Tycho, driven by necessity, had observed altitudes of the new star with a sextant, and as the planets never could attain an altitude above 60°, he found a sextant a convenient instrument for many purposes, and specially mentions that it was easily taken asunder and transported wherever required. Though Tycho believed himself to be the inventor of this instrument, he had been anticipated by the Arabs, as Al Chogandi in 992 at Bagdad erected a sextant (which is even said to have been of sixty feet radius) for measuring the inclination of the ecliptic.[1]
The sextant was with Tycho Brache a favourite instrument, which he had already constructed for Paul Hainzel in 1569 for measuring the angular distances of stars. At Hveen he constructed three large sextants for this purpose. One of these, which was placed in the great northern observatory, and was made entirely of brass, was on the same plan as the Augsburg instrument, the arc being attached to the end of one arm, the two arms being placed at the proper angle by a screw, the eye of the observer being at the hinge on which the arms turned.[2] The second was placed in one of the crypts of Stjerneborg, and was a solid sextant of wood, covered with painted canvas, and a brass arc 512 feet radius, braced with stays and supported on a globe sheeted with copper, which enabled the two observers to place it in the plane through the two stars to be measured, while they steadied it in the position required by two long rods with pointed ends which rested on the floor. One of the observers sighted one star through a fixed sight at one end of the arc
- ↑ L. A. Sédillot, Mémoire, p. 204; Matériaux pour servir à l'hist. des sciences chez les Grecs et les Orientaux, i. p. 358. Prolégomènes (1847), p. xlii. Sarafedaula, who founded the Bagdad observatory, was not a Chaliph, as supposed by Bailly and Wolf, but Emir-ul-umara.
- ↑ Sextans chalybeus, used already in 1577; Mechanica, fol. E.; De mundi aeth. rec. phen., p. 460. The sextant at Cassel (constructed from Wittich's description) also required one observer only, who placed his eye at the centre of the arc.
