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Cassini, Giovanni Domenico

observed satellites paris planets

[ka see nee] (1625–1712) Italian–French astronomer: greatly enhanced knowledge of the planets.

Born in Italy, Cassini became director of the Paris Observatory in 1669 and never returned to Italy. He added greatly to our knowledge of the planets of the solar system. It was he who worked out the rotational periods of Jupiter, Mars and Venus and tabulated the movement of the Jovian satellites discovered by subsequently used his results to calculate the speed of light). Between 1671 and 1674 he discovered four new satellites of Saturn (Iapetus, Rhea, Dione and Tethys) and in 1675 observed the gap in Saturn’s ring system first noted 10 years before by William Balle and now known as the Cassini division. Most importantly, he was able to calculate the first reasonably accurate figure (only 7% low) for the Earth’s distance from the Sun (the astronomical unit). To do this he observed Mars from Paris at the same time as Jean Richer (1630–96) observed it from Cayenne in French Guiana, 10 000 km away. (Jupiter’s satellites provide a universal clock; when they are seen in the same positions at both sites, the time is the same.) The parallax gave the distance of Mars, and Third Law then gave the distances of the other planets. In later life he attempted to measure the shape of the Earth but concluded incorrectly that it was a prolate spheroid. Three generations of his descendants succeeded him as director of the Paris Observatory; all were highly conservative astronomers, resisting major new theories.

Cassini, Oleg [next] [back] Cassilly, Richard

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