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CASSINI,G.M.
By Pierre Joppen
Aug 29, 2005, 08:22

Cassini, a native of the County of Nice, is one of the greatest astronomers of his time, and the creator of French astronomy, " the one who picked up the torch of Astrophysics, fallen off Galileo's hands" (Andre Danjon). He was born, on June 8, 1625, in Perinaldo, a village of the hinterland between Vintimiglia and Bordighera, today in the Ligurian province of Imperia (Italy), but then in the County of Nice. After his studies at the Jesuit College at Genoa, he was named professor at the University of Bologna in 1651. His first observations were devoted to the Sun, and carried out with the meridian line, that he had built in San Petronio church.
 

His results obtained from 1656 to 1659, made him famous as a mathematician and as an astronomer. He gave a new value of the inclination of the ecliptic of 23°29' 15 ' ( the best at his time!), a table of the atmospheric refractions that remained the most accurate for over a century, and he confirmed that the orbital velocity of the Earth was not uniform, in line with Kepler's laws of planetary motion,. The latter went against aristotelician physics that assumed uniform movements in the sky.
He was sent to Pope Alexander VII, by the Senate of Bologna to arrange the difficulties that had arisen between Bologna and Ferrara about the navigation and of the courses of the rivers Po and Reno(1657-59), later on he was named superintendant of the fortifications of Fort Urban and of the fortress of Perugia(1663), and then he was delegated for solving the problems about the course of river Chiana, alternate affluent of the Tiber and Arno(1663-67), which brought into conflict the Pope and the Grand Duke of Tuscany.
All this time, he continued his astronomical work, and devised a method for the determination of longitudes through observing the solar eclipses (1661), the publication of which was censored by the Inquisitor of Modena. It will be published later in France (1670).
Using one of the first simple long focal lenses, manufactured by Giuseppe Campani, he turned to the study of planets. He discovered the shadows of Jupiter's satellites on that planet, which one took for spots, and deduced their rotational period; he observed an "exceptional" spot (the Red Spot), that he called the "big permanent spot" and which enabled him to evaluate Jupiter's rotational period (9h56')(1665). He noticed the flatness of the disc and started to draw up tables of Jupiter's satellites(1668), for the determination of longitudes, which will be improved in France (1693). He also evaluated Mars's rotational period (24h40) (1666) and tried to find the more tricky one of Venus (23h20')(1667).


With all this work Cassini became famous beyond the limits of Italy. Louis XIV, who wished " to make France as well flourishing and illustrious by the letters as it was it by the weapons", entrusted his minister Colbert with inviting him to join the recently formed Académie Royale des Sciences, and urging him to come to Paris, at the Observatory which was in construction. The Pope (Clement IX) , his employer agreed to lend him to France, temporarily.
 

Cassini reached Paris in April 1669, and two years later, he went to live in the Observatory. In 1673, at his request he obtained the French nationality. He never returned to Italy, except for personal trips, or observations. The same year he married Genevieve de Laistre, daughter of the Count de Clermont's lieutenant-general, who was a King's adviser, and bought the castle of Thury, near Beauvais, which became his family residence and whose area was crossed- amazingly!- by the Paris meridian.


As soon as Cassini arrived at the Observatory, he began a series of observations of the lunar surface which was to lead to the realization of an Atlas (1678), a large Map(1692), and to a theory of the libration and three laws of the Moon rotation, that bear his name. At the same time as Richer in Cayenne(1672), he observed the planet Mars at Paris Observatory , and thanks to his excellent tables of refractions, he measured the Mars parallax and deduced from it the Sun parallax to 9' ' 5 (correct to within 8 °/° ). This value increased considerably the scale of the solar system (1674), and gave to Jupiter and Saturn huge sizes. One still find the echo of that event seventy years later, in the Voltaire's Micromégas.
 

But, above all, his name remains associated with the planet Saturn. Huygens had discovered the first Saturn satellite, Titan (1655). Hardly settled at the Observatory, Cassini discovered two others, Iapetus(1671) and Rhea(1672), then he discovered that the breadth of the ring was divided into two parts, now known as 'Cassini's Division'(1675), he saw for the first time, a cloudy strip parallel to its equator (1677), the flatness of the disc and two other satellites, Dione and Thetys (1684).


Finally, he suggested that the ring, considered as solid, could be formed " of a swarm of very small satellites with various motions that cannot be seen separately" (1705). This opinion was not adopted, even though it was reformulated in the XVIIIth century by Thomas Wright and despite Laplace's mathematical theory of a subdivision in many narrow rings. The Huygens' hypothesis, of the solidity and flatness of the ring prevailed until the mid-nineteenth century.

He gave birth to a dynasty of astronomers and Cassini's name remained attached to the history of Paris Observatory during more than one century.
The CASSINI's dynasty
  • Jean Dominique(Giovanni Domenico), ou Cassini I (1625-1712) (1669-1712)*
  • Jacques, ou Cassini II (1677-1756) (1712-1756)
  • César François Cassini III de Thury (1714-1784) (1756-1784)
  • Jean Dominique Cassini, ou Cassini IV(1748-1845) (1784-1793) 
     


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