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Orazio Grassi

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Born
1 May 1583
Savona, Liguria, Republic of Genoa, now Italy
Died
23 July 1654
Rome, Papal States, now Italy
Summary
Orazio Grassi was an Italian mathematician, Jesuit priest and architect who became involved in an argument with Galileo regarding the nature of comets. He taught mathematics at the Collegio Romano and lecture notes from his 1623 course are still extant.
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Biography

Orazio Grassi was the son of Camillo Grassi. Orazio was brought up in Savona where he received his early education. Savona is on the north west coast of what today is Italy, not far from France. When he reached the age of seventeen he went to Rome to become a candidate to joint the religious order of the Jesuits, the Society of Jesus. The Jesuits had been founded in 1540 by Ignatius of Loyola and six companions, with the approval of Pope Paul III. It required a noviciate to train its candidates, and in 1566 it acquired the old church on the Quirinal Hill, the highest of the seven hills of Rome. This was rebuilt and a convent building was added. It was here that Grassi began his training on 18 October 1600.

Completing his studies on the Quirinal Hill and having taken his first religious vows, in April 1603 Grassi began the three year philosophy course at the Collegio Romano in Rome. This Jesuit college had been founded by Ignatius of Loyola in 1551 and in 1553 became a university. For the Jesuits, education was an important part of their beliefs and they produced a plan of study which reached its final form in the Jesuit Ratio Studiorum Ⓣ in 1599. Mathematics was an important part of the syllabus set out in the Ratio Studiorum Ⓣ. It was taught in the second of the three years and included a study of Euclid's Elements and the Sphere of Sacrobosco. Christopher Clavius played a key role in developing the mathematics syllabus in the Ratio Studiorum Ⓣ and he taught at the Collegio Romano for 37 years. Astronomy, which at that time was basically geometry of the heavens, played a large role in the Ratio Studiorum Ⓣ.

Grassi studied the mathematics course at the Collegio Romano in 1603-04 and showed such talent that he was told to take the two year "mathematical academy" course in 1604-06. This higher mathematics course at the mathematical academy was given by Christoph Grienberger (1561-1636) and Odo van Maelcote (1572-1615) but directed by Christopher Clavius. Grassi continued to study at the Collegio Romano until the autumn of 1610. Over the following three years it is unclear exactly what Grassi did, but it appears that he spent some time as Grienberger's assistant in the mathematical academy and also served a probationary year.

In January 1614 Grassi returned to Savona; it was a long and difficult journey from Rome. He remained in Savona until the autumn of 1614 when he was sent to Genoa to take up the position of spiritual assistant to the novices at the Jesuit College. The Jesuits had had a monopoly on higher education in Genoa since 1554. Grassi wanted to return to Rome and, after two years in Genoa, he was appointed to the chair of mathematics in the Collegio Romano, the chair which had been held first by Christopher Clavius and then by Christoph Grienberger. His appointment was from 8 September 1616 and, with except for the two years 1625-27, he held this appointment until 1628.

While he was professor of mathematics at the Collegio Romano, Grassi became involved in a controversy with Galileo. It is this controversy which is today the thing for which Grassi is most remembered. This is illustrated by the fact that many of the references we have found are in works about Galileo. Let us give a little background information before explaining the controversy.

Galileo believed in the heliocentric model of the solar system as proposed by Copernicus, but the Inquisition had met on 24 February 1616, taken evidence from theological experts, condemned the teachings of Copernicus, and Galileo was forbidden to hold Copernican views. Grassi followed the model put forward by Tycho Brahe which had the sun and moon in orbit round a stationary earth, while the planets were in orbit round the sun. The only way to chose between these two models was to measure the parallax for a close star against distant stars taking observations six months apart. Not until the 19th century would instruments be good enough to make such observations possible. Galileo had made many discoveries with his telescope and had lectured about these at the Collegio Romano in 1611; his lecture had been highly praised.

Three comets had been visible to the naked eye in 1618, the first in October, the second in the middle of November and the brightest near the end of November. These were the first comets to be seen after the invention of the telescope so were of special interest. The Collegio Romano held four separate conferences by the professors of mathematics, philosophy, rhetoric and theology. Grassi published details of the mathematics conference in De tribus cometis anni 1618 disputatio astronomica publice habita in Collegio Romano Societatis Iesu ab uno ex patribus eiusdem Societatis Ⓣ. Observations of the comets made in various Jesuit colleges in Italy and abroad were discussed. From parallax calculations, they deduced that the comets were beyond the moon, but closer than the sun. The observations led to the belief that the comets followed circular orbits round the sun although this is likely to be influenced by the long held belief in circular (i.e. perfect) orbits. Observations with telescopes showed, they believed, little magnification and this confirmed their belief that they were beyond the moon. Certainly they were beyond the moon, but this argument is quite false. Grassi's document also claimed, without substantiation, that this study of comets added to the belief that Tycho Brahe's model of the solar system was the true one. Jesuits had adopted this model after the Inquisition banned belief in a heliocentric model.

Giovanni Battista Rinuccini (1592-1653) was a Roman Catholic priest and scholar who had been proposed by Galileo for membership of the Accademia dei Lincei in 1616. He wrote to Galileo from Rome on 2 March 1619 telling him about Grassi's document [19]:-
The Jesuits have composed a Problem, which is in press, and firmly maintain that the comet is beyond the sky of the Moon; and some apart from the Jesuits are spreading word that this overthrows the system of Copernicus ...
Galileo received a copy of Grassi's document and was angered by it. A copy still exists with Galileo's comments scribbled on it in his own hand. These comments include [36]:-
... 'pezzo d'asinaccio' ('piece of utter stupidity'), 'bufolaccio' ('buffoon'), 'villan poltrone' ('wicked idiot'), 'balordone' ('bumbling idiot'), ...
Prompted by Galileo, Mario Guiducci delivered the lecture entitled Discourse on the comets at the Academy of Florence in May 1619; it was published not long after he had delivered it with a date 8 June 1619. The content was basically written by Galileo who thought it politically wise to have it appear under the name of one of his disciples. The Discourse argued that comets were caused by sunlight reflecting off vapours in the earth's atmosphere:-
I shall not believe that parallax has really any place in comets until it is first proved that comets are not reflections of light, but are unique, fixed, real and permanent objects.
The Discourse strongly attacked the argument in De tribus cometis anni Ⓣ that comets added weight to a belief in Tycho Brahe's model. It claims that Grassi's explanation is "false and vain." Grassi responded to the Discourse by publishing Libra astronomica ac philosophica qua Galilaei Galilaei opiniones de cometis a Mario Guiduccio in Florentina Academia expositae, atque in lucem nuper editae, examinantur a Lothario Sarsio Sigensano Ⓣ in October 1619 [37]:-
While the title seemed to imply calm, thoughtful consideration (an attitude claimed by the author at every step) in weighing the various theories on comets, in reality the work breathed rancour from every comma. Although justly noting certain logical inconsistencies found in the 'Discourse', which was indeed casual with regard to the development of the argument, the 'Libra' was still based on the usual scholastic canons, and thus had few means apart from verbal aggression of combating the deep-seated objections cited by Galileo and Guiducci.
Grassi says that the observations of the comet can only be explained by it having an orbit round the sun and, with the aid of mathematics, one can see that only two possible models of the solar system satisfy this. Since the only model other than Tycho Brahe's is a "recently condemned hypothesis," says Grassi, being good Catholics, the authors of the Discourse will have to admit they are wrong.

Of course neither Grassi nor Galileo was correct in their ideas about comets but nevertheless Grassi was closer to being correct than Galileo. Although at first Galileo did not realise that Lotario Sarsi of Siguenza was Grassi, it was soon pointed out to him that name was essentially the Latin version of Grassi's name with the letters permuted. Other Jesuits supported Grassi, for example Niccolò Cabeo, a professor of mathematics in the Jesuits Colleges of Parma and Genoa. Galileo finished his response to Sarsi, whom he now knew was Grassi, by December 1621 and showed it to members of the Accademia dei Lincei who were enthusiastic. After receiving these comments, Galileo sent his manuscript to Rome for printing in October 1622 but it had to wait for the approval of the Church and it was not until October 1623 that Il Saggiatore Ⓣ was published.

There is no doubt that Il Saggiatore Ⓣ is a masterpiece in describing the scientific method and its claim that the book of nature is to be read with mathematical tools. It attacks Grassi, however, more by insults rather than trying to prove scientifically that he is wrong. Galileo writes (see for example [21]):-
Sarsi reminds me of what the witty poet [Aristo] said: "For the sword of Orland, which they do not possess and will probably never possess, they are fighting like crazy."
There is the quote from Galileo's Il Saggiatore Ⓣ which contains the very frequently praised phrase "Nature is written in the language of mathematics." This phrase occurs as part of Galileo's attack on Grassi (see for example [27]):-
It seems to me that I discern in Sarsi a firm belief that in philosophising it is essential to support oneself upon the opinion of some celebrated author, as if when our minds are not wedded to the reasoning of some other person they ought to remain completely barren and sterile. Possibly he thinks that philosophy is a book of fiction created by some man, like the 'Iliad' or 'Orlando Furioso' - books in which the least important thing is whether what is written in them is true. Well, Sig Sarsi, that is not the way matters stand. Philosophy is written in this grand book - I mean the universe which stands continually open to our gaze, but it cannot be understood unless one first learns to comprehend the language and interpret the characters in which it is written. It is written in the language of mathematics, and its characters are triangles, circles, and other geometrical figures, without which it is humanly impossible to understand a single word of it; without these, one is wandering about in a dark labyrinth. Sarsi seems to think that our intellects should be enslaved to that of some other man (I shall disregard the fact that in thus making everyone, including himself, an imitator, he will praise in himself what he has blamed in Sig Mario), and that in the contemplation of the celestial motions one should adhere to somebody else.
James Maclachlan adds [27]:-
The whole burden of the paragraph is to encourage readers to think for themselves, by poking fun at Sarsi/Grassi who seems unwilling to do so. Galileo did it by urging the reading of the book of nature instead of some fictional philosophical work. And, he added by the way, "the book of nature is written in mathematical symbols." Can anyone miss the irony of Galileo telling that to the chairman of the department of mathematics of the Collegio Romano, the chief institution of higher learning of the Society of Jesus?
Maclachlan suggests this is being used by Galileo to make fun of Grassi, while he gives other examples where Galileo plays down the importance of mathematical explanations.

Il Saggiatore Ⓣ produced a response from Grassi, namely Ratio ponderum librae et simbellae in qua quid e Lotharii Sarsi libra astronomica, quidque e Galilei Galilei symbolatore, de Cometis statuendum fit, collatis utriusque rationum momentis, Philosophorum arbitrio proponitur Ⓣ (1626). This attacks Galileo's belief in a corpuscular theory of matter which Grassi claims unacceptable since it is contrary to the Catholic belief in Eucharistic transubstantiation.

We are very fortunate that notes taken by a student in a mathematics course given by Grassi at the Collegio Romano in 1623 are still in existence. These are very interesting as they give us an accurate view of the teaching of the time by a leading mathematician. The manuscript has four sections: Tractatus de sphaera Ⓣ; De spiritalibus Ⓣ; De geometria practica Ⓣ; and De mensuris corporum et solidorum Ⓣ. The Treatise on the Sphere is based on the astronomy book written by Johannes de Sacrobosco in 1220. Sacrobosco's Sphere was prescribed for teaching astronomy courses by the Jesuit Ratio Studiorum Ⓣ. Christopher Clavius had written a commentary on Sacrobosco's book which, almost certainly, had been the basis of the course on astronomy that Grassi himself had followed at the Collegio Romano.

Sacrobosco's book was based on Ptolemy's geocentric theory of the solar system based on spheres and epicycles. Grassi, however, presented four different models of the solar system in his lectures. The first model has the sun, moon and planets in orbit round the earth but, instead of Ptolemy's spheres and epicycles, it proposes a fluid medium. It [22]:-
... postulates that the space between the heaven and the earth is fluid and controls the paths of the planets. They say that it is entirely ridiculous that there are so many spheres in the heaven and to make epicycles, since everything can be explained much more easily if we say that each planet is moved either by its individual firmament or by an assisting intelligence.
The second model is the one in Sacrobosco's Sphere based on Ptolemy's spheres and epicycles. Grassi's third model has the sun at the centre and the planets and the earth orbit the sun. Grassi does not mention Copernicus is relation to this model, probably because of the Church's edict forbidding holding Copernican views. The fourth model of the solar system is Tycho Brahe's model with the sun and moon in orbit round the earth, with the planets in orbit round the sun. He cites Brahe for this model and also gives Brahe's observations of Mars which, he claims, show that Ptolemy's spheres cannot exist since these observations show they would intersect.

Grassi, in the second chapter of the Treatise on the Sphere, discussed the importance of observations and, in particular, explained how parallax can be used to determine distances. Despite the controversy he has been involved in over parallax measurements of comets, Grassi does not mention these. In the third chapter he discusses definitions such as right ascension and declination following Sacrobosco. He also discusses, however, the recent astronomical discoveries made with telescopes. He discusses the phases of Mercury and Venus, the appearance of spots on the sun, and the surface of the moon. He says that the phases of Mercury and Venus prove they are in orbit round the sun. He explains about the four moons of Jupiter, and also says that Saturn has two moons. Poor quality telescopes of the time meant that Saturn's ring system was mistakenly thought to be two moons. Grassi also explains how telescopes had shown that the Milky Way contains many stars not visible to the naked eye. The thesis [2] gives more details of Grassi's Treatise on the Sphere.

We mentioned above that Grassi was away from his mathematical duties at the Collegio Romano in 1625-27. He was for a few months in Siena as rector of the college of San Virgilio, then back in Rome as prefect of the library of the Collegio Romano. He had expertise in architecture and while in Siena he had been involved in advising on the refurbishment of San Virgilio church. In 1627 he returned to his mathematics teaching but he was also made pro-rector of the College, a position he held until 1633.

Grassi left Rome in 1633 when he was sent to his home town of Savona to work in the Jesuit College there. While in Rome, Grassi had held the role of reviser of the scientific works of the Jesuits and he continued to hold this role after going to Savona. Considered as a leading expert on engineering and architecture, he was often called by the Republic of Genoa for his advice on such matters. Clearly he was considered an expert engineer for, in 1638, a knight of the Order of Malta travelled to Savona to get Grassi's advice on how to strengthen the fortifications of Malta.

An important role as an architect had been given to Grassi while he was at the Collegio Romano. Ignatius of Loyola, the Jesuit founder, had been canonised in 1622 and Pope Gregory XV proposed that a new church be erected at the Collegio Romano dedicated to Ignatius of Loyola. He approached Ignatius of Loyola's nephew, Cardinal Ludovico Ludovisi, with the idea and suggested he find a suitable architect. A number of architects, including Grassi, were asked to draw up plans and, after considering all the proposed plans, the one by Grassi was chosen by Ludovisi. Building began on 2 August 1626 but was delayed since an old church had to be demolished. Twenty years later the church was still under construction and, in early 1645, Grassi, now in Savona, was shown alterations to his plan which he felt unacceptable. He travelled to Rome and produced a document protesting at the changes to his plan.

The Church of St Ignatius of Loyola was certainly not Grassi's only architecture project. Over many years he was involved in other architectural projects such as the church of San Giovanni attached to the Jesuit College in Florence, and the church of St Peter and Paul in Sezze. After ending his time as a professor of mathematics, he did not give up his scientific work and in 1644 he carried out experiments trying to repeat Evangelista Torricelli's recent experiments that demonstrated atmospheric pressure by inverting a tube filled with mercury. He was involved in discussions over the following couple of years as to whether this proved the existence of a vacuum.

In July 1646 Grassi became rector of the Jesuit College in Genoa. In 1651 he returned to the Jesuit College in Savona, then in 1653 he returned to the Collegio Romano, again advising on the building of the Church of St Ignatius of Loyola. He was working on plans for the College in Parma early in 1654 but died following a heart attack on 23 July of that year.

Other Mathematicians born in Italy
A Poster of Orazio Grassi

References (show)

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Written by J J O'Connor and E F Robertson
Last Update June 2025