Ringing in the Spring

Saturn is well placed in the evening sky, already high in the sky as night falls. It transits at about 8pm and 50 degrees up, high enough to be in fairly clear air so we can get a good look at it.

Its rings are tilted about seven degrees to us – starting to open wide enough to show the structure of the rings.

If you’re new to Saturn observing, the rings are the place to start.

With any telescope, no matter how small, you’ll usually be able to see two rings with a gap between them: the thinner “A ring” on the outside, the thicker “B ring” inside it. The gap between them is called the Cassini division (named for the astronomer Giovanni Domenico Cassini – the spacecraft came later). It’s almost 3,000 miles wide. Its inner edge is caused by a resonance with Saturn’s moon Mimas, which orbits with exactly half the period of particles at the inner edge of the division.

But within the gap is more complex structure, so there’s a lot more going on than just the effects of one moon.

Inside the B ring is another ring, the C or “Crepe” ring. Much less bright than A and B, the C ring can be a challenge to see. It doesn’t take a big telescope – I’ve seen it with an 80mm refractor, and I’m sure it’s possible to see it with even smaller optics – but its faintness makes it curiously vulnerable to sky glow and bad seeing. If you don’t see it at all at first, don’t despair – keep trying, and one night it will pop out and you and you’ll wonder what all the fuss was about.

The C ring, curiously, probably has about the same particle density as the Cassini division, and they both have a richly complex structure in photos from the Cassini spacecraft, especially the ones that are backlit, showing the rings as the sun shines through them from the other side. Not a view we’re able to get with our earthbound telescopes, alas.

Your next challenge is at the outer edge of the A ring: the thin, dark hairline crack most commonly known as the Encke division. But it was probably seen first by James Edward Keeler or Francesco deVico, and there’s some controversy over whether ever Encke saw the feature that now bears his name. Whatever you call it, it’s a challenge to observe: steady seeing and a reasonably large telescope (say, a good 6” or larger) make it easier, but again, it’s definitely possible with a smaller scope with a good night, a good eyepiece and some patience. This gap is created by a small moon named Pan which orbits within the gap itself, clearing out ring particles as it goes.

Of course, Saturn has an atmosphere too, with colored cloud bands similar to Jupiter’s ... but much more subtle. You probably won’t see the swirls and festoons you see when looking at its larger neighbor, but study it anyway: every now and then a storm or other interesting feature shows up.

Around the middle of the month, Saturn passes by the close double star Porrima (Gamma Virginis), with their approach on the 14th. Porrima is a tough target these days, but you might have a chance with a big telescope and steady air.

Aside from Saturn and the moon, there’s not much going on in our evening shallow skies. Jupiter, Mars, Venus, Uranus and Neptune are all up in the morning, while Mercury sneaks back into the early evening sky by the end of the month. Pluto, though, is visible all night, transiting a few hours after midnight. It never gets very high, only about 33 degrees, so it’s still not an ideal time to chase the faint planet – but if you do go after it, give a thought to New Horizons, which passed the orbit of Uranus a few months ago while winging its way out to distant Pluto. It’s now over 2/3 of the way there, but it’ll still be several more years before it gets there.