65
See also: Galileo’s telescope 56–63 ■ The origin of the moon 186–87 ■
Huygens (Directory) 335W
orking at the Panzano
Observatory near
Bologna, Italian
astronomer Giovanni Cassini was
provided in 1664 with a state-of-
the-art refracting telescope made by
Guiseppe Campini of Rome. With it,
he discovered the bands and spots
on Jupiter, measured the planet’s
spin period and polar flattening,
and made observations of the orbits
of Jupiter’s four known moons.Observing Saturn
Cassini’s reputation as a brilliant
observer led to an invitation to
oversee the completion of the new
Paris observatory. There, he turned
his telescope on Saturn, the largest
moon of which, Titan, had been
discovered in 1655 by Christiaan
Huygens. Cassini discovered two
more satellites: Iapetus in 1671 and
Rhea in 1672. In 1675, he noticed
a large gap in the Saturnian rings
and concluded, correctly, that the
rings were not solid but made up
of a multitude of small orbiting
bodies. In 1684, he discovered two
fainter satellites, Tethys and Dione.With these observations, Cassini
single-handedly nearly doubled
the number of known satellites
in the solar system. This number
has since increased dramatically.
Jupiter and Saturn have more
than 60 known satellites each.
The gas giants in the outer solar
system have two types of moon—
large ones that were formed at the
same time as the planet and smaller
ones captured from the asteroid belt.
In the inner solar system, Mars has
two small captured asteroidal moons,
while Mercury and Venus have no
moons. Earth has one huge moon,(^1) ⁄ 81 its mass, and astronomers are
still not certain how it formed. ■
THE TELESCOPE REVOLUTION
NEW MOONS
AROUND SATURN
OBSERVING SATURN’S RINGS
IN CONTEXT
KEY ASTRONOMER
Giovanni Domenico Cassini
(162 5 –1712)
BEFORE
1610 Galileo announces
the discovery of four moons
around Jupiter.
1655 Christiaan Huygens
discovers Titan, a moon orbiting
Saturn that is 50 percent larger
than Earth’s moon.
AFTER
1801 The first asteroid
is discovered in an orbit
between Mars and Jupiter.
1859 Scottish physicist
James Clerk Maxwell proves
that Saturn’s rings cannot be
solid, since they would break
apart under the force of gravity.
1960s onward In recent
decades, spacecraft have gone
into orbit around Jupiter and
Saturn, and Voyager 2 flew by
Uranus and Neptune. Large
numbers of moons have
been discovered.
The largest gap in Saturn’s rings,
called the Cassini Division, separates
the outer A-ring from the inner B-ring.
It is 3,000 miles (4,800 km) wide.