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ASTRONOMY. ated at opposite sides of its orbit around the sun. In oilier words, we employ the diameter of the earth's orbit as a sort of base-line. And though this base-line is 18.5,000,000 miles long, so great are tlic stellar distances that no star has" yet been found for which this great orbit diameter subtends an angle greater than about one second of arc. (See Parallax.) With refer- ence to their brilliancy, the stars have been classified in groups, and the term magnitude has been used to designate brilliancy. Thus a star of the first magnittule means one whose light is so brilliant as to place it among, say, the twenty brightest stars. Of course, the num- ber of stars of eacli magnitude increases very rapidly as we come to the classes of lower bril- liancy. Strangely enough, the magnitudes of the stars are not constant. There is, for instance, one star in the constellation Argo that has diminished so much in brightness that it is at present no longer visible to the naked eye, though about the middle of the past century it was at least the equal of any star in the sky. To this class of variable stars belong also the 'temporary stars,' which blaze up now and then like a great conflagration, only again to sub- side into invisibility after a short time. The chemical composition of all these stars admits, like that of the sun, of being studied to a cer- tain extent by means of a spectroscope. It is found that the stars are probably composed of the same elements that go to make up the sun and the earth.

Frequently we find a couple of stars very close together on the sky. Indeed, often they are so near that the eye fails to distinguish them, and it requires a powerful telescope to separate their light. It was thought at first that these double stars (see Douhle Stars) were accidental mere!)', and that while the two stars had nearly the same direction in space, they might never- theless be separated by an immense linear dis- tance. But it lias been found that some such doubles are true binaries, showing an orbital revolution about the common centre of gravity of the two component stars. Sometimes such stellar Systems have three or more components. Even entire clusters may have a physical connec- tion within themselves, and it is not improbable that the forces of gravitational attraction com- pel and control the complex movements of such aggregations of stars, just as they do the lesser intricacies of our own solar system. A magnifi- cent field for speculation is opened by a consid- eration of the vast possibilities of such mighty stellar groups — true universes within the uni- verse.

NEBULE. In addition to the stellar systems, we have in the sky another interesting class of objects, the nebulte (q.v.). These appear in the telescope as hazy clouds of light, usually con- densing here and there into brighter nuclei. It is impossible to escape the conviction that these nebulae are gaseous, and that they furnish a sort of key to the life history of the ordinary stars. Indeed, spectroscopic evidence tends to show that many of the nebulae are composed of matter in the form of incandescent gas. What, then, can be more plausible than to see in the stars the result of a gradual process of cooling in nebulous matter? One can imagine readily that con- densation and contraction might result in the formation of a single central sun, or that there might be two or more such centres. Indeed, the sky actually does show some eases of 'double nebula'.' Recent mathematical researches have indicated that the tidal effects seen on a small scale in the case of the earth and moon would jilay a much more important ))art in double nebuls while still in the plastic condition. The mighty cosmic tides that would there be set up might undoubtedly produce perturbations great enough to account for the most complete changes in the character of motion within the system. An account of tliis study of the past and future history of cosmic s3-stems may be found in the article Ccsmogony.

Bibliography. The following books will be found useful in a more detailed study of the science of astronomy :

Grant, History of Phijsical Astronomu (London, 1852) ; Chauvenet, Manual of Spherical and Prac- tical Astronomy (Philadelphia, 1803) ; Young, The Sun (New York, 1881) ; Newcomb. Popular As- tronomy (New Y^ork, 1882) : Doolittle, Treatise on Practical Astronomy Applied to Geodesy andyavi- gation (New Y'ork, 1885) : Y'oung, Text-book of General Astronwny (New Y'ork, 1888) : Tisserand, Traits de mecaniqiie celeste. 4 vols. (Paris, 1889- 96) ; Kayser and Rnnge, Ueber die Spectren der Eleniente, 7 vols. (Berlin, 1888-93) ; Gierke, Sys- tems of Stars (New York, 1890) ; Oppolzer, Lehrbuch zxtr Bahnbestimmung (Leipzig, 1882) ; Scheiner, Die Spektral analyse der Gestirne (Leipzig, 1890) : Ball, Atlas of Astronomy (New York, 1893) ; Valentiner, Handirorterbuch der Astronomie (Karlsruhe, 1896) ; Berry, Short Tlistory of Astronomy (London, 1898) ; Darwin, The Tides, and Kindred Phenomena in the Solar System (Boston, 1898) ; Neweomb, Elements of Astronomy (New York, 1900); Neweomb, The Stars (New York, 1901) ; Gierke, Popular His- torxi of Astronomy in the Nineteenth Century (New York, 1893).

AS'TROPHEL. The name under which Sir Philip Sidney disguised his identity in writing the sonnets to "Stella," 1575-83. These poems appeared in 1591, and are considered the author's most remarkable achievement. Also the title of Spenser's elegy on Sidney.

AS'TRO-PHOTOG'RAPHY (Gk. oarpoi; as-iron, star + photography). In no department of astronomy has development in recent years been so marked as in the application of photographic processes of observation. If photography could accomplish nothing more than what it has done and is doing for descriptive astronomy alone, we should still rank it among the most powerful weapons in the armory of the astronomer. It is but necessary, for instance, to examine a series of eye and hand drawings of some total eclipse, executed by different skilled observers at the same time, to come at once to the conclusion that it is well-nigh impossible to obtain correct information in this way; even the outlines of the corona and prominences will sometimes differ so greatly that one would scarcely believe an attempt had been made to delineate the same objects. Photography, of course, gives us a far more faithful picture, and thus furnishes observational material of specially high value. But even more important, perhaps, are the services it is capable of rendering to the astronomy of precise measurement, and photographic processes