Cassini, Giovanni Domenico

Italian Astronomer 1625-1712

Born in Perinaldo, Italy, Giovanni^* Domenico Cassini (1625-1712) was an astronomer best known for his discoveries connected with the planet Saturn. At the age of twenty-five, Cassini was named chair of astronomy at the University of Bologna and held that position for nineteen years. He determined the rotation rates of Jupiter in 1665, of Mars in 1666, and of Venus (erroneously) in 1667. In 1668, Cassini computed tables that predicted the motion of Jupiter's four known moons. This led directly to Danish astronomer Ole (or Olaus) Römer's determination of the speed of light in 1675.

In 1669, King Louis XIV of France invited Cassini to Paris to direct the city's observatory. At the Paris Observatory, Cassini, now using Jean Dominique as his first name, continued his astronomical observations, at times using the extremely long "aerial telescopes" developed by Dutch astronomer Christiaan Huygens.

In Paris, Cassini discovered the second satellite of Saturn, Iapetus, in 1671 and correctly explained its brightness variations. He found another satellite of Saturn, Rhea, in 1672. In 1675 Cassini observed a band on Saturn and found that its ring had a division, now named the Cassini Division. Cassini discovered two more of Saturn's satellites, Tethys and Dione, in 1684.

Among his other projects, Cassini used innovative methods to make the best measure—at the time—of the astronomical unit (the average distance between Earth and the Sun). Cassini also studied atmospheric refraction and conducted a geodetic survey .

Cassini is the namesake of a joint program of the National Aeronautics and Space Administration, the European Space Agency, and the Italian Space Agency to study the Saturn system beginning in 2004.

see also Huygens, Christiaan (volume 2); Jupiter (volume 2); Saturn (volume 2); Small Bodies (volume 2).

Stephen J. Edberg

Bibliography

Abetti, Giorgio. The History of Astronomy, trans. Betty Burr Abetti. New York: Henry Schuman, 1952.

Beatty, J. Kelly. "A 'Comet Crash' in 1690?" Sky and Telescope 93, no. 4 (1997):111.

Berry, Arthur. A Short History of Astronomy (1898). New York: Dover Publications, Inc., 1961.

Bishop, R., ed. Observer's Handbook, 2000. Toronto: Royal Astronomical Society of Canada, 1999.

^*Giovanni Domenico Cassini is sometimes known as "Gian Cassini."