Ancient Astronomical Instruments

The oldest astronomical instrument is the gnomon, a vertical situle that casts its shadow on a horizontal plane. The altitude of the sun is determined by the ratio of the length of the shadow to the middle of the gnomon. Once the midday line (direction of the shortest shadow) had been determined, Bea observed the time of the true midday (highest position of the sun) and the culmination height of the sun on every sunny day. From the observations at the time of the two Solstitial one finds the aquatic height of the observation site (the arithmetic mean of both culmination heights) and the obliquity of the ecliptic (half the difference of both heights). (Such teachings were given by the chinese Emperor Chu-Kong around 1100 BC. The gnomons of the modern age were often installed in churches in order to gain a significant audience.) opening the image of which was observed on the floor or an opposite wall. Of such kind are that by Toscanelli in 1468 in the Dorn in Florence, by Danti in 1576 in the Church of St. Petronino in Bologna, by Cesaris and Reggio in 1786 in the Milan Dom gnomon, among others.
Making a small opening in the upper part of the shadow-casting rod, the image of which in the Se had to be considered instead of the extreme shadow limit that was uncertain as a result of the fall shadow, was already known to the Chinese around 500 B.C. known. But besides the gnomon, angle-measuring instruments were also employed, especially since the flourishing of astronomy in the school of Alexandria. The measurement is made either directly, as in our present instruments equipped with divided circles, or indirectly, in that the angles to be determined appear in triangles, the sixtons of which have known lengths from which the angles can be found by calculation. The latter instruments are the altern, consisting of several rulers forming a variable triangle, one side of which is scaled. The triquetrum and the Jacob’s staff belong here.

The triquetrunt (equatorial ruler, Ptolemaic rule) was already described by Ptolemy in the Almageste, but was still used by Copernicus.
Fig.1 shows the instrument of Copernicus, which later came into the possession of Tycho Brahe and was described and illustrated by him. The same consists of three rulers, which are equal in size form a triangle. One of the same legs, AB, is vertical, the other, AC, rotatable about the upper end point of the first, is provided with sights and is directed towards the star to be observed; on the third ruler BD, provided with a graduation, the length of the unequal side B C is measured, and thereby the angle at A, d. h. is the zenith distance of the stars.
A similar instrument is the geometrical square, which was used by the Arabs and later used in the West, notably by Purbach. It consisted of a square, usually depicted on a brass plate, whose sides were placed horizontally or vertically. A ruler provided with diopters moved around the upper corner of the square over the two opposite graduated sides of the square. If the ruler was directed towards a star, the reading of the division of the horizontal Belie (Latus rectus), at smaller heights that of the vertical Seito (Latus versus) yielded the height of the star.
From the middle of the 15th century, the Jacob’s staff (Baculus astronomicus, Gradstock, French arbalestrille, English cross-stall) came into general use for measuring angles, mainly due to Regiomontan. It consists of a long rod AB (Fig. 2)

on which a cross rod CD was attached in its center E so that it could be moved. Visors were attached in A, C, D , that one sees, for example, one of two stars in the direction of the other to AD, the angular distance between the two CADs was given by the equation Ā½ CAD=EC/AE . The rod AB had a division, by which the stretch AE was read, which, in conjunction with the known length of the crossbar, yielded the angular distance sought. In order to measure angles of different graphics, crossbars of different lengths could usually be used. Until the middle of the 18th century, the Jacob’s staff was the main instrument of the Sailors to determine time and latitudeĀ°.

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