Jacques Cassini was the son of Jean-Dominique Cassini and Geneviève de Laistre. To avoid confusion with other members of the Cassini family (his father, son and grandson), he is sometimes known as Cassini II. Jean-Dominique Cassini was head of the Paris Observatory at the time of his marriage to Geneviève in 1674 and, two years previously, had become a French citizen, changing his name from Giovanni Domenico Cassini. Geneviève was the daughter of the lieutenant general of the compté of Clermont and they owned the Château de Thury in the Oise which became their summer residence. Jacques, the second of his parents two sons, was born at their home at the Observatory in Paris. It was a wealthy family and Jacques received the finest education possible.
The Paris Observatory where his family lived provided an excellent place to educate a young boy with enthusiastic interests in science. This is where Jacques had his earliest education, after which he studied at the Collège Mazarin in Paris. Varignon had been appointed professor of mathematics at the Collège Mazarin in 1688 and he supervised Cassini's thesis on optics. In August 1691, at the age of fourteen, Cassini defended his thesis. An early biographer  states that, when he was fifteen years old, he dedicated a thesis on mathematics to the Duc de Bourgogne. It is unclear whether these two references to theses refer to the same work or to two separate ones.
In 1694 Cassini was admitted to the Académie des Sciences and began to undertake scientific work on projects which his father was carrying out. The following year he travelled with his father in Italy and they made numerous geodesic observations, as well as visiting Bologna where they repaired the gnomon at the Church of San Petronio which Cassini's father had designed nearly thirty years before. Cassini, like his father, was interested in making both astronomical observations and also interested in accurate surveys. After the Italian trip, Cassini visited Flanders and then England in around 1698. While in England he met Newton, Flamsteed, and Halley and was elected to the Royal Society of London. After returning to France he published the astronomical and geodesic data which he had gathered on his travels.
In 1700 Cassini's father undertook a project to measure the meridian from Paris to Perpignan, which is 13 km west of the Mediterranean coast. Cassini assisted his father on this project and they obtained results which wrongly suggested that the Earth was elongated at the poles. In 1713 he proposed a new method for determining longitude by means of the eclipses of the stars and planets by the moon. Applying this method, and using data from the 1700 Paris to Perpignan survey, he claimed to have proved :-
... that the degrees of the terrestrial meridian grow smaller from the equator towards the pole.It was unfortunate that Cassini resolutely stuck to this position throughout his life, refusing to acknowledge the flattening despite the scientific evidence which was put forward. Whether this was due to a false sense of patriotism, believing that he was supporting the French view against the English view which was a consequence of Newton's gravitational theory (which Cassini never accepted), or whether it was through a false sense of family loyalty supporting his father's views, we shall never know. Perhaps indeed both may have contributed without Cassini being fully aware that they were affecting his scientific judgement, or perhaps he was so convinced that the results of the 1700 survey were correct that he could never accept the contrary. The real problem with the data from the survey was, as pointed out in 1733 by Giovanni Poleni, that both the elongation proposed by Cassini or the flattening proposed by others, fell within the experimental error of the instruments used.
Cassini had some legal training but not so much that one would expect him to play a substantial role in this field but, in addition to his scientific work, he did undertake a second parallel career. He was appointed to the Chambre des Comptes in 1706, which was a financial court with administrative and legal duties relating to the King's accounts and in particular to the land owned by the Crown. He gained a high reputation for honesty in this position but was sometimes criticised for being indecisive. Later, in 1716 he was an advocate in the Court of Justice, and in 1722 he became a Councillor of State.
His scientific role was one of major importance, playing a major role in the Académie des Sciences and taking over as head of the Paris Observatory from his father. Around 1709 his father's health began to fail and Cassini took on more and more of the duties of head. It is worth noting that at this time there was no official post of Director of the Observatory, which was in theory run by the Académie des Sciences, but Cassini like his father before him effectively ran it. In the following year 1710 he married Suzanne-Françoise Charpentier from Charmois. They had three sons, Dominique-Jean, César-François who was later known as Cassini III or Cassini de Thury, and Dominique-Joseph, and two daughters Suzanne-Françoise and Elisabeth-Germaine. The eldest son Dominique-Jean followed his father into the Chambre des Comptes while the youngest son followed a military career.
Cassini continued both his astronomical and surveying work. He undertook the measurement of the Paris meridian north to Dunkerque in 1718. He published the results, which again supported his incorrect theory of elongation at the poles, in De la grandeur et de la figure de la terre in 1722. This important treatise surveyed the results which had been obtained on measuring the Earth over the preceding fifty years. However, those like Maupertuis who believed that the Earth was flattened at the poles argued ever more strongly against Cassini's theory and, in an attempt to gain further evidence to support his case, Cassini organised another project in 1733, this time to measure the perpendicular to the meridian from Saint-Malo to Strasbourg. As he had accompanied his father when he was a young man, on this measuring project Cassini had the assistance of his own son César-François Cassini (who appears in this archive under the name Cassini de Thury). The data they collected during the years 1733-34 seemed to support the elongation theory but this only encouraged those members of the Académie such as Maupertuis who supported the Newtonian view, to organise further scientific expeditions in an attempt to settle the argument in their favour. By 1738 the geodesic measurements carried out in Peru by Bouguer and La Condamine in 1735 and Lapland by Maupertuis in 1736 to measure the length of a meridian degree had produced very strong evidence for the flattening at the poles. Although Cassini never deviated from his belief, he began to scale back his scientific work from 1740.
As to his contributions to astronomy, Cassini published many papers in the journals of the Académie des Sciences and two major treatises in 1740, namely Elements of Astronomy and Astronomical Tables of the Sun, Moon, Planets, Fixed Stars, and Satellites of Jupiter and Saturn. These showed his excellent abilities as an observer and he made impressive contributions with his study of the moons of Jupiter and Saturn and his study of the structure of Saturn's rings. Most notable of his discoveries was that of the proper motions of the stars which he made in 1738. Here for the first time the ancient belief in the unchanging sphere of the stars had been shown to be incorrect by direct measurement.
Cassini's work was not restricted to that of astronomy and geodesy. He wrote papers on a variety of other topics on the applications of mathematics, particularly on electricity, barometers, the recoil of firearms, and mirrors. Although after 1740 his scientific activity decreased, he did assist his son Cassini de Thury in his project to produce an accurate map of France.
Let us end by stressing that Cassini made an extremely important contribution to the major scientific debates of his time. Although he supported an incorrect hypothesis regarding the shape of the Earth, nevertheless his contribution is extremely important. It is in the nature of scientific progress that hypotheses get put forward and tested. We should in no way consider Cassini's contribution any the less important because he was on the wrong side in the debate.
On 15 April 1756 when he was on a visit to his Château at Thury, his carriage overturned and he died as a result of this accident on the following day. He was buried in the Church at Thury.
Article by: J J O'Connor and E F Robertson