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Mercury, September/October 1996 Table of Contents

by James C. White II, Middle Tennessee State University

(c) 1996 Astronomical Society of the Pacific

Saturn's delicate rings have caught human eyes for over three centuries. This month, the bejeweled planet will be at its brightest of the year -- an ideal time to observe it.

This Month's Project: coming to know Saturn's rings

At observatory open houses, the most common statement I hear from people viewing the ringed planet through a telescope for the first time is, "Wow. That really is Saturn?!" It is as if we are all reluctant to believe that such an exotic planet could actually exist. Mighty Jupiter may be the king of planets, but only Saturn wears a crown.

In July 1610, the very first telescope observer, Galileo Galilei, got tired of watching Jupiter's moons and decided to look at Saturn -- the farthest known planet from the Sun. Until then, Saturn had been just a bright point of light in the sky. Through Galileo's telescope, it became a world. Yet it was unlike Jupiter and all the other worlds Galileo had seen. "The star of Saturn is not a single one," Galileo wrote, "but an arrangement of three that almost touch each other." The poor quality of the lenses in his telescope was distorting Saturn's image to such a degree that he couldn't recognize the planet as we do today. It was almost 50 years later that Christian Huygens realized that the three worlds Galileo had seen were actually one planet and its rings.

Orbiting 10 times farther from the Sun than Earth, Saturn is a frigid world 10 times larger in diameter than our puny planet. It is noticeably squashed. Because the planet rotates very quickly -- once every 10 hours -- its midriff bulges out. Planetary love- handles are nothing to snicker about, though. Just ask the Earth; our blue planet is a little paunchy, too. Beneath Saturn's relatively thin hydrogen atmosphere swirls a deep sea of molecular hydrogen. At bottom is a rocky core, itself larger than Earth.

Although Saturn is not the only ringed planet -- Jupiter, Uranus, and Neptune are also members of the club -- its adornments are the most extensive. Until the mid-19th century, people imagined them to be solid, but that idea was shattered when James Clerk Maxwell (of Maxwell's equations fame) showed that solid rings can't exist stably around a planet: The planet's gravity would destroy them. Now we know, thanks to the Voyager space probes, that Saturn's rings consist of thousands of "ringlets," each made of chunks of ice ranging in size from dust to boulders [see "Rally Around the Ring," March/April 1995, p. 10].

And do those chunks move! They zip around the planet at high speeds, occasionally crashing into each other. In the absence of the injection of new ring material -- from a wayward comet, say, or a crumbling moonlet -- these collisions will eventually wipe the rings out and leave Saturn naked. That doesn't sound too good, but it's not something you or I have to worry about. The rings have a knack of regenerating themselves -- and even if they don't, we'll leave a ringless Saturn to our remote descendants.

Observing Guidelines

The appearance of Saturn's rings to us ground-hugging Terrans is changing constantly. The rings are tipped nearly 30 degrees to our orbital plane, affording us an endless variety of perspectives as our own planet whips around the Sun.

This past February we saw the rings edge-on, and until 2009 we'll view their southern side. On Sept. 26, the ringed planet will reach opposition -- and its largest size for the year: 20 arcseconds. Even from urban areas it will be easy to spot amidst the dim stars of Pisces [see SkyChart, July/August, p. C-3].

Your mission, should you decide to accept it, is to familiarize yourself with Saturn and its rings. For this project, you will need a telescope with moderate to high magnification. A 8-inch scope with a 12-millimeter eyepiece would be ideal, but other combinations work well, too. After you locate Saturn, take some time to study it. Describe it as best you can:

• What colors are the rings?
• Are the colors in the rings the same as those in the planet's atmosphere?
• Is the color uniform? If not, how does it vary?
• Can you discern the Cassini Division, the dark area between the outer, dim A ring and inner, bright B ring?
• Can you detect ring shadows on Saturn?
• How wide are the rings compared to the diameter of the planet. Saturn's diameter is 120,000 kilometers, so how large are the rings? Amazing, huh?

We are happy to announce that we have arranged a special treat for you. We've been able to schedule a total lunar eclipse on Sept. 27 [see SkyTalk, July/August, p. C-4]. It'll be most exciting for those in North America and Europe. (We'll plan something else special for other parts of the world in upcoming columns.) During totality, which will last 70 minutes, Saturn will be only about 3 degrees from the dimmed Moon. What an incredible coincidence: Luna at opposition and Saturn within hours of opposition.

Incorporate your observations into a document with the following information: name of the project (such as, "Observing Saturn's Rings"), your name or the name of your group, details of the observing location, mailing address, telephone number, and email address, if available. We welcome reports from observers of all ages in all countries. We want to know where you were, when you were there, and what the weather and skies were like. This helps us as we select the Guest Observer, and it helps readers understand what you studied.

Please submit your completed report by Nov. 30, 1996 by email to jisles@voyager.net or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203-3630.

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. His astronomical research focuses on cataclysmic variables. White writes a monthly astronomy column carried by newspapers in Tennessee. His email address is jwhite@physics.mtsu.edu.