Sky and Telescope - June 2016 - 50

OBSERVING

Celestial Calendar

Jupiter's Moons
June 1
2
3

EAST

WEST

5
6
7
8
9
10
11
12
13

Europa

14
15

Callisto

the Encke Gap shimmering in the seeing
at your highest power?
As opposition comes and goes, watch
for the Seeliger eﬀect: a brightening of the
rings (but not the globe) for a few days,
caused by the dusty ring particles backscattering sunlight toward the Sun. We
intercept this backscattering when we are
very close to the Sun-Saturn line around
opposition. This same "opposition eﬀect"
also slightly brightens Mars and the full
Moon. On a sunny day, you sometimes
see it as a slightly brighter glow around the
shadow of your head when your shadow
falls on dusty material of the right texture.
The globe of Saturn itself is a more subdued version of Jupiter. You'll probably be
able to see at least the bright Equatorial
Zone and the darker North Equatorial Belt.
Any more belts? A rare white spot? The tilt
of the whole Saturnian system this year
shows us the North Polar Region unusually well. Read more about Saturn observing in the June 2015 S&T, page 50.

17
18

Ganymede

19
20

Moons Galore
The planet with the obvious rings oﬀers the
most moons for amateur telescopes, and
that's no coincidence. Rings and moons
form out of an abundance of the same orbit-

ing material. Over cosmic time, they can
even morph from one into the other.
Take this opportunity to add to your
life tally of moons you've observed. Titan
shows up in a 3-inch scope, and a 4-inch
may reveal the orange color of its thick
atmosphere. A 4- or 6-inch will add Rhea,
Dione, and (maybe with diﬃculty) Tethys.
An 8- or 10-inch may get you Enceladus.
Weird Iapetus is not shown on the
orbit diagram below. Its wide orbit keeps
it mostly far outside the frame. Iapetus
varies from an easy magnitude 10 when
farthest west of Saturn to a darker magnitude 12 when farthest east. (Remember
this by "East is least, west is best.")
Moons don't count toward your lifetime
tally unless you can say which is which!
Identify the ones you see, or locate in
advance exactly where to look, using
skyandtelescope.com/satmoons or our
SaturnMoons app from the iTunes App
Store. For Iapetus you'll need a full-up
sky-charting program.
If you want to read more about building your lifetime tally of moons, see our
article on this topic in the February 2015
issue, page 28. It builds on the familiar
ones for amateur telescopes to add a truly
challenging table of 20 more.

21
22
23

North

Orbits of
Saturn's Moons

24
25

Titan

Dione
Tethys
East

26
27

Rhea

Enceladus
West

28
29
30
July 1

South
The wavy lines represent Jupiter's four big satellites. The central
vertical band is Jupiter itself. Each gray or black horizontal band is
one day, from 0h (upper edge of band) to 24h 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.

June 2016 sky & telescope

Big Titan, magnitude 8.4, takes 16 days to complete its wide orbit around Saturn. Little
Enceladus, magnitude 11.8 and deep in the planet's glare, whips around the planet in
just 1.4 days. Wide-ranging Iapetus is not shown.

http://www.skyandtelescope.com/satmoons

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016