Contemplative soul to view the starry Realms above, and commune with its God. How wond'rous are thy works, Thou Great First Cause, More wonderous far to sight seraphic, Who sees ten thousand worlds unknown to man. (With mind eclipsed by vanity, conceit, But 'tis not thus with astronomic sage, From Heaven above, the Christian wakes, In realms of light, to everlasting joy. Rev. W. Munsey. COMETARY ASTRONOMY. Motions of Comets. The motions of comets through the system may be divided into apparent and true, being compounded of the earth's motion, and the motion of the comet, each in its respective orbit. When the two bodies are proceeding in different directions, the apparent motion of the comet will exceed its true motion; when moving the same way, its apparent motion will be less than its absolute motion. The apparent velocity has, in some instances, been very great, the comet of 1770 described an arc of 50° in 24 hours; the apparent swiftness will also be augmented in proportion to the proximity of a comet to the earth, for the arc through. which it moves, in any given time, like all other objects, appears larger the nearer it is to the eye. With respect to the true motions of comets, it may be said, in general, that when at their perihelion, or nearest the sun, they move swiftest, and when at their aphelion, or farthest from the sun, they move slowest. The velocity of some, when at their perihelion, is almost inconceivably great: the comet of 1680, which approached within a very short space of the solar orb, so very near, indeed, that it may be said that its place of perihelion was in the globe of the sun,-this comet went half round the sun in ten hours! A striking illustration is afforded of the inequality of the motions of cometary bodies, when at these opposite points of their orbits, by the comet of 1811: the period of which, as computed by Lemaur, is 4237 years; the time taken up in traversing the half of its ellipse nearer the sun is only 775 years; the other 3462 years of its revolution being employed in traversing the most remote half. The velocity of the comet of 1680 has been referred to its middle distance from the sun is more than 5000 millions of miles, and its greatest distance may be stated as twice as much, (for its nearest is only a 20,000th part of its most remote distance,) so that in its whole revolution it is subject to the greatest extremes. It sees the sun's orb as a vast globe filling the heavens, and, in a lapse of 287 years, it beholds it as dwindled to a point! It bathes its glowing globe in the full effulgence of the solar light, and gradually retreats, till that light almost sinks into the subdued brilliancy of sur rounding suns! It whirls round the sun with tremendous rapidity, and, by degrees, lags in its pace, and, comparatively, creeps in its path! The centrifugal and the centripetal forces are, at one period of the revolution, so intense, that it may be conceived to be possible that one principle would be sacrificed to the other, and the comet either rush to the sun, or fly off never to return! At the further extremity of its long travel, these forces are so exhausted, that the wanderer glides slowly along its course with dimmed splendor, destitute of its brilliant train, and seems abandoned, in the vast fields of ether, to the random attraction of any neighbouring sun, yet these forces are so adjusted, (though a cannon ball would not reach the aphelion of the comet of 1680 in less than 2260 years,) that the comet can neither rush to the sun when near him, nor leave him when most remote, but is bound to his glorious chariot, and reined in ultimately, to return in its appointed period of time. To ascertain the orbit of a comet is the most difficult problem in astronomy; chiefly because no comet is visible through the whole of its course, and rarely seen in those points of its orbit, which enable us, as with the planets, to determine, with accuracy, the figure of the path described. Comets are seldom seen either in opposition, conjunction, or the plane of the ecliptic, which latter being the place of the node, is the most important particular for calculating the elements. According to the relation which the centrifugal and centripetal forces bear to each other, will be the nature of the path of a moving body, a circle, an ellipse, a parabola, or a hyperbola; if the velocity of the earth were about 101,000 feet per second, the orbit would be circular; its actual velocity, when nearest the sun, is 102,300 feet per second, by reason of which, it is constrained to move in an ellipse; were the projectile force increased to 144,700 feet, the earth's path would be parabolic, and should this intensity be at all augmented, the orbit would be hyperbolic. There can be but little doubt that an ellipse is the figure of the paths of comets as well as of those of planets; discoveries of recent date confirm the supposition; we know of no heavenly body that moves in a perfect circle, and if the motion were in a parabolic or hyperbolic curve, when once the comet had passed its perihelion, it would continue to recede from the sun, and never return again,-this is evident from the nature of these sections of a cone; but elliptic orbits return into themselves, and the re-appearance of the Encke comet, in the very places predicted, is a proof that this is the curve in which these bodies move. It is usual, however, with astronomers, to calculate the orbit of a comet on the parabolic scheme, which very well represents the motions of a comet, during the time it is visible, though it leaves uncertain the length of the major axis of the ellipse, and, consequently, the period it occupies in revolving round the sun. The elements of the orbit of a comet are six : 1st. The perihelion distance. 2nd. The situation of the perihelion. 3rd. The place of the nodes. 4th. The inclination of the orbit to the ecliptic. 5th. The time of the perihelion passage. And 6th. The period of the comet's revolution. Respecting the last particular-the periodic time of a comet—an error, of only a few seconds, will cause a difference of many hundred years. Some astronomers, in calculating the orbit of the great comet of 1680, have found the length of its greater axis 426 times the earth's distance from the sun, and, consequently, its period 8792 years; whilst others estimate the greater axis 430 times the earth's distance, which alters the period to 8916 years. The celebrated astronomers, Newton and Halley, judged that this comet completed a revolution in 575 years. A similar disagreement exists respecting the comet of 1811, as determined by different astronomers, its period was, according to Calandrelli, 3056 years; Bessel, 3383 years; Lemaur, 4237 years; Ferrer, 3757 years. Beneath thine all-directing nod, Both worlds and worms are equal, God! And spreadst yon cowslip's glittering cup. |