Advice and tools for skywatchers; The Practical Astronomer, by Colin A. Ronan. New York: Macmillan. 153 pp. $20 .; Whitney's Star Finder, by Charles A. Whitney. New York: Alfred A. Knopf. 102 pp. $8.95.; A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations, by P. Clay Sherrod with Thomas L. Koed. Englewood Cliffs, NJ: Prentice-Hall. 319 pp. $10.95.

These three useful books span the spectrum of skywatchers you may have on your Christmas list, ranging from those who like to look at the stars occasionally to the advanced, or would-be advanced, amateur.

Colin Ronan, editor of the British Astronomical Association Journal, has come up with an intriguing concept - help people understand what they see in the sky using simple experiments anyone can perform with homemade tools. It's the milk-carton, drinking-straw, scrap-lumber approach to astronomy. This can be a good deal of fun for everyone in a family. But it's especially well adapted to helping children grasp basic astronomical concepts.

In ''The Practical Astronomer,'' subjects range over all that is easily seen in the sky - including comets, meteors, and the moon - plus simple gravitational and optical physics. Star maps and a chart of the moon help you orient yourself. The charts, discussions, and experiments are too simplified for a serious amateur observer. For others, though, they would be quite useful.

''Whitney's Star Finder,'' on the other hand, should interest the gamut of skywatchers. What more can one say when a highly useful classic is revised, updated, and expanded than ''hurray,'' unless it has been spoiled in the process. That hasn't happened here, so ''hurray'' it is.

The Star Finder is a book plus a plastic tool for locating ''every prominent star in the sky at any point in the world,'' as the book cover puts it. Useful tables with times of eclipse events, phases of the moon, and the positions of planets and 30 prominent stars are good from 1982 through 1985. However, the main strength of this observing tool is the easy-to-use star finder wheel. It's a universal star chart that shows you the general appearance of the sky at any time and any season in either the Northern or Southern Hemispheres.

This star finder is easy for the casual star watcher to use, yet useful enough to aid even experienced observers.

For those who are or would like to become serious observers, P. Clay Sherrod has produced a valuable book in ''A Complete Manual of Amateur Astronomy.'' It is a condensation of experience gained in more than a decade of observing and covers all aspects from how to pick the right telescope to reporting astronomically useful observations of comets.

This is a detailed, technical book that has many practical hands-on tips. But there is one, in my opinion, serious deficiency. Little is said about the use of hand-held calculators or home computers in amateur astronomy. These now are becoming so ubiquitous and are so useful it is hard to understand how they could have been overlooked. However, with this caveat, the book can still be strongly recommended.

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