Sky & Telescope - July 2020 - 53

Ironically, one of the most prominent skeptics of Ring C anomalies,
Lick Observatory astronomer Edward
Emerson Barnard, may have provided
the most compelling evidence that they
had a basis in reality. On the night of
November 1-2, 1889, Barnard watched
Saturn's satellite Iapetus emerge from
behind the globe of the planet and
slowly pass through the shadow cast
by the translucent C ring. Using a
12-inch refractor under excellent seeing conditions, he made 80 estimates
of Iapetus's ﬂuctuating brightness to
determine the C ring's structure and
optical density. The ﬁ xed brightness of
the nearby moons Tethys and Enceladus served as references.
McDonald Observatory astronomer
James Bryan published a painstaking
review of Barnard's data in the October
2007 issue of the Journal of Astronomical History and Heritage. He found that
Barnard's light curve contains a curious
shadow that ". . . appears to be consistent with observations of transitory
dark markings on the C ring in the
late 1880s . . . Apparently, a condition
existed in the inner ring system on 1-2
November 1889 that does not, for whatever reason, exist now."
Bryan speculated that the shadow
might have been cast by a "spoke,"
the radial streaks that appear in the B
ring, even though NASA's Voyager and
Cassini spacecraft never recorded these
ephemeral features in Ring C. In the
faint C ring, visual observers might per-

March 21

p Thomas Gwyn Elger depicted strange serrations on the inner edge of Ring C on two nights in
March 1888.

ceive radial spokes as the indentations
and serrations described in the 19thcentury reports.
The Cassini spacecraft recorded
several clouds of ejecta produced by the
impact of meter-sized meteoroids with
particles in Ring C. Far too small in
scale to be detected with any telescope,
these events seem to be encounters with
compact streams of Saturn-orbiting
material derived from the prior breakup
of larger bodies rather than random collisions with single objects.
Cassini images revealed the presence of vertical ripples or warps in the
C ring and in the even more tenuous
innermost D ring. According to Jeffrey
Cuzzi of NASA's Ames Research Center,
a leading expert on the rings of Saturn,
these corrugated structures ". . . suggest
that Saturn's D and inner C rings were
tipped relative to its equator several

q This widely reproduced lithograph accompanied François Terby's 1887 report of anomalous
dusky patches in the western (left) ansa of the C ring.

TERBY DR AWING: THE OBSERVATORY; ELGER
DR AWING: ROYA L ASTRONO MICA L SOCIE T Y

March 27

times over the last millennium, and as
recently as a few decades ago. Impacts
by rubble streams produced by disrupted
objects are the most likely cause."
I can't help but wonder if the
strange appearances in the C ring
reported late in the 19th century
were produced by a series of unusually intense bombardments. There are
tantalizing clues of others.
In 1913 and 1914 the French
astronomers Georges and Valentin
Fournier at the Jarry-Desloges Observatory in Algeria found Ring C appearing
"granular" and of varying intensity
with dusky knots and brighter areas. In
1992 the eagle-eyed observer Stephen
James O'Meara glimpsed a pair of
"dark interludes in brightness" in Ring
C through the 1-meter Cassegrain
reﬂector at the Pic du Midi Observatory
(S&T: Jan. 1993, p. 20). These delicate features were ". . . slightly darker
features against a relatively dark ring,
more blocky and broad" than the arced
wisps of the B ring spokes and required
averted vision to see.
Visual observers of Saturn seldom
give the C ring more than a passing
glance. The observational record suggests that it deserves careful inspection,
particularly while the rings are favorably tipped at opposition this month.
¢ Contributing Editor THOMAS A.
DOBBINS was awarded the Donald E.
Osterbrock Prize of the American Astronomical Society Historical Astronomy
Division in 2017.
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

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Sky & Telescope - July 2020

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