Although all the planets gravitate to the sun, yet the centre of the sun is not the centre of gravity of the whole solar system. The centre of the sun is, however, never distant from that point so much as his own diameter, consequently the centre of gravity of the whole system is always within the body of the sun. But as this point and the centre of the sun do not coincide exactly, and as the gravitation of the planets to the sun must be accompanied by the gravitation of the sun to the planets, from the quality of action and re-action, it follows that the sun must have a motion in a small orbit round the centre of gravity of the whole system. The form of this orbit is, however, very complicated, on account of the disturbing forces of so many planets, which are sometimes exerted toward one side and sometimes toward another; and even unequally exerted on different sides at the same time, according to the situation of the planets in their orbits. Thus we see the extraordinary and universal principle called gravitation has not only been the means of making us acquainted with a great number of inequalities in the motion of the heavenly bodies, which it would have been impossible to have discovered by observation, but it has furnished us with the means of subjecting these motions to precise and certain rules. The motion of the earth, which had obtained the assent of astronomers, on account of the simplicity with which it explained the celestial phenomena, has received a new confirmation, which has carried it to the highest degree of evidence of which physical science is susceptible. Without the knowledge of this universal principle, the ellipticity of the planetary orbits; the laws which the planets and comets obey in their revolution round the sun; their secular and periodical inequalities; the numberless inequalities of the moon, and the satellites of Jupiter; the precession of the equinoxes; the nutation of the earth's axis; the motions of the lunar axis; and the ebbing and flowing of the sea, would only be insulated facts, and unconnected phenomena. It is, therefore, a circumstance which can scarcely be sufficiently admired, that all these phenomena, which at first sight appear so unconnected, should be explained by one principle-that of the mutual gravitation of all bodies with forces directly as their quantities of matter, and inversely as the squares of their distances. OF THE PRINCIPAL SYSTEMS OF ASTRONOMY, WHICH HAVE BEEN PROPOSED TO ACCOUNT FOR THE CELESTIAL PHENOMENA. After the description which has been given of the various phenomena of the heavens, both as viewed with the naked eye and the telescope, it may not be unnecessary, nor unacceptable to the reader, to give a short account of the principal theories, or systems, which have been formed at various periods to account for some of these appearances, and particularly for the apparent motions of the celestial bodies. The explanation of the celestial motions which naturally occurred to those who began the study of the heavens, was, that the stars are so many luminous points fixed in the surface of a sphere, having the earth in its centre, and revolving on an axis passing through that centre, in the space of twenty-four hours. When it was observed, that all the stars did not partake of this diurnal motion in the same degree, but that some were carried slowly towards the east, and that their paths estimated in that direction, after certain intervals of time, returned into themselves, it was believed that they were fixed in the surfaces of spheres, which revolved westward more slowly than the sphere of the fixed stars. The spheres were supposed transparent, or made of some crystalline substance, and from this arose the name of the crystalline spheres, by which they were distinguished. Though this system grew more complicated, as the number and variety of the apparent phenomena increased, yet it was the system of Aristotle and Eudoxus; and, with few exceptions, of all the philosophers of antiquity.* But when the business of observation came to be regularly pursued, little was said either of the fixed stars, or of the crystalline spheres; astronomers being chiefly bent on ascertaining the laws or general facts connected with the motions of the planets. To do this, however, without the introduction of hypothesis, at this period, was scarcely possible. The simplest and most natural hypothesis was, that the planets moved eastward in circles, at a uniform rate. But when it was found that, instead of moving uniformly to the eastward, every one of them was subject to great irregularity, the motion eastward becoming slower, at certain periods, and at length vanishing altogether, so that the planet became stationary, and afterwards acquiring a motion in the contrary direction, and proceeded for a time to the westward, it was far from obvious that all these appearances could be reconciled with the idea of a uniform circular motion. The solution of this difficulty is attributed to Apollonius Pergaus, one of the most celebrated mathematicians of antiquity. He conceived that each planet moved in a small circle, and that the centre of this small circle moved in the circumference of a large circle, which had the earth for its centre. The first of these circles was called the epicycle, and the second the deferent; and the motion in the circumference of each was supposed uniform. Lastly, it was conceived that the motion of the centre of the epicycle in the circumference of the deferent, and likewise the motion of the planet in that of the epicycle, were in opposite directions; the first being towards the east, and the scond towards the west. In this way, the change * Although it is said that Pythagoras taught that the earth was a planet, and that the sun was fixed in the centre of the planetary system; that the apparent revolution of the heavens was produced by the diurnal revolution of the earth; and that the apparent annual motion of the sun was occasioned by the earth moving round him like the other planets; yet this doctrine was never taught publicly, and in a very short time it was completely forgotten. from progressive to retrograde, as well as the intermediate stationary points, were readily explained. But, notwithstanding the accomplishment of this important object, and some further applications of the method of epicycles by Hipparchus, to account for the inequality of the sun's apparent motion round the earth, no regular system of astronomy appears to have been framed or taught by any individual till the appearance of the celebrated Ptolemy, who has always been reckoned the prince of ancient astronomers, not so much on account of being the founder of the system which goes by his name, as for the number of observations which he made, and the extent of his astronomical writings. PTOLEMAIC SYSTEM. Ptolemy supposed the earth to be fixed immoveably in the centre of the universe, and that the sun, moon, and planets move round it in the following order; viz. The Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, as represented by the following figure. P.M II.C I.C. Above these he placed the firmament of fixed stars, then two crystalline spheres; all of which were included in what he called the primum mobile, which was by some unaccountable means turned round once in twenty-four hours, carrying all the rest along with it. From this arrangement, in which the sun is placed between Venus and Mars, it appears, that he was acquainted with the common distinction of inferior and superior planets. It appears, also, that the arrangement of the planets in the above order, was made on considering the time which each of them required to perform a complete circuit of the heavens; hence we find the moon placed nearest to the earth. Although he could explain the respective motions of the planets round the heavens, from west to east, as well as their diurnal motion round the earth, yet he was obliged to have recourse to the epicycles of Apollonius to explain the apparent irregularities in their motions, such as appearing to be stationary, retrograde, &c. In order to account for the inclination of the respective orbits of the planets to the ecliptic, it was said, that the deferent and epicycle were in different planes from the ecliptic. And when any new and anomalous motion was detected, another epicycle was immediately added to explain the appearance. In this way, by means of the most extravagant suppositions, and complicated machinery, a system was at length formed, which explained, in a plausible but conjectural manner, most of the constant phenomena of the heavens. But the awkward and complicated machinery by which it is supported, when compared with the simplicity of the Copernican system, is almost sufficient to cause it to be rejected. Besides the whole theory is entirely hypothetical; and that the existence of the agents employed to produce such mighty effects has never been attempted to be proved. But, independent of these objections, this system is now found to be at complete variance with many of the modern discoveries which have been made, respecting comets; for the orbits of these bodies are known to cross those of the planets in every direction; a fact which is quite incompatible with the existence of crystalline spheres. And the constant periodical motions and appearances of the two inferior planets, Mercury and Venus, completely refute the whole system. For if this system were true, these two planets could never be hid behind the sun, because their orbits are included in his, according to Ptolemy's hypothesis. These motions, too, would always be direct, and they would appear as often in opposition to, as in conjunction with, the sun but the contrary of all this takes place; for these two planets are just as often behind the sun as before him-appear as often to move backward as forward—and, instead of being seen at any time in that side of the heavens which is opposite to the sun, they were never yet seen a quarter of a circle distant from him; which certainly proves that the Ptolemaic system is contrary to what actually takes place in nature: and yet it continued to receive the sanction both of the learned and unlearned for nearly fourteen hundred years, without the truth of it ever being publicly called in question. COPERNICAN SYSTEM, The celebrated astronomer Copernicus, was the person who gave the death-blow to the system of Ptolemy, by conceiving the bold system which removes the earth from the centre of the world, and ascribes to it a two-fold motion,* Copernicus was first led to turn his attention to the framing of a new theory, from a perception of the complexity, or rather absurdity of the doctrine of epicycles. In the early part of his life he began to consider whether a more rational and satisfactory manner of accounting for the apparent motions of the heavenly bodies could not be discovered, than that which was given by Ptolemy. By intense application to the subject, and a few obscure hints obtained from the ancients, he at last deduced a complete system, capable of solving every phenomena in a more simple and satisfactory manner than was ever done before, In this system the sun is placed in the centre, and the planets move round him in the following order; viz. Mercury, Venus, the Earth, Mars, Jupiter, Saturn; and far beyond the orbit of Saturn are placed the fixed stars, which form the boundary of the visible creation.+ This is the system which is now generally received by all modern astronomers, and the one upon which all the phenomena, mentioned in the preceding pages, have been accounted for and explained. It may, perhaps, be proper to notice some objections which have repeatedly been advanced against it, especially as these were not noticed when treating of the annual and diurnal motions of the earth. This we shall however defer till we have given a short account of the Tychonic System; because its author was one of the most ingenious and powerful opponents that ever appeared against the Copernican System. TYCHONIC SYSTEM. Although the Copernican System was received by most men of science then living, yet there were some who would never assent to it. The motion of the earth was so contrary to what they were always accustomed to hear on the subject, and, as they thought, to appearances, that they could never agree to support such doctrine. Among those who opposed the system of Copernicus, was Tycho Brahe, a Danish nobleman, who was born in the year 1546, and who devoted the whole of his life to the study of astronomy. As he could not entirely adopt the Ptolemaic system, being convinced that the earth is not the centre about which the planets revolve, and being a man of genius, he invented a new system, which was a kind of mean between the Ptolemaic and the Copernican, The diurnal and annual. See pages 120 and 123. |