Giovanni Domenico Cassini

1625-1712

Italian-French Astronomer

Giovanni Domenico Cassini made the first accurate determination of the dimensions of the solar system. A gifted observationalist, he was an extremely conservative theorist, refusing to accept Nicolaus Copernicus's (1473-1543) heliocentric view and opposing Isaac Newton's (1642-1727) gravitational theory.

Cassini was born June 8, 1625, in Perinaldo, near Nice (then in Italy). His early education was completed at Genoa. He later became Professor of Astronomy at the University of Bologna (1650), where his scientific reputation was established through a series of solar and planetary observations. This prompted an invitation to France, where he became director of the Royal Observatory (1671). He observational work and duties as director ceased after he went blind in 1710. He died September 11, 1712, in Paris.

At Bologna, Cassini produced new tables of the Sun's motion (1662). In 1664 he determined Jupiter's rotational period to within a few minutes and detected bands and a red spot on the Jovian surface. After detecting markings on the surface of Mars, he determined that planet's rotational period to within three minutes of its presently accepted value (1666). In 1668 Cassini produced accurate tables of the motions of Jupiter's satellites, which were widely used for determining terrestrial longitude.

His growing reputation brought him to the attention of the French finance minister Jean Baptiste Colbert, who was working to attract prominent scientists to France. Colbert nominated Cassini for membership in the newly established Académie des Sciences and invited him to Paris to oversee the establishment of the Royal Observatory. Cassini accepted, arriving in 1669 for what was to be but a temporary stay. When the Observatory opened in 1671 he accepted the directorship and in 1673 became a French citizen, changing his name to Jean Dominique Cassini.

In 1671 Cassini discovered a second satellite of Saturn, Iapetus, and correctly attributed variations in its brightness to always having the same face turned towards Saturn. He later discovered three more Saturnian satellites, Rhea (1672), Tethys (1684), and Dione (1684). In 1675 he drew attention to the dark gap—today referred to as the Cassini Divide—splitting Saturn's ring in two and postulated each part was composed of minute particles behaving like small satellites. This hypothesis has since been corroborated. Cassini also made extensive observations of the Moon's surface (1671-79), which culminated in his magnificent lunar map presented to the Academy in 1679. In addition, he carried out the earliest continuous observations of the zodiacal light and produced improved tables of atmospheric refraction.

Cassini's most significant work is associated with the Académie-sponsored astronomical expedition to Cayenne off the coast of French Guiana (1672-73). Measurements of Mars's opposition by Cassini and others in France in conjunction with those made by expedition leader Jean Richer (1630-1696) allowed Cassini to determine the astronomical unit. His value of 87 million miles was the first fairly accurate estimate of the Earth-Sun distance.

Richer also observed that pendulums beat slower in Cayenne than in France. Newton argued this was due to a decreased gravitational attraction at the equator. This suggested, in conformity with his gravitational theory, that Earth was an oblate spheroid—bulging equator and flattened poles. Supporting the Cartesian view (named after French philosopher René Descartes) that Earth was a prolate spheroid—elongated along the polar axis—Cassini maintained that temperature differences explained the effect. In 1683 Cassini undertook to measure an arc of the meridian between the northern and southern French borders to settle the issue. Completed in 1700 and published by his son Jacques (1677-1756) in 1718, the measurements seemingly supported Descartes. However, expeditions to Peru (1734-1744) and Lapland (1736) later settled the issue decisively in Newton's favor.

STEPHEN D. NORTON