The magnificent ringed planet Saturn, the jewel in the Solar System’s crown, comes to opposition on the night of 20/21 July. At around this time, the ringed wonder offers its best observing circumstances for 2020, as it’s observable all night. Saturn comes to opposition about every 378 days, so opposition occurs about two weeks later every year.
If quizzed, most astronomers rate Saturn as the most alluring of all the planets through the eyepiece, as in our Solar System its’ unique for hosting a marvellous system of rings that are easily seen through even a small telescope. Mars, the red planet, may be the most tantalising planet, making observers wait patiently before giving up its secrets, and Jupiter the most observationally rewarding on a regular basis, but when you see Saturn float into view through a high-powered eyepiece of a moderate- to large-aperture telescope, you’ll swear nothing comes close to matching the thrill.
The ringed-wonder shines at magnitude +0.1, brighter than all of the stars currently visible in the sky apart from Arcturus, Vega and Capella. However, it will be overshadowed by the brilliance of Jupiter, its fellow gas giant planet that lies just seven degrees to the west. You may be able to resolve Saturn’s 18.5 arcsecond-sized disc (or, with the rings, nearly 42 arcseconds) through mounted, large image-stabilised binoculars, but a small telescope (~80mm) operating at 50x magnification should make short work of the job. If you can resolve Saturn’s disc, you should notice it’s not a perfect sphere but, like Jupiter, a distinctly flattened, or oblate, one.
Like Jupiter, Saturn does display dark belts and brighter zones. However, it lies twice as far away from the Sun as its fellow gas giant so it receives much less energy to enliven its atmosphere, resulting in a much more subtle effect. Today’s startlingly good amateur images reveal their true extent, but all that’s likely to be on show visually while Saturn lies at such a disappointing altitude is an equatorial belt and a dark polar hood.
When to look for it
On the night of 20/21 July, Saturn rises at about 9pm BST, from London, just as the Sun sets. By 11pm BST (midnight BST from Edinburgh), the Sun’s lingering glow in the north-north-western to north-western sky, manifest by the seemingly endless, lingering twilight, gives way to a workable darkness (the end of astronomical twilight). At this time Saturn lies almost 12 degrees above the south-eastern horizon, among the stars of the far-western reaches of Sagittarius, the spectacular southern constellation. Together with Jupiter, the unmistakable pair will be easy to see with the naked eye if you’re looking out across an uninterrupted southern horizon.
Saturn transits the southern meridian (culminating, or reaching its highest point above the local horizon) shortly after 1am BST, having climbed to an altitude just short of 18 degrees (14 degrees, from Edinburgh at 1.20am BST). While it is placed as low in the night sky for observers based at mid-northern latitudes (which includes the UK), it’s best to try to observe it within an hour or so of culmination. You’ll not get as good a view as when the planet is placed much higher in the sky, but in moments of steadier seeing, you may be surprised at what you can see or image.
By the month’s end, Saturn can be seen by about 10pm BST and culminates at around 12.20am BST.
See Saturn’s majestic rings
Saturn’s overwhelming observational attraction is its stunning system of rings. From Earth, three distinct, major rings can be seen encircling Saturn’s equator. The outer ring, Ring A, is divided about 20 per cent of the way in from its outer edge by the elusive 325-kilometre-wide Encke Gap, or Division. The middle ring, Ring B, is the broadest (width 25,500 km) and brightest ring and is separated from Ring A by the famous 3,000-kilometre-wide Cassini Division. Inside Ring B is the dusky and very faint Ring C, or Crêpe Ring. The rings are composed overwhelmingly of water-ice with some rocky content, the individual particles ranging from micron- to metre-size.
A huge telescope isn’t required to see the rings, as a small- to medium- sized telescope – say in the 70–150mm class – has sufficient power to give fine views, making this magical sight accessible to everyone. At this year’s opposition, the resolving power of a 150mm (six-inch) telescope should be sufficient to see the Cassini Division, but a 250mm (ten-inch) ’scope may be required to bag the Encke Gap. Ring C is a notoriously difficult visual target at the best of times. Look out too for the fascinating occurrence of the ‘opposition effect’, also known as the Seeliger effect, a dramatic brightening of the rings for a few days either side of opposition.
The ring’s changing angle
The rings offer a changing aspect year on year. Over intervals of 13.75 and 15.75 years, alternately, the Earth passes through the plane of the rings and at such times when the rings are visible from Earth, they are presented very close to or precisely edge-on to our line of sight. The last time a so-called ring plane crossing occurred was in 2009 and the rings were last fully open (tilted by around 26 to 27 degrees towards us) not too long ago, in 2017.
Since then the rings have been closing. Currently they, along with the planet’s north pole, are titled towards us by a pretty healthy 21.6 degrees. The next ring plane crossing occurs in March 2025, but Saturn will then lie too close to the Sun to be visible.
Observe Saturn’ family of Moons
Seven of Saturn’s impressive retinue of 62 moons can be observed through amateur instruments when the planet is best placed in the night sky. Titan, it’s giant moon, which is the second largest satellite in the Solar System, shines at magnitude +8.4 and is visible through a small telescope at this opposition throughout its 15.94-day orbit around Saturn. At opposition it lies close to or at its greatest eastern elongation.
Iapetus, Saturn’s third-largest moon, is a very curious object that shows two vastly different hemispheres. The leading side is dark, while the following side is bright, causing it to appear over two magnitudes brighter (magnitude +10.5–12.7) when its brighter side faces us (when it lies at greatest western elongation from Saturn rather than at greatest eastern elongation). At this opposition, Iapetus lies on the eastern side of its orbit.
Rhea shines at magnitude +9.7 and orbits its parent in 4.5 days, while Dione is a little fainter at magnitude 10.4 and lies closer to Saturn, with an orbital period of 2.7 days. Tethys lies closer still, with a 1.88-day period and rivals Dione in brightness. See the graphic here for the positions of Saturn’s major moons on opposition night.