known that this planet was at that time west of the sun. Soon after sunset a splendid tail, 22° in length, revealed itself, and showed that a new comet had appeared. It was situated just above the head of Sagittarius, with the slightly curved tail pointing towards the horns of Capricornus, and it moved towards Pegasus, in which constellation it was last seen on the 26th January 1578. During the time it was visible Tycho observed it diligently, measuring with a radius and a sextant the distance of the head from various fixed stars, and occasionally also with a quadrant furnished with an azimuth circle (four feet in diameter), the altitude and azimuth of the comet. The sextant, which afterwards was placed in the large northern observing room at Uraniborg,[1] was constructed on the same principle as the one which Tycho had made at Augsburg in 1569, and was mounted on a convenient stand, which enabled the observer to place it in any plane he liked; the arms were about four feet long. The quadrant was about 32 inches in semi-diameter, and the arc was graduated both by the transversals nearly always employed by Tycho, and by the concentric circles on the plan proposed by Nunez; and on the back of the quadrant was a table, by means of which the readings of the latter could be converted into minutes without calculation.[2] When observing this comet, Tycho had not yet at his disposal as many instruments and observers as in after years, nor had he as yet perceived the necessity of accurate daily time determinations by observing altitudes of stars, but merely corrected his clocks by sunset.[3] The observa-
- ↑ Figured in De Mundi Æth. Rec. Phenom., p. 460, and Astr. Inst. Mech., ol. D. 6 verso.
- ↑ The quadrant is figured in De Mundi Æth. Rec. Phen., p. 463, and Mech., fol. A. 2.
- ↑ The orbit of the comet of 1577 was computed from Tycho's sextant observations by F. Woldstedt, "De Gradu Præcisionis Positionum Cometæ Anni 1577 a celeberrimo T. B. . . . determinatarum et de fide elementorum orbitæ," &c. Helsingsfors, 1844, 15 pp. 4to.
