Astronomy produces calculations concerning the magnitudes, distances, and revolutions of the planets, and their respective satellites, which, to the uninformed, appear absurd, chimerical, and presumptuous; while, probably, they laugh at such notions as were received among men, when even the wisest of them were weak enough to believe, that the earth was an immense plain, situated in the centre of the universe; that the vault of heaven was of crystal; and that the sun was no other than a plate of red hot iron, about as large as the Peloponnessus. The following thoughts, communicated by my much esteemed friend Thomas Exley, A. M. may assist such persons to entertain more favorable sentiments of the science of astronomy, and also serve to enlarge their views of the Supreme Being.
“Many persons who have not had the advantages of proper instruction in mathematical science, cannot be persuaded that it is in the power of man to ascertain the distances of the sun, moon, and planets, and, of course, pay little regard to the assertions of astronomers on this subject. Sometimes, they are bold enough to say the thing is impossible, because no one has ever been to any of those bodies. Let such persons consider, that it is not necessary to go to a remote object in order to measure its distance; for that purpose, it will be sufficient to know the length of a line at the place of the spectator, and the inclination of this line to two others directed from its extremities to the object; for, on the length of this line, and the position of the two others, depends the distance of the object from the ends of that line.
“Thus, if I wish to know the distance of a neighboring tower, or other object beyond a river, or in some other way inaccessible; I measure any convenient line terminating in my station, and by some instrument proper for measuring angles, I ascertain the position of my measured line to the lines connecting its extreme points and the object. On these data depends the distance, and from this line and these angles accurately measured, the exact distance may be with great ease truly found. It is on similar principles that astronomers investigate the distances of the heavenly bodies. They take as the given or measured line, which may be called the base, some line on the earth, the semi-diameter for instance, as being the most convenient. The angle formed, or rather contained by two lines drawn from the sun or planet to the ends of the semi-diameter of the earth, is called the parallax, because it shows the difference of the apparent situation of the object as seen from the extremities of the semi-diameter, that is, it measures the arc of a great circle in the heavens contained between its two apparent places. Hence to ascertain its parallax, or difference of the apparent place when the object is viewed from the other end of the semi-diameter, becomes a problem of great importance in astronomy; for this being truly discovered, the distance of the planet will be obtained with the utmost exactitude. If any other line besides the semi-diameter of the earth, whose length and position are known, be used as a base, and the parallax in respect of this line be found, the same conclusions will follow. The chief difficulty in this affair arises from the smallness of the angle to be measured, which is a consequence of the greatness of the distance in respect of the earth’s semi-diameter. Several ingenious methods have been proposed and employed by astronomers to discover the distances of the sun and planets, but nothing serves this purpose so well as the transits of Venus over the sun’s disk. At certain periods, which can be foretold by astronomers, this planet passes exactly between us and the sun, and is seen as a dark round spot for some hours, moving in a line across the sun’s face or disk. The observer should be furnished with a good chronometer, or pendulum clock with seconds, to note the time of the transit; and good instruments, to take the apparent diameters of the sun and Venus, and her greatest distance from the sun’s limb while passing over his disk: from these observations, and the known phenomena of the motions of the earth and Venus, the parallax may be found. But if two observers, at very distant places of the earth properly chosen, make these observations, the parallax may be obtained with much greater ease and nicety; because the distance of the apparent tracks of Venus across the sun as seen from the two places, and also the difference of the time of the passage, arises from the parallax of Venus and that of the sun. The two last transits, which happened in the years 1761 and 1769, were carefully observed for this purpose; and it is to the results of these observations that the present astronomers are indebted for their more accurate knowledge of the distances of the planets, and the dimensions of the solar system.
“It should be observed, that if the parallax, and consequently the distance of any one of the planets by any means becomes known, the same is easily obtained for each of the other planets, from the relation which has been clearly discovered to subsist between the periodical times of revolution of the planets round the sun, and their distances from that central luminary. Astronomers have most decidedly proved that the square of the time in which any planet revolves is to the square of the time in which any other revolves, as the cube of the distance of the first, is to the cube of the distance of the other; and since all the times are known from observation, if the distance of any one be determined, there is no difficulty at all to find the distances of all the other planets from the sun.
“It has also been matter of great surprise to the unlearned, that astronomers should pretend to tell the magnitudes of the sun and planets. But this is no difficult problem when the distance is known. The apparent diameter is readily found from observation, and on this and the distance depends the true diameter. If the apparent diameters of two objects be equal, the true diameter of the one will be greater as it is more remote; and the apparent diameter of any object will increase as the distance of it from the observer diminishes. From this every one sees, that a knowledge of the distance of the object is an indispensible element for finding its bulk; and, according to the accuracy of the measure of the distance, will be that of the measure of the magnitude, provided the apparent distance be truly taken; and this, in the present improved state of our instruments, presents no obstacle. There can be no doubt but that astronomers are very near the truth in the numbers which they now give us for expressing the distances and magnitudes of the sun and planets.
“The telescope has been of singular use to the astronomer; it has shown him many phenomena of the heavenly bodies, concerning which he would otherwise have been totally ignorant. It is by the assistance of this noble instrument that we have attained to the knowledge of the rotations of the sun and planets, the phases of Venus and Mercury, Saturn’s ring, and many other particulars exceedingly interesting. The telescope has discovered several planets which otherwise would have revolved in their courses unknown and unnoticed by the inhabitants of this globe; it has informed us that several of the planets have moons moving round them, as our moon revolves round the earth; besides, it has presented to our view an innumerable multitude of fixed stars which without this assistance we should never have seen.
“It is no wonder that great efforts have been made to improve this excellent instrument; these efforts have been attended with great success, and what may be further done in this respect we cannot tell; however, there is a limit to the improvements of the telescope, for after it has attained a certain degree of magnifying power, the motes and vapors in the atmosphere would be so magnified as to occupy its whole field of view, and thus render it a useless incumbrance.”
Who can contemplate the power which produced the solar system, at once so magnificent, beautiful, and delightful, without astonishment and admiration? The planets are kept in a regular motion, and retained in an invariable course round the sun, by the power of this luminary’s attraction or gravity. These bodies have a projectile force, being propelled forwards in a right line, which is the nature of all simple motion; but the sun’s attraction combining with their own projectile force, withdraws them from their rectilineal courses, and preserves the most perfect harmony in the system. This wonderful mechanism was originally impressed on the system by its infinitely wise and omnipotent Creator; to which primary impulse it has with undeviating uniformity adhered, having never suffered in its operations from the greatest distance of space, or intervals of time!
Surely no power less than that which at first gave existence and modification to matter, is equal to the government of the world. The solar orb and the planetary bodies could no more subsist in their present form and order, without a Divine, supporting, and directing hand, than they could at the beginning make themselves. What is that general law or force called gravitation, without which the whole frame of nature would soon be dissolved? Is it not a power constantly issuing from the Deity, and which if he should suspend but for one moment, the whole creation would sink into ruins? How inconceivably great and operative must that power be, that is present throughout the universe, with all the heavenly orbs to preserve them in their courses; and on this earth, with every creature, and every particle of matter, to preserve its present form!
In addition to the planets and their satellites, there are Comets, which revolve round the sun, and, consequently, are a part of the solar system. They have often a long tail, in appearance resembling hair, issuing from that side which is turned away from the sun. Comets are popularly divided into three kinds, namely, bearded, tailed, and hairy: but this arrangement seems to apply rather to the different circumstances of the same comet, than to the phenomena of several. Thus, when a comet is eastward of the sun, and moves with him, it is said to be bearded, because the light precedes it in the manner of a beard: but when it is westward of him, it is said to be tailed, because the train of light follows it in the manner of a tail: and, lastly, when the sun and comet are diametrically opposite, the earth being between them, the train is hid behind the body of the comet, excepting the extremities, which being broader than the body of the comet, appear round it like a border of hair (coma), from which circumstance it is said to be hairy, and is denominated a comet.