Sky and Telescope - June 2015 - 51

Learn how to use detailed star charts to ﬁnd the faintest things
with your telescope: skypub.com/charts.

Alan MacRobert

Rings Past Poles
This particular subtlety may be lost in
the fuzz of atmospheric seeing on most
nights. But Saturn is always a work of
jeweler's art regardless. Here are more
aspects to look for.

Markings on the Globe
Saturn's rings were tilted 25.0° last
February 26th, enough to extend beyond
Saturn's poles. The shadow of the globe
fell well to the west on the rings behind
(lower right of the globe here) but did not
yet quite break the rings over the north
pole (bottom). Cameras can bring out not
just details but color contrasts better than
the eye; note the whiteness of the icy rings
compared to the yellowish planet. Darryl
Milika and Pat Nicholas in Australia used a
Celestron 14 scope and a ZWO ASI174MM
planetary video camera for this image.

The tilt of the rings and Saturn's axis
will increase only slightly in the next two
years (with minor seasonal ﬂuctuations),
to a maximum of 27° in 2017.
°
As for those minor seasonal ﬂuctuations, the tilt is currently decreasing
very slightly, from 24.6° on May 1st to a
minimum of 24.05° in mid-July. Normally
that would be insigniﬁcant. But it's not
quite enough for Saturn's south pole to
start peeping above the outer edge of the
A ring; that happens at ring tilt 24.0°. In
the 2014 pictures at left and lower right,
by comparison, the rings were tilted only
21° or 22°.
You may, however, detect something
else around the sunlit north pole at this
opposition. Before opposition, Saturn's
globe always casts its shadow westward
onto the rings behind it (to the left in
south-up images like these). The width
of the shadow visible next to the globe
shrinks in the months and weeks as
opposition approaches. Afterward, the
shadow on the rings grows wider just
oﬀ the globe's eastern edge. And around
opposition itself this year, the north pole
of the planet will cast its shadow northward just enough to break the outer edge
of the rings barely above the pole.

Like Jupiter, Saturn is gas planet with
banded cloudtops. But Saturn is not only
smaller and farther than Jupiter, its markings are also more deeply veiled under
high-altitude haze. Even so, my 6-inch
reﬂector almost always shows some of
Saturn's banding: the bright Equatorial
Zone, the slightly darker North Equatorial
Belt, and the dusky North Polar Region.
Subtler banding is sometimes detectable
in the mid-latitudes.
In late 2010 Saturn erupted with a
record-breaking white storm at about 45°
north latitude that became easy to see in
amateur scopes. The planet's fast upper
winds tore a turbulent trail of white
clouds from the storm's active head that
eventually ran completely around the
globe. Might such a thing happen again,
now that Saturn's northern hemisphere is
approaching its midsummer?
Large scopes and sharp imaging more
often show tinier white and dark spots:
smaller round or oval storms, visible only
during very steady seeing.

Rings Show Oﬀ
The smallest astronomical telescope
should reveal the rings easily and the
dark Cassini Division between the A and

May 11, 2014

B rings with a little more eﬀort.
The dusky C ring is more of a challenge
where it appears against the dark-sky background. Its dark shading is easier to see
where it crosses Saturn's bright face just
inside the B ring. This season, the C ring
in front of Saturn may seem especially
noticeable. That's because it blends with
a thin line of the rings' shadow extending
into view at the same Saturnian latitude.
Seeing any further detail in the rings
takes upwards of 200× on a high-quality
8-inch or larger scope on a night of topnotch seeing. On such rare nights I think
I detect occasional ﬂickers of Encke Gap
with my 12.5-inch reﬂector.
Very subtle banding in the rings is
also occasionally reported under nearperfect conditions. The fabled ring
spokes, diﬀ use radial bands on the wide
B ring, have even lower contrast, eluding all but the most experienced visual
observers with large scopes. And usually they aren't there at all. Fortunately,
stacked-video imaging has brought such
elusive details within reach of backyard
observers. To capture spokes if they're
present, shoot short video clips no more
than 90 seconds long to stack. Any longer
and the rapid orbital motion of the ring
material will blur away any spokes. And,
be very careful not to overprocess your
images or false features will creep in.
This is more of a problem with Saturn
than the other planets because of all the
sharp lines between bright and dark that
the rings add to the scene.

May 15, 2014

May 23, 2014

The Seeliger eﬀect, the brightening of the rings near opposition, is subtle in images. But
visual observers who become familiar with Saturn notice it fairly easily. Opposition fell on
May 10th in 2014. Trevor Barry in Australia recorded a series of images in the following days
using a 16-inch reﬂector with a ZWO ASI120MM planetary video camera.

Sk yandTelescope.com June 2015

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Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015