Neptune’s got ’em. Jupiter’s got ’em. Uranus has them, too. But no planet has a set of rings like Saturn. While the others are spindly and faint, Saturn’s are rich in water ice ranging in size from tiny grains to big boulders. This year, those rings will be tipped wide open, as far as they can go. And there’s no better time to see them in a telescope than now, when the planet’s at opposition and brightest and closest to the Earth.
Opposition occurs tomorrow (June 15), when Saturn will be directly opposite the sun in the sky, rising at sunset and setting at sunrise Friday morning. I’ve been keeping my eye on the planet and can tell you it first appears in late twilight low in the southeastern sky. It’s bright at magnitude +0.0 or equal to Vega, the brightest star in the Summer Triangle figure. For skywatchers living in mid-northern latitudes, Saturn never gets very high, maybe 2-3 fists above the southern horizon when it hits peak altitude after midnight.
Now through much of the fall, Saturn moves slowly across the southern reaches of Ophiuchus the Serpent Holder about a fist and a half to the left of the bright red star, Antares, in Scorpius. An equal distance to the east will bring you to Sagittarius, better known as the “Teapot”.
Earth’s axis is tilted 23.5°, about a quarter of the way from vertical; Saturn’s a touch more at 27°. Exactly the same way that Earth’s tilt brings us the four seasons, so too Saturn. Because Saturn takes 29.5 years to make one trip around the sun, its seasons each last more than 7 years. Right now, the ringed planet’s northern hemisphere is tipped our way, so it’s summer there just like its summer here. Only a bit colder. Saturn’s ammonia ice crystal cloud tops register a frigid 288° below zero (–178° C).
15 years from now, Saturn will have moved to the other end of its orbit. At that future opposition, we’ll see their south face fully open. Viewing the rings is as easy as dragging out that scope someone gave you for Christmas 8 years ago. Even a rickety scope as small as 2.4-inches (60 mm) working at just 30x will show the globe of the planet surrounded by a bright ring. A 6-inch or larger scope at 100-150x will show that the one ring is really three: the narrow, outer A-ring; the bright and broad B-ring and the translucent C-ring, visible as a gray band just inside the B ring where it crosses in front of the planet.
The Cassini spacecraft, in orbit about the planet until mission end on September 15, subdivides the rings further into thousands of individual ringlets. All these fine ringlets blend together from our distant perspective into what look like solid rings through the telescope. Just remember when you view them, that each is comprised of billions of tiny moonlets, each describing its own orbit about the mighty planet. Imagine sitting atop one of these icy boulders and looking around at countless other dusty-icy rocks all glinting in the sunshine like a field of daisies. I don’t expect we’ll be there soon. Maybe in a 100 years?
The gap separating the A and B rings is named for the 17th century Italian astronomer Giovanni Cassini, who discovered four of Saturn’s moons and was the first to note the division that now bears his name. It’s 3,000 miles wide (4,800 km) but seen from so far away looks as thick as a hair. But with the rings tipped out, it’s more obvious than ever. Much more difficult to see is the Encke Gap, located 80% of the way from outer edge of the B-ring into the A-ring. Only a couple hundred miles wide, you’ll need a good telescope, excellent seeing and a practiced eye to spot it.
Each time you look at Saturn, you’ll inevitably see a small “star” near it. That’s actually a moon, the planet’s largest, called Titan. It shines at magnitude +9, so it’s easy to see in any scope. Titan’s 3,201 miles (5,151 km) across or about half again as large as our moon and large enough (and cold enough) to maintain an an atmosphere. It’s composed of nitrogen and laced with methane-derived compounds that create a smog-like, orange haze. Take a close look at Titan if you have a 6-inch or larger scope, and you’ll see the orange hue.
Rhea, Tethys and Dione orbit closer to the planet, but a 6-inch will show them fairly easily. Enceladus, the geyser-spouting moon, is much closer in; you’ll need an 8-inch or large scope to see it best. Iapetus orbits much farther out, completing a run around the planet in 79 days. One of its sides is covered in bright ice, the other in organic compounds as dark as coal. When Iapetus is east of Saturn, as it is this month, we see the dark side, and the moon shines dimly at magnitude +12. About 6 weeks later, at the western end of its orbit, we catch the icy side and it’s two magnitudes brighter. Right now, Iapetus is faint, but come mid-to-late July, when it’s west of Saturn, it will be super-easy to see.
To locate and identify five of Saturn’s moons any time of night, check out Sky & Telescope’s Saturn’s Moons site. Just enter the time, and you’re golden.
Saturn is a singular planet. It connotes the wonder and mystery of outer space better than just about anything else out there. Spend a few minutes with this amazing world the next clear night. And if don’t have a telescope but have been looking for an excuse to get one, the time is ripe.