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May is prime time for observing Saturn and rings
BY MARK ARMSTRONG
ASTRONOMY NOW

Posted: 6 May 2014


There is no finer sight in the night sky than the beautiful ringed 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.

Moons in focus

Saturn has over 60 moons with eight of them visible in amateur equipment. Titan is by far the largest and brightest, shining at magnitude +8.3 at opposition with an diameter of 5,150 kilometres, making it the second largest moon in the Solar System. It can be spotted in 10 x 50 binoculars from a dark site and it orbits Saturn every 16 days with a maximum distance from Saturn of some 170 arcseconds.


How Saturn will appear (north is up; inverting telescopes will show this upside down) on 10 May. The moons Mimas and Enceladus will be detectable in medium-sized telescopes. Astronomy Now graphic by Greg Smye-Rumsby.
 
Among Saturn's family of moons that can be observed through a 200mm telescope are Rhea (magnitude +9.7), Tethys (magnitude +10.2), Dione (magnitude +10.4), Iapetus (magnitude +10.2 to +11.9), Enceladus (magnitude +11.7) and Mimas (magnitude +12.9).

Imaging Saturn

Saturn is not an easy planet to image. Its low surface brightness means at least a 150-mm 'scope should be pressed into service. If you are going to attempt filter work then a larger aperture is an advantage due to the light-loss incurred. With Saturn at a low altitude for UK observers, red filters are very useful. Try combining these images with those shot through green and blue filters within a few minutes imaging window. An infra-red blocking filter must be employed too.

North America

Observers in Canada and the United States have a significant advantage from those of us in the UK and northern Europe. Saturn is higher in the southern sky at culmination, which also gives a longer observing window. From New York latitudes, Saturn rises on 10 May at 7.45pm EDT, attains 20 degrees altitude by 10pm and it at its highest above the southern horizon, a favourable 34 degrees, just before 1am. By the time it sinks below 20 degrees at 4am, Saturn enthusiasts have hopefully had a cloud-free observing session lasting six hours, all in an astronomically dark sky too. Further south from Miami latitudes, the ecliptic (the apparent annual path of the Sun to which the planets remain very close to) lies perpendicular to the east-south-east horizon and Saturn climbs steeply away from the muck and turbulence at the horizon to be 20 degrees up by 9.30pm. The ringed planet peaks at almost 50 degrees altitude at 1.20am, and after nearly eight hours it sinks below 20 degrees at 5.15am.

An occultation down under!

Saturn is superbly placed for observation, culminating over 70 degrees above the northern horizon around midnight from Sydney around opposition. The ringed planet is above 20 degrees altitude from 6.45pm until 5am, giving visual observers and imagers alike a great opportunity to secure some decent observations. Circumstances are not quite as favourable from New Zealand but there is still a ten hour observing window in Wellington from 7.15pm to 5.30am, the ringed planet culminating at 64 degrees.

There is a spectacular event on the night of 14/15 May when Saturn is occulted by the full Moon. This is another in a series of Saturn occultations in 2014 and the best one visible in Australia and New Zealand. It's only the northern part of Australia that will miss out. Binoculars and small telescopes are all you need to observe the ringed planet and its giant moon Titan disappear behind the dark lunar limb (as the Moon is hours away from full, it's almost fully illuminated). Astro-imagers should be able to secure some marvellous images.

Here are the local timings for the major cities (always start observing well in advance):



City............TDD.......SDD.......Alt.......TRB.......SRB.......PA.......Alt.

Sydney.......8:47p.......8:54p.....47°.......9:52p......9:58p.....323°.....60°

Melbourne...8:42p.......8:49p.....40°.......9:53p......9:59p.....303°.....53°

Brisbane.....8:54p.......9:03p.....51°.......9:30p......9:33p.....358°.....60°

Adelaide.....8:03p.......8:09p.....34°.......9:10p......9:16p.....303°.....48°

Perth.........6:20p.......6:25p.....12°.......7:18p......7:25p.....283°.....25°

Wellington..11.37p......11.44p....63°......12.44a.....12.50a....321°.....63°

TDD = Titan disappearance at dark limb
SDD = Saturn disappearance at dark limb
TRB = Titan reappearance at bright limb
SRB = Saturn reappearance at bright limb
Alt. = degrees above horizon
PA = position angle where body reappears at the lunar limb (0 degrees is north, 90 degrees is east, 180 degrees is south and 270 is west)

 

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