astronomer who discovered four moons of Saturn and the gap in the rings
of Saturn now called the Cassini division.
Cassini was born near Nice (then in Italy). Having assisted two astronomers
at an observatory near Bologna, Cassini was made professor of astronomy
at the University of Bologna at the age of 25. In 1669 he departed for
France at the invitation of King Louis XIV, to construct and run the
Paris Observatory. When he went blind 1710, his son Jacques Cassini
(1677-1756) succeeded him.
Cassini refused to accept the Copernican cosmological model and rejected
the concept of a finite speed of light, although its proof was demonstrated
by Danish astronomer Ole Römer using Cassini's own data.
During 1664-67 Cassini determined the rotation periods of Mars, Jupiter,
and Venus. In 1675 he distinguished two zones within what was thought
to be the single ring around Saturn. Cassini correctly suggested that
the rings were composed of myriads of tiny satellites.
In the 1670s Cassini made many observations of details on the lunar
surface. He also took advantage of a good opposition of Mars 1672 to
determine the distance between the Earth and that planet. He arranged
for Jean Richer (1630-1696) to make measurements from his base in Cayenne,
on the NE coast of South America, while Cassini made simultaneous measurements
in Paris, which permitted them to make a triangulation of Mars. From
the result, Cassini was able to deduce many other astronomical distances.