December's Sky Features Saturn At Its Best


by Carol Ryback

Saturn is December's showpiece for observing. Nothing quite matches the feeling of beholding it in a telescope for the first time -- yet it holds a certain fascination even for experienced observers. Watch for it in the east/northeast as it rises about 45 minutes after sunset. It's easy to find: Just seek out the yellowish "star" that outshines the other stars, except for Sirius and Canopus.

As the second-largest planet in the solar system, Saturn beats Earth in both diameter (75,000 miles, or nine times) and mass (95 times). It also trumps Earth with the speed of its rotation. In less than 11 Earth hours, Saturn spins all the way around. This may be aided in part by the fact that gaseous Saturn has a density less than that of water: Theoretically, if you could find a bowl large enough, Saturn would float!

Instead, this beauty appears to float in space, buoyed by its rings. At their fullest -- when the rings are tilted most toward our line of sight -- they reflect more light than the disk of the planet itself. In Saturn's 29 Earth-year-long journey around the sun, the rings slowly change their orientation to us in a 14-year cycle. By 2009, Saturn's rings will orient edge-on to Earth and nearly disappear. However, the present tilt is large enough to reveal divisions in the rings. In September of 2001, Saturn tilted its rings toward Earth at about 26 degrees, about the maximum possible. They will remain at this angle for the rest of the year, providing an excellent opportunity to study their structural nuances.

As you gaze upon Saturn through a telescope, you may notice a dark gap in the rings. The largest of these dark splits, the Cassini Division, separates rings A and B. It was first noticed in the 17th Century by Giovanni Domenico Cassini. Another dark, thin line completely within the A ring is known as the Encke Gap. The C ring -- also called the crepe ring -- is darker, and D darker still, and both are between the bright B ring and Saturn itself. The remaining rings (labeled through G) are identified in their order of discovery rather than placement. They are actually farther out from Saturn than the A ring and cannot be seen from Earth.

Astronomers believe this dynamic ring system holds clues to the evolutionary processes of the solar system. Ring elements range in size from microscopic particles all the way to kilometer-sized chunks (composed mostly of water ice). Pioneer 11 as well as the Voyager 1 and 2 probes during the late 1970s and early 1980s revealed a variety of structural features, such as sharp edges, arcs, waves, clumps, and kinked and braided configurations. Occasionally, spokes may appear in the lighter B ring, caused by very small particles (1 mm) that reflect light differently.

Observers should also watch for some of Saturn's many moons, which at last count stood at 30. Titan, the only known moon with a dense atmosphere, is half the size of Earth. Other large satellites include Rhea, Iapetus, Dione, Tethys, and Enceladus. Smaller but still sizable are Hyperion, Phoebe, Janus, Epimetheus, Mimas, and Prometheus; all are difficult to observe from Earth. Another of Saturn's moons -- Pan -- is among those embedded in the ring system.

Any telescope that magnifies at least 30x provides a good view of Saturn, while views through instruments in the 4-inch to 8-inch range are often exquisite. And don't forget to look for the shadow cast by the planet onto its rings, which lends an awesome 3-D quality to the display. Only observers with high-resolution telescopes will get to enjoy features on Saturn's cloud decks. If you are among the lucky, you may also spot the pale equatorial region, a dusky equatorial band, and the darker polar regions. And if you experience excellent seeing conditions, you may even get a glimpse at Saturn's belt system.

Equipment considerations aside, anyone who makes the effort to catch a view of Saturn this month won't be disappointed. It's one of the most compelling sites in the December sky.

Note to editors: Please see Astronomy Magazine's website at http://www.astronomy.com/content/static/pressroom/.

Contact: Richard Talcott, senior editor
Phone: (262) 796-8776 ext. 566
E-mail: rtalcott@astronomy.com

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