Sky & Telescope - August 2022 - 33

ANTONIADI SKETCH: WILLIAM SHEEHAN / PUBLIC DOMAIN; CASSINI VIEW OF
THE SPOKES: NASA / JPL / SPACE SCIENCE INSTITUTE
MISSPOKEN TESTIMONY Famed observer Eugène Michel Antoniadi
drew Saturn on July 30, 1899, using the 9½-inch refractor at Camille
Flammarion's observatory near Paris. Antoniadi's skill was legendary, but
he has placed spokelike features in the A ring not the B ring. A version
of this drawing appeared in Arthur Francis O'Donel Alexander's book
The Planet Saturn: A History of Observation, Theory and Discovery.
ring spokes. Famed astronomer Clyde Tombaugh, Smith's
erstwhile mentor at New Mexico State University, regularly
saw them in a 16-inch reflector from his backyard in Las
Cruces, New Mexico. The 3.6-meter Canada-France-Hawaii
telescope atop Mauna Kea obtained the first Earth-based
images in 1996, while master planetary imager Donald C.
Parker often recorded them at his home observatory in Coral
Gables, Florida.
The Hubble Space Telescope also regularly captured the
spokes from 1995 until 1998, when they dropped from sight
for several years. Presumably the increasing inclination of
the rings rendered them invisible. There was every confidence
that the Cassini spacecraft would see the spokes again when
it entered orbit around Saturn in 2004. It didn't.
Finally, in 2006, the features reappeared, and a secret
regarding their visibility came to light. It seems that the
spokes may only form when the Sun's angle over the ring
plane is less than 17°. O'Meara's observations between 1976
and 1980, which took place when the Sun was between 3°
and 16.5° above the ring plane, backed this up. With this
finding in mind, historians reviewed a handful of earlier
drawings, such as those by Stuyvaert, in which spokelike
markings appear. However, none of the drawings were
produced when conditions were right. It seems we still don't
know enough about the true nature of the spokes to make
definitive statements about their occurrence. Or to rule
out the possibility they might also appear in the other rings
(especially ring A, as seen by Antoniadi). Nineteenth-century
observers may be vindicated yet.
The Sun's angle over the ring plane became less than 17°
once again in January this year, though at the time of writing
(April 2022) we know of no sightings or images showing
the spokes. However, they're expected to become visible by
the time Saturn reaches opposition this month. That's when
STEADY SATURN The author's drawing of Saturn made on August 1,
1992, depicts the planet's stunning appearance in the 1-meter Cassegrain
at Pic du Midi Observatory. The site's steady seeing allowed productive
use of magnifications as high as 1,200×. In addition to subtle spokes
in the B ring (seen also by O'Meara), the intense blue hue of the planet's
southern hemisphere was striking.
observers have the chance to confirm once again William
Herschel's maxim: " When our particular attention is once
called to an object, we see things at first sight that would
otherwise have escaped our notice. "
¢ WILLIAM SHEEHAN co-authored (with Stephen James
O'Meara) Mars: The Lure of the Red Planet. Among his recent
books is Saturn, published in 2019.
THE SPOKES RETURN After vanishing for several years, Saturn's ring
spokes were recovered by the Cassini spacecraft in 2006. Although the
formation mechanism of the spokes is still debated, their visibility seems
to depend on the Sun's elevation above the ring plane. In this Cassini
image from January 27, 2010, the spokes stretch from left to right across
the finely pleated B ring.
sk yand tele scope .o r g * AUGUST 2022 33
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