Saturn and its rings reach opposition in April
BY MARK ARMSTRONG
Posted: 19 April 2013
Saturn is unquestionably the solar system's showpiece planet. The sixth world from the Sun has been known since ancient times but its true majesty was not fully realised until the invention of the telescope.
This month there's a great chance to observe Saturn as it comes to opposition on 28 April and will be observable all night, gradually moving retrograde (westwards) through western Libra before re-entering Virgo next month. At opposition, it will shine at magnitude +0.1 and have an apparent diameter of 18.9 arcseconds.
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 11 degrees.
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. 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. 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.
The ring master
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 18 degrees. A small telescope is needed to see the rings but through mounted binoculars Saturn's elongated shape is apparent. A good quality 60mm 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 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 telescope under good conditions and perhaps a violet-blue filter.
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. 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).
Saturn is not an easy planet to image, its low surface brightness means at least a 150-mm telescope 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, then red filters are very useful, and try combining these images with those shot through green and blue filters within roughly a five-minute imaging window. An infrared blocking filter must be employed too.
The ringed planet's visibility
Saturn is well south of the celestial equator in Libra, which means this is not a favourable opposition for the UK in terms of altitude. The ringed planet lies a hand's width (15 degrees) east (right) of Spica (mag. +1.0, alpha Virginis) and half of that north-west of Zubenelgenubi (mag. +2.7, alpha Librae). On opposition night, Saturn rises at 8pm and transits at 1am, with the time when Saturn is above 20 degrees being between 10:45pm and 3:15am. This gives an very handy observing window of over four hours, despite the low altitude.
For observers in the USA, Saturn is better placed the further south you live. On opposition night from New York, Saturn rises at 7:30pm EDT but it's not until 9:30pm that it's sufficiently clear of the horizon to observe. It transits at 1am, not far short of 40 degrees altitude above the southern horizon and sinks to 20 degrees by 4:15am. From Houston, Saturn rises at 7:45pm and can be observed from 9:30pm to 5:15am, culminating at a respectable 50 degrees. This is a very favourable opposition for observers in Australia and New Zealand, the observing window extending to ten hours by opposition night with Saturn rising at 5:20pm and can be observed from 7pm until 5am, at its best around midnight around 70 degrees up.
For an in-depth look at Saturn, with observing advice from experienced observers, including imaging tips from number one imaging guru Damian Peach, pick up a copy of the May issue of Astronomy Now. Please send any images or sketches of the ringed planet you wish to share with our readers to firstname.lastname@example.org.
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