Saturn, considered by many the most beautiful sight in the sky, comes to opposition this week with its rings in full tilt. You won't want to miss it.
It happened again just last week. I was showing a group of people sky sights on a windy June night. Saturn had just cleared the trees, so I pointed my 10-inch Dob that way. "Yeah, I see it!" "Wow!" said a young woman when the ringed wonder greeted her eye.
Saturn's back and how glad we are that it is. It reaches opposition to the Sun tomorrow (June 15th) and spends all summer in the southern reaches of the constellation Ophiuchus, offering a perfect opportunity for sidewalk astronomers to mix their astronomy with a little astrology. The Sun, Moon, and planets spend nearly as much time here as in Cancer, another zodiacal constellation, begging the question as to why Ophiuchus couldn't be drafted into a modern astrology as the 13th sign. But I'm just making trouble!
At declination –22° south, Saturn hangs low in the southern sky for skywatchers at mid-northern latitudes this apparition. That means we should take every opportunity to observe the planet to increase our chances of catching it on a night of good seeing, the better to appreciate its arresting beauty and the subtleties of ring divisions, faint moons, and delicate cloud belts.
Oh, those rings! It's summertime in Saturn's northern hemisphere, so we see their north face fully tipped into view, all of 26.6° at the moment. In October, they'll open to their maximum 27°. Amateurs can spot the three major rings — A, B, and C — even in telescopes as small as 4-inches. A-ring is the outermost and separated from B-ring, the brightest and widest, by the prominent dark gap called Cassini's Division.
The innermost C-ring begins along the inner edge of the B-ring and appears translucent compared to the solid-looking A- and B-rings; that's the reason it's also known as the Crepe Ring. Because the rings are tilted far forward, the Crepe Ring stands out plainly as a gray arc set against Saturn's bright disk (see photo diagram below). In calm air, you can also see it off to either side of the planet, tucked within the arc of the B-ring.
Ring lovers will find this opposition the best time to look for Cassini's Division, a hair-like gap separating the narrower A-ring from the broad, brighter B-ring. The gap is easiest to make out at the east and west extremities of the ring plane, known as the ansae, in a good 3-inch telescope. Use a magnification of 100× to 200× to see it best.
In steady seeing, using a 6-inch or larger telescope you can almost follow Cassini's full circle around the planet. During a spell of calm air several nights ago, I discovered that the tippy-top of Saturn's globe notches out a small section of the much-foreshortened ring plane on the planet's backside. About ~8° is missing. Maybe early this fall, when the rings reach their maximum tilt, we'll be able to see the full 360°.
Tugs from the moon Mimas clear the 4,800 km (3,000 mile) wide Cassini's Division, while tiny Pan, which orbits within the A-ring, sweeps ring particles hither and thither to carve out the 325 km (202 mile) wide Encke Division, located about 80% of the way from the Cassini Division to the outer edge of the A-ring.
Observers using 10-inch telescopes in crystal sharp seeing have momentarily spotted this 0.35″ gap, and I've heard of at least one observation made with a 6-inch aperture. Me? Not yet, but it's definitely on my bucket list. Catching sight of the divide requires perfect seeing and the highest magnification that conditions will allow. Like Cassini's, it is most obvious in the outer ansae.
One of my favorite Saturn pastimes is comparing and contrasting the color and brightness of the planet vs. the rings. Made of billions of bits of reflective water ice ranging in size from tiny grains to large boulders, the A- and B-rings — particularly the latter — appear brilliant white with maybe a hint of yellow. If you study them for a few minutes and then shift your focus to the globe, it looks strikingly tan or even brown in comparison.
The difference between rings and disk can be stark around the time of opposition due to the Seeliger Effect. At opposition, the Sun strikes the ring particles squarely from our perspective, so the shadows they cast are hidden behind their tiny bulks. Without shadows to ameliorate the light, the rings surge in brilliance. It's quite obvious at the moment. When you observe the planet, make a mental note of the rings' appearance and then compare the view to their appearance in a few weeks.
Like Jupiter, Saturn has a diverse family of moons. The brightest, largest and most atmospheric is Titan at magnitude +9. It seems no matter what scope you use or on what night you point to the planet, Titan's always nearby like a dog on a leash. 8-inch and larger telescopes will reveal the orange hue of its smoggy atmosphere. Next in brightness is Rhea (REE-uh) at magnitude +10.3, then Tethys (TEE-thiss) at +10.9, and finally Dione (dee-OH-Nee) at +11.0. All orbit interior to Titan and are visible in a 6-inch.
Joining the fab four, though fainter and orbiting even closer to the planet — and more affected by the glare from the maxed-out rings — is magnitude +11.4 Enceladus, a moon famous for its misty geysers and salt-water interior. An 8-inch will deliver this icy point of light to your eye. Be sure to use Sky & Telescope's interactive Saturn's Moons site so you know how to find all five any night of the year.
Let's not forget the Jekyll and Hyde moon, Iapetus. With one side coated in coal-black organics and the other in bright ice, we see alternate faces as it swings around Saturn every 79 days. At western elongation, the icy side shines at +10.2 magnitude and becomes the planet's second brightest moon. But 6 weeks later at eastern elongation, it only manages magnitude +12. Not terribly faint, but the two-magnitude difference dramatically affects its appearance in small telescopes.
Two additional moons, Mimas and Hyperion, will tax a 12-inch scope. Mimas (magnitude +13.6) never gets farther than about one ring-plane width from the rings, making it a difficult catch in their glare. Hyperion orbits further out but at magnitude +14.9, you'll need a good chart to find it. Or an app. Check out SaturnMoons for iPhone from Sky & Telescope or Moons of Saturn for Android.
This year, both Saturn and Earth are seasonally in sync, with summer underway in both planets' northern hemispheres. Like soldiers at attention, the twin north polar axes tilt sunward in solidarity in salute of the season. Join the fun by tilting your telescope Saturn-ward the next clear night.