To expand search, see Astronomy and Measurement. Laterally related topics: The Balance and the Measurement of Weight, The Measurement of Area and Volume, The Measurement of Distance, Leveling, and Angular Measure.
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Lorch, Richard. The sphera solida and related instruments. Special issue dedicated to Olaf Pedersen on his sixtieth birthday. Centaurus 24 (1980), 153--161. (Reviewer: K.-B. Gundlach.) SC: 01A99 (85-03), MR: 82a:01057.
The sphera solida or "solid sphere" is "essentially a globe, on which the stars and principal celestial circles are depicted, and a frame of horizon and meridian circles." Related instruments include the astrolabe, and particularly the spherical astrolabe. On the other hand, the sphera solida should not be confused with the armillary sphere. As an example how the sphera solida was used, the author explains that "To align the sphere with the Heavens in the daytime, and so obtain the configuratio celi, a pin is stuck into the degree of the sun in the ecliptic and the sphere is turned until the pin has no shadow. At night the same can be achieved by the less spectacular method of taking the altitude of a known star and shifting the sphere till the representation of the star has the same altitude--just as in a plane astrolabe." (p. 157) Much of the article focuses on the literary sources on the sphera solida, which are "at least as old as the fourteenth century." The author concludes that the ultimate source may be Arabic, and mentions a related Islamic globe made in 1279. "But unfortunately there is no clear Arabic exemplar for the text of the Sphera solida." This article has a rather scholarly tone, was doubtless difficult to research; it ends with the unusual note "Finit tractatus. Deo gratias." Closely related topics: Medieval Europe and The Islamic World.
North, J. D. Astrolabes and the hour-line ritual. J. Hist. Arabic Sci. 5 (1981), no. 1-2, 113--114. SC: 01A99, MR: 84h:01102.
The author examined 132 astrolabes in the Museum of the History of Science in Oxford, and concluded that they were of less value than one might expect for timekeeping: "Our of 132 astrolabes examined, 41 instruments have the unequal-hour lines, and yet only four could have been used in at best a rough and ready way to find unaided the unequal hour." Equally interesting, the author observes that "not a single medieval instrument has survived in a form which would suggest that the unequal-hour lines were used meaningfully." All this is in spite of the fact that the author observed that "it seems to be commonly believed that a standard part of the engraving of the back of an astrolabe is a set of hour-lines forming, as it were, a double horary quadrant." Closely related topic: The Reckoning of Time.