Australian Sky Telescope MayJune 2017

http://www.skyandtelescope.com.au 39

NASA / JPL-CALTECH

banding — the bright Equatorial Zone,

the slightly darker North Equatorial

Belt (the South Equatorial Belt is

behind the rings now), and the dusky

North Polar Region. Subtler banding

is sometimes detectable in the mid-

latitudes.

Can you detect any sign of white

spots? In 2010 Saturn broke out with a

massive white storm at about 45° north

latitude that became obvious through

amateur scopes. But such events are

rare. Lesser, subtle spots are somewhat

more frequent, but they generally

require a large scope in good seeing, or

stacked-video imaging.

Rings on parade

A 60- or 75-mm scope should reveal

the rings easily, and the dark Cassini

Division between the A and B rings

with a little more effort. Both the A and

B rings brighten toward their edges

bordering the Cassini Division.

The dusky C ring is more of a

challenge where it appears against the

dark sky background. It’s often fairly

plain through my 32-cm but rarely in

my 15-cm. Its dim grey veiling is easier

to see where it crosses Saturn’s bright

face, just inside the B ring. Sometimes

the C ring in front of Saturn’s face is

indistinguishable from the rings’ dark

shadow falling at the same latitude. But

this season the rings’ shadow is hidden

behind the rings themselves.

The hair-thin Encke Gap, near the

outer edge of the A ring, is a severe

challenge under any conditions. I can’t

say for sure that I’ve ever seen it with

my 32-cm even in excellent seeing.

With the rings wide open, you

have your best chance to see Saturn’s

surface through the Cassini Division.

In very steady air, detecting the planet’s

light through the division — as opposed

to black sky behind the division

elsewhere — can emphasise the three-

dimensional appearance of the view.

For several nights around Saturn’s June

15 opposition, watch for the Seeliger

effect — a noticeable brightening of

the rings with respect to the globe. This

is caused by the solid ring particles

‘backscattering’ sunlight in the direction

it came from — the inner Solar System,

home of Earth — more effectively than

the planet’s cloudtops do.

In the weeks and months away from

opposition, note the shadow of the

planet’s globe on the rings. It’s the

narrow black gap right where the rings

pass behind the globe’s celestial west

(preceding) side before opposition,

and the east (following) side after

opposition.

Marvelous night

for a moon fance

Saturn and its rings are surrounded by

more moons for amateur scopes than

any other planet.

Even a 60-mm scope will usually

reveal Titan, a frigid, haze-shrouded

world half again as big as our Moon.

A 15-cm shows Titan’s orange colour:

Its haze is photochemical smog, thick

enough to hide Titan’s rainclouds,

rivers, dark hydrocarbon mudflats and

lakes of liquefied natural gas.

A 10- to 15-cm scope will also show

Iapetus, Rhea, Dione and (maybe with

difficulty) Tethys. Fainter Enceladus

closer in almost always eludes my

15-cm despite my best efforts, but the

32-cm fishes it up when the seeing is

especially good. It helps to know in

advance precisely where a difficult object

you’re looking for ought to be. You can

identify the moons, or find exactly

where to look for them, at any time and

date using the interactive observing aid

at skyandtelescope.com/satmoons

SSaturn’s rings were tilted 25°, almost as

much as now, when Damian Peach took this

very sharp image on March 10, 2015, using a

planetary video camera on a 14-inch Schmidt-

Cassegrain scope. South is up.

North Polar Region

RINGS

ABC

C ring on

globe

Equatorial

zone

Cassini

Division

Encke

Gap

North

Equatorial

Belt

S If you could view through a 4,500-metre telescope above Earth’s atmosphere, you could resolve

features in Saturn’s rings and on the 8-km-wide moonlet Daphnis as sharply as the Cassini Saturn

orbiter did on January 16, as seen above. Daphnis keeps the 42-km-wide Keeler Gap clear. It has

collected a ridge-like pileup of dust around its equator, and its gravity raises waves in the gap’s

edges. The Keeler Gap, a black chasm here, went undetected by the best telescopes on Earth.