Parallax

April 30, 2001
Measuring distances to stars and planets by parallax observation—that is, by noting "the apparent shift in an object's position when the object is viewed from different vantage points"—was based on a simple, accurate and archaic theory, known since Archimedes; however, putting the theory into successful practice was a 3,000-year exercise in frustration and ruthless competition for astronomers, generations of whom were driven to distraction as seemingly fixed, finite numbers shifted minutely with each technological advance. Archimedes, Galileo and Copernicus slowly completed the first familiar laps of the astronomic race. According to Hirshfeld, director of astronomy at University of Massachusetts, Dartmouth, the pace quickens by the 1800s, as lesser-known astronomers focus on near stars, and concludes dramatically with two German and one English observer neck and neck as they finish rough proofs on different stars within months of one another. Hirshfeld breathlessly annexes familiar astronomical legends ("Imagine yourself in Aristarchus's sandals"), and his social history, though somewhat thin, engages. For instance, a teenage Wilhelm Struve, forebear of modern astronomy, was kidnapped into Napoleon's army but escaped out a second-story window, freeing himself to pursue parallax. The book comes just as the cosmic map begins to emerge in three dimensions: totally reliable parallax measurements were achieved only recently with satellite observations (in 1989, fewer than 1,000 stars were accurately mapped; now the number is 22,000). Some day, interstellar travelers will remember the stars of Hirshfeld's book—Thomas Henderson, Friedrich Bessel and Struve—the way geographers honor John Harrison, the man who first determined longitude. Illus. not seen by PW.