Giovanni Domenico Cassini

This article is about the Italian-born astronomer. For his French-born great-grandson, see Dominique, comte de Cassini.

Giovanni Domenico Cassini
Born	(1625-06-08)June 8, 1625 Perinaldo, Republic of Genova
Died	September 14, 1712(1712-09-14) (aged 87) Paris, France
Nationality	Italian (1625-1673) French (1673-1712)
Alma mater	The Jesuit College at Genoa
Known for	Cassini Division, Cassini's Laws, Cassini oval
Scientific career
Fields	Mathematics, astronomy, engineering, and astrology
Institutions	University of Bologna

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.

Life

Time in Italy

"The first of a family of astronomers who settled in France and were prominent in directing the activities of the French school of astronomy until the Revolution, Cassini was the son of Jacopo Cassini, a Tuscan, and Julia Crovesi. Cassini accepted a position at the observatory at Panzano, near Bologna, to work with Marquis Cornelio Malvasia, a rich amateur astronomer, in 1648 initiating the first part of Cassini's career." During Cassini's time at the Panzano Observatory, Cassini was able to complete his education under the scientists Giovanni Battista Riccioli and Francesco Maria Grimaldi. In 1650 the senate of Bologna appointed Cassini as the principal chair of astronomy at the University of Bologna. Cassini remained in Bologna working until Colbert recruited him to come to Paris to help set up the Paris Observatory. Cassini departed from Bologna on 25 February 1669.

Moving to France

An engraving of the Paris Observatory during Cassini's time. The tower on the right is the "Marly Tower", a dismantled part of the Machine de Marly, moved there by Cassini for mounting long focus and aerial telescopes.

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. 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.

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 and geodesist under the name of Cassini II."

In 1711 Cassini went blind, and died on September 14, 1712 in Paris at the age of 87.

Astronomer

Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was appointed professor of astronomy at the University of Bologna in 1650 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 observed and published surface markings on Mars (earlier seen by Huygens but not published), determined the rotation periods of Mars and Jupiter, and discovered no fewer than four satellites of Saturn, Iapetus and Rhea in 1671 and 1672, and Tethys and Dione (1684)." Cassini was the first to observe these four Saturn's moons, which he called Sidera Lodoicea (the stars of Louis), 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. "In 1659 he presented a model of the planetary system that was in accord with the hypothesis of Tycho Brahe. In 1661 he developed a method, inspired by Kepler’s work, of mapping successive phases of solar eclipses; and in 1662 he published new tables of the sun, based on his observations at San Petronio." 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.

Cassini was also the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock.

In 1683, Giovanni Domenico Cassini presented the correct explanation of the phenomenon of zodiacal light. 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. "Cosmic dust particles in interplanetary space as a contributor to the zodiacal light are constantly produced by asteroid collisions and liberated from the sublimating icy surfaces of comets."

Astrologer

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.

Engineering

In 1653, Cassini, wishing to employ the use of a meridian, sketched a plan for a new and larger meridian 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.

Cassini was employed by Pope Clement IX in regard to fortifications, river management, and flooding of the Po River. "Cassini composed several memoirs on the flooding of the Po River and on the means of avoiding it; moreover, he also carried out experiments in applied hydraulics." In 1663 he was named superintendent of fortifications and in 1665 inspector for Perugia. 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

References

http://www.astronomy-for-kids-online.com/giovanni-cassini-biography.html

http://www.esa.int/esaSC/SEMJ2T57ESD_index_0.html

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)

External links

• 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)

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