ASTRONOMY ANCIENT ASTRONOMY.
IT has already been remarked, that the ancients made more considerable advances in as tronomy than in almost any other of the physical sciences. They applied themselves dili gently to observe the heavens, and employed mathematical reasoning to connect together the insulated facts, which are the only objects direct observation. The astronomer dis corers nothing by help of his instruments, but that, at a given instant, a certain luminous point has a particular position in the heavens. The application of mathematics, and par ticulirly of spherical trigonometry, enables him to trace out the precise tract of this lumi -nous spot ;• to discover the rate of its motion, whether varied or uniform, and thus to re solve the first great problem which the science of astronomy involves, viz. to express the positions of the heavenly bodies, relatively to a given plane in functions of the time. The problem thus generally enunciated, comprehends all that is usually called by the name of descriptive or mathematical astronomy.
The explanation of tile 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 sur face of a sphere, hiving 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 in.
tervals of time, returned into themselves, it was believed that they were fixed in the stir faces of spheres, which revolved westward, more slowly than the sphere of the fixed stars. These spheres must be transparent, or made of some crystalline substance, and hence the name of the crystalline spheres, by which they were distinguished. This system, though it grew more complicated in proportion to the number and variety of the phenomena ob served, was the system of Aristotle and Eudoxus, and, with a few exceptions, of all the philosophers of antiquity.
But when the business of observation came to be regularly pursued ; when Tim ocharis and Aristillus, and their successors in the Alexandrian school, began to study the pheno mena of the heavens, little was said of these orbs ; and astronomers seemed only desirous of ascertaining the laws or the general facts concerning the planetary motions.
To do this, however, without the introduction of hypothesis, was certainly difficult, and probably was then impossible. The simplest and most natural hypothesis was, that the ,planets moved eastward in circles, and 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 irre gularity, the motion eastward becoming at certain periods slower, and at length vanish ing altogether, so that the planet became stationary, and afterwards acquiring a motion in the contrary direction, proceeded for a time toward the west, it was far from obvious how all these appearances could be reconciled with the idea of a uniform circular motion.
The solution of this difficulty is ascribed • to Apollonius Pergseus, one of the greatest geometers of antiquity. He conceived that, in the circumference of a circle, having the earth for its centre, there moved the centre of another circle, in the circumference of which the planet actually revolved. The first of these circles was called the deferent, and the second the epicycle, 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 of the planet in that of the epicycle, were in opposite directions, the first being towards the east, and the second towards the west.. In this way, the alterations from progressive to retrograde, with the intermediate stationary points, were readily ex plained, and Apollonius carried his investigation so far as to determine the ratio between the radius of the deferent, and that of epicycle, from knowing the stations and retrograda tions of any•particular planet.