graduated to single minutes. The arms were four cubits, or about five and a half feet long,[1] three inches broad, and two inches thick; and to steady the instrument an undivided arc was attached to the arm which held the graduated arc, about eighteen inches from the centre, and passing loosely through a hole in the other arm, where it could be clamped by a small screw. This undivided arc and the long screw which served to separate the arms steadied the instrument, and kept its various parts in one plane. The graduated arc was not, as in his later instruments, subdivided by transversals, and the two sights were still of the usual kind, which he afterwards discarded, viz., two square metallic plates with a hole in the centre. The error of excentricity, caused by the unavoidable position of the observer's eye slightly behind the centre of the arc, was duly tabulated and taken into account.
With this instrument Tycho measured the distance of the star from the nine principal stars of Cassiopea. We can easily picture to ourselves the impatience with which he must have awaited the next clear night, in order to see whether this most unusual phenomenon would still appear, or whether the star should have vanished again as suddenly as it had revealed itself. But there the star was, and continued to be for about eighteen months, north of the three stars (now called β, α, γ Cassiopeæ) which form the preceding part of the well-known W of this constellation, and forming a parallelogram with them. It was only a degree and a half distant from a star (κ) of the 412 magnitude. Tycho continued, while the star was visible, to measure its distance from the other stars of Cassiopea; and in order to find whether it had any parallax, he repeated these measures from time to time during the night, and even left the
- ↑ One Tychonic cubit is=16.1 English inches (D'Arrest, Astr. Nachr., No. 1718, p. 219).
