Sky and Telescope - April 2017 - 49

Jupiter's Moons
grabbing the instants when the atmosphere is steadiest, you have to be alert to
grab them with your eye and brain. But
there's nothing like observing the real
thing. Your skill in discerning planetary
detail develops with time, so don't think
you've reached your limit anytime soon.
Here are things to look for on Jupiter,
in rough order of difﬁculty - depending, of course, on what the planet is
actually displaying when you look.
Easiest: Jupiter isn't round. Its fast
rotation period (9 hours 56 minutes,
but 5 minutes shorter around its
midsection) distorts it to be 9% wider
across its equator than between the
poles. Even the humblest telescope will
show Jupiter's spun-up elliptical shape.
Belts and zones. Belts are dark, zones
are bright. A small scope usually shows
the North and South Equatorial Belts
even if nothing else. Can you detect
differences between the two - in width,
darkness, color, or irregularities?
These and other belts and zones are
labeled in the images at left. The naming is somewhat ﬂuid; belts and zones
not only come and go, but they can also
move up and down a bit in latitude.
Limb darkening. Near Jupiter's edges,
sunlight strikes the planet's cloudtops
at a grazing angle, and both the sunlight
and our own line of sight pass through
more of Jupiter's atmosphere. So Jupiter
always looks dimmer near its limb. This
is especially obvious in photos, which
are more sensitive to differing brightness levels than your eye is.
Around opposition, the limb darkening is uniform. At times away from
opposition, the side of Jupiter farthest
from the Sun in our sky is darkened
more. This effect is strongest when Jupiter is at quadrature, 90° from the Sun,
which will occur on July 7th.
Medium hard: Belt irregularities.
Often, the finer details that you're most
likely to see are irregularities in edges of
the two broad equatorial belts.
Great Red Spot. This trademark of
Jupiter has been colorful enough in
recent years to actually live up to its
name. It's more vivid now than the
appearance most of us grew up with.
Someday it will fade back to pale

tan-white, so make the most of the
opportunity now. The timetable on page
50 tells when the Red Spot faces Earth
most directly during March and April.
Jupiter's rotation. If you can see
discrete details, then you should notice
the planet's rapid rotation within 10
minutes or so. Features rotate from
celestial east ("following," f) to west
("preceding," p). It takes 50 minutes for
a feature to rotate from Jupiter's central
meridian halfway to the preceding limb.
Harder: The Red Spot Hollow (RSH)
is the indentation in the South Equatorial Belt where the Great Red Spot
lives. Sometimes it's white, sometimes
gray, sometimes hardly visible.
White ovals. These are especially
common in the South Temperate Belt
(STB). About 11 of them currently reside
there, spaced all around the planet.
Oval BA, "Red Spot Junior," is the
long-lived product of the merger of three
white ovals in the STB that had lasted for
decades before. It's very pale tan-pink.
Blue patches often mark the edges
of the dark equatorial belts against the
lighter Equatorial Zone. They seem to be
atmospheric clearings between clouds,
blue for the same reason our daytime
sky is blue. But the color is subtle.
Festoons extend from blue patches
into the Equatorial Zone, where they're
blown sideways like banners in the wind.
The Equatorial Band is a thin, subtle
line sometimes seen along the equator,
like the combined tails of many festoons.
Red ovals are small but often quite
dark red.
Barges, just as narrow and dark, are
elongated east-west into line segments.
White outbreaks. Tiny, bright-white
dots of fresh, upwelling clouds may
evolve in days into thin white streaks;
they occasionally morph into large,
white, turbulent areas extending east
and west far along a belt.

Apr 1
2
3

EAST

WEST

4
5

Io

6
7
8
9
10
11

Callisto

12
13
14
15
16
17
18
19
20
21
22

Ganymede

23
24
25
26
27

Europa

28
29
30
May 1

April's Jupiter Events
Of course, you can hardly miss Jupiter's
ever-changing Big Four moons! Any
telescope shows them. Spot Io, Europa,
Ganymede, and Callisto at any date and
time using the diagram at right.
With either luck or planning, you

The wavy lines represent Jupiter's four biggest satellites.
The central vertical band is Jupiter itself. Each gray or
black horizontal band is one day, from 0 h (upper edge of
band) to 24 h UT (GMT). UT dates are at left. Slide a paper's
edge down to your date and time, and read across to see
the satellites' positions east or west of Jupiter.

s k y a n d t e l e s c o p e .c o m

* APRIL 2017

49
Sky and Telescope - April 2017

Table of Contents for the Digital Edition of Sky and Telescope - April 2017
