Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/767

OBSERVATORY 709 of the heavens then taken were only made by individuals and at intervals, employing the simplest possible appara tuses. But, when philosophical speculation had exhausted its resources, and an accumulation of facts was found to be necessary before the knowledge of the construction of the universe could advance farther, the first observatory was founded at Alexandria, and continued in activity for about four hundred years, or until the middle or end of the 2d century of the Christian era. It was here that Hipparchus, the founder of modern astronomy, by repeat ing observations made by his predecessors, discovered the precession of the equinoxes, and investigated with consi derable success the motions of the sun, moon, and planets. His work was continued by more or less distinguished astronomers, until Ptolemy (in the 2d century A.D.) gave the astronomy of Alexandria its final development. When science again began to be cultivated after the dark ages which followed, we find several observatories founded by Arabian princes; first one at Baghdad (and possibly one at Damascus), built by the caliph Al-Mamun early in the 9th century, then one on the Mokattam near Cairo, built for Ibn Yunis by the caliph Hakim (about 1000 A.D.), where the Hakimite tables of the sun, moon, and planets were constructed. The Mongol khans followed the ex ample ; thus arose the splendid observatory at Meragha in the north-west of Persia, founded about 1260 A.D. by Hulagu Khan, where Nasir al-din Tusl constructed the Ilohkhanic tables ; and in the 1 5th century the observa tory at Samarkand was founded by Ulug Begh, and served not only in the construction of new planetary tables but also in the formation of a new catalogue of stars. With the commencement of scientific studies in Europe in the 15th century the necessity of astronomical ob servations became at once felt, as they afforded the only hope of improving the theory of the motions of the celestial bodies. Although astronomy was taught in all univer sities, the taking of observations was for two hundred years left to private individuals. The first observatory in Europe was erected at Nuremberg in 1472 by a wealthy citizen, Bernhard Walther, who for some years enjoyed the co-operation of the celebrated astronomer Regiomontanus. At this observatory, where the work was continued till the founder s death in 1504, many new methods of observing were invented, so that the revival of practical astronomy may be dated from its foundation. The two celebrated observatories of the 16th century, Tycho Brahe s on the Danish island of Huen (in activity from 1576 to 1597) and that of Landgrave William IV. at Cassel (1561-97), made a complete revolution in the art of observing. While the credit of having vastly improved the astronomical in struments perhaps should be divided equally between Tycho Brahe and the landgrave s astronomer Biirgi, the former may claim the honour of having been the first to see the necessity of carrying on for a number of years an extensive and carefully-planned series of observations with various instruments, worked by himself and a staff of assistants. In this respect his observatory (Uraniburgum) resembles our modern larger institutions more closely than do many observatories of much more recent date. The mighty impulse which Tycho Brahe gave to practical astro nomy at last installed this science at the universities, among which those of Leyden and Copenhagen were the first to found observatories. We still find a large private observa tory in the middle of the 17th century, that of Johannes Hevelius at Dantzic, but the foundation of the royal observatories at Paris and Greenwich and of numerous university observatories shows how rapidly the importance of observations had become recognized by governments and public bodies, and it is not until within the last hundred years that the development of various new branches of astronomy has enabled private observers to compete with public institutions. The instruments employed in observatories have of course changed considerably during the last two hundred years. When the first royal observatories were founded, the principal instruments were the mural quadrant for measuring meridian zenith distances of stars, and the sextant for measuring distances of stars inter se, with a view of determining their difference of right ascension by a simple calculation. These instruments were intro duced by Tycho Brahe, but were subsequently much im proved by the addition of telescopes and micrometers. When the law of gravitation was discovered it became necessary to test the correctness of the theoretical con clusions drawn from it as to the motions within the solar system, and this necessarily added to the importance of observations. By degrees, as theory progressed, it made greater demands for the accuracy of observations, and accordingly the instruments had to be improved. The transit instrument superseded the sextant and offered the advantage of furnishing the difference of right ascension directly ; the clocks and chronometers were greatly im proved ; and lastly astronomers began early in the 19th century to treat their instruments, not as faultless appara tuses but as imperfect ones, whose errors of construction had to be detected, studied, and taken into account before the results of observations could be used to test the theory. This century has also witnessed the combination of the transit instrument and the mural qiiadrant or circle in one instrument, the transit or meridian circle. While the necessity of following the sun, moon, and planets as regularly as possible increased the daily work of observatories, other branches of astronomy were opened and demanded other observations. Hitherto observations of the "fixed stars" had been supposed to be of little importance beyond fixing points of comparison for obser vations of the movable bodies. But when many of the fixed stars were found to be endowed with "proper motion," it became necessary to include them among the objects of constant attention, and in their turn the hitherto totally neglected telescopic stars had to be observed with precision, when they were required as comparison stars for comets or minor planets. Thus the field of work for meridian instru ments became very considerably enlarged. In addition to this, the increase of optical power of tele scopes revealed hitherto unknown objects double stars and nebulae and brought the study of the physical con stitution of the heavenly bodies within the range of obser vatory work. Researches connected with these matters were, however, for a number of years chiefly left to amateur observers, and it is only within the last fifty years that many public observatories have taken up this kind of work. The application of spectrum analysis, photo graphy, photometry, &c., in astronomy has still more increased the number and variety of observations to be made, so that it has now become necessary for most observatories to devote themselves to one or two special fields of work. It would be difficult to arrange the existing observatories into classes either according to the work pursued in them or their organization, as the work in many cases at different times has been directed to different objects, while the organization depends mostly on national and local circum stances. As already alluded to above, one of the principal characteristics of the larger observatories of the present day is the distribution of the work among a number of assistants under the general superintendence of a director. This applies principally to the great observatories, where the sun, moon, planets, and a limited number of fixed stars are without interruption being observed, but even among