The sun is the largest body yet known in the universe; its diameter being 887,693 English miles, its circumference 2,800,000, and its bulk about 1,400,000 times that of the earth. The quantity of matter compared with that of the earth has also been stated by La Place to be 337,422 times that of the earth. The ancients believed that the sun, and all the other celestial bodies, moved round the earth as their common centre; but since the days of Copernicus, this supposition has been daily losing ground, and has now completely given way to the more rational and wellestablished theory, taught by all the astronomers of the present day; namely, that the sun is the centre of the orbits of all those bodies which compose the solar system. It has, however, been discovered that the sun has a motion on its axis, which is completed in about 25 days 10 hours, as appears from the maculæ, or spots, which have been discovered on his disc by means of the telescope. Besides this motion round his axis, the sun has a small motion round the centre of gravity of the system; but if he has any motion in the immensity of space, it must be exceedingly small, as it has never yet been discovered. As for the apparent annual motion of this great luminary round the earth, it can easily be shown, that the real annual motion of the earth round the sun is quite sufficient to cause such an appearance. NATURE OF THE SUN. For many ages the sun was believed to be a globe of fire; but the majority of modern astronomers have rejected this opinion, and several of them have published very ingenious hypotheses on this curious subject. One of the most plausible and ingenious theories on this subject is given by Dr. Herschel in the Philosophical Transactions of the Royal Society. He supposes the sun has an atmosphere resembling that of the earth, and that this atmosphere consists of various elastic fluids, some of which exhibit a shining brilliancy, while others are merely transparent. Whenever the luminous fluid is removed, the body of the sun may be seen through those that are transparent. In like manner, an observer placed in the moon will see the solid body of the earth only in those places where the transparent fluids of our atmosphere will permit him. In others, the opaque vapours will reflect the sun's light, without permitting his solid body to be seen on the surface of our globe. In the same manner the Doctor illustrates the various appearances of spots in the sun, some of which have the following forms : Such appearances, he thinks, may be easily and satisfactorily explained, if it be allowed that the real solid body of the sun itself is seen on these occasions, though we seldom see more than its shining atmosphere. He apprehends that there are considerable inequalities in the surface of the sun, and that there may be elevations, not less than 500 or 600 miles in height; that a very high country, or chain of mountains, may oftener become visible by the removal of the obstructing fluid than the lower regions, on account of its not being so deeply covered by it. In the year 1779, the Doctor observed a spot on the sun large enough to be discerned by the naked eye; for it extended more than 50,000 miles. The following are sections of these, with their various formations of nuclei and umbrae: He also says, that he observed a fine large spot in 1783, which he followed up to the edge of the sun's limb; that he plainly perceived it to be depressed below the surface of the sun, and that it had very broad shelving sides. This appearance, he says, may be explained by a gentle and gradual removal of the shining fluid which permits us to see the globe of the sun. The Doctor also says, that on the 26th of August, 1792, he examined the sun with several powers, from 90 to 500, and that it evidently appeared that the black spots were the opaque ground, or body of the sun; and that the luminous part was an atmosphere, which, being interrupted, or broken, gave a transient glimpse of the sun himself. He farther adds, that with his seven feet reflector, which was in excellent perfection, he could see the spots, as on former occasions, with the same telescope, much depressed below the surface of the luminous part. On the 8th of September, 1792, he made a small speculum, which he brought to a perfect figure on hones, without polish; this had the effect of stifling a great part of the sun's rays; and on this account the object speculum admitted a greater aperture, which enabled him to see with more comfort and less danger. He then discovered that the surface of the sun was unequal, many parts of it being elevated, and others depressed: but this inequality was in the shining surface only; for he . thinks the real body of the sun can seldom be seen otherwise than in its black spots, which resemble the following: އއ As light is a transparent fluid, it may not be impossible that the sun's real surface may be now and then perceived; as the shape of the wick of a candle may sometimes be seen through its flame, or the contents of a furnace in the midst of the brightest glare of it. But this the Doctor thinks can only happen where the luminous matter of the sun is not very accumulated. From these appearances Dr. H. draws the following conclusions : that the sun has a very extensive atmosphere, which consists of various elastic fluids, that are more or less lucid and transparent, and that the lucid one is that which furnishes us with light; that the generation of this lucid fluid on the solar atmosphere is a phenomenon similar to the generation of clouds in our atmosphere, which are produced by the decomposition of its constituent elastic fluids; but with this difference, that the continual and very extensive decompositions of the elastic fluids of the sun are of a phosphoric nature, and attended with lucid appearances, by giving out light. To the objection that such decompositions, and consequent emissions of light, would exhaust the sun, Dr. H. replies, that, in the decomposition of phosphoric fluids, every other ingredient except light may return to the body of the sun; and, besides, the exceeding subtilty of light is such, that, in ages of time, its emanation from the sun cannot very sensibly lessen the size of so great a body. From the atmosphere, the Doctor next proceeds to state that the body of the sun is opaque, of great solidity, and its surface diversified with mountains and valleys; that the sun is nothing else but a large, eminent, lucid planet, evidently the first, or, strictly speaking, the only primary one of our system, all others being truly secondary to it. Its similarity to the other globes of the solar system, with regard to its solidity, its atmosphere, and its diversified surface, the rotation on its axis, and the fall of heavy bodies, lead to the supposition that it is inhabited, like the rest of the planets, by beings, whose organs are adapted to the peculiar circumstances of that vast globe. Should it be objected that the heat of the sun renders it unfit for a habitable world, Dr. H. answers, that heat is produced by the sun's rays only when they act on a calorific medium, and that they are the cause of the production of heat by uniting with the matter of fire which is contained in the substances that are heated. The Doctor also suggests other considerations intended to invalidate the objection; but these are too abstruse and extended to be given in this work. After the Doctor thinks he has shown that the heat of the sun is not so great as to prevent it from being inhabited, he then deduces from analogy a variety of arguments to confirm the notion of the sun being a habitable body; and then infers, that, if the sun be capable of accommodating inhabitants, the other stars, which are suns, may be appropriated to the same use; and thus, says he, we see at once what an extensive field for animation thus opens to our view. The late Dr. Wilson, Professor of Astronomy, Glasgow, supposes the spots of the sun are depressions, or excavations, rather than elevations, and that the dark nucleus of each spot is the opaque body of the sun seen through an opening in the luminous atmosphere with which he is surrounded. To this opinion of Dr. Wilson's, several celebrated astronomers have started objections; among others, M. Lalande, a French astronomer, contends that the spots are appearances arising from dark bodies like rocks, which, by an alternate flux and reflux of the liquid igneous matter of the sun, sometimes raise their heads above the general surface. That part of the opaque rock which at any time thus stands above, gives the appearance of the nucleus, while those parts that lie only a little under the igneous matter appear to us as the umbra which surrounds the dark nucleus. Respecting the nature of fire, and the heating power of the sun's rays, philosophers have been much divided in their opinions; but these opinions are both too numerous and too vague to be noticed in a work like the present. In one particular they all agree, namely, that the sun is either composed of, or surrounded by, some substance that has a very powerful effect in heating or raising the temperature of bodies exposed to the rays which proceed from it. Many experiments have been made both in this country and on the continent, to determine the intensity of those rays when concentrated in the focus of a lens, or by reflecting mirrors. Among the most powerful of these may be mentioned the mirror constructed by M. Villette, a French artist at Lyons, and publicly exhibited in England. This mirror was 47 inches in diameter, and its focal distance 38 inches. From the experiments of Mr. Harris and Dr. Desaguliers, it appears that it melted a silver sixpence in 7 seconds; a fossil shell was calcined in 7 seconds; a copper halfpenny melted in 20 seconds; iron ore melted in 24 seconds; a great fish's tooth melted in 32 seconds; tin melted in 3 seconds; and bone was calcined in 4 seconds. Sir I. Newton presented a burning glass to the Royal Society, consisting of 7 concave glasses, so placed that the focus of each united in one point. This glass vitrifies brick or tile in 1 second, and melts gold in 30 seconds. So powerful are the sun's rays when condensed, that it is said Archimedes set fire to the Roman fleet, at the siege of Syracuse, by a combination of burning glasses; and Buffon, in the year 1747, constructed a reflecting mirror of 168 plane glasses, moveable on hinges, with which he set wood on fire at the distance of 150 feet, and melted lead at 145 feet. The time which the light of the sun takes to arrive at the earth is eight minutes and thirteen seconds; but this will be more fully treated of in another part of this work. 20 OF THE PLANETS. The Sun revolving on his axis turns, CHATTERTON. Beside the Earth and Moon, ten of the stars have motions eastward peculiar to themselves. These are called Planets,* and are distinguished by particular names; which, taken in the order of their proximity to the sun, or of the celerity of their motions round that body, are, Mercury, Venus, Mars, Juno, Vesta, Ceres, Pallas, Jupiter, Saturn, and Uranus,+ or Herschel. The first two of these perform their revolution round the sun in less than a year; and as their orbits are included in that of the earth, they are called inferior planets. The rest require a longer period than a year to complete their revolutions round the sun; and as their orbits include that of the earth's, they are called superior planets. Five of the planets; viz. Mercury, Venus, Mars, Jupiter, and Saturn, are very conspicuous, and have been known from the earliest ages. The other five are visible only through the telescope, and have been very lately discovered: Uranus, by Dr. Herschel, in 1781; Ceres, by Piazzi, in 1801; Pallas, by Ölbers, in 1802; Juno, by Harding, in 1803; Vesta, by Olbers, in 1807. The planets have also particular characters, by which they are distinguished; these, in the order in which they have been enumerated, are ŏ ? ô ‡ ✪ ? £ 2 h H. The planets distinguished by these characters are termed primary planets, because they perform revolutions round the sun in their respective periodic times; but besides these there are a number of other small planets, that circulate round several of the primary ones, and on that account are called secondary planets, or satellites. * From a Greek word signifying to wander, because these bodies are continually changing their places. + Those planets that are nearest the sun move quickest in their orbits. |