compasses was all he had to begin with; by holding the centre close to the eye, and pointing the legs to two stars or a planet and a star, he was able to find their angular distance by afterwards applying the compasses to a circle drawn on paper and divided into degrees and half degrees. His first recorded observation was made on the 17th August 1563,[1] and on the 24th of August in the morning he noted that Saturn and Jupiter were so close together that the interval between them was scarcely visible.[2] The Alphonsine tables turned out to be a whole month in error, while the Prutenic ones were only a few days wrong as to the moment of nearest approach. Tycho continued his observations, partly with the above-mentioned tool, partly using eye-estimations as to which stars formed with a planet a rectangular triangle, or which stars were in a right line with it. But in the following year he provided himself with a "radius," or "cross-staff," as it used to be called in English, one of the few instruments employed by the intrepid navigators who discovered the new worlds beyond the ocean.[3] It consisted of a light graduated rod about three feet long and another rod of about half that length, also graduated, which at the centre could slide along the longer
- ↑ Die 17, H. 13, M. 15, Erat ♂ in 7 Gr. 8 lat. Mer. 3 Gr. ad fixas.
- ↑ "Intervallum ♄ et ♃ matutino tempore vix observatione oculari notari potuit: in hac nocte enim uterque se invicem obumbrabat suis radiis sed latitudo ipsorum diversa adhuc erat, ♄ enim meridionalior ipso ♃ erat. Die 27 (astron. 26) Mane vidi ♃ cum ♄ obtinere eandem alt. ab horizonte, hinc licet conjicere eorum ☌ jam præteriisse sed propius erant ab invicem dispositi quam ante triduum: quare etiam tempus συζυγίας propinquius huic 27 Aug. quam priori 24 fuisse manifestum est. In utroque autem die paulo ante ortum ʘ distantiam ipsorum observavi."—MS. volume of observations, 1563-1581 incl., in the Royal Library at Copenhagen (Gamle Kongelige Samlinger, 4to, No. 1824). The early observations (up to 1577) only exist in this copy, the originals would seem to be lost, at least they are not at Copenhagen.
- ↑ In French called arbalète or arbalestrille, in Spanish ballestilla, in German Jacobsstab. It seems to have been invented by Regiomontanus, and is described in his Problemata XVI. de Cometæ Longitudine, written in 1472. and printed in 1531 in Schoner's Descriptio Cometæ.
