This is a short account of the discoveries made with the telescope. Well might Hugenius congratulate the age he lived in, on such a great acquisition of knowledge: And recollecting those great men, Copernicus, Regiomontanus, and Tycho, so lately excluded from it by death, what an immense treasure, says he, would they have given for it. Those ancient philosophers too, Pythagoras, Democritus, Anaxagoras, Philolaus, Plato, Hipparchus; would they not have travelled over all the countries of the world, for the sake of knowing such secrets of nature, and of enjoying such sights as these?
Thus have we seen the materials collected, which were to compose the magnificent edifice of astronomical Philosophy; collected, indeed, with infinite labour and industry, by a few volunteers in the service of human knowledge, and with an ardour not to be abated by the weaknesses of human nature, or the threatened loss of sight, one of the greatest of bodily misfortunes! It was now time for the great master-builder to appear, who was to rear up this whole splendid group of materials into due order and proportion. And it was, I make no doubt, by a particular appointment of Providence, that at this time the immortal Newton appeared. Much had been done preparatory to this great work by others, without which if he had succeeded, we should have been ready to pronounce him something more than human. The doctrine of atoms had been taught by some of the ancients. Kepler had suspected that the planets gravitated towards each other, particularly the earth and moon; and that their motion prevented their falling together: and Galileo first of all applied geometrical reasoning to the motion of projectiles. But the solid spheres of the ancients, or the vortices of Des Cartes,[[A28]] were still found necessary to explain the planetary motions; or if Kepler had discarded them, it was only to substitute something else in their stead, by no means sufficient to account for those grand movements of nature. It was Newton alone that extended the simple principle of gravity, under certain just regulations, and the laws of motion, whether rectilinear or circular, which constantly take place on the surface of this globe, throughout every part of the solar system; and from thence, by the assistance of a sublime geometry, deduced the planetary motions, with the strictest conformity to nature and observation.
Other systems of Philosophy have been spun out of the fertile brain of some great genius or other; and for want of a foundation in nature, have had their rise and fall, succeeding each other by turns. But this will be durable as science, and can never sink into neglect, until “universal darkness buries all.”
Other systems of Philosophy have ever found it necessary to conceal their weakness, and inconsistency, under the veil of unintelligible terms[[A29]] and phrases, to which no two mortals perhaps ever affixed the same meaning: But the Philosophy of Newton disdains to make use of such subterfuges; it is not reduced to the necessity of using them, because it pretends not to be of nature’s privy council, or to have free access to her most inscrutable mysteries; but to attend carefully to her works, to discover the immediate causes of visible effects, to trace those causes to others more general and simple, advancing by slow and sure steps towards the great First Cause of all things.
And now the Astronomy of our planetary system seemed compleated. The telescope had discovered all the globes whereof it is composed, at least as far as we yet know. Newton with more than mortal sagacity had discovered those laws by which all their various, yet regular, motions are governed, and reduced them to the most beautiful simplicity: laws to which not only their great and obvious variety of motions are conformable, but even their minute irregularities; and not only planets but comets likewise. The busy mind of man, never satiated with knowledge, now extended its views further, and made use of every expedient that suggested itself, to find the relation that this system of worlds bears to the whole visible creation. Instruments were made with all possible accuracy, and the most skilful observers applied themselves with great diligence to discover an annual parallax, from which the distances of the fixed stars would be known. They found unexpected irregularities, and might have been long perplexed with them to little purpose, had not Dr. Bradley happily accounted for them, by shewing that light from the heavenly bodies strikes the eye with a velocity and direction, compounded of the proper velocity and direction of light, and of the eye, as carried about with the earth in its orbit; compared to which, the diurnal motion and all other accidental motions of the eye, are quite inconsiderable. Thus, instead of what he aimed at, he discovered something still more curious, the real velocity of light, in a way entirely new and unthought of.
All Astronomical knowledge being conveyed to us from the remotest distances, by that subtle, swift and universal messenger of intelligence, Light; it was natural for the curious to enquire into its properties, and particularly to endeavour to know with what velocity it proceeds, in its immeasurable journeys. Experimental Philosophy, accustomed to conquer every difficulty, undertook the arduous problem; but confessed herself unequal to the task.[[A30]] Here, Astronomy itself revealed the secret; first in the discovery of Roemer, who found that the farther Jupiter is distant from us, the later the light of his satellites always reaches us; and afterwards in this of Dr. Bradley, informed us, that light proceeds from the sun to us in about eight minutes of time.[[A31]]
As the apparent motion of the fixed stars, arising from this cause, was observed to complete the intire circle of its changes in the space of a year, it was for some time supposed to arise from an annual parallax, notwithstanding its inconsistency in other respects with such a supposition. But this obstacle being removed, there followed the discovery of another apparent motion in the heavens, arising from the nutation of the earth’s axis; the period whereof is about nineteen years. Had it not been so very different from the period of the former, the causes of both must have been almost inexplicable. This latter discovery is an instance of the superior advantages of accurate observation: For it was well known that such a nutation must take place from the principles of the Newtonian Philosophy; yet a celebrated astronomer had concluded from hypothetical reasoning, that its quantity must be perfectly insensible.
The way being cleared thus far, Dr. Bradley assures us, from his most accurate observations, that the annual parallax cannot exceed two seconds, he thinks not one; and we have the best reason to confide in his judgment and accuracy. From hence then we draw this amazing conclusion; that the diameter of the earth’s orb bears no greater proportion to the distance of the stars which Bradley observed, than one second does to the radius; which is less than as one to 200,000. Prodigiously great as the distance of the fixed stars from our sun appears to be, and probably their distances from each other are no less, the Newtonian Philosophy will furnish us with a reason for it: That the several systems may be sufficiently removed from each other’s attraction, which we are very certain must require an immense distance; especially if we consider that the cometic part, of our system at least, appears to be the most considerable though so little known to us. The dimensions of the several parts of the planetary system, had been determined near the truth by the astronomers of the last age, from the parallax of Mars. But from that rare phenomenon the transit of Venus over the sun’s disk, which has twice happened within a few years past, the sun’s parallax is now known beyond dispute to be 8 seconds and an half, nearly; and consequently, the sun’s distance almost 12,000 diameters of the earth.
If from the distances of the several planets, and their apparent diameters taken with that excellent instrument, the micrometer, we compare their several magnitudes, we shall find the Moon, Mercury, and Mars, to be much less than our Earth, Venus a little less, but Saturn many hundred times greater, and Jupiter above one thousand times. This prodigious globe, placed at such a vast distance from the other planets, that the force of its attraction might the less disturb their motions, is far more bulky and ponderous than all the other planets taken together. But even Jupiter, with all his fellows of our system, are as nothing compared to that amazing mass of matter the Sun. How much are we then indebted to Astronomy, for correcting our ideas of the visible creation! Wanting its instruction, we should infallibly have supposed the earth by far the most important body in the universe, both for magnitude and use. The sun and moon would have been thought two little bodies nearly equal in size, though different in lustre, created solely for the purpose of enlightening the earth; and the fixed stars, so many sparks of fire, placed in the concave vault of heaven, to adorn it, and afford us a glimmering light in the absence of the sun and moon.
But how does Astronomy change the scene!—Take the miser from the earth, if it be possible to disengage him; he whose nightly rest has been long broken by the loss of a single foot of it, useless perhaps to him; and remove him to the planet Mars, one of the least distant from us: Persuade the ambitious monarch to accompany him, who has sacrificed the lives of thousands of his subjects to an imaginary property in certain small portions of the earth; and now point it out to them, with all its kingdoms and wealth, a glittering star “close by the moon,” the latter scarce visible and the former less bright than our Evening Star:—Would they not turn away their disgusted sight from it, as not thinking it worth their smallest attention, and look for consolation in the gloomy regions of Mars?[[A32]]