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A backyard guide to Saturn: shining in the east, Saturn may offer more than you expect.

LATE THESE EVENINGS, as the Orion family of constellations moves west, bright Saturn rises in a good telescope often draws gasps from visitors.

But you can never see Saturn as well as you want! Its globe is only 20 arcseconds wide this season, the same size that Mars appeared at its best last fall. And while Saturn's ring system is 2.27 times as wide as the globe, that's still only about as wide as Jupiter near opposition. Saturn is indeed a jewel: exquisite but tiny.

However, with time, patience, and a top-quality 4-inch or larger telescope, you can tease out more of the planet's secrets than you may suspect.

What to Look For

The rings should be visible in even the smallest telescope at 25?. A good 3-inch scope at 50??can show them as a distinctly separate structure from the ball of the planet. The rings are currently tilted about 19[degrees] from our line of sight, less than in recent years. We see their south face, as well as the southern hemisphere of Saturn's globe. With the ring tilt decreasing year by year, the planet's north polar region is now in view too.

Saturn has a more three-dimensional appearance than any other object in the sky--at least that's how it looks to me with a 6-inch scope on a night of fine seeing. The edges of the planet are limb-darkened, making Saturn look like a yellow-brown marble rather than just a disk, while the rings encircling it show no such effect and look as flat as a paper cutout. The planet's black shadow on the rings, which is visible barely to one side of the globe when Saturn is away from opposition, adds to the 3-D appearance.

The rings' thin shadow on the planet is subtler. From our earthly viewpoint, it shifts from the inside edge to the outside edge of the ring system about every six months. In 2006 the shadow is on the rings' inside edge from late January to early August. I think Saturn is prettier when the shadow is on the outside edge, as in the image above; a sharp black line then divides the outer edge of the rings from the ball, improving the 3-D effect.

Details in the rings can be viewed with a small scope during spells of good seeing. The plainest is the black Cassini Division between the A and B rings. Its clarity is a fine test of atmospheric steadiness and the telescope's optical quality. Shading within the rings is even easier to discern. The broad B ring is plainly brighter than the narrower A ring outside it. To me, both the A and B rings seem to brighten smoothly to a maximum at the edges of the Cassini Division.

Dark belts and bright zones can often be made out on the ball of the planet. They're much vaguer than the similar belts and zones of Jupiter, but it's rare when my 6-inch reflector shows Saturn completely blank.

And, of course, there are Saturn's many moons. A 2-inch scope will show Titan. A half dozen are sometimes within reach of a 10-inch. You can use our graphical almanac of Saturn's satellites on page 56 to create a map of where to look for the moons in your telescope. Or try our handy applet at SkyandTelescope.com/satmoons.

That's about all Saturn displays to most observers. But there's more.

Looking Deeper

In a high-quality planetary telescope of at least 6- or 8-inch aperture on a night of excellent seeing, the rings become more interesting. Just inside the A ring's outer edge is the extremely thin Encke Division or Keeler Gap (usage varies)--an extreme test for any telescope. Only on rare nights when the seeing is so steady that I can use 450x on my 12.5-inch reflector have I even suspected seeing it during moments when the air is especially steady.

The rings also contain thin, grayish brightness minima at various radii. Julius Benton, the longtime Saturn section coordinator for the Association of Lunar and Planetary Observers (ALPO), says that these brightness minima are reported to change somewhat in both prominence and location.

Ring C, also called the crepe ring, can be either difficult or easy to make out. Many have seen it without knowing it. Evidence of the C ring is easy to distinguish when the rings' shadow on the ball appears on the rings' outside. At such times the duskiness you see against the planet just inside the B ring is purely the semitransparent C.

Changes in the belts and zones become apparent, even obvious, to regular Saturn watchers--one of the benefits of long-term study. The larger and better your scope the more likely you are to see enough details to note changes in them.

Spots and other markings occasionally appear amid the belts and zones. Major white eruptions happen about every 30 years (once per Saturnian year); great white outbreaks were seen in 1876, 1903, 1933, 1960, and 1990, around the start of each Saturnian northern-hemisphere summer. Lesser bright and dark spots appear much more often. As of last November, writes Benton, Saturn was showing "only sporadic, poorly defined activity in the form of very small and diffuse white ovals and dusky features, all of them very transient."

To predict when a spot will return to the same location on the disk from night to night, remember that Saturn's equatorial parts rotate once in 10 hours 14 minutes. Higher latitudes rotate more slowly, in 10 hours 38 minutes.

Colors change on Saturn too, but only subtly. The best way to pin them down is to note the relative brightnesses of different areas as seen through red, green, and blue filters. Oddly, the two ends (ansae) of the ring system sometimes appear to differ slightly in color. Using red and blue filters, note whether one end looks brighter or dimmer than the other.

Overall, I find that a yellow filter sharpens up the whole planet a trace, probably by suppressing atmospheric color dispersion and differences in atmospheric turbulence at the red and blue ends of the spectrum. (Red and blue images quiver and shimmer out of phase with the yellow near the middle of the spectrum. This is the same effect that causes Sirius to twinkle in vivid colors.) A light green filter may slightly improve contrast in the planet's belts and zones. Webcams and video-frame stacking have made it easy not only to take fine Saturn pictures but to document effects that were once the sole province of the skilled visual observer (S&T: December 2005, page 94, and June 2003, page 117).

If you think you'd like to try these sorts of observations on a regular basis, the ALPO Saturn section wants you. "We're getting requests for observations from the Cassini people--imaging at different wavelengths and things like that," says Benton. Observations made from Earth while the Cassini spacecraft is studying Saturn from up close serve to calibrate and give meaning to Saturn observations going back well over a century. "We need all the observers we can get doing good quality imaging," says Benton, "and to encourage visual observers not to neglect drawing the planet and making brightness intensity estimates" of features in Saturn and in the rings.

For more about the ALPO Saturn section, and to get report forms and instructions, go to the Saturn page on the ALPO Web site, www.lpl.arizona.edu/alpo. You can also read and join the Yahoo Saturn Group discussions at saturn-alpo@yahoogroups.com.

Alan MacRobert has been a Saturn watcher for 1.3 Saturnian years.
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Title Annotation:celestial calendar
Author:MacRobert, Alan M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Feb 1, 2006
Words:1270
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