Giovanni Domenico Cassini, generally referred to as Cassini I

The Cassini and Maraldi families of Perinaldo were both destined to make names for themselves in the scientific fields of astronomy and, in the case of the Cassinis, also in cartography. Both disciplines that had, at that time, political overtones.

The most famous of the group was Giovanni Domenico Cassini who came to be known as Cassini I.

Giovanni Dominico Cassini was born on the 8th June 1625 to Giacomo Cassini, a Tuscan, and Julia Crovesi (or Tullia Crovese or Croese), daughter of a Notary Public. His birth was recorded, though difficult to read, in the archives of Perinaldo’s parish church of San Nicolò as:

Alli 10 di d.ᵒ Gio. Dominico figlio de Giac. Cassino e di Tullia sua moglie è stato battezato da me sud.ᵒ Ber.ᵈᵒ curato, tenendolo m. Ant.ᵒ Maria Crovese e Battista Cassina q. Antonio, natto dui giorni inanti.

This might be loosely translated as:

On the aforementioned 10th June, Giovanni Domenico, son of Giacomo Cassini and his wife Tullia, was baptized by me, curate Bernardo, Antonio Maria Crovese and Battista Cassina being godparents, and Giacomo being born two days previously.

The building he was born in was, most probably, what is now known as the Maraldi castle or house in the centre of Perinaldo and which belonged to his mother’s family. It is worth mentioning at this point that not only are there a number of variations in the spelling of names but also, because of Giovanni Domenico Cassini’s move to France, a French form of many of the names. Generally I have used the names I have researched, and not sought to change them.

Giovanni Domenico’s godparents at his christening in the Parish church in Perinaldo, San Nicolo, were Antonio Maria Crovese, his maternal uncle, and Battista Cassina.

go to top of page

Education

Gian Domenico Cassini was not brought up by his parents but by Antonio Maria Crovese, a brother of his mother Julia Crovesi and, like his father, a Notary Public. Antonio Maria was also Gian Domenico’s first tutor. Gian Domenico Cassini was evidently a bright child. After spending two years being educated by Father Aprosio of Vallebone, Gian Domenico entered the Jesuit College at Genoa in 1644 where he studied under Casselli at the invitation of the senator, Marquis Cornelio Malvasia who was constructing the Panzano Observatory.

The marquis had an interest in astrology which probably accounts for the invitation, but Gian Domenico’s interest in astrology was quickly supplanted by astronomy where he benefitted considerably by association with the outstanding Jesuit scientists Giovanni Battista Riccioli (1598-1671) and Francesco Maria Grimaldi, the latter of whom later discovered diffraction. After this he went on to study at the abbey of San Fructuoso. It was in these early years he developed interests in poetry, mathematics and astronomy having by chance come across books on astrology.

At some stage Cassini and Philippe de la Hire, a mathematician, studied under the Jesuit scientist, Honoré Fabri at the Collège de Trinité in Lyon, though I have not yet worked out when this was.

On the 12th April 1651 he was officially appointed for five years to be the professor of astronomy at the University of Bologna at an annual salary of six hundred Bolognese Liri, the position having been vacant since the death in 1647 of Bonaventura Cavalieri. Here Gian Domenico both taught and carried out research.

One of his first achievements was, in 1653, the re-establishment and improvement of the gnomon and the meridian, traced by Ignazio Danti in the church of St. Petronius, Bologna, for the purpose of fixing the time of the solstices and reforming the calendar. Work on the church had rendered the gnomon unusable, and Gian Domenico designed a new opening in the roof with corresponding brass meridian plate on the floor of the north aisle enabling him to use the new system for his studies.

But he was not just interested in mathematics and astronomy, rather his investigations ranged through a number of scientific areas as was common in those days. In addition to an interest in the circulation of blood and in entomology, he was directed from 1657 fo 1659 by Pope Alexander VII to look at issues such as those relating to the fortifications of Rome and Perugia, the navigation and courses of the rivers Reno and Po and, later, the dispute between the Pope and the Grand-Duke of Tuscana on the path of the river Chiana from 1663 to 1667. It is notable that, while carrying out work for Pope Alexander VII he sent a manuscript to the Jesuit Riccioli, a friend and astronomer at Bologna, dealing with the Immaculate Conception, and recommending that it be celebrated as a special feast.

In addition to this, his interest and studies in hydraulic engineering obtained for him the position of inspector of water and waterways. Later, in 1663, he was appointed by Mario Chigi, the brother of Alexander VII, to be superintendent of the fortifications of Fort Urban and Perugia.

go to top of page

Move to Paris

His scientific reputation obviously travelled at least as far as Paris where Jean Baptiste Colbert, comptroller general of finances to the French King, Louis XIV, was approached by the Abbé Picard, Prior of Rillé in Anjou. The Abbé was the successor to Gassendi in the Chair of Astronomy at the College de France, and recommended Cassini to Colbert. At this time the King wished to attract scholars who would improve the intellectual credibility of France in the world. It appears that three astronomers were identified – Christian Huygens of Holland, Ole Roemer of Denmark, and Cassini, and all were invited and came to Paris.

Following discussions and, perhaps, with some trepidation, Pope Clement IX agreed to lend Cassini to France for a short stay when he would give his advice on scientific issues. On this understanding Gian Domenico travelled to Paris, leaving Bologna on the 25th February 1669 with Colbert paying 1,000 Écus for his travel and arranging a pension of 9,000 Livres. It was anticipated that Gian Domenico would remain in Paris for a period of eight or nine months.

But it was not just an astronomical interest which caused the King to invite Cassini to Paris, he also had an interest in geometry in its widest sense and would have realised that with Cassini as his geometer, he would be able to reinforce his place at the centre of France.

Geometrical determinism had developed through intellectual argument beginning with Euclid then developing in 14th century Italy to suggest that geometry was the key binding the world together and, by extension, those who ruled demonstrating their geometrical place at the centre of this geometry. At its simplest it needs to be understood that Louis XIV focussed his geometrical position in his palace and gardens at Versailles with the court operating within an extraordinary framework of coded behaviours, again relating to geometrical and, therefore, social order.

It was within this framework, probably not understood by him at the time, that Gian Domenico arrived in Paris on the 4th April 1669. Two days later he was presented to the King and involved himself in the work of the Académie des Sciences as well as beginning to adapt the plans of Perrault to improve the capability of the Observatory for astronomical observations. In this he was helped by the high quality of the lenses manufactured by Campani in Rome.

Three years later, in 1672, Louis ordered a map to be prepared of the whole of France with the greatest precision possible, and under the supervision of the Académie des Sciences. Thus began the process of triangulation establishing what was considered to be the most accurately produced map at that time and, in the process and to the irritation of the King, demonstrating that France was not quite as large as he had thought. Four generations of the Cassini family were involved in this exercise.

As might be anticipated, some of the Academicians did not take well to him because of his nationality, hesitant French, authoritarian character and apparent closeness to the King. However, finding his life in France increasingly agreeable, he worked hard on the research carried out by the Académie and gradually gained their essential support.

go to top of page

Marriage

Two years later Gian Domenico moved from the accommodations he had first occupied in the Louvre, to a first floor apartment of the Paris Observatory which was then being constructed under his supervision. In April 1673 King Louis agreed to his taking French nationality, though this followed heated discussions between Colbert and the University of Bologna who still wished to see his return. The papers for his French naturalisation were posted on the 14th June 1673, signed by the King. This may well have been the result of his meeting Geneviève Delaistre, daughter of Lieutenant-Général au baillage de Clermont en Beauvoisis, though I have also seen it suggested that it was King Louis who encouraged this change of nationality, Giovanni’s name becoming Jean Dominique Cassini. He and Geneviève were married in 1673, the civil marriage contract being signed on the 10th November 1673 with the marriage following on the 20th November in Clermont. As an indication of their social positions, Jean Dominique’s witnesses were the King and Colbert who both attended the wedding, while his wife brought sixteen witnesses illustrating her connections with legal and political circles as well as the nobility. Her dowry, a modest one of 5,000 Livres, reflected her parents’ need to provide for their large family. Geneviève’s younger sister, Madeleine-Françoise, was betrothed two days later to a nobleman.

Lieutenant-Général Delaistre owned the Château de Thury in the Oise, north-west of Paris, and this became the summer residence of the newly married couple. It was sold to Jean Dominique in 1701.

Jean Dominique and Geneviève had two sons, both of whom were mathematicians and a daughter:

• Jean Baptiste Cassini, who was born in 1674, and died young as a midshipman at the naval battle of La Hougue in Normandy, 23rd May 1692, during the Nine Years War,
• Jacques Cassini, born on the 8th or 16th February 1677, who became known as Cassini II, and
• Anne-Tullie Cassini, born on the 2nd June 1678, and baptised with the names of her grandmothers in the church of Saint Jacques du Haut-Pas, in the Observatory parish, sadly dying young, perhaps the same year.

Now known as Jean Dominique Cassini he continued with his work where he might be characterised as principally an observer. He was said to be kind and generous with a deeply religious nature and, despite his growing blindness, he continued to work for the King while training astronomers and organising the progress of work in astronomy and cartography.

go to top of page

Professional work

Jean Dominique made many discoveries in his lifetime. His calculation of the rotation periods of Jupiter was extremely significant. In observing the motions of the shadows of its moons against its surface it could be argued as being analogous to the disputed motion of the earth.

He followed this work in 1666 with observations of Mars, measuring that planet’s rotational period accurate to within 3 minutes of the time now known. He also studied the more difficult rotational period of Venus. Two years later he compiled a table of the positions of Jupiter’s moons that was used in 1675 by another astronomer to establish that the speed of light is finite.

In 1675 Jean Dominique discovered the dark gap between the main rings of Saturn, this gap of about 3,500 kilometers becoming known as the Cassini Division. His theory that Saturn’s rings were swarms of tiny moonlets too small to be seen individually has since been confirmed.

With the aid of Giuseppe Campani’s long refractor telescope, brought to Paris in 1671, he added four satellites of Saturn to that which had been discovered by Huyghens, Titan. Giapeto was discovered in 1671, Rhea in 1672, with Tetide and Dione following in 1684.

In conjunction with Jean Richter (1630-1696) in 1672 he made parallel observations of Mars from Paris, France and Cayenne, French Guyana, using this observation for the indirect determination of the solar parallax – the virtual shift of the Sun in the sky when seen from two different positions near the Earth’s equator. Their measure of 9.5 arc seconds can be compared with the real mean value of the solar parallax, 8.8 arc seconds, and can be used for the correct determination of the distance of Earth from the Sun.

Jean Dominique must have kept in touch with family and science in Italy as, in 1687 he invited his nephew, Giacomo Filippo Maraldi – later known as Jacques Phillipe Maraldi or Maraldi I – to join him in Paris on the completion of the latter’s studies in the classics and mathematics.

Jean Dominique continued his researches and studied the causes of the librations of the moon – the swinging motion of the moon which allows us to see more than the half normally visible. He observed the zodiacal light – a sky illumination on the ecliptic plane caused by interplanetary dust and the reflected sunlight within the dust. And, in addition to all this, he developed a theory of the motion of comets.

Jean Dominique’s wife, Geneviève, died a year before him in Paris on 17th September 1708. Married for over thirty years, by all accounts theirs was a happy marriage with it being said that Jean Dominique had affectionately depicted her as a small detail in Cassini’s depiction of the moon, drawn and engraved by Jean Patigny – 1647-1679.

Jean Dominique Cassini became infirm in the latter years of his life. By 1709 his infirmity was marked and, by 1711 he was blind. Following two days’ illness he died at the Paris Observatory on the 14th September 1712 and is buried in the church of St Jacques du Haut Pas, Paris, France. A eulogy was read to the Academy by Bernard de Fontenelle, on the 16th November 1712.

Jean Dominique was honoured with the naming in 1997 by the ESA-NASA of the Mission Cassini to Saturn.

Delambre, a well respected mathematician and astronomer in his own right, believed that Cassini’s contribution to astronomy was over-rated. To some extent this was due to Cassini taking positions based on traditional science, and at odds with progressive views. However, this has to be seen against his work in establishing and directing the work of the Paris Observatory, together with his other, various discoveries.