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|Giovanni Domenico Cassini
(aka Jean-Dominique Cassini) mathematician, astronomer, engineer, and astrologer
Giovanni Domenico Cassini was born in Perinaldo, Republic of Genoa, on June 8, 1625. Little is known about his parents, Jacopo Cassini and Julia Crovesi, as he was raised by an uncle, a brother of his mother. He spent two years studying at Vallebone before entering the Jesuit College at Genoa and then the abbey of San Fructuoso.
Cassini was very interested in astrology and read widely on the subject, and this knowledge led the Marquis Cornelio Malvasia, a senator from Bologna with a great interest in astrology, to offer Cassini a position in the Panzano Observatory, which he was constructing at that time. Cassini accepted the offer, and began there in 1648. In 1650, Cassini became professor of mathematics and astronomy at the University of Bologna.
Cassini observed a comet in 1652-3, and he subsequently published an account of his observations that clearly showed his belief in an Earth-centered solar system, with comets beyond Saturn but originating from the Earth. Further observations, however, led him to accept the model of the solar system proposed by Tycho Brahe and, in 1659, he presented an Earth-centered system with the moon and sun orbiting the Earth and the other planets orbiting the sun. Later he came to accept a version of the Copernican model.
One of Cassini's predecessor's as professor of mathematics and astronomy at Bologna had been Egnatio Danti, who had built a gnomon at the Church of San Petronio in Bologna, one of the largest Christian churches ever built. A small hole allowed the rays of the sun to enter the church. They formed a small image on a scale on the floor which allowed the position of the sun to be accurately determined. That gnomon had been rendered unusable by expansion of the church before Cassini's arrival, however. In 1653, Cassini, wishing to employ such an instrument, sketched a plan for a new and larger one, the building of which earned him a brilliant reputation. He made many important observations with his new gnomon, which he published in Specimen observationum Bononiensium ... (1656).
In 1664 Cassini observed a comet which led him to propose a theory that comets travelled in circular orbits around the sun with the center of the orbit in the direction of the star Sirius. In that same year, he also measured the period of rotation of Jupiter on its axis, discovered the bands and spots on the planet, and saw that the planet was flattened at its poles. In 1666 he measured the period of rotation of Mars on its axis, getting a value within three minutes of the correct one, and observed surface features. He published detailed series of observations of the moons of Jupiter in 1668.
Cassini's brilliant discoveries led to him being invited to Paris by Louis XIV in 1668. The construction of the Paris Observatory had just begun, and Cassini was offered a generous salary, free accommodation, and a good travel allowance to oversee the project. Cassini became head of the Paris Observatory in 1671, and became a French citizen two years later. It was at this time that he changed his name to Jean-Dominique Cassini. In 1674 he married Geneviève de Laistre, with whom he had two sons.
At the Paris Observatory, Cassini became the first to observe four of Saturn's moons -- Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684). He discovered the gap in the ring system of Saturn now known as the Cassini division in 1675, and correctly proposed that the rings were composed of large numbers of tiny satellites each orbiting the planet. In 1679 he presented a large Chart of the Moon to the Académie des Sciences that remained the best available until the invention of photography for astronomical purposes.
Cassini's tables of Jupiter's moons were used to determine longitudes by providing a universal time with which to compare the local time at various positions on the Earth. While French expeditions measured the longitudes of numerous places, Cassini remained in Paris coordinating their data and making his own measurements. In 1672 Jean Richer made measurements of Mars from Cayenne, French Guyana, while Jean Picard and Cassini made measurements in Paris. From their data the first accurate value of the solar parallax was found, giving the distance from the Earth to the sun. Another measurement made by Jean Richer, namely that a pendulum with a period of one second is shorter in Cayenne than Paris, led him to suggest that the Earth was flattened at the poles. This supported theoretical proposals by Isaac Newton and Christiaan Huygens, but Cassini did not accept Jean Richer's explanation. In order to determine the shape of the Earth, Cassini proposed measuring an arc of the meridian from the north of France to the south. The project was begun in 1683, but cancelled for financial reasons in 1684. It was revived in 1700, but the results obtained by Cassini wrongly suggested that the Earth was elongated at the poles.
From around 1709 Jacques Cassini gradually took over his father's duties as head of the Paris Observatory. Cassini's health began to deteriorate, and his eyesight became so poor by 1711 he was nearly completely blind. He died in Paris on September 14, 1712.
The MacTutor History of Mathematics Archive www-history.mcs.st-andrews.ac.uk
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