THE SOLAR SYSTEM
Of the two main rings, B is the brighter. The Cassini
Division is very conspicuous when the system is fav-
ourably tilted to the Earth, and before the Pioneer and
Voyager missions several minor divisions had been
reported, though only one (Encke’s Division, in Ring A)
had been confirmed, and it was believed that the other
divisions were mere ‘ripples’ in an otherwise fairly regular
and homogeneous flat ring.
In 1907 the French observer G. Fournier announced
the discovery of a dim ring outside the main system;
though at that time confirmation was lacking, and it
became known as Ring F. There were also reports of
a faint ring between the Crêpe Ring and the cloud-tops,
and this was usually referred to as Ring D, though again
positive confirmation was lacking.
The Cassini Division was thought to be due mainly
to the gravitational pull of the 400-kilometre (250-mile)
satellite Mimas, which had been discovered by William
Herschel as long ago as 1789. A particle moving in the
Division would have an orbital period exactly half that
of Mimas, and cumulative perturbations would drive it
away from the ‘forbidden zone’. No doubt there is some
substance in this, though the Voyager revelations showed
that there must be other effects involved as well. The rings
turned out to be completely different from anything which
had been expected.DISTANCES AND PERIODS OF RINGS
AND INNER SATELLITES
Distance from
centre of Saturn, Period,
km h
Cloud-tops 60,330 10.66
Inner edge of ‘Ring’ D 67,000 4.91
Inner edge of Ring C 73,200 5.61
Inner edge of Ring B 92,200 7.93
Outer edge of Ring B 117,500 11.41
Middle of Cassini Division 119,000 11.75
Inner edge of Ring A 121,000 11.92
Encke Division 133,500 13.82
Pan 133,600 14
Outer edge of Ring A 135,200 14.14
Atlas 137,670 14.61
Prometheus 139,350 14.71
Ring F 140,600 14.94
Pandora 141,700 15.07
Epimetheus 151,420 16.65
Janus 151,420 16.68
Inner edge of Ring G 165,800 18
Outer edge of Ring G 173,800 21
Inner edge of Ring E 180,000 22
Mimas 185,540 22.60
Enceladus 238,040 32.88
Tethys 294,760 1.88d
Dione 377,420 2.74
Outer edge of Ring E 480,000 4
Rhea 527,040 4.52 Saturn, imaged by Damian
Peach on 13 January 2005,
with a 9-inch reflecting tele-
scope. The minor divisions inthe rings are clearly shown.
Note also the belts on the
disk, and the disk’s polar
‘hood’. Saturn from Voyager 1.
This mosaic of images was
taken on 30 October 1980 at a
range of 18 million km(11 million miles). Saturn’s
satellites Tethys and
Enceladus can be seen at top
right. At bottom left is Mimas.108-151 Atl of Univ Phil'05 7/6/05 12:15 pm Page 111