This week marks a special time for Saturn that happens just once a year. It’s called opposition. On Friday evening May 22, Earth and Saturn line up on the same side of the Sun. Face to face with each other across a distance of 833 million miles, it’s as close as they’ll get in 2015.
When Earth’s on the opposite side of the Sun from Saturn, it puts an additional 186 million miles between us and the ringed planet. When you add distance in astronomy, things get smaller and fainter. That means this week Saturn will also shine brightest and largest for the entire year. At magnitude 0.0, the lord of the rings outdoes Vega by a hair, the 5th brightest star of the night.
When a planet’s in opposition, it’s “opposite” the Sun in the sky behind the Earth. On Friday, Saturn will rise in the constellation Libra the Scales around sunset, pass due south at 1 a.m. local daylight time and set at sunrise. A perfect case of planetary insomnia. Because the Sun sets late and twilight lasts around 2 hours, Saturn first comes into view around 9:30-10 p.m. low in the southeastern sky. Around 11 o’clock local time, it’s high enough for a telescopic viewing, which we’ll discuss in just a moment.
When the planet does appear, you’ll notice it has an entourage. By good fortune, Saturn lies not far from the bright “head” of Scorpius the Scorpion. Just 2° away is a Beta Scorpii or Graffias, a fabulously fine double star for small telescopes. 10° (one fist at arm’s length) further down the line you bump into Mars-red Antares, Scorpius’ brightest star. They’re all grouped together with Saturn brightest of all, so the planet’s easy to find even if it’s low in the sky from mid-northern latitudes.
So what’s to see in a telescope? Rings! Saturn’s got going for it more than anything else. Try to point to another object in nature where a ball is surrounded by a ring nowhere touching. The sight is quintessential. It defines the essence of what makes astronomy cool, accessible and fascinating to so many people. Yet the average person’s first impression on seeing the planet in a scope is one of disbelief. It can’t be real! Aren’t we lucky it is.
Even in 10x binoculars Saturn looks strangely oval because its rings are tilted near their maximum. Any small telescope magnifying at least 30 x will show a pale, cocoa-toned globe surrounded by at least one ring. 6-inch and larger scopes on a night when the air is calm and steady reveal three — Rings A, B and C.
The broad, bright B ring is easiest to spot. A narrow, black gap called Cassini’s Division separates it from the outer, narrow A ring. Both rings appear white and bright because they’re made of millions of tiny, icy satellites, chunks of mostly water ice ranging in size from dust particles to mountains. From a great distance, the individual pieces blend together to form what appear to be smooth, solid rings. The C ring is likewise composed of icy chunks but more challenging to see because it’s translucent and so close to Saturn’s bright globe.
Let’s return to Cassini’s Division, named for Giovanni Cassini, a 17th century Italian mathematician and astronomer who discovered four of Saturn’s moons and the division that bears his name. Regular Saturn observers always keep an eye out for this gap which neatly cleaves the A from the B ring. Coincidentally, its width is 2,920 miles (4,700 km), very nearly the same as the width of the United States (2,892 miles). How’s that for a sense of scale.
Jupiter has two prominent dark cloud belts or stripes; Saturn has just one that’s currently visible called the North Equatorial Belt or NEB. There are more belts ringing the ringed planet but they’re muted by a deep layer of haze that engulfs the globe and only visible in larger amateur telescopes. Need more challenges? Use the photo above as a key to seeing additional details like Encke’s Gap and the globe peeking through Cassini’s Division.
Saturn has 62 known moons, five of which are visible in a 6-inch telescope — Titan, the largest and brightest, Rhea (REE-uh), Dione (Die-OH-nee), Tethys (TEE-thiss) and Iapetus (eye-AP-eh-tuss). In a 3-inch scope, Titan at 9th magnitude and smaller Rhea (mag. 10) are easy to see. So is Iapetus when located at the far west end of its orbit. It shines at magnitude 10 then because its bright, icy hemisphere faces us. When it swings around to the east of Saturn, it fades to 12th magnitude as its dark, residue-covered hemisphere faces our way.
Tethys and Dione are dimmer at magnitude +11 and nearer the planet. You’ll see them through Saturn’s glare if you look closely at medium and high magnifications. If you have a 10-inch or larger telescope, try tackling even closer-in Mimas and Enceladus as well as faint Hyperion. Free software programs like Stellarium will help you track all of these moons down any time of night anywhere on Earth. Also check out Sky and Telescope’s Saturn Moons page to get an instant diagram showing the positions of five of the brightest moons.
Saturn remains visible throughout the spring and summer, so if you have bad weather the next few nights, not to worry. Many viewing opportunities lie ahead.