Observatory, an institution supplied with instruments for the regular observation of natural phenomena, whether astronomical, meteorological, or magnetical. In some observatories all three classes of observation are carried on, but in most cases special attention is paid to astronomy alone, and only such meteorological observations are taken as are required for the calculation of the effect of atmospheric refraction on the position of a heavenly body; there are, however, several observatories which are devoted solely to meteorological or magnetical observations.
The most important work which is carried out in public astronomical observatories is the determination of the movements of the sun, moon, and planets among the stars; and, as a corollary to this, the relative positions of the stars to which the other heavenly bodies are referred. In early times the Greek astronomers fixed these positions by means of Armillary Spheres (q.v.) and Astrolabes (q.v.). Ptolemy made use of a Quadrant (q.v.); and many centuries after Tycho Brahe converted this form of instrument into an Altazimuth (q.v.), with which he made a long series of observations of the altitudes and azimuths of the heavenly bodies at the observatory which the king of Denmark erected for him; and he also measured with great assiduity their angular distances from each other by means of a Sextant (q.v.). It was not till the middle of the 18th century that the improvement of the clock by Graham enabled astronomers to rely on it for the determination of right ascensions by the times of passage across the meridian by means of a quadrant. A pair of such instruments pointing respectively north and south were erected at the Royal Observatory, Greenwich, and used by Bradley and his successors from 1750 till they were displaced by the Mural Circle (q.v.). At the same time the accuracy of readings was greatly increased by the invention of the micrometer-microscope, which made it possible to measure spaces to th of an inch. Neither the quadrant nor the mural circle, however, could be relied upon for accurate motion in the plane of the meridian, but Römer remedied this defect by inventing a separate instrument, the Transit (q.v.). With the transit and quadrant Bradley commenced that series of observations of the positions of the sun, moon, and planets, and of stars for reference, which has been continued ever since at Greenwich, and on which, in combination with less extensive series at Paris and Königsberg, all our tables of the motions of the heavenly bodies are founded. In modern observatories the transit and mural circle are combined into one instrument, the Transit-circle (q.v.). An important auxiliary to the transit-circle is the chronograph, an American invention, which, in various forms, is now found in all well-equipped observatories, the principle in all cases being the same—viz. the registration on a revolving cylinder of paper of the times of transit across the system of spider-lines of the transit-circle, as well as of the seconds of the sidereal clock, by means of electric currents, which pass through electro-magnets when the circuit is closed either by the observer or the clock, thus causing a momentary attraction of a piece of soft iron, and producing a corresponding mark on the paper either with a pen or a steel point. This system, while improving somewhat the accuracy of the individual observations, admits of a large number being made at intervals of two or three seconds, and leaves the observer free to make several observations of zenith distance during the passage of a star across the field of view. The observations of stars at the observatories of Greenwich, Paris, Washington, and Oxford are mainly directed to the most accurate determination of the places of a limited number, and the deduction of their proper motions by comparison with the results obtained by Bradley, Piazzi, and Groombridge; at other observatories differential or zone observations of large numbers of stars have been made, with the object of making a complete and tolerably accurate survey of the heavens.
A large number of observatories, chiefly in Germany and America, are devoted to a very different class of observations—viz. differential observations with the Equatorial (q.v.) of comets and small planets as referred to comparison-stars, and the search for such objects; whilst at other observatories, among which that of Pulkova may be mentioned, the measurement of double stars with the micrometer is laid down as the chief object. Of late years two new subjects have been introduced in the routine of observatory work—photography and spectroscopy. The former was carried on for many years at Kew Observatory under Mr De La Rue's auspices, and at his private observatory at Cranford, and the work is now being continued at many observatories, both public and private; the latter has been taken up at a number of Italian observatories, and particularly at Rome by P. Secchi, and it now forms part of the regular system at Greenwich; whilst the observatories at Paris, Berlin, and Vienna are equipped for these physical observations, and in America and Australia they are vigorously carried on at several observatories—Melbourne, in particular, being provided with a four-feet equatorial reflector for this purpose, as well as for the examination of nebulae. The most important work of an observatory, however, consists not in making observations which are easily multiplied, but in reducing and publishing them—a task of far greater labour, and requiring far higher qualifications. However various may be the observations, the mathematical method of eliminating their errors is the same in all cases; and it is when such treatment is required in any inquiry that it should be undertaken at a public observatory, where this rigorous method will be applied.
In addition to regular astronomical observations of all kinds, national observatories are usually charged with the distribution of time signals, and the rating of chronometers for the navy—matters of great practical importance, especially in Great Britain, where Greenwich time is communicated directly by telegraph to more than 600 towns.
There were observatories amongst ancient
Babylonians and Egyptians. Of modern observatories that at Cassel dates from 1561, Tycho Brahë's at Uranienburg from 1576; that of Greenwich (q.v.) was founded in 1675. The oldest in the United States is the Hopkins Observatory, Williams College, erected in 1836; now there are upwards of sixty, of which those of Washington and Harvard, and the Lick (q.v.) Observatory are the chief. The highest in the world is that on Mont Blanc, at a height of 14,470 feet.