and Phoenicia, it is probable he might receive an account of this hypothesis from thence; but whether he did or not, we have now no means of knowing, neither is it of any importance. Certain it is, however, that this opinion did not prevail in his days, nor for many ages afterwards. 213. In the second century the Pythagorean hypothesis was superseded by a system erected by the famous geographer and astronomer, Claudius Ptolemæus. This system, which commonly goes by the name of the Ptolemaic, he seems not to have originally invented, but adopted as the prevailing one of that age; and he, perhaps, made it somewhat more consistent than it was before. He supposed the earth at rest in the centre of the universe. Round the earth, and the nearest to it of all the heavenly bodies, the moon performed its monthly revolutions. Next to the moon was placed the planet Mercury; then Venus; and above that the Sun, Mars, Jupiter, and Saturn, in their proper orbits; then the sphere of the fixed stars; above these, two spheres of what he called chrystalline heavens; above these was the primum mobile, which, by turning round once in twenty-four hours, by some unaccountable means or other, carried all the rest along with it. This primum mobile was encompassed by the empyrean heaven, which was of a cubic form, and the seat of angels and blessed spirits. Besides the motions of all the heavens round the earth once in twentyfour hours, each planet was supposed to have a particular motion of its own; the moon, for instance, once in a month, performed an additional revolution, the sun in a year, &c. See Plate VII. fig. 4. was invented by Tycho Brahe, who supposed that the earth was at rest, and that the moon and sun revolved about it; the moon in a month, and the sun in a year; and at the same time, that the rest of the planets, Mercury, Venus, Mars, Jupiter, and Saturn, revolved round the sun; the three last also encompassing the earth. Besides these motions, he supposed them all to have a diurnal motion round the earth, as well as all the stars, 217. The semi-Tychonic system supposed the planets to revolve round the sun, while the sun and moon revolve about the earth as their centre of motion; and it supposed the earth to move about its axis from west to east in twentyfour hours. This system differs from the Tychonic only in this, that it supposes a diurnal motion in the earth, but, like the Tychonic, denies an annual one. 218. The Cartesian system, so named from its author, Des Cartes, supposes a variety of vortices or whirlpools, in which the motions of the heavenly bodies are performed, being carried round the sun in a vortex of ethereal matter, in different times, proportioned to their distances; and each planet having also a particular vortex of its own, in which the motions of its satellites are performed. From the laws of motion it will easily appear, that the irregular motions of the planets cannot be accounted for by these vortices; and besides, the supposition of an ethereal matter to perform the operations, is without any foundation, or analogy in nature. SECT. II.-OF THE COPERNICAN, OR ThUE SYSTEM OF ASTRONOMY. 214. It is evident, that on this supposition, the complicated motions of the planets already 219. The Ptolemaic system had gained unidescribed could never be accounted for. Had versal credit, when Copernicus began to entertain they circulated uniformly round the earth, their doubts of its truth, and to try if a more satisapparent motion ought always to have been equal factory method of accounting for the apparent and uniform, without appearing either stationary motion of the celestial bodies, might not be obor retrograde in any part of their courses. In tained. He had recourse to every author upon consequence of this objection, Ptolemy was the subject, but obtained no satisfaction, till obliged to invent a great number of circles, he found from Cicero, that Nicetas, the Syinterfering with each other, which he called racusan, had maintained the motion of the earth; epicycles and eccentrics. These proved a ready and from Plutarch, that Pythagoras and others and effectual salvo for all the defects of his sys- of the ancients had been of the same opinion. tem; as whenever a planet was deviating from the course it ought on his plan to have followed, it was then only moving in an epicycle or an eccentric, and would in due time fall into its proper path. As to the natural causes, by which the planets were directed to move in these epicycles and eccentrics, it is no wonder that he found himself much at a loss, and was obliged to have recourse to divine power for an explanation, or, in other words, to own that his system was unintelligible. It, however, continued to be in vogue till the beginning of the sixteenth century, when it was superseded by the Copernican, of which afterwards. 215. The only other systems worth mentioning, besides the true system, are the Tychonic, the semi-Tychonic, and the Cartesian; all of which have gained proselytes, though none of them were ever so universally received as the Ptolemaic and Copernican. 216. The Tychonic system, late VII. fig. 7, 220. From these small hints, this great genius deduced a most complete system of astronomy, capable of solving every phenomenon in a satisfactory manner :-a system which has been more and more confirmed by the discoveries and improvements that have been made in astronomy and mathematics, since his time; as well as by the use of telescopes, which have discovered numerous celestial phenomena formerly quite unknown. Like all important discoveries, however, when they run counter to general prejudices, the Copernican system was at first much opposed; and by none more than the celebrated Tycho Brahe, who could never assent to the motion of the earth, and who invented the system described in the last section, with a view to supersede the necessity of it. 221. But while philosophers were divided between the Ptolemaic, the Tychonic, the Cartesian, and Copernican systems, Sir Isaac Newton laid down the laws of nature and motion, and, comparing all the phenomena in the heavens, discovered the true system of the universe, confirmed the Copernican system of astronomy, and demonstrated its truth by unanswerable arguments, drawn from the most obvious laws of nature. This system, which is founded on a basis not to be shaken, is as follows: 222. The sun, which to us is the fountain of light and heat, is an immense spherical body, which revolves on its own axis in about twentyfive days; and is the centre round which eleven other bodies, called planets, are known to revolve at different distances and in different periods. The planes in which the planets revolve all pass through the centre of the sun, and they are in general inclined to each other in very small angles. They are called primary planets, and some of them are attended by smaller ones, called satellites, which revolve round them in the same manner as they revolve round the sun. See plate III. 223. The sun and the planets are called the solar system. The orbits of the planets are not strictly circular, but elliptical or oval, and the sun is situated in a focus of the ellipse; so that the planets, at one period of their revolution, are nearer to the sun than at another. 224. Besides the periodical revolution round the sun, each of the planets has a uniform rotatory motion round an imaginary line, called the axis, passing through the centre; and, during the whole of any planet's revolution, its axis of rotation preserves the same parallel position. In consequence of this rotation, the different parts of the surfaces of the planets are presented to the sun in succession; but it has not been observed, that the axis round which any planet rotates, is perpendicular to the plane in which it revolves round the sun; therefore, at one period of the revolution, one extremity of the axis and the adjacent parts of the surface will be inclined towards the sun, and the other at the opposite period. 225. There is a class of bodies called comets, which also revolve round the sun, and appear to be governed in their motion by the same laws that regulate the motions of the planets. Their orbits are greatly elongated, and they come towards the sun from all quarters of the heavens, differing in this respect from the planets, which revolve pretty nearly in the sun's plane. They are further distinguished from the other stars, by a luminous stream of light which they emit when they come near the sun. 226. The earth, on which we live, is one of the planets; it revolves round the sun in a year, and performs its rotation on its axis, from west to east, once in a day. The moon is a satellite attending the earth, round which it revolves from west to east in about twenty-seven days eight hours. 227. The planets, in the order of their distance from the sun, are: Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel, Uranus, or the Georgium Sidus. Mercury, and Venus, which are nearer the sun than the earth, are called inferior planets; and those which are more distant are called superior planets, as Jupiter, Saturn, and Herschel. These fatter, indeed, are also by far the largest. Venus, Juno, Ceres, and Pallas, which are all nearly at the same distance from the sun, and all lately dis covered, are so small that they are generally called asteroids. Jupiter has four satellites; Saturn seven; and Herschel six: Saturn is besides surrounded with a thin, broad, and beautiful ring, perfectly detached from his body. When an inferior planet is between the earth and the sun, its dark side being turned towards the earth, it cannot of course be seen by us, except as a spot apparently passing over the surface of the sun; but it can only be so seen when it passes the sun in one of those points in which its orbit enters into the plane of the earth's orbit. These points are called the nodes of the planet's orbit. For the characteristic marks of the sun and planets, see plate VIII. fig. 8. 228. The fixed stars are at an immense distance; for it has not yet been determined, by the nicest observations, that they have any annual sensible parallax; that is, they appear to the earth, when on different sides of its orbit, to be exactly in the same places, the earth's orbit seen from a fixed star, appearing only as a point. Consequently, the fixed stars all shine with their own native light: for it would be impossible for light, transmitted from the sun, ever to render them visible, as it would be infinitely weak at so immense a distance. 229. The distance of the sun is immensely great, in comparison with that of the moon, although it is almost nothing with respect to that of the fixed stars. For the sun's diurnal parallax, that is, the apparent semidiameter of the earth seen from the sun, is so small, that no instruments could be so exactly made as to find it. Hence, it is inferred, that the sun's magnitude is vastly greater than the earth's. For, supposing the sun's parallax to amount to as much as a minute, then, since the apparent diameter of the sun is half a degree, this would make the sun's diameter fifteen times as big as the earth's; but the sun's parallax has been found not to exceed 87", which will make the sun's diameter 100 times as great as the earth's. That the sun is of a globular form, is plain from the apparent motion of the spots upon its surface; for while the sun moves uniformly about its axis, the spots in the middle of the disk move very quickly, and near the edges more slowly, agreeably to the motion of a globe about its axis. By observations on these spots, the sun is found to revolve about its axis in twenty-five or twenty-six days. 230. None of the celestial bodies in our planetary system shine with their own native light, except the sun; so that all the planets, both primary and secondary, are opaque bodies, that have no other light but what they receive from the sun, and reflect it back towards the earth and other planets. This is evident from the moon; for only that side of her is observed to shine which is directly opposed to the sun; but the other side, which is from the sun, is quite dark, except so far as it is illuminated by the reflection from the earth; for the more of the illuminated side that is turned towards the earth, the more we see her enlightened, the rest being dark; and the more of her dark side that is turned towards the earth, the more of her appears dark. Thus, at the full, she appears all enlightened, and at her change, all dark. |