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Star Spangled Banter

The Case of the Disappearing Rings

These are not rings you can smuggle in your purse or wallet. They are not the material for an Agatha Christy novel. They are famous, however, known the world over for more than four centuries, and everyone recognizes them. Yet somehow from time to time they disappear, and mysteriously reappear a short time later.

I am talking about the rings of Saturn, which are composed of huge aggregates of bodies ranging in size from a two-garage house right down to that of a smoke particle; each particle or hunk, as the case may be, is an individual satellite of Saturn; each travels high above Saturn’s equator; each rides in its own special orbit. What are these rings, and why do they pull the routine of disappearing and reappearing?

When first discovered by Galileo in 1610, they were not recognized as rings. His “spyglass” was not of sufficiently high quality to tell him about their structure. He could only see that this planet, unlike the others, did not appear round, but elongated, with protuberances on either side. At various times he described them as “ears,” “jug handles,” or as separate spheres (Saturn to his perception, appeared as three spheres, lined up like beads on a string, with the central bead larger than the outer two). He wrote his Medici patron about this “very strange wonder,” but possibly felt there was egg on his face a few years later, when they disappeared. Whether to save face or perhaps just as a lucky guess, he predicted they would be back, and surely enough, a few years later they reappeared.

It was not until after Galileo’s death that this mystery was explained by Christian Huyghens, who had better optics at his disposal. The protuberances were actually a system of rings, situated high above Saturn’s equator, and “nowhere touching.” He surmised that Saturn’s orbit was tilted with respect to ours, so that during half of Saturn’s 30-year orbit we view the rings from slightly above and the other half from below. When the ring plane crosses our orbit we view the rings edgewise, and being quite thin, they seem to disappear at such times. This will happen again on September 4, 2009.

To form some idea of the size of the rings, ponder this: A particle in the outer edge of the rings travels in an orbit almost 170,000 miles in diameter (that’s about 2/3 the distance from the earth to the moon), yet the rings are only about 300 feet thick. Yes, not miles, and not light years, but feet. In other words, their thickness is only about the size of a football field. Viewed from at least a billion miles away, it is no wonder that they are not seen at all!

To make a model that is truly scale, use onion-skin paper and cut out a ring large enough to include both San Francisco and New York. How can such an immense structure have any stability at all?

The answer is simple. Each particle in the rings behaves as a separate body, traveling in its separate orbit, so structural stresses in the entire ring system do not build up. Saturn’s equatorial diameter is about 12% larger than its polar diameter – the largest bulge of any planet in our solar system. The extra mass of this bulge produces a slightly stronger gravitational field above the equator that pulls any stray particles that wander north or south back into the most stable orbit, namely the one high above the bulge. Thus, each particle orbits high above Saturn’s mid-section, and as a corollary, stability is built into the system.

Something else happens during the ring plane crossings. At such time any satellites that happen to be in the right place become more easily visible. It is usually very difficult to see satellites that are relatively close to their mother planet because they are hidden by the rings. When the rings offer no competition it is the ideal time to find previously undiscovered bodies, which follow the same rules as the ring particles. They orbit above Saturn’s equator, but considerably farther from Saturn than the rings. Currently, NASA has assigned names to 60 Saturnian satellites, but new ones are still being discovered. In fact, NASA’s Cassini Imaging Team announced the discovery of moon #61 on March 3, 2009. And now we have placed a satellite of our own (named Cassini) into a polar orbit around Saturn, so it is possible to view the planet and its rings from above, and from other vantage points never before available to earthlings. We’ve come a long way since Galileo first noticed that Saturn is not round!

© April, 2009, by Nathan B. Miron, Ph.D.


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