SJAA Ephemeris November 2002 | SJAA Home | Contents | Previous | Next

The Shallow Sky

Saturn spoken here

Akkana Peck


Saturn sketch by Jane Houston Jones, from observations on October 5, 2002, using an AP180EDT f/9 refractor at 480x.

 

It's time to dust off that telescope and have a look at Saturn!

The ringed planet is visible most of the night throughout November, hanging in Taurus off the end of the bull's left horn. And it's nearly as far north as it ever gets — that means, to those of us in the northern hemisphere, that it rises higher in the sky, where there's less atmosphere waving and rippling and obscuring our view of the details of the planet and its ring system.

What can you expect to see? Well, first of all, any telescope will show you Saturn's rings — and that's an unreal sight the first few times you see them. Come to think of it, it's still an unreal sight decades later. Since Saturn is not yet at opposition (the point at which it's on the side of us opposite the sun), from our vantage point the planet is casting a shadow onto its ring system, which makes the view look eerily three-dimensional. (Around opposition, which is December 17, this shadow will get difficult to see, then gradually it will reappear on the east side of the planet.)

Look closer at the rings, and in nearly any telescope, unless the air is extremely unsteady, you'll see that it's really two rings: a broad inner ring, covering about 2/3 the width of the ring system, called the "B" ring, and a thinner outer "A" ring. The gap separating the A and B rings is called Cassini's division, after Giovanni Cassini, who first noticed the gap in 1675.

Look closely at the area just inside the B ring. There's actually a third ring there, called the "Crepe ring" or just the C ring. It's different from the other two: It reflects a lot less light than the other two, so it appears as a shadowy grey band, and it's easy to miss. The crepe ring is a funny thing — sometimes it shows up easily in an 80mm telescope, and other times it's hard to see in a 10". I suspect it's a function of the sky's transparency, since the crepe ring is rather dim. Sometimes it seems to show up better in refractors than in reflectors, but that's not always the case. Don't be discouraged if you don't see the crepe ring right off; keep trying, and remember to look for it whenever you look at Saturn. When you do see it: what happens where the crepe ring crosses in front of the planet? Do you see the planet showing through the ring? Do you see the ring at all, standing out from the planet? This, too, may vary with your sky conditions, and it's an interesting feature to watch.

While you're looking at the planet, don't forget to look for markings on its surface! Saturn is a much more subtle planet than Jupiter, with bands of yellows and off-whites and light-browns rather than Jupiter's reds and creams and blue-greys. The features, too, are a lot more subtle — few festoons or swirls or lingering storms -but often you can pick up white spots in the polar region (the south pole is the one visible to us now, since Saturn is on the more northern side of its orbit) or in the equatorial bands. A relatively large telescope with excellent contrast will help here — notice some of the details in the rings that Jane Houston Jones picked up in her sketch early last month through a 7" refractor. This month the planet will be both higher and closer: if you find yourself armed with good seeing and a good telescope, do look at the planet's globe!

You're seeing the Cassini division and the Crepe ring easily, and some details on the planet, and you want a little more challenge? There's lots more to see on Saturn. First, there's the gap in the A ring. There's some controversy over this gap: spacecraft photos show a hair-thin gap which is named after Johann Franz Encke; but what amateur observers from earth see is usually a much broader and less sharply defined dark area. This area was seen by several observers, such as Keeler and deVico, long before Encke's A ring observations; and Encke's sketches also show this gradual darkening rather than the sharp gap which is named after him. This darkening we see with earth-based telescopes doesn't correspond very well to what shows up on spacecraft photos. Is it an optical illusion caused by almost seeing the sharp Encke gap, or are we seeing something else entirely? The mystery continues, and many Saturn observers refer to the "Encke smudge", or, sometimes, the "Keeler or deVico smudge" to distinguish it from the sharp gap that appears in photos.

Want more? You've probably seen NASA photos of Saturn showing radial spokes in the B ring. What you may not know is that amateurs reported those spokes from telescopic observations before they showed up in spacecraft photos. It's somewhat rare to have a night good enough to see this level of detail, but it does happen, and it doesn't take a huge amount of aperture (again, see Jane's sketch for an example of what can be seen with an excellent 7").

This should be enough to get started ... get out the telescope and take a look! Oh, I'll be updating my Saturn's moons program soon; check shallowsky.com for more information.

In other planetary news: Uranus and Neptune are still well placed for evening observations, in Capricornus. Jupiter rises a few hours after Saturn, and is always an impressive sight. (I'll talk more about Jupiter next month.) Venus and Mars are both in the predawn sky, Venus showing a slim crescent that should offer a nice reward to early risers. Mercury and Pluto are too close to the sun for good observation this month.

 


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