The Astronomical Observatories of Jai Singh c6.s1.t1.47

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³ Figure 23 shows how these scales are constructed. There are generally two kinds of scales—one for a 7-unit gnomon and the other for a 12-unit gnomon. The semi-circle a c e is bisected at c, and ac and ce are bisected at b and d. The line bd, which is bisected at c¹, is the basis for both scales: the part bc¹ is divided into 12 equal divisions and c¹d into 7 equal divisions and these graduations are continued on their respective sides, as far as is convenient. Lines joining the points of division with the centre cut the arcs and the small central rectangle, and form on them the shadow scales. On most astrolabes the 12-unit scales are on a b c and e g, etc.; and the 7-unit scales on c e and i l. The Zarqālī astrolabe has, however, in place of the central rectangle, the smaller semi-circle, fj, graduated. Variations occur, e.g., the basis line (b c¹) may be placed vertically through b, and then graduated.