While in his position in Bologna, he observed and wrote a book on thecomet of 1652. He was also employed by the senate of Bologna as ahydraulic engineer, and appointed byPope Alexander VII inspector of fortifications in 1657. He was subsequently director of waterways in the papal states.[8]
The pinhole-projected image of the Sun on the floor atFlorence Cathedral. Cassini measured a similar image over a year at San Petronio Basilica to try to prove the Earth orbited the Sun.
In San Petronio, Bologna, Cassini convinced church officials to create an improved sundialmeridian line at the San Petronio Basilica, moving thepinhole gnomon that projected the Sun's image up into the church's vaults 66.8 meters (219 ft) away from the meridian inscribed in the floor. The much larger image of the Sun's disk projected by thecamera obscura effect allowed him to measure the change in diameter of the Sun's disk over the year as the Earth moved toward and then away from the Sun. He concluded the changes in size he measured were consistent withJohannes Kepler's 1609 heliocentric theory, where the Earth was moving around the Sun in an elliptical orbit instead of thePtolemaic system where the Sun orbited the Earth in an eccentric orbit.[9]
Cassini remained in Bologna working untilColbert recruited him to come to Paris to help set up theParis Observatory. Cassini departed from Bologna on 25 February 1669.[7]
An engraving of theParis Observatory during Cassini's time. The tower on the right is the "Marly Tower", a dismantled part of theMachine de Marly, moved there by Cassini for mounting long focus andaerial telescopes.
Cassini's determinations of the rotational periods of Jupiter and Mars in 1665–1667 enhanced his fame, and in 1669, with the reluctant assent of the Pope, he moved to France and through a grant fromLouis XIV of France helped to set up theParis Observatory, which opened in 1671;[8] he would remain the director of the observatory for the rest of his career until his death in 1712. For the remaining forty-one years of his life Cassini served asastronomer/astrologer toLouis XIV ("The Sun King"); serving the expected dual role yet focusing the overwhelming majority of his time on astronomy rather than the astrology he had studied so much in his youth.Cassini thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini, although that is also the name of his great-grandson,Dominique, comte de Cassini.
During this time, Cassini's method of determining longitude was used to measure the size of France accurately for the first time. The country turned out to be considerably smaller than expected, and the king quipped that Cassini had taken more of his kingdom from him than he had won in all his wars.
On 14 July 1673 Cassini obtained the benefits of French citizenship. In 1674 he married Geneviève de Laistre, the daughter of the lieutenant general of the comté of Clermont. "From this marriage Cassini had two sons; the younger,Jacques Cassini, succeeded him as astronomer andgeodesist under the name of Cassini II."[7]
In 1711, Cassini went blind, and he died on 14 September 1712 in Paris at the age of 87.[2]
Cassini observed and published surface markings onMars (earlier seen byChristiaan Huygens but not published), determined the rotation periods of Mars andJupiter, and discovered four satellites ofSaturn:Iapetus andRhea in 1671 and 1672, andTethys andDione (1684).[10] Cassini was the first to observe these fourmoons, which he calledSidera Lodoicea (the stars of Louis), includingIapetus, whose anomalous variations in brightness he correctly ascribed as being due to the presence of dark material on one hemisphere (now calledCassini Regio in his honour). In addition, he discovered theCassini Division in the rings of Saturn in 1675.[7] He shares credit withRobert Hooke for the discovery of theGreat Red Spot on Jupiter (ca. 1665). Around 1690, Cassini was the first to observedifferential rotation within Jupiter'satmosphere.
In 1672, he sent his colleagueJean Richer toCayenne,French Guiana, while he himself stayed in Paris. The two made simultaneous observations of Mars and, by computing theparallax, determined its distance from Earth. This allowed for the first time an estimation of the dimensions of theSolar System: since the relative ratios of various Sun-planet distances were already known from geometry, only a single absolute interplanetary distance was needed to calculate all of the distances.
Cassini's map of the Moon
In 1677, the English philosopherJohn Locke visited Cassini in Paris. He writes, "At the Observatory, we saw the Moon in a twenty-two foot glass, and Jupiter, with his satellites, in the same. The most remote was on the east, and the other three on the west. We also saw Saturn and his rings, in a twelve-foot glass, and one of his satellites."[11]
Cassini initially held the Earth to be the centre of the Solar System, though later observations compelled him to accept the model of the Solar System proposed byNicolaus Copernicus, and eventually that ofTycho Brahe. "In 1659 he presented a model of the planetary system that was in accord with the hypothesis of Nicolaus Copernicus. In 1661 he developed a method, inspired by Kepler's work, of mapping successive phases ofsolar eclipses; and in 1662 he published new tables of the sun, based on his observations at San Petronio."[7] Cassini also rejected Newton'stheory of gravity, after measurements he conducted which wrongly suggested that the Earth was elongated at its poles. More than forty years of controversy about the subject were closed in favour of Newton's theory after the measurements of theFrench Geodesic Mission (1736 to 1744) and the Lapponian expedition in 1737 led byPierre Louis Moreau de Maupertuis.
Cassini was also the first to make successful measurements oflongitude by the method suggested byGalileo, using eclipses of theGalilean satellites as a clock.
In 1683, Cassini presented the correct explanation of the phenomenon ofzodiacal light.[8] Zodiacal light is a faint glow that extends away from the Sun in the ecliptic plane of the sky, caused by dusty objects in interplanetary space.
Cassini is also credited with introducingIndian Astronomy to Europe. In 1688, the French envoy toSiam (Thailand),Simon de la Loubère, returned to Paris with an obscure manuscript relating to the astronomical traditions of that country, along with a French translation. The Siamese Manuscript, as it is now called, somehow fell into Cassini's hands. He was intrigued enough by it to spend considerable time and effort deciphering its cryptic contents,[citation needed] also determining on the way that the document originated inIndia.[12] His explication of the manuscript appeared in La Loubère's book on the Kingdom of Siam in 1691.[13][non-primary source needed]
Attracted to the heavens in his youth, his first interest was inastrology. While young he read widely on the subject, and soon was very knowledgeable about it; this extensive knowledge of astrology led to his first appointment as an astronomer. Later in life he focused almost exclusively onastronomy and all but denounced astrology as he became increasingly involved in theScientific Revolution.
In 1645 the MarquisCornelio Malvasia, a senator ofBologna with a great interest in astrology, invited Cassini to Bologna and offered him a position in the Panzano Observatory, which he was constructing at that time. Most of their time was spent calculating newer, better, and more accurateephemerides for astrological purposes using the rapidly advancing astronomical methods and tools of the day.
In 1653, Cassini, wishing to employ the use of a meridian line, sketched a plan for a new and larger meridian line but one that would be difficult to build. His calculations were precise; the construction succeeded perfectly; and its success gave Cassini a brilliant reputation for working with engineering and structural works.[7]
Cassini was employed byPope Clement IX in regard tofortifications,river management, and flooding of thePo River. "Cassini composed several memoirs on the flooding of thePo River and on the means of avoiding it; moreover, he also carried out experiments in applied hydraulics."[7] In 1663 he was named superintendent offortifications and in 1665 inspector forPerugia.[7] The Pope asked Cassini to takeHoly Orders to work with him permanently but Cassini turned him down because he wanted to work on astronomy full-time.
^Augusto De Ferrari (1978),"Cassini, Giovan Domenico"Dizionario Biografico degli Italiani21 (Rome: Istituto dell'Enciclopedia Italiana).
^Gandolfo, Andrea.La provincia di Imperia: storia, arti, tradizioni. Blue Edizioni, 2005.
^"c11371-07".British Library Images. Retrieved24 October 2024.
^abcdefghiCassini, Gian Domenico (Jean-Dominique) (Cassini I).Complete Dictionary of Scientific Biography. Detroit: Charles Scribner's Sons, 2008. pp. 100–104. retrieved 30 May 2013.
Barkin, Iu. V. (1978). "On Cassini's laws".Astronomicheskii Zhurnal.55:113–122.Bibcode:1978SvA....22...64B.
Connor, Elizabeth (1947). "The Cassini Family and the Paris Observatory".Astronomical Society of the Pacific Leaflets.5 (218):146–153.Bibcode:1947ASPL....5..146C.
Cassini, Anna,Gio. Domenico Cassini. Uno scienziato del Seicento, Comune di Perinaldo, 1994. (Italian)
Cuesta, JAF,Cassini y el origen de la astronomía experimental, in La cosmovisión de los grandes creadores de la ciencia moderna: convicciones éticas, políticas, filosóficas o religiosas de los protagonistas de la renovación del saber en los siglos XVI y XVII / coord. por Juan Arana Cañedo-Argüelles, 2023. (Spanish)
Giordano Berti (a cura di),G.D. Cassini e le origini dell'astronomia moderna, catalogo della mostra svoltasi a Perinaldo -Im-, Palazzo Comunale, 31 agosto – 2 novembre 1997. (Italian)
Giordano Berti e Giovanni Paltrinieri (a cura di),Gian Domenico Cassini. La Meridiana del Tempio di S. Petronio in Bologna, Arnaldo Forni Editore, S. Giovanni in Persiceto, 2000. (Italian)
De Ferrari, Augusto (1978)."Cassini, Giovan Domenico".Dizionario Biografico degli Italiani (in Italian). Enciclopedia Italiana. Retrieved2 December 2016.
Narayanan, Anil,History of Indian Astronomy: The Siamese Manuscript, Lulu Publishing, 2019.
