Sky & Telescope - May 2019 - 53

SOUTH

JUPITER DIAG R A M: G REGG DINDER M A N / S&T;
JUPITER: DA MIA N PE ACH / CHILESCOPE

The Great Red Spot

Dance of the Galilean Moons
Beyond the planet itself, Jupiter is
attended by the four bright Galilean
moons. All four are unique worlds in
their own right and would be easily
visible to the naked eye if they weren't
closely orbiting the gas giant. While
visible with any binoculars or telescope,

S. S. Temperate Belt

Zones (bright)
S. Temperate Zone
S. Tropical Zone
Equatorial Zone
N. Tropical Zone
N. Temperate Zone

Direction of
rotation

S. Temperate Belt

Central meridian (CM)

Of all of Jupiter's swirling cloud features, the long-lived Great Red Spot
(GRS) is best known. This immense
anticyclonic storm resides in a notch
within the South Equatorial Belt known
as the Red Spot Hollow. Astronomers
have observed this iconic feature continuously since the mid-19th century,
but over the past century this gigantic
system has been shrinking at an everaccelerating rate. Spanning a massive
40,000 km during the late 1800s, it has
slowly condensed to roughly 16,000 km,
or slightly larger than the Earth (S&T:
Mar. 2016, p. 18).
Although the GRS is relatively stable,
it can often ﬂuctuate in color and
contrast. Throughout the late 1990s and
early 2000s, it appeared a pale orange
that was difﬁcult to detect with small
apertures. But in the past few years it
has regained its distinctive, reddishorange color, which makes it easy to
spot in telescopes with apertures as
small as 80 millimeters. Additionally,
the whitish Red Spot Hollow helps to
boost its contrast with the surrounding
SEB. Your best bet for catching sight of
this famous storm is to look within an
hour of the time it transits the planet's
central meridian. A table with the GRS
transit times for May can be found on
pages 50-51.
Slightly farther to the south of the
GRS in the South Temperate Belt is Oval
BA. This storm system is the result of
three white oval mergers occurring in
the late 1990s. Noted planetary imager
Christopher Go noticed in early 2006
that Oval BA had changed to orangered. Recently, Oval BA's color has been
fading to the point of appearing white
at the start of the apparition this year.
See close-up views of Jupiter in our
feature beginning on page 14.

Belts (dark)
South Polar Region

Great Red Spot
S. Equatorial Belt
Equatorial Band
N. Equatorial Belt
N. Temperate Belt
N. N. Temperate Belt
North Polar Region

NORTH

p The ever-changing features within Jupiter's cloudtops are identifiable by the belt or zone where
they generally reside. South is up to match the view in most telescopes. Features rotate across the
face of the planet from celestial east to west.

these moons yield subtle, dusky albedo
markings with larger instruments of 10
inches or more under exceptional seeing.
Typically, smaller scopes will permit observers to detect the size of each
moon, ranging from Ganymede, the
largest satellite in the solar system, to
ice-covered Europa, which is slightly
smaller than our Moon. A 6-inch scope
under good conditions will resolve each
into tiny disks of differing albedo and
hue, ranging from yellowish Io closest
to the gas giant at 1.1″ across, brilliant
white Europa (1.0″), dusky Ganymede
(1.7″), and grayish Callisto spanning
1.5″ farthest from the planet.
The best time to hunt for albedo
markings on the Galilean satellites is
when they transit the disk of Jupiter.
The planet's bright clouds and belts help
to signiﬁcantly reduce the contrast of
these icy moons and the background
q Recent changes in the gas giant's Equatorial Zone have turned the normally white zone a
yellowish-tan hue. South is up in this image.

sky, permitting brief glimpses of their
varied surfaces. A table of Jupiter's moon
phenomena can be found on page 51.
Speaking of transits, a particularly
spectacular one occurs within a few
days of opposition on June 12 at 2:20
UT, beginning with Io and its shadow
passing in front of the North Equatorial Belt, followed by Ganymede cutting
across the north polar regions approximately one hour later. Occurring so
close to opposition, Io's disk will overlay
a portion of the moon's shadow. The
timing of this event makes it particularly favorable for observers in western
Europe, western Africa, eastern North
America, and eastern South America.
Jupiter's dynamic atmosphere and
attendant moons offer observers an
endless source of activity to witness
unlike anywhere else in our skies. The
constant evolution and appearance
of ovals, barges, festoons, and loop
structures make the largest gas giant a
popular target for imagers and observers
alike. Additionally, comets and asteroids
have struck the planet several times
over the past 25 years, the most recent
occurring in March 2016. There's a
small chance you might witness a strike
yourself. So be sure to turn your scope's
gaze toward the king of the solar system
early this year. You just might have a
hard time turning it away again.
¢ RICHARD JAKIEL observes and photographs the night sky from his home in
the suburbs of Atlanta, Georgia.
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

http://www.skyandtelescope.com

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019