ASTRONOMICAL INSTRUMENTS On errors with F Measuring instruments A “Since in the manufacture of measuring instruments it is not possible to achieve the desired precision, be it with the evenness of the surfaces or with the marking of the divisions or holes at the right place, it is but natural that errors occur, as with the adjustment of the instruments. In almost every construction inaccuracies exist, whether visible or hidden. If the instrument is made of wood, then it will warp, particularly when it stands at a place exposed to Sun and humidity. The errors are larger or smaller, according to the theoretical knowledge, craftsmanship and experi- ence. Added to this, there is the expertise of the observer in setting up and measur- ing, the precision of the adjusting apparatus and much more. Whosoever believes that anybody can execute measurements on order and without previous practice, and that each measuring instrument delivers correct results, is in error. Whosoever wishes to achieve this must, first of all, spend a long time on the study of the instru- ments and on the practice in measuring, until finally his measurement rests on the knowledge of the precision of his instrument and on his experience in measuring.” Ibn Yūnus1 (d. 399/1009); translation after Eilhard Wiedemann2. 1 Kitāb az-Zī™ al-kabīr al-ºākimī, extracts ed. and translated into French by A.-P. Caussin de Perceval in: Notices et Extraits des manuscrits de la Bibliothèque nationale et autres bibliothèques (Paris) 7, 12/1803-04/16-20, here pp. 80/81-82/83 (reprint in: Islamic Mathematics and Astronomy, vol. 24, Frankfurt 1997, pp. 54-278, ici pp. 118/119-120/121). 2 Zur islamischen Astronomie, op. cit. p. 122 (reprint in: Gesammelte Schriften, vol. 2, p. 906 ASTROLABES 79 The astrolabe THE ASTROLABE, the most wide−spread and most pop− through a type of Cardanic suspension. One of its ular instrument in the history of astronomy, reached main parts is a stationary disc on which the horizon the Arabic−Islamic world from Persian, Syrian and is projected with its parallel circles and vertical other centres of science in the eastern Mediterrane− circles (muqanflara and azimuthal circles) from a an area, where Greek sciences were cultivated prior point, mostly from one of the celestial poles. The to Islam and in early Islamic times. In its simplest horizontal line divides the disc into two parts, into form it was already known to the Greeks presum− an upper part with the projections of the muqanflara ably already in the 2nd, perhaps even already in the and azimuthal circles, which corresponds to the half 4th c. B.C. The names Hipparchus (2nd c. B.C.), of the celestial sphere above the earth, and into a Apollonius (2nd c. B.C.), or Eudoxus (4th c. B.C.) lower part that corresponds to the half of the ce− are linked with its invention. In any case, it is lestial sphere beneath the earth. On this lower part mentioned by Ptolemy in his tract on the projection several arcs are drawn from the centre of the disc of the spherical surface on the plane surface. The up to the rim; these are designated as hour lines. It astrolabe also seems to have passed through a cer− has to be kept in mind that the counting of the hours tain process of development in Late Antiquity. The begins with sunrise, according to ancient customs. science historian Ibn an−Nad¬m (4th/10th c.) knew The other main part of the instrument is a movable a work by Theon of Alexandria (4th c. A.D.) on the disc which, however, is not solid but an open−work use of the astrolabe (Kit®b al−‘Amal bi−l−asflurl®b).2 piece. On it are seen the projection of the eclip− This seems to be identical with a book which was tic (of the zodiac) which, corresponding with the translated in the 2nd/8th c. under the title Kit®b number of the signs of the zodiac, is divided into 12 f¬ †®t a◊−◊af®’iΩ wa−hiya l−asflurl®b as a work by parts; these are further subdivided into 30 degrees. Ptolemy and which was described as such in detail There are also the projections of a number of the by the historian al−Ya‘q‚b¬ (3rd/9th c.).3 largest and most well−known fixed stars.” The astrolabe itself must have been known in the “The movable disc, called spider or net [‘ankab‚t or Arabic−Islamic area, if not already in the 1st/7th ·abaka], can be rotated around an axis at its centre century, then in the first half of the 2nd/8th century. upon the stationary disc. By rotating the spider, The Arabic titles of books known to us, the extant the daily rotation of the heavenly bodies at a given fragments and books convey the impression that the local horizon can be simulated. If the spider is set books on astrolabes which originated in the Islamic up in a particular position, it is possible to read world in the 2nd/8th and 3rd/9th century contrib− off the altitude above the horizon and the azimuth uted to the development of a constantly improving directly on the disc which is under the spider, for literature on applied astronomy. The theoretical each of the stars and signs of the zodiac represented element preserved in this literature shows that we on the spider, for the Sun and, in a certain sense, can place the beginning of the creative period of the the planets included, and can read off the hours, Arabic−Islamic culture in the history of the astro− which have elapsed since sunrise or sunset, from labe in the first half of the 3rd/9th century. the intersection with the hour lines of the point of “The astrolabe is a portable instrument which the zodiac sign where the Sun is situated just then, adjusts itself into an exactly vertical position or of the point in the zodiac diametrically opposite to the Sun …” “The astrolabe makes it possible to directly deter− v. Josef Frank, Zur Geschichte des Astrolabs, Erlangen mine the stars in the following main positions. It 1920 (reprint in: Islamic Mathematics and Astronomy, vol. 35, is only necessary to see which heavenly body lies, Frankfurt 1998, pp. 1–33), p. 6. 2 v. F. Sezgin, Geschichte des arabischen Schrifttums, vol. 6, with a [80] particular position of the spider, on p. 102. the eastern or western part of the horizon, on the 3 v. ibid., vol. 5, p. 173, 180. 80 ASTRONOMY upper or lower part of the meridian line, which is The signs of the advanced development which the the vertical diameter of the disc. In order to be able astrolabe underwent in the Arabic−Islamic period to situate the spider at a position corresponding include its numerous variants. The types known with the given position of the celestial sphere, it until the turn of the 4th/10th to the 5th/11th century is necessary to know one of the above−mentioned are described by Abu r−RaiΩ®n al−B¬r‚n¬ in his book astronomical data, be it e.g. the altitude of a star Ist¬‘®b al−wu™‚h al−mumkina6 in which he leans or of the Sun above the horizon, be it the hour that heavily on a book by his teacher Ab‚ Sa‘¬d AΩmad has elapsed since sunrise. By rotating the spider, b. MuΩammad as−Si™z¬7 (2nd half 4th/10th c.). the star is placed on the muqanflara in accordance From the studies on the various types of astrolabes with its altitude, or at night by giving the hour, and done so far, it is evident that there is a connection that is to say the hour of the night, the position of between their origin and the concept of the mixed the Sun in the zodiac, and with the time of the day, astrolabes (miz®™ al−asflurl®b). This has to do with the point situated diametrically opposite the Sun’s the combination of the features of the northern and position upon the respective hour line. The spider the southern astrolabe in a single one. As early as then shows the desired position. Besides these few in the first half of the 3rd/9th century, the Arabs problems mentioned, quite a number of other astro− were—in the words of J. Frank8—”not content with nomical and astrological problems can be solved the form adopted from their predecessors where the mechanically with the astrolabe, almost without part of the celestial sphere to the north of the tropic calculation”. of Capricorn is projected upon a plane parallel to The enormous development in professional, techni− the celestial equator or upon itself from the south cal, artistic and literary respects which this chief pole. They also attempted stereographic projec− instrument of Arabic−Islamic astronomy underwent tion from the north pole of that part of the celestial through the centuries has been dealt with more sphere which lay south of the tropic of Cancer and comprehensively by modern research than most called an astrolabe thus produced the southern other topics of Islamic history of science. astrolabe, as distinct from the northern astrolabe. The common astrolabe or planispheric astrolabe, When exactly the southern one originated cannot Arabic asflurl®b musaflflaΩ or asflurl®b saflΩ¬, has one be ascertained any more, but in any case before to nine discs (◊af¬Ωa, pl. ◊af®’iΩ), which are valid Far∫ânî, who also provides the theory for this astro− for the degrees of latitude of those places whose labe.” horizontal coordinates are engraved.