![[Image of G.D. Cassini]](../../Pics/Hi-res/cassini-big.jpg)
Giovanni Domenico (or, in French, Jean Dominique) Cassini studied mathematics
and astronomy at the Jesuits and became professot of astronomy at Bologna at
age 25. There, he determined the obliquity of the ecliptic at 23 deg 29' and
studied the effects of refraction and the Solar parallax, worked on planetary
theory and observed the comets of 1664 and 1668 (according to Kenneth Glyn
Jones, also already one in 1652).
In 1665, he determined the rotation periods of Jupiter (as 9h 56m) and Mars (24h 40m) with remarkable acuracy, by observing Jupiter's Great Red Spot, discovered by Robert Hooke (1635-1703) in 1664, and moving surface contours on Mars (such as Syrtis Major which Huygens had discovered in 1659).
On invitation of the French King, Louis XIV, Cassini moved to Paris where he arrived on April 4, 1669, joined the newly founded Academie Royale des Sciences (Royal Academy of Sciences), and became the first director of the Paris Observatory. He used very long air telescopes, and collaborated with Christiaan Huygens in many astronomical projects. In 1673 became a naturalised Frenchman. In this year, he married a Frenchwoman.
In France, Cassini observed more comets: those of 1672, 1677, 1698, 1699, two in 1702, and those of 1706 and 1707.
In 1772, he measured the distance of Mars by triangulation with the help of observations of Jean Richer (1630-96); this enabled him to refine the dimensions of the Solar System, i.e. determine the value of the astronomical unit (AU). His value was just7 % short.
He discovered Saturn's moons Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684). In 1675, Cassini discovered that Saturn's rings are separated into two parts by a gap, which is now called Cassini Division in his honor.
Cassini was the founder of a dynasty of four astronomers in Paris: His son Jaques Cassini (Cassini II, 1677-1756), his grandson César François Cassini (Cassini III, 1714-84) and his grand-grandson Jean Dominique Cassini (Cassini IV, 1748-1845) followed him as directors of the Paris Observatory. In 1711 Cassini got blind, and died in 1712.
He is reported by his son, Jacques Cassini, to have discovered a "nebula" somewhen before 1711 in the area between Canis Major and Canis Minor and "which was one of the finest to be seen in the telescope". Messier was not able to verify this discovery, but it may be that Cassini had found M50 (according to Messier's list of unverified nebula discoveries).
G.D. Cassini (or Cassini I) was multiply honored by the astronomical community: Nasa/ESA's Cassini spacecraft to Saturn.
