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 ½ 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 latitude°.    In addition to these instruments with rectilinear divisions, ancient astronomers also used instruments with circular divisions. Soho instruments are the armillary spheres (s. d.), which are to be regarded as forerunners of the iquatoriala (s. d.) and may have been used by Timocharis and Aristyllos around 300 BC. BC were used to determine the position of the fixed stars at the equator. We know with greater certainty from Eratosthenes that around 220 BC. at Alexandria with armilla of great size. These instruments date back to the 17th century and have been in use and substantially perfected by Tycho Braise.

Fig. 3 shows an equatorial armillary sphere he made at his Uraniborg observatory on the island of Hven.

The circle BCDE, resting on a fixed base, carries the circle MESR, which is perpendicular to it, and the axis CAD, which is again perpendicular to the latter, around which the circle KLMN can rotate; all circles are provided, with the exception of the circle FGHI, which is firmly connected to M ES R and only serves as a handle. O Q P R are rear sights that can be moved on the KLMN and MESR circles. On the CAD axis, at the center point of all the circles, is the sight A. The instrument is placed in such a way that the circle BCDE falls in the plane of the meridian and the CD axis is parallel to the tide, which is done with the help of the BT plumb line; then the circle MESR will represent the equator and KLMN a declination circle. If one now turns the circle KLMN and the diopter so that a star appears in the line of sight OA, the arc OM is equal to the declinal ion and the arc E S equal to the hour angle of the star, which magnitudes are found by reading the circles. To determine the longitude and latitude of the stars, Hipparchus used an ecliptic (zodiacal) armillary sphere, which Ptolemies described as an astrolabe. The same arises from the above equatorial – annillary sphere if one places the circle BCDE in the color of the solstices, ME SR in the plane of the ecliptic. If one rotates the circle KLMN, which then represents a circle of latitude, and the diopter O so that a star appears in the sighting line OA, then the arc OM is equal to the latitude of the star and the arc ES is equal to the longitude increased by 90° of the star. Another instrument with circular graduations, which was used especially by seafarers, is the astrolabe planisphaerium, invented by Hipparchus, with which on the one hand high measurements could be taken, but on the other hand it was used for almost all tasks related to time determination as a result of skillful use allowed the stereo-graphic projection to be easily loosened.

Fig. 4 shows such an astrolabe made by Vincenzo Dante dei Rinaldi at the end of the 15th century, which has a diameter of 27.6 cm. There are four parts to be distinguished with this instrument: the mater, the actual planisphitrium, the rete and the dorsum. The first three parts form the deepened disc on the front side of the astrolabe, the dorsum on the back side. The mater is the deepened disc in which the planispharium is firmly inserted, the rete is placed over it and a rotating radius is placed on top of it

 The planispharium is a dark metal disk on which a projection of the main circles in the sky, parallel circles, almucantharates, vertical circles, is drawn for a certain polar height. Usually there are several planispheres for different poles in one astrolabe. The rete is a usually cut out and elaborately decorated disk, on which the ecliptic, the pole, as well as a number of honored stereo are indicated. The dorsum forms the back of the mater and contains a circular graduation over which moves the diopter ruler (alidade rule), with which, if you hold the instrument on the ring attached to the top with one hand free, can determine the height of the sun or the stars. In the free inner space of the dorsum there is usually also a Purbach-sehesgeometric square, in addition, the months and anniversaries are divided in a concentric ring in such a way that zero corresponds to the circular division of the vernal equinox, so that each can directly read off the length of the sun corresponding to the day. The use of the astrolabe is very varied. Do you have e.g. If, for example, a sun’s height is measured and the corresponding sun’s length is read off the dorsum, one looks for the latter on the ecliptic of the rete and, by turning the rete, brings the point obtained to the Almucantharat corresponding to the measured height, sets the rotating radius to the point found , the tip of the radius then indicates the solar time of the observation at the hour circle of the Mater. the material indicates the solar time of the observation. In a similar way the sidereal time can be determined. Among the instruments with circular division is the torquetum constructed by Regiomontane, which served to determine the longitude and latitude of the stars. A rotating circle was placed parallel to the aqua gate, while a second rotating circle was inclined about the squint of the ecliptic, representing the ecliptic, and a third circle (latitude circle) was placed vertically on top of it, with a diopter ruler moving around it. The use of the instrument was very similar to that of the armillary spheres. As already briefly mentioned, in order to fix exactly the direction in which a star was to be seen, the instrument was equipped with the usual ruler, which was used for sighting, and equipped at each end with a small pierced attachment (diopter) and looked through both openings, or a circle, which could be rotated within the divided circle, and placed on it at two diametrically opposed points. Instead of these sights, needles were sometimes used, e.g. by Regiomontan, the tips of which marked the sighting direction. For solar observations, a small cylinder was also placed in the center , which cast its shadow on the divided circle. The instruments described so far were essentially in use both among the Alexandrians and in the Middle Ages among the Arabs and in the West. The Arabs, however, took great care in the making and setting up of their instruments, and provided them with circles of considerable radius, on which the graduations were engraved on metal. Due to the difficulty of making larger full circles, quarter circles or quadrants were used early on, From the description of a quadrant of 5 Arabian cubits in radius on the observatory of Meragah (13th century), intended for the measurement of culminating heights and firmly erected on the east side of a vertical wall going from S. each N., one can see that the Arabs already had the value instruments that are firmly set up in the meridian, and that they are to be regarded as the actual inventors of the wall quadrant. Tycho Brahe in particular raised these wall quadrants to the highest level of perfection that could ever be achieved before the telescope was used.

Fig. 5, based on an illustration in Tychos Astronomiae instauratae mechanica (Wandsbek 1598), shows its large wall quadrant, the Quadrans muralis sive Tichonicus, the largest instrument of the Uranienburg observatory erected on the island of Hveen. B C is the brass-made, 5 inch wide quadrant of about 3 m radius, graduated from degree to degree, but readable to sixths of a minute by means of transversals. This quadrant is fixed to a wall so that the plane of its division exactly coincides with the meridian plane. Two indexed diopters D and E can be moved on the partition, and in the center of the quadrant is the fixed diopter A in a wall perpendicular to the quarant wall. Three people are required to operate the instrument: the assistant F, the actual Be -observer, move the eyepiece diopter E so worldly until the star to be observed appears in the direction EA and read off the culmination height of the star at the graduation, at the moment of the star’s passage it gives a signal, the time of which is the second Assistant H reads the dials I and K of the clock, the third assistant G finally enters the information given by both of them in the observation book. Tycho himself directs the observations. The difference in transit times of two. Stars gave their right ascension difference, while the elevation read at the quadrant, after subtracting the equator elevation of the observation site, gave the declination. The use of the clock to determine the differences in right ascension was attempted by Walther in Nurnberg towards the end of the 18th century, but without success because of the irregular rate of the clocks of the time; It was introduced to the art of astronomical observation by Landgrave Wilhelm IV of Hesse, who had good watches from his assistant Burgi at his disposal. The wall quadrants remained in use until the end of the 18th century, only instead of the rear sight they were fitted with a telescope that could be rotated around the centre. Furthermore, quadrants were used, which, being rotatable around a vertical column, allowed observations in any azimuth.

Fig. 6 shows such a large quadrant that Hevel set up at his observatory in Danzig. The setup is essentially similar to the Tychonic quadrant. Large octants and sextants were used to measure the angular distance between two stars. Per large octant from Hevel (Fig. 7) was set up on a vertical column and could be tilted by means of various chains with counterweights in such a way that the two stars fell into its plane. In the center of the octant, as well as at the zero point of the graduation, and on a ruler movable over the graduation, diopters were attached. Two observers are needed to carry out a measurement; one (Hevel’s wife) adjusted one star so that it appeared in the midpoint-zero diopter direction, while at the same time the other observer (Revel) moved the moveable ruler so that the other star appeared in the midpoint-ruler-direction. Diopter appeared. The reading of the position of the ruler on the division then directly gave the angular interpretation of the two stars.

This article was translated from a 1888 German Publication. Meyers Konversations