, Carlo, Ortensia, Lucrezia, and Quinto, while his sister Dorotea did not reach adulthood. The date of Antonio's birth is uncertain for we only know that he was ten years old in 1588, meaning he was born in 1577 or 1578. It was certainly not a happy family for three of the brothers met violent deaths after becoming involved in quarrels, while the youngest brother Quinto was banished from the Republic of Venice after committing various crimes. The ban was lifted only after Carlo and Benedetto, as the subject of this biography was called by this time, paid a large sums of money to allow Quinto to return.

Castelli joined the Benedictine order in the monastery of Saints Faustino and Giovita in Brescia on 4 September 1595. He began studying mathematics while in Brescia but, after he was transferred to the monastery of Saint Giustina in Padua, he came into contact with Galileo. Galileo had been appointed as professor of mathematics at the University of Padua (the university of the Republic of Venice) in 1592 and he lived near to Saint Giustina. Galileo was a personal friend of the abbot and frequently met with the monks in the monastery. Although exact details are unclear, we know that before 1604 Castelli had attended lectures by Galileo and watched him making various experiments, particularly with the thermometer. Castelli later described what he saw in 1603:-

[Galileo] took a small glass flask, about as large as a small hen's egg, with a neck about two spans long and as fine as a wheat straw, and warmed the flask well in his hands, then turned its mouth upside down into a vessel placed underneath, in which there was a little water. When he took away the heat of his hands from the flask, the water at once began to rise in the neck, and mounted to more than a span above the level of the water in the vessel. Galileo had then made use of this effect in order to construct an instrument for examining the degrees of heat and cold.

At first Castelli was certainly one of Galileo's students, but later the two became close friends. When Castelli was sent to the Benedictine abbey of La Trinità della Cava in the Finestre Hills outside Cava dei Tirreni in 1607 he corresponded regularly with Galileo and, when he was in Brescia in 1610, he wrote to Galileo thanking him for the copy of Sidereus Nuncius. By the time he wrote the letter he had already read, and greatly enjoyed, Galileo's book. On 5 December 1610 Castelli wrote to Galileo (see [21]):-

If the position of Copernicus, that Venus revolves around the sun, is true (as I believe), it is clear that it would necessarily sometimes be seen by us horned and sometimes not, even though the planet maintains the same position relative to the sun. ... Now I want to know from you if you, with the help of your marvellous glasses, have observed such a phenomenon, which will be, beyond doubt, a sure means to convince even the most obstinate mind. I also suspect a similar thing with Mars near the quadrature with the sun; I don't mean a horned or non-horned shape, but only a semicircular and a more full one.

It is now impossible to prove whether this idea occurred to both Galileo and Castelli at the same time, or whether this letter of Castelli made Galileo turn his telescope on Venus to see if it showed phases. Certainly by 11 December Galileo had discovered that Venus did indeed appear as a crescent for on that day he wrote to Giuliano d'Medici expressing the discovery in code. It is of little consequence which scenario is correct, for in either case Castelli came up with one of the most important ideas of the time. To complete the story of this episode, let us quote from Galileo's reply to Castelli written towards the end of December [21]:-

O how many consequences and ones of such import have I deduced, my Master Benedetto, from these and from my other observations. You almost made me laugh when you said that with these manifest observations the obstinate could be convinced. Well then, don't you know that to convince those capable of reason and anxious to know the truth the other demonstrations already produced were enough, but to convince the obstinate who care only for the empty applause of the stupid and dull crowd, the testimony of the stars themselves, come down to earth to discuss themselves, would not suffice? Let us then endeavour to learn something for ourselves and rest satisfied with this alone, but as for advancing ourselves in popular opinion or gaining the assent of philosophers in books, let us give up the desire and the hope.

Castelli continued to study mathematics with a view to becoming a teacher, making a deep study of the works of Euclid, Ptolemy, Theodosius and Archimedes. He was particularly keen to return to Padua so that he could learn more from Galileo but, around this time, Galileo resigned his post at Padua and moved to Florence becoming Chief Mathematician at the University of Pisa (without any teaching duties) and 'Mathematician and Philosopher' to the Grand Duke of Tuscany. Castelli, therefore, requested a transfer to Florence where Galileo was based and, in 1611, he went to Santa Maria di Firenze, the Benedictine abbey (called the Badia) located in the centre of Florence. He discussed with Galileo his observations of sunspots and it was Castelli who suggested that Galileo might find them easier to observe by using a telescope to project the sun's image onto a sheet of paper. While in Florence, Castelli gave private mathematics lessons.

In 1613 Castelli was appointed as professor of mathematics at the University of Pisa, succeeding Antonio Santucci, having been recommended by Galileo. He quickly gained a reputation as a fine teacher, and students came from many different regions to study with him. A significant event occurred in December 1613, not long after he took up the appointment. In a letter he wrote to Galileo on 14 December, he described an invitation to breakfast at the Tuscan court which was meeting in Pisa:-

On Thursday morning I had breakfast with our Lordships, and, when asked about school by the Grand Duke Cosimo II de' Medici, I gave him a detailed account of everything, and he seemed to be very satisfied. He asked me whether I had a telescope, and I said "Yes" and so began to tell him about the observation of the Medicean planets [the four moons of Jupiter discovered by Galileo a few years earlier] which I had made just the previous night. Then Her Most Serene Ladyship, the Grand Duchess Dowager Christina [mother of the Grand Duke Cosimo II] inquired about their position and began saying to herself that they had better be real and not deceptions of the instrument. So their Highnesses asked Cosimo Boscaglia [a Platonist and Professor of Logic and Philosophy at Pisa], who answered that in truth their existence could not be denied. I used the occasion to add whatever I knew and could say about your wonderful invention and your proof of the motions of these planets. ... As soon as I had come out of the palace, the porter of Her Most Serene Ladyship caught up with me and called me back. However, before I say what followed, you must know that at the table Boscaglia had been whispering for a long time in Her Ladyship's ear; he admitted as true all the celestial novelties you have discovered, but he said that the earth's motion was unbelievable and could not happen, especially since the Holy Scripture was clearly contrary to this claim. Now, to get back to the story, I entered the chambers of Her Highness, where I found the Grand Duke, Her Ladyship, the Archduchess [the Grand Duke Cosimo II's wife, Maria Maddalena of Austria], Don Antonio, Don Paolo Giordano, and Boscaglia. At this point, after some questions about my views, Her Ladyship began to argue against me by means of the Holy Scripture. I first expressed the appropriate disclaimers, but then I began to play the theologian with such finesse and authority that you would have been especially pleased to hear. ... The Grand Duke and the Archduchess were on my side and Don Paolo Giordano came to my defence with a very appropriate passage from the Holy Scripture. Only Her Ladyship contradicted me, but in such a way that I thought she was doing it in order to hear me. Cosimo Boscaglia remained silent. ... As I was leaving ... [Don Antonio] explicitly uttered these words: "Write to Galileo about my having become acquainted with you and about what I said in the chambers of Her Highness." I answered that I would report to you about this beautiful opportunity I have had to serve you. ...

Galileo replied to Castelli in a letter written on 21 December. What he wrote in this letter was significant since it, and his subsequent Lettera a Madama Cristina di Lorena Granduchessa di Toscana (1615) in which he further amplified his views on science and religion, would be used by the Inquisition in their case against him. There was no Benedictine convent so Castelli lived in Pisa in the Jesuati monastery of San Girolamo. In 1616 Bonaventura Cavalieri came to live in the San Girolamo monastery and he was taught mathematics by Castelli who introduced him to the ideas of Galileo. Castelli was careful in his teaching in Pisa, where he was forced to teach that the earth was stationary. From 1617 on, he corresponded with Cardinal Federico Borromeo, and was also employed by the Tuscan Court on various committees. He was ordered by the Grand Duke of Tuscany to make a sea voyage with the aim of testing the celatone, a device invented by Galileo for determining longitude using observations of the moons of Jupiter. This was an unpleasant experience for Castelli, who suffered severely from seasickness, and, despite his best efforts, the celatone did not achieve success. In 1618 three comets appeared and Archduke Leopold V of Austria demanded to know about comets. Castelli, and later Galileo, gave the Archduke their opinions.

In 1624 Castelli's appointment as professor at Pisa was confirmed as one for life. However, he only remained there for two more years before moving to Rome. In fact he had visited Rome in 1623 when Cardinal Maffeo Barberini became Pope Urban VIII. At this time he was appointed to take care of the waterways of Ferrara and Bologna. He was already interested in hydraulics and, after returning to Pisa, undertook new research on the topic. He made frequent trips to Florence to discuss hydraulics with Galileo, and also corresponded regularly with him on this topic. He left his professorship in Pisa in 1626 when the pope called him to Rome as an adviser on waterways and on mathematics, as well as employing him as a tutor to his young nephew, Taddeo Barberini. His pupil's elder brother, Francesco Barberini, arranged for Castelli's appointment as reader in mathematics at La Sapienza university in 1627. His students in Rome included Giovanni Alfonso Borelli and Evangelista Torricelli.

In 1628 Castelli published Della misura dell'acque correnti, his famous book on hydraulics. The work is in two parts, one dedicated to pope Urban VIII, the other to Taddeo Barberini. Many historians suggest this work inaugurated modern hydrodynamics. Maffioli writes [20]:-

Castelli's 'Della misura dell'acque correnti' (1628) stood out from contemporary engineering practices and deep-rooted philosophical beliefs such as that of the condensation of water. This second notion has not yet been fully acknowledged because Castelli only hinted at it by criticizing Giovanni Fontana's measurements of the Tiber and his use of the term 'acqua premuta'. The acknowledgment of the incompressibility of water was nonetheless an essential prerequisite of Castelli's mathematical formulation of the continuity law of running water.

Galileo was in Rome in May-June 1630 attempting to get Dialogue Concerning the Two Chief Systems of the World - Ptolemaic and Copernican published; he was made very welcome by Castelli during his visit. In 1631, while in Pesaro, Castelli wrote to Galileo. It tells us something of the society that Castelli moved in at this time and also of his continued support for Galileo and his ideas [15]:-

He writes from Pesaro, where the court was sojourning while Cardinal Antonio Barberini [another brother of Taddeo Barberini and Francesco Barberini] supervised the transfer of the vacant duchy of Urbino to the Papal State. He remarks with wry amusement on the gay times had by the many knights and gentlefolk in the cardinal's entourage, while he devoted himself instead to the solution of hundreds of equations. One day he was approached by a group of literate gentlemen who wanted to be taught the principles of geometry. Some of these dilettantes he found to be unusually intelligent. But when he recounted the merits of Galileo's work they were at first stunned, and were "most contrary to your name and your work, of which either they knew nothing at all or which they had learned in a distorted fashion. But now they are studying away with gusto."

Galileo was summoned to Rome in 1633 and at this time Castelli was sent to Brescia to support his brother Quinto who was sentenced to seven years in prison. By the time Castelli returned to Rome, he was very upset to discover that Galileo had been found guilty and condemned to lifelong imprisonment. Castelli became increasingly uncomfortable in his position in Rome. He continued to correspond with Galileo but his letters written in this period are sad and melancholy. We should understand, of course, why the Inquisition found it necessary to proceed against Galileo. It was not really about his belief in the heliocentric system. For example Castelli was a believer in the heliocentric system yet the Inquisition did not proceed against him. Rather it was all about authority - the Church saw in Galileo someone who had challenged their authority. Unhappy and out of favour in Rome, Castelli attempted to move to be near to his friend Galileo. For example, a letter written by Castelli (published in [2]) shows how he felt [17]:-

The single letter to Galileo (reproduced in [2]) happens to be one of the longest in the volume. Dated 13 June 1637, it is a detailed report on the failure of Castelli's apparent attempt to move out of Rome and be closer to Galileo. For about a dozen years Castelli had been teaching at the University of Rome under the patronage of the pope (Maffeo Barberini) and his nephew (Cardinal Francesco Barberini), but at a general convention of Benedictines that year Castelli (who belonged to this order) had been appointed abbot of a convent near Padua with the duty of being in residence there; however, the decision was not implemented because of the opposition of the Barberinis.

To keep him in Rome he was entrusted, in a purely formal capacity, with the four abbeys of S. Benedetto in Foligno, of S. Grisogono of Zara in Verona, of S. Maria of Praglia in Monreale, and of SS. Benedetto and Luigi in Palermo. Despite having to remain in Rome, Castelli continued to do fine science and to keep in contact with Galileo. For example, in a 1637 letter to Galileo, Castelli describes experiments where a brick was half painted white and half black, then exposed to heat from a fire and to light from the sun. He summarised his important findings [7]:-

... the heat which proceeds from light only, heats up much more noticeably the black than the white. On the other hand, the heat from fire without light heats up equally the white and the black, while heat which originates from fire with light heats up a little more the black than the white. From a careful observation of these data and from other findings in nature, we see a great difference in the operation of these heats.

Among his other achievements, Castelli claimed first that the intensity of light varies as the square of the distance, and was the first person recorded to have invented a rain-gauge in 1639. In the same year he wrote another remarkable work [10]:-

In 1639-1640 Benedetto Castelli wrote a treatise on the loadstone which is quite unlike any of its contemporaries. In it are the origins of the notion of elementary magnets sharing a common alignment, the idea that all materials are magnetic in different ways, and the first intimation of the concept of magnetic domains. Castelli did not publish his treatise. Nevertheless his work was noted during his lifetime, and may have exerted an influence on the development of magnetic theory in the 17th century. The treatise was published in 1883.

1. S Drake, Biography in Dictionary of Scientific Biography (New York 1970-1990).

Books:

1. M Bucciantini (ed.), Benedetto Castelli, Carteggio (Leo S Olschki, Florence, 1988).
2. P de la Fuente, X Granados and F Reus (eds.), Letters from Galileo Galilei, member of the Accademia dei Lincei, to Benedetto Castelli and to Cristina de Lorena, Grand Duchess of Tuscany (Spanish) (Editorial Alhambra, S.A., Madrid, 1986).
3. M J Lewis, Galileo in France: French reactions to the theories and trial of Galileo (Peter Lang, 2006).
4. G L M Zannini, La vita di Benedetto Castelli (Camera di commercio, industria e agricoltura di Brescia, Brescia, 1961).

Articles:

1. P E Ariotti, Benedetto Castelli and George Berkeley as Anticipators of recent Findings on the Moon Illusion, Journal of the History of Behavioral Sciences 9 (1973), 328-332.
2. P E Ariotti, Benedetto Castelli: Early Systematic Experiments and Theory of the Differential Absorption of Heat by Colors, Isis 63 (1) (1972), 79-87.
3. P E Ariotti, On the Apparent Size of the Projected After-Image: Emmert's or Castelli's Law? A Case of 242 Years Anticipation, Journal of the History of Behavioral Sciences 9 (1973), 18-28.
4. P E Ariotti, A Little Known early 17th-Century Treatise on Vision: Benedetto Castelli's 'Discorso sopra la Vista' (1639, 1669): Translation and Critical Comments by Piero E Ariotti, Ann. of Sci. 30 (1973), 1-30.
5. P E Ariotti, Benedetto Castelli's Discourse on the Loadstone (1639-1640): The Origin of the Notion of Elementary Magnets Similarly Aligned, Ann. of Sci. 38 (2) (1981), 125-140.
6. G Arrighi, Benedetto Castelli: considerazioni e proposte, in G Arrighi (ed.), La scuola galileiana: prospettive di ricerca : atti del Convegno di studio di Santa Margherita Ligure, 26-28 ottobre 1978 (La nuova Italia, Florence, 1979), 3-11.
7. G Arrighi, Benedetto Castelli: il primo discepolo, in C Maccagni (ed.), Saggi su Galileo Galilei 3 vols. (Barbèra, Florence, 1972), 625-636.
8. A K Biswas, The Automatic Rain-Gauge of Sir Christopher Wren, F.R.S., Notes and Records of the Royal Society of London 22 (1-2) (1967), 94-104.
9. M Bucciantini, Aspects of geometry and theology in the Galilean philosophy of Benedetto Castelli, in Geometry and atomism in the Galilean school (Italian) (Bibl. Nuncius Studi Testi, X, Olschki, Florence, 1992), 171-191.
10. J Connors, S Ivo Alla Sapienza: The First Three Minutes, Journal of the Society of Architectural Historians 55 (1) (1996), 38-57.
11. A De Ferrari, Benedetto Castelli, Dizionario biografico degli Italiani 21 (Rome, 1978), 686-690.
12. M A Finocchiaro, Review: Benedetto Castelli, Carteggio edited by Massimo Bucciantini, Isis 81 (2) (1990), 344-345.
13. W Fox, Benedetto Castelli, in The Catholic Encyclopedia (Robert Appleton Company, New York, 1908). 
    http://www.newadvent.org/cathen/03408d.htm
14. W Herbert, Grabbing the Horns of Venus, Science News (124) (22) (1983), 347
15. C S Maffioli, 'Acqua premuta'. Benedetto Castelli and the incompressibility of water, Boll. Stor. Sci. Mat. 28 (1) (2008), 9-50.
16. R S Westfall, Science and Patronage: Galileo and the Telescope, Isis 76 (1) (1985), 11-30.