ASTRONOMICAL

ESSAY S.

ESSAY I

PART I

MANKIND have in all ages been desirous of forming rational conceptions of the nature and motion of those bodies that appear in the vast concave above their heads. Amidst the infinite variety of objects which surround them on every side, the heavenly bodies must have been amongst those which first attracted their attention. They are of all objects the most conspicuous, the most important, and the most beautiful.

Astronomy instructs us in the laws, or rules, that govern and direct the motions of the heavenly host. It weighs and considers the powers by which they circulate in their orbs. It enables us to discover their size, determine their distance, explain their various phenomena, and correct the fallacies of the senses by the light of truth.

Astronomy is not merely a speculative science; its use is as extensive as its researches are sublime. Navigation owns it for its guide; by it, commerce has been extended and geography improved. It is astronomical observations that form the basis of geography. Thus, it has co-operated with other causes in the greatest of all works, the diffusion of knowledge and the civilization of man.

As, in order to attain an accurate idea of any piece of mechanism, it is best to begin our investigation by an examination of those parts which give motion to the rest, the primary causes of those effects for which the machine was made; so the young pupil will more easily gain a just idea of the motion of the heavenly bodies, by considering them as seen from. the sun, the centre of our system, and the princi pal agent used by the LORD OF NATURE for conducting and regulating the planetary system.

It will not be difficult, after this, to inform him how those appearances are to be accounted for, that arise from his particular situation; whence he views the heavens from a point which is not in the centre of the system, and is consequently the source of many apparent irregularities. This knowledge attained, it will be easy to prove to him, that the real and apparent motions of the heavenly bodies are frequently the reverse of each other. For, being by this means put into possession of the universals of this science, the knowledge of particulars will be rendered facile and clear.

OF THE SOLAR SYSTEM AS SEEN BY A SPECTATOR SUPPOSED TO BE PLACED IN THE SUN.

As the centre of the system is the only place from which the motion of the planets can be truely seen, let us suppose an observer placed in the centre of the sun. In this situation he will see at one view all the heavens, which will appear to him perfectly spherical, the stars being so many lucid points in the concave surface of the sphere, whose centre is the sun, or, in the present instance, the eye of the observer.

Our spectator will not, however, immediately conclude from appearances, either that the heavens are really spherical, or that the sun is in the centre of that sphere, or that the stars are all at an equal distance from him; having been previously taught by experience and observation, that while he remains in the same place, he cannot judge properly of the distance of the surrounding objects, at least of those which are placed beyond the ordinary reach of view. When objects are removed beyond the distances we are accustomed to, the principles by which we form our general judgment fail us; and we can only tell which is nearest, or which is furthest, either by our own motion, or that of the objects.

To illustrate this, let us suppose a number of lamps to be placed irregularly, at different distances from the eye, in a dark night. Now, if in this case we suppose the darkness to be so complete, that no intermediate objects could be seen, no difference in

colour discerned, nor any convergence towards the point of sight be perceived; our judgment could not assist us in distinguishing the distance of one from the other, and they would therefore all seem to be at an equal distance from the spectator.

For the same reason, the sun and moon, the stars and planets, appear to be all at an equal distance from us, though it is highly probable, that some of the stars are many millions of times nearer to us than others. The sun is demonstrated to be nearer than any of the stars. The moon and some of the planets are known by ocular proof to be nearer to us than the sun, because they sometimes come between it and our eye, and hide the whole, or a great part of his disc, from our view. They all, however, appear equally distant, and as if placed in the surface of a sphere, whereof our eye is the centre. In whatever place, therefore, the spectator resides, whether it be on this earth, in the sun, or in the regions of saturn, he will consider that place as the middle point of the universe, and the centre of the world; for it will be to him the centre of a spherical surface, in which all distant bodies seem to be placed.

These things being rendered plain, the pupil may proceed to consider the observations of the solar spectator; to whom, as we have already observed, the heavens will appear as the surface of a concave sphere, concentrical to his eye: in this surface he will discover an innumerable host of fixt stars, which will for some time engage his attention, before he discovers that they may be distinguished into two

kinds; the one, dispersed through the whole heavens, differing in their degree of brightness, but remaining always at the same relative distance from each other; these he will therefore call fixed stars, or only stars. Besides these, he will find some others moving among the foregoing with different velocities, which he will call wandering stars or planets.

OF THE CELESTIAL SIGNS AND CONSTELLATIONS.

Having proceeded thus far, our spectator will endeavour to find out some method of distinguishing the stars from each other; concluding, that as they do not change their relative positions one to the other, he may easily make an exact description of them, and by repeated observations determine the position and order which subsist among them.

That he may avoid confusion in description, and be able to point out any particular star, without being obliged to give a name to each, he will divide them into several parcels; to each of these he will assign a figure at pleasure; these assemblages or groups of stars, he will call constellations. Thus, a number of stars near the north pole is called the Bear, because the stars which compose it are at such distances from each other, that they may fall within the figure of a bear. Another constellation is called the Ship, because that collection of stars which compose it is represented upon a celestial globe as com prized within some part of the figure of a ship.