planet Saturn in the eyepiece. The gas giant comes to opposition -- the
planet directly opposite the Sun -- among the stars of Libra on 10 May,
when it's 1,331 million kilometres from Earth and observable for much of
the night.
Where to find Saturn, over the southern horizon in Libra, on opposition on 10 May. Astronomy Now graphic by Greg Smye-Rumsby.
Saturn shines like a bright star with a noticeably yellow-hue at
magnitude +0.1, brighter than nearby, white Spica in neighbouring Virgo
and a small telescope will easily show Saturn's crowning glory, its
unique major ring system and the giant moon Titan.
Saturn is the second largest planet with an equatorial diameter
of 120,536 kilometres (if we count the rings, from one tip of the A-ring
to the other, this extends to 274,000 kilometres) with an average
distance from the Sun of 1.4 billion kilometres (9.58 astronomical
units). Saturn is the most oblate of the planets with its polar diameter
(107,566 kilometres) 90 percent that of its equatorial diameter, giving
it an even more flattened appearance than Jupiter.
The ringed planet takes 29.4 years to complete one orbit around
our star, moving 12.2 degrees eastwards along the ecliptic each year.
This year observers in the Southern Hemisphere are favoured with Saturn
lying among the stars of Libra at a southerly declination of 15 degrees.
This has not been an easy apparition for Saturn observers as it
lies well south of the celestial equator. This means Saturn is never
more than 25 degrees above the southern horizon at best from the UK and
the period when it's pulled sufficiently clear of local horizons to
limit the turbulence is limited to around three and a half hours around
opposition.
However the rewards for seeing Saturn float into your telescope's
field of view or securing those decent frames far outstrip the
inconveniences I've mentioned; if you've never seen Saturn through a
telescope this is your chance and it's a never to be forgotten sight!
You will need a decent south-eastern through to south-western horizon to
make the most of observing the ringed planet.
Saturn rises at 8.15pm BST due east-south-east and takes three
hours to haul itself twenty degrees up, by then lying in the
south-south-east. The sky is just about astronomically dark from London
and the south of England (Sun more than 18 degrees below the horizon) by
this time too. Saturn is at its highest above the southern horizon (24
degrees) at 1am and remains above 20 degrees altitude until 3am, setting
at 5.45am. Saturn remains well placed through the rest of May and it's
not until late September that it starts to become difficult to observe.
Observing Saturn
Saturn is a gas giant like Jupiter so there is no solid surface
to speak of; the face we see is the top of its tumultuous atmosphere.
Apart from the magnificent rings the planet's most striking feature is
its flattened shape, its oblateness.
This
portrait looking down on Saturn and its rings was created from images
obtained by NASA's Cassini spacecraft on Oct. 10, 2013. It was made by
amateur image processor and Cassini fan Gordan Ugarkovic. Credit:
NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic.
Saturn rotates on its axis every 10 hours and 14 minutes, and although
marginally slower than Jupiter, this rapid spin coupled to its lower
density (it would float if there were an ocean large enough!) and
gravity causes the obvious equatorial bulge. It does have belts and
zones but they are much less pronounced than on Jupiter and require
generally telescopes in the 150-200mm range to see well.
Astronomy Now's Peter Grego reports that the North Equatorial
Belt is nicely presented, appearing very broad and is more sharply
delineated on its southern margin where it meet the brighter Equatorial
Zone. Colour filters will make it easier to see more subtle features on
the globe and the rings; try a light blue filter to increase the
visibility of boundaries between belts and zones and a red or orange
filter to to make the belts darker. Saturn usually has a number of
low-contrast spots and projections and a light magenta filter can help
here. Advanced CCD imagers embracing the marvellous camera technology
and image processing techniques now available are brilliantly placed to
capture any short-lived atmospheric phenomena, despite Saturn's less
than generous placing in Northern Europe.
Wondrous rings
Saturn's rings are beautiful and unique are appearing to 'open
out' since appearing to be edge-on to us in 2009. At opposition the
northern side of the rings faces us at a tilt of 21.7 degrees. A small
telescope is needed to see the rings but through mounted binoculars
Saturn's elongated shape is apparent.
The Seeliger Effect will cause Saturn's rings to brighten on opposition night.
A good quality 60-80-mm refractor will easily show the brighter and
larger inner B-ring, the outer A-ring and the Cassini Division between
them in the 'ansae', the broadest part of the ring. The Encke Gap in the
outer part A-ring can be snared through a 100-mm (four-inch) apochromat
or a 150-mm (six-inch) Newtonian. The dusty inner C or Crepe ring is
semi-translucent and very hard to see even through large amateur
telescopes. As the rings open out the C-ring becomes easier to see; try a
200-mm 'scope under good conditions and perhaps a violet-blue filter.
There are a number of notable changes to Saturn's 'normal'
appearance either side of opposition that the experienced Saturn
observer is familiar with but can be readily viewed by a beginner. The
shadow of Saturn's globe can usually be seen through a small telescope;
before opposition appearing on the far side of the rings to the west of
the planet. Those observers who have been keeping Saturn under
observation these past few months will have noticed this shadow getting
ever-smaller. At opposition the shadow is almost completely hidden as
Saturn is being fully illuminated by the Sun and we are in line. After
opposition the shadow will fall to on the rings far side again, but this
time ever-widening to the east of Saturn's globe.
A really startling effect occurs right at opposition, when the
rings very noticeably brighten visually and in images. This is the
Seeliger Effect caused by the tiny ring particles being lit full on. The
phase angle, defined as the angle between the observer, the observed
object and the source of light (the Sun), is zero causing the shadows
normally cast to disappear and a temporary brightening occurring.
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