Book excerpt: "Observing Visual Double Stars, " written by an astronomer who has discovered almost 2,000 of them, opens the way to amateur astronomers who wish to make a direct and real contribution to science through their avocation.Double or binary stars--pairs of stars that revolve around one another--were once thought to be rare, anomalies among the vast number of normal, isolated stars, like our sun. Now, however, it is believed that many if not "most" stars are mated in binary systems. The visual binaries are those whose component stars are rather distant from each other and require decades or even centuries to complete their orbits. Few professional astronomers devote their time to making the observations needed, over these extended periods, to determine the characteristics of even a small sample of these systems. Thus, if any sizable number of double stars are to be closely scrutinized, their periodic variations plotted, and their orbits and masses calculated, the host of amateur astronomers will have to come to the aid of the professionals by making patient, systematic, night-after-night, year-after-year recorded observations."Observing Visual Double Stars" is designed to train amateurs to become such lookouts. After a historical account of the discovery of binaries (from the sighting of the first in 1650, through the work of the Herschels and the Struves in the eighteenth and nineteenth centuries, to the present), the author describes the various classes of telescopes and other instruments and the relevant optical principles. This is followed by practical advice on how to use this apparatus to identify double stars and measure their variations over time.The heart of the book--and its technically most advanced section--presents the mathematical techniques that will allow the observer to calculate orbits and masses from the variables that have been measured. A chapter entitled "Voyage to the Country of Double Stars" describes a binary system as it might appear to an observer within it. The book also explains the use of star catalogues and presents its own catalogue of 744 double stars accessible to the amateur observer.