Update on the solar system; Orbiting the Sun, by Fred L. Whipple. Cambridge, Mass.: Harvard University Press. $20.; The New Solar System, edited by J. Kelly Beatty, Brian O'Leary, Andrew Chaikin. New York: Cambridge University Press.$19.95

The Voyager 2 spacecraft has closed in on Saturn. Its sweep past the ringed planet marks the last planetary survey by a US probe until mid-decade, when a Voyager should encounter Uranus. This is a good time to take stock of what a decade and a half of firsthand solar system exploration has produced.

These two timely books provide an excellent opportunity to do it. In fact, either one of them would probably provide most of what a general reader would want to know on the subject. They focus on the knowledge gained, not on the technology of space exploration. Although they cannot take account of Voyager 2 's Saturn encounter, they do contain information, and some striking pictures, gathered by Voyager 1, when it went by Saturn last fall. Beyond this, the two volumes are distinctly different.

Veteran astronomer Fred Whipple has done more than update his earlier book "Earth, Moon, and Planets." He has integrated a vast amount of new knowledge into his text to produce what amounts to a new book. It is part of the long-playing series of Harvard books on Astronomy that have been available for several decades, periodically updated or rewritten as necessary. Other volumes cover such things as stars, galaxies, or the Milky Way. They have always been good value for the general reader, and this volume is no exception.

Whipple is telling the story of the planets and their satellites, not just reviewing scientific findings. He traces both the evolution of the planetary system itself and of humanity's evolving understanding of that system. This helps you relate what the space probes show to what you can see in the sky with the unaided eye or through small telescopes.

The book is well, but not lavishly, illustrated. There are only four pages of color plates, which show close-ups of Jupiter and some of its moons. One wishes there could have been more of those magnificent color views sent back from space. But that would have put the price up, and the book aims to be informative, not merely entertaining. Indeed, Whipple tries, among other things , to provide readers with a "basic foundation in astronomy, through the simple presentation of scientific principles." In this, he succeeds. And one of the valuable aspects of the book is the set of simple charts and tables with which you can find the planets in the sky. These are good for a decade -- 1981 through 1992 -- which gives the book a reasonable shelf life.

"The New Solar System" is quite a different kind of volume.Well turned out with an abundance color illustrations, it is a report on space-age planetary exploration by a number of the scientists directly involved. Their respective essays cover the entire solar system -- including comets, meteorites and asteroids, the interplanetary medium, and the sun itself. This gives it broader scope than Whipple's more tightly focused book.

Also, unlike the Whipple book, this is not a tutorial. It doesn't aim to give readers a basic grounding in astronomy along with an overview of planetary science.It is a detailed discussion of the latest information by the experts directly involved. While it is written for general readers, it is pitched at the level of, say, Scientific American magazine and, within the limits of its subject, is encyclopedic in scope.

This book, too, is good value for the money. However, its very timeliness, which highlights the latest speculations and perplexities of the researchers, also gives it a certain ephemeral quality. A few years from now, after a scientists have digested their findings a bit more thoroughly, the volume will still be a magnificent memento and still have some value as a reference. But it also will likely have an outdated flavor. Whipple's book, which puts more emphasis on giving readers a grasp of basic knowledge and underlying principles, is less likely to suffer this way.

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