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on Saturn over the past 140
years, alternating between the
equator and midlatitudes. Up
until 1990, all the storms had
occurred on a roughly 30-year
cycle, but a surprise Great
White Spot broke the cycle,
occurring at an unexpected
time and latitude.
The storms usually occur
when Saturn’s northern hemi-
sphere is most tilted toward
the Sun near the moment of
saturnian midsummer. The
spots appear to be caused by
erupting plumes of warm
moist air that rapidly bring
ammonia ice crystals into the
planet’s upper atmosphere,
triggering thunderstorms.
Three types of storms have
been recognized to date: small
white storms (like those that
occurred in 1994 and 2006)
that measure about 1,240 miles
(2,000 kilometers) across and
appear as bright clouds for a
few days; Great White Spots
(like those of 2010 and 1990)
that are up to 10 times larger
than the small storms and last
for months every two to three
decades; and midsized storms
(like those detected near
Saturn’s north pole in 2018)
that can last anywhere from
10 to more than 200 days.
Will a Great White Spot
appear on its normal 30-year
cycle in 2020? The answer
remains unknown. In fact,
much remains uncertain
about these storms, such as:
How periodic are they? Can
they occur at any latitude?
How are they triggered?Will we see
Saturn’s spokes?
This year, the north face of
Saturn’s rings will be inclined
21° from edge-on. That’s
just 6° from their maximum
opening, or within 1° of the
ring angle (20°) when spokes
may occur.
Spokes are tiny particles
levitated above the ring plane
by electrostatic forces. Some
astronomers believe they are
connected to massive thun-
derstorms in Saturn’s atmo-
sphere. According to one
theory, lightning strikes —
10,000 times more energetic
than those on Earth —
release beams of electrons
that surge up from Saturn’scloud tops like sprites, zap-
ping the rings and blasting
out jets of electrically charged
dust that get trapped in the
planet’s electromagnetic field
and appear as spokes.
Some models of spoke for-
mation, which take into
account plasma density in the
rings, say that spoke activity
switches on when the angle
between the rings and the Sun
is between 0° and 20°. When
that happens, we may expect
spoke activity for about eight
years. The spoke situation
should be in full swing by
Saturn’s August 2021 opposi-
tion, when the rings will be
angled 18° to our line of sight.
As these predictions are only
based on models, be sure to
start looking now.
Given that this is a good
season for white spots and
thunderstorm activity to
occur, will the spokes mani-
fest in concert with increased
white spot activity?Shadow play
Around the time of opposi-
tion, the shadow of Saturn’s
globe plays a game of hide-
and-seek with observers. At
opposition, the shadow lies
directly behind the globe,
which hides it from view.
But most observers will not
see Saturn at that exact time.
Careful observations in the
days and hours around oppo-
sition may reveal the shadow
as a thin thread of darknessadjacent to the planet, appear-
ing very straight or with a
gentle curve toward the globe.
In the weeks before opposi-
tion, all manner of fun optical
illusions related to the shadow
may have you scratching your
head. Irradiation, simultane-
ous contrast, and effects
within Earth’s atmosphere
can make the shadow appear
to bulge toward the Cassini
Division, develop a notch
(blackdrop effects), or even
reverse its curvature.
Finally, don’t forget to look
for another whisker of shadow
— that of Saturn’s rings. You’ll
find it adjacent to the A Ring,
where it appears projected
against the globe of the planet.
It’s a hyperfine detail that
requires the best of atmo-
spheric seeing. Be patient, take
your time, and let the planet
reveal its secrets. Now, more
than ever, observations of
Jupiter and Saturn by amateur
astronomers are needed to
round out what we know
about these two planets. Why
not take a look at these mag-
nificent worlds, record what
you see, capture images of
them, and help astronomers
untangle the mysteries that
still linger over the solar
system’s largest worlds?Stephen James O’Meara is a
contributing editor of Astronomy
and the author of numerous
books on backyard observing,
including planetary viewing.On February 25, 2011, astronomers captured the 2010/11 Great White Spot
on Saturn with the Cassini spacecraft, about 12 weeks after the storm
began. The storm clouds by this time had formed a tail that had wrapped
around the planet. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTESaturn as rendered by Étienne Léopold Trouvelot, displaying notched
optical effects in the shadow of the ball of Saturn on its rings. Trouvelot
observed this phenomenon on November 30, 1874. WIKICOMMONS