Die Entwicklung de Meridiankreises, 1700-1850: Genesis eines astronomischen Hauptinstrumentes unter Berucksichtigung des Wechselverhatnisses zwischen Astronomie, Astro-Technik und Technik (review)

In the middle of the nineteenth century, every self-respecting observatory had to have a meridian circle as its main instrumental facility. The choice of a principal instrument depended first on the research program of the astronomer in charge. Next to purely scientific motives, there were technological, traditional, and other, more irrational, influences that played a role. Thus it happened that the first meridian circle was constructed by Repsold in Germany in 1803, whereas the Royal Astronomer at Greenwich in 1812 still preferred a fixed mural circle. Only in 1850 did the Royal Observatory receive a proper meridian circle, the so-called Airy transit circle, from Troughton and Simms. More significantly, almost a whole century had passed before the prototype of the meridian circle, Römer's 1704 invention of the rota meridiana, received the recognition among astronomers that it deserved.

Why did it take so long? This question is the main subject of Die Entwicklung des Meridiankreises. Its author, Klaus-Dieter Herbst, has tried to find an answer by studying the dominant role of the mural quadrant in the eighteenth century in relation to several technical problems, such as improving the accuracy of the scale graduations of either quadrants or circles by the transition from hand graduation to the use of the dividing engine. Other technological improvements, such as the achromatic lens and the use of reading microscopes, also in one way or another contributed to the evolution of the early rota meridiana into the typical nineteenth-century meridian circle.

Herbst shows further that the need for an instrument such as the meridian circle arose only by the end of the eighteenth century. A crucial moment was William Herschel's lecture “On the Parallax of the Fixed Stars,” given in 1781 for the Royal Society. Herschel demanded better instruments for measuring extremely small angles with great accuracy. Interestingly, the response to this demand for more precision for stellar parallax measurements is actually embodied in another instrument, the Fraunhofer heliometer. This refractor was used successfully by Bessel at the Königsberg Observatory for his determination of the stellar parallax of 61 Cygni. The discussion of the need for better parallaxes would therefore seem unrelated to the introduction of the meridian circle. However, Herschel's lecture signals an important transition in the history of astronomy, namely, that from solar to stellar research. The structure of the universe as outlined by the stars, the dynamics behind their small proper motions, and their distribution in space would occupy the minds and the nights of most astronomers for the next century. The quest for the structure of the Milky Way required the type of observation that could best be obtained with the help of the meridian circle.

Against this background, it seems clear that technological innovations were not the only consideration in the choice between a meridian circle and, for instance, a fixed mural quadrant or mural circle. Astronomers were increasingly aware that a discrepancy existed between instrumentally feasible precision and that actually achieved, as shown by comparing measurements obtained by different observers. This discrepancy resulted in new conceptions about accuracy, a development that was certainly aided by the new ideas about statistics developed at the same time by mathematicians such as Gauss and Bessel. Bessel especially emphasized that considerable effort had to be made to control the so-called systematic errors involved in positional astronomy. Thus, instead of believing that it was possible to make instruments without constructional shortcomings (such as bending by their own weight), it was assumed that such unavoidable errors could be eliminated by observation (for instance, by comparing the data obtained in a direct and a reversed position of the instrument). The very notion of elimination of systematic errors by observational manipulation was decisive...