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No. 8: Fall 1979

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Rings of uranus: invisible and impossible?

Now that they have discovered nine rings around Uranus, astronomers are having trouble explaining them. First, if they are made up of small chunks of matter, the laws of celestial mechanics dictate that they should quickly spread out radially into much wider rings in just a decade or two. In other words, if the rings are ancient they should not have maintained their present form. Second, the rings are invisible when one would expect them to be bright like Saturn's. Yet, they reflect less light than the blackest coal dust. T.C. Van Flandern proposes that each ring is actually a single satellite, so small that we cannot see it, and that it sheds gases as it orbits. This small solid body would make the celestial mechanics people happy, and the gases would be invisible to the eye but still absorb light, making the ring of gases detectable when Uranus occults a star.

(Van Flandern, Thomas C.; "Rings of Uranus: Invisible and Impossible?" Science, 204:1076, 1979.)

Comment. An alternative explanation is that the rings are recently acquired and will soon disappear. An 1847 observation of a ring around Uranus exists, but a datum this old carries little weight. See our Catalog: The Moon and the Planets for this old sighting. This book is described here.

From Science Frontiers #8, Fall 1979. 1979-2000 William R. Corliss