Giovanni Domenico Cassini
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|Giovanni Domenico Cassini|
June 8, 1625|
Perinaldo, Republic of Genova
|Died||September 14, 1712
|Fields||Mathematics, astronomy, engineering, and astrology|
|Institutions||University of Bologna|
|Alma mater||The Jesuit College at Genoa|
|Known for||Cassini Division, Cassini's Laws, Cassini oval|
Giovanni Domenico Cassini (June 8, 1625 – September 14, 1712) was an Italian/French mathematician, astronomer, engineer, and astrologer. Cassini, also known as Gian Domenico Cassini or Jean-Dominique Cassini, was born in Perinaldo, near Imperia, at that time in the County of Nice.
Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was a professor of astronomy at the University of Bologna and became, in 1671, director of the Paris Observatory. He 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.
Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea, including Iapetus, whose anomalous variations in brightness he correctly ascribed as being due to the presence of dark material on one hemisphere (now called Cassini regio in his honour). In addition he discovered the Cassini Division in the rings of Saturn (1675). He shares with Robert Hooke credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.
In 1672 he sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris. The two made simultaneous observations of Mars and, by computing the parallax, determined its distance from Earth. This allowed for the first time an estimation of the dimensions of the solar 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 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 by Tycho Brahe, and eventually that of Nicolaus Copernicus. Cassini also rejected Newton's theory of universal gravitation, after an experiment he conducted which wrongly suggested that the Earth was elongated at its poles.
Attracted to the heavens in his youth, his first interest was in astrology. While young he read widely on the subject of astrology, 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 on astronomy and all but denounced astrology as he became increasingly involved in the Scientific Revolution.
In 1645 the Marquis Cornelio Malvasia, a senator of Bologna 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 accurate ephemerides for astrological purposes using the rapidly advancing astronomical methods and tools of the day.
Moving to France 
In 1669 Cassini moved to France and through a grant from Louis XIV of France helped to set up the Paris Observatory, which opened in 1671; Cassini would remain the director of the observatory for the rest of his career until his death in 1712. In 1673 he became a French citizen. For the remaining forty-one years of his life Cassini served as astronomer/astrologer to Louis 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.
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. Cassini went blind in 1711.
Cassini was employed by Pope Clement IX in regard to fortifications, river management, and flooding of the Po River. The Pope asked Cassini to take Holy Orders to work with him permanently but Cassini turned him down because he wanted to work on astronomy full-time.
In the 1670s, Cassini began work on a project to create a topographic map of France, using Gemma Frisius's technique of triangulation. The project was continued by his son Jacques Cassini and eventually finished by his grandson César-François Cassini de Thury and published as the Carte de Cassini in 1789 or 1793. It was the first topographic map of an entire country.
See also 
- Aerial telescope – large telescopes used by Cassini
- Cassini–Huygens Mission to Saturn
- Cassini Division in Saturn's rings
- Cassini Regio, dark area on Iapetus
- Cassini (Martian crater)
- Cassini (lunar crater)
- Cassini's Laws
- 24101 Cassini, an asteroid
- Cassini's identity for Fibonacci numbers
- Cassini oval
- "Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712)". Messier Seds.org. Retrieved 31 October 2012.
- "Giovanni Domenico Cassini: The rings and moons of Saturn". Surveyor in Berlin.de. Retrieved 31 October 2012.
- See Encyclopædia Britannica.
- See this site.
Further reading 
- Barkin, Iu. V. (1978). "On Cassini's laws". Astronomicheskii Zhurnal, Soviet Astronomy 55: 113–122. Bibcode:1978SvA....22...64B.
- Connor, Elizabeth (1947). "The Cassini Family and the Paris Observatory". Astronomical Society of the Pacific Leaflets 5: 146–153. Bibcode:1947ASPL....5..146C.
- Cassini, Anna, Gio. Domenico Cassini. Uno scienziato del Seicento, Comune di Perinaldo, 1994. (Italian)
- 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)
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- Catholic Encyclopedia article
- O'Connor, John J.; Robertson, Edmund F., "Giovanni Domenico Cassini", MacTutor History of Mathematics archive, University of St Andrews.
- Domenico Cassini – complete digitization of 14 volumes belonging to the Old Fund's Department of Astronomy, University of Bologna, held to mark the celebrations of the Cassini in 2005 (website in Italian)