FIXED STARS.
Various opinions have been entertained by astronomers respecting the tails of comets. Tycho Brahe and Ap pian imagined, that the tail was occasioned by the rays of the sun transmitted through the nucleus of the comet, Which they believed to be transparent like a lens. Kepler thought that it was the atmosphere of the comet driven behind it by the impulsion of the solar rays. Descartes ascribes the phenomenon to the refraction of the nu cleus. Sir Isaac Newton maintained, that the tail of a comet is a thin vapour ascending by means of the sun's aheat, as smoke does from the earth. Euler supposes that the tail is produced by the impulsion of the solar rays driving off the atmospheres of the comets, and that the curvature of the tail is the combined effect of this im pulsive force, and the gravitation of the atmospherical particles to the nucleus of the comet. Dr Hamilton sup poses them to be streams of electric matter.
We cannot stop to notice the wild and extravagant opinions which several astronomers have entertained re specting the nature and use of comets. To maintain that these bodies are employed to convey back to the planets the electric fluid which is perpetually dissipating, that they occasioned the great deluge which seems to have overrun our own globe, and that they are intended to supply the fuel wasted by the sun, is to give loose reins to licentious conjecture, without contributing to the pro IN order to distinguish the stars from one another, the ancients divided the heavens into different spaces, called Constellations, which they supposed to be occupied by the. figures of animals and other objects. The various names of these constellations, and the longitudes and lati tudes of the stars which they contain, will be found in the Catalogue at the end of this article. A star situated in that part of the heavens which is covered by the right foot of the constellation, called the Lion, is distinguished from other stars, by calling it a star in the right leg of the Lion. As there may be several stars, however, in the same place, astronomers have divided the stars that are visible by the naked eye into six magnitudes; the bright est being called, stars of the first magnitude ; the next brightest, stars of the second magnitude ; and so on with the rest. The stars of each constellation arc also dis
tinguished from one another, by prefixing the first let ter of the Greek alphabet to the name of the constella tion, for the brightest star, and the second letter for the second brightest. Thus the brightest star of the con stellation Leo, is called a ',COWS ; the second brightest, Leonis ; and the third brightest, y Leonis. The Greek letters, however, do not indicate the magnitude of the stars which they represent, but merely the relative magnitude of those in the same constellation. us a Virginis is a star of the first magnitude ; a Libra a star of the second magnitude ; and a Aguarii a star of the third magnitude. All the stars above the sixth magnitude, which cannot be seen by the naked eye, are called teles copic stars ; and those which lie in the spaces between each constellation, are called unformed stars.
It has been one of the principal labours of practical astronomers, to determine the position of all the stars in the heavens, for the purpose of detecting the changes in which they may be liable.
The various catalogues founded on new observations, the time in which they were published, the number of stars which they contain, and the works in which the catalogues may be found, will be seen in the following enumeration.
The catalogue which we have given at the end of this article, contains the greater part of the catalogues of Hevelius, Flamstead, La Caille, and Bradley. The longitudes and the latitudes given in the Table are the mean of the longitudes and latitudes of the stars ac cording to these four astronomers. The subsequent columns contain the differences, which when added to the mean longitude or latitude, gives the longitude or latitude according to any one of the astronomers. Thus the mean longitude and latitude of Capella, in the cata logue, are, The relative positions of the stars in their respective constellations have been represented on the surface of globes, or on maps called planispheres, and atlases. The best planispheres are those of Senex ; and the most correct celestial atlases are those of Flamstead, Fortin, and Bode, the last of which contains twenty sheets, each sheet being twenty-eight inches by twenty. See PLA- NISPHERE.