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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 is known for his work in the fields of astronomy and engineering. Cassini most notably discovered four satellites of Saturn, and noted the division of the rings of Saturn. Giovanni Domenico Cassini was also the first of his family to begin work on the project of creating a topographic map of France.
Cassini was an astronomer at the Panzano
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 is known for his work in the fields of astronomy and engineering. Cassini most notably discovered four satellites of Saturn, and noted the division of the rings of Saturn. Giovanni Domenico Cassini was also the first of his family to begin work on the project of creating a topographic map of France.
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 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."
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