The Measurement of Starlight

Reviewed by: The Measurement of Starlight: Two Centuries of Astronomical Photometry * Jon Agar (bio) The Measurement of Starlight: Two Centuries of Astronomical Photometry. By J. B. Hearnshaw. New York: Cambridge University Press, 1996. Pp. xiv+511; illustrations, notes, index. $89.95. J. B. Hearnshaw wisely prefaces this book with a warning that neither science historians nor amateur astronomers are its primary audience. The [End Page 764] Measurement of Starlight is aimed at the practicing astronomer interested in the development of methods and instruments used to measure the magnitudes and colors of stars. Hearnshaw divides the history of photometry into four periods marked by “revolutions” in technology—visual, photographic, photomultiplier, and photometry with charge-coupled devices—but discusses only the first three in depth. The thesis that will interest readers of Technology and Culture is that “photometry . . . was tied almost slavishly to detector technology and instrumentation, to such an extent that the subject was practically started from scratch after each major revolution in detectors” (p. x). While the methods changed, the products of photometrists remained fairly stable over the two centuries: catalogs of star magnitudes and sequences of standard stars. The latter Hearnshaw accredits to William Herschel, “the first astronomer to produce reliable naked-eye estimates of stellar brightness,” along with rules of thumb to guide later observers. From the mid-nineteenth century, prizes encouraged technical innovations, such as Friedrich Zöllner’s “reliable and accurate” astrophotometer in which a star’s light could be compared to that from a standard kerosene flame. The spread of photometers clearly expanded the number of sites from which trusted data were produced, although the author does not discuss in detail the crucial question of how astronomers convinced each other of a particular photometer’s reliability and accuracy. During the first period several important catalogs, notably the Revised Harvard Photometry and the Potsdamer Durchmusterung, were amassed. The catalogs produced by visual methods demanded an immense commitment of effort and resources, as did photographic catalogs produced from the early twentieth century onward. Harvard College Observatory astronomer Edward Pickering, for example, made almost half a million photometric settings on thirty thousand stars between 1891 and 1898. International photographic catalogs such as the Astrographic Catalogue (begun in 1887 and only completed in 1962) and the Carte du Ciel (never completed) were among the most ambitious and resource-intensive scientific projects of the period. Hearnshaw is strong on the technical descriptions of these projects, somewhat at the expense of helping the reader understand why such scientific and cultural programs were deemed so important. The same general criticism stands for the later chapters, in which the author expertly describes how the catalogs built from new techniques and standardized stars, such as the North Polar Sequence, were established, used, criticized, and replaced. The Measurement of Starlight shines in its account of early-twentieth-century developments. Hearnshaw, for example, usefully stresses the technological underpinnings of the shift to astrophysics. The book, however, is less satisfactory outside this period. The earlier photometrists particularly suffer from the lack of contextualization, and the most recent history of photometry is only very briefly sketched. [End Page 765] While there is plenty of rich material here for the historian of astronomy, the unwillingness to ask broader questions can also be frustrating. I have already mentioned the question of why so many resources were devoted to the routinized collection of accurate star positions and magnitudes. More analysis of the development of the physiology of the observer would also have been welcome (although The Measurement of Starlight does contain fascinating material on the difficulties of color estimation), as would the subject of variable stars in general. Perhaps the biggest problem I have with the book is that it takes as axiomatic the possibility of assigning a fixed magnitude to a star, when it was the greatest achievement of the people, instruments, and institutions that Hearnshaw discusses to establish and sustain such artifacts. Many of these criticisms stem from the conflict between the book’s different audiences’ expectations. Although the historian may feel occasionally uncomfortable with the methodologically unfamiliar—quantified estimates of past “errors” in catalogs, for example—the book covers a previously thinly described but important subject area. Jon

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