The Essays of Adam Smith

by Adam Smith

If this new hypothesis thus connected together all these appearances as happily as that of Ptolemy, there were others which it connected together much better. The three superior Planets, when nearly in conjunction with the Sun, appear always at the greatest distance from the Earth, are smallest, and least sensible to the eye, and seem to revolve forward in their direct motion with the greatest rapidity. On the contrary, when in opposition to the Sun, that is, when in their meridian about midnight, they appear nearest the Earth, are largest, and most sensible to the eye, and seem to revolve backwards in their retrograde motion. To explain these appearances, the system of Ptolemy supposed 358 each of these Planets to be at the upper part of their several Epicycles, in the one case; and at the lower, in the other. But it afforded no satisfactory principle of connection, which could lead the mind easily to conceive how the Epicycles of those Planets, whose spheres were so distant from the sphere of the Sun, should thus, if one may say so, keep time to his motion. The system of Copernicus afforded this easily, and like a more simple machine, without the assistance of Epicycles, connected together, by fewer movements, the complex appearances of the heavens. When the superior Planets appear nearly in conjunction with the Sun, they are then in the side of their orbits, which is almost opposite to, and most distant from the Earth, and therefore appear smallest, and least sensible to the eye. But, as they then revolve in a direction which is almost contrary to that of the Earth, they appear to advance forward with double velocity; as a ship, that sails in a contrary direction to another, appears from that other, to sail both with its own velocity, and the velocity of that from which it is seen. On the contrary, when those Planets are in opposition to the Sun, they are on the same side of the Sun with the Earth, are nearest it, most sensible to the eye, and revolve in the same direction with it; but, as their revolutions round the Sun are slower than that of the Earth, they are necessarily left behind by it, and therefore seem to revolve backwards; as a ship which sails slower than another, though it sails in the same direction, appears from that other to sail backwards. After the same manner, by the same annual revolution of the Earth, he connected together the direct and retrograde motions of the two inferior Planets, as well as the stationary appearances of all the Five.

There are some other particular phenomena of the two inferior Planets, which correspond still better to this system, and still worse to that of Ptolemy. Venus and Mercury seem to attend constantly upon the motion of the Sun, appearing, sometimes on the one side, and sometimes on the other, of that great luminary; Mercury being almost always buried in his rays, and Venus never receding above forty-eight degrees from him, contrary to what is observed in the other three Planets, which are often seen in the opposite side of the heavens, at the greatest possible distance from the Sun. The system of Ptolemy accounted for this, by supposing that the centres of the Epicycles of these two Planets were always in the same line with those of the Sun and the Earth; that they appeared therefore in conjunction with the Sun, when either in the upper or lower part of their Epicycles, and at the greatest distance from him, when in the sides of them. It assigned, however, no reason why the Epicycles of these two Planets should observe so different a rule from that which takes place in those of the other three, nor for the enormous Epicycle of Venus, whose sides must have been forty-eight degrees distant from the Sun, while its centre was in conjunction with him, and whose diameter must have covered 359 more than a quadrant of the Great Circle. But how easily all these appearances coincide with the hypothesis, which represents those two inferior Planets revolving round the Sun in orbits comprehended within the orbit of the Earth, is too obvious to require an explanation.

Thus far did this new account of things render the appearances of the heavens more completely coherent than had been done by any of the former systems. It did this, too, by a more simple and intelligible, as well as more beautiful machinery. It represented the Sun, the great enlightener of the universe, whose body was alone larger than all the Planets taken together, as established immovable in the centre, shedding light and heat on all the worlds that circulated around him in one uniform direction, but in longer or shorter periods, according to their different distances. It took away the diurnal revolution of the firmament, whose rapidity, upon the old hypothesis, was beyond what even thought could conceive. It not only delivered the imagination from the embarrassment of Epicycles, but from the difficulty of conceiving these two opposite motions going on at the same time, which the system of Ptolemy and Aristotle bestowed upon all the Planets; I mean, their diurnal westward, and periodical eastward revolutions. The Earth’s revolution round its own axis took away the necessity for supposing the first, and the second was easily conceived when by itself. The Five Planets, which seem, upon all other systems, to be objects of a species by themselves, unlike to every thing to which the imagination has been accustomed, when supposed to revolve along with the Earth round the Sun, were naturally apprehended to be objects of the same kind with the Earth, habitable, opaque, and enlightened only by the rays of the Sun. And thus this hypothesis, by classing them in the same species of things, with an object that is of all others the most familiar to us, took off that wonder and that uncertainty which the strangeness and singularity of their appearance had excited; and thus far, too, better answered the great end of Philosophy.

Neither did the beauty and simplicity of this system alone recommend it to the imagination; the novelty and unexpectedness of that view of nature, which it opened to the fancy, excited more wonder and surprise than the strangest of those appearances, which it had been invented to render natural and familiar, and these sentiments still more endeared it. For, though it is the end of Philosophy, to allay that wonder, which either the unusual or seemingly disjointed appearances of nature excite, yet she never triumphs so much, as when, in order to connect together a few, in themselves, perhaps, inconsiderable objects, she has, if I may say so, created another constitution of things, more natural, indeed, and such as the imagination can more easily attend to, but more new, more contrary to common opinion and expectation, than any of those appearances themselves. As, in the instance before us, in order to connect together some seeming irregularities in the motions of 360 the Planets, the most inconsiderable objects in the heavens, and of which the greater part of mankind have no occasion to take any notice during the whole course of their lives, she has, to talk in the hyperbolical language of Tycho Brahe, moved the Earth from its foundations, stopped the revolution of the Firmament, made the Sun stand still, and subverted the whole order of the Universe.

Such were the advantages of this new hypothesis, as they appeared to its author, when he first invented it. But, though that love of paradox, so natural to the learned, and that pleasure, which they are so apt to take in exciting, by the novelties of their supposed discoveries, the amazement of mankind, may, notwithstanding what one of his disciples tells us to the contrary, have had its weight in prompting Copernicus to adopt this system; yet, when he had completed his Treatise of Revolutions, and began coolly to consider what a strange doctrine he was about to offer to the world, he so much dreaded the prejudice of mankind against it, that, by a species of continence, of all others the most difficult to a philosopher, he detained it in his closet for thirty years together. At last, in the extremity of old age, he allowed it to be extorted from him, but he died as soon as it was printed, and before it was published to the world.

When it appeared in the world, it was almost universally disapproved of, by the learned as well as by the ignorant. The natural prejudices of sense, confirmed by education, prevailed too much with both, to allow them to give it a fair examination. A few disciples only, whom he himself had instructed in his doctrine, received it with esteem and admiration. One of them, Reinholdus, formed, upon this hypothesis, larger and more accurate astronomical tables, than what accompanied the Treatise of Revolutions, in which Copernicus had been guilty of some errors in calculation. It soon appeared, that these Prutenic Tables, as they were called, corresponded more exactly with the heavens, than the Tables of Alphonsus. This ought naturally to have formed a prejudice in favour of the diligence and accuracy of Copernicus in observing the heavens. But it ought to have formed none in favour of his hypothesis; since the same observations, and the result of the same calculations, might have been accommodated to the system of Ptolemy, without making any greater alteration in that system than what Ptolemy had foreseen, and had even foretold should be made. It formed, however, a prejudice in favour of both, and the learned began to examine, with some attention, an hypothesis which afforded the easiest methods of calculation, and upon which the most exact predictions had been made. The superior degree of coherence, which it bestowed upon the celestial appearances, the simplicity and uniformity which it introduced into the real directions and velocities of the Planets, soon disposed many astronomers, first to favour, and at last to embrace a system, which thus connected together so happily, 361 the most disjointed of those objects that chiefly occupied their thoughts. Nor can any thing more evidently demonstrate, how easily the learned give up the evidence of their senses to preserve the coherence of the ideas of their imagination, than the readiness with which this, the most violent paradox in all philosophy, was adopted by many ingenious astronomers, notwithstanding its inconsistency with every system of physics then known in the world, and notwithstanding the great number of other more real objections, to which, as Copernicus left it, this account of things was most justly exposed.

It was adopted, however, nor can this be wondered at, by astronomers only. The learned in all other sciences, continued to regard it with the same contempt as the vulgar. Even astronomers were divided about its merit; and many of them rejected a doctrine, which not only contradicted the established system of Natural Philosophy, but which, considered astronomically only, seemed, to them, to labour under several difficulties.

Some of the objections against the motion of the Earth, that were drawn from the prejudices of sense, the patrons of this system, indeed, easily enough got over. They represented, that the Earth might really be in motion, though, to its inhabitants, it seemed to be at rest; and that the Sun and Fixed Stars might really be at rest, though from the Earth they seemed to be in motion; in the same manner as a ship, which sails through a smooth sea, seems to those who are in it, to be at rest, though really in motion; while the objects which she passes along, seem to be in motion, though really at rest.

But there were some other objections, which, though grounded upon the same natural prejudices, they found it more difficult to get over. The earth had always presented itself to the senses, not only as at rest, but as inert, ponderous, and even averse to motion. The imagination had always been accustomed to conceive it as such, and suffered the greatest violence, when obliged to pursue, and attend it, in that rapid motion which the system of Copernicus bestowed upon it. To enforce their objection, the adversaries of this hypothesis were at pains to calculate the extreme rapidity of this motion. They represented, that the circumference of the Earth had been computed to be above twenty-thousand miles: if the Earth, therefore, was supposed to revolve every day round its axis, every point of it near the equator would pass over above twenty-three thousand miles in a day; and consequently, near a thousand miles in an hour, and about sixteen miles in a minute; a motion more rapid than that of a cannon ball, or even than the swifter progress of sound. The rapidity of its periodical revolution was yet more violent than that of its diurnal rotation. How, therefore, could the imagination ever conceive so ponderous a body to be naturally endowed with so dreadful a movement? The Peripatetic Philosophy, the only philosophy then known in the world, still further confirmed 362 this prejudice. That philosophy, by a very natural, though, perhaps, groundless distinction, divided all motion into Natural and Violent. Natural motion was that which flowed from an innate tendency in the body, as when a stone fell downwards: Violent motion, that which arose from external force, and which was, in some measure, contrary to the natural tendency of the body, as when a stone was thrown upwards, or horizontally. No violent motion could be lasting; for, being constantly weakened by the natural tendency of the body, it would soon be destroyed. The natural motion of the Earth, as was evident in all its parts, was downwards, in a straight line to the centre; as that of fire and air was upwards, in a straight line from the centre. It was the heavens only that revolved naturally in a circle. Neither, therefore, the supposed revolution of the Earth round its own centre, nor that round the Sun, could be natural motions; they must therefore be violent, and consequently could be of no long continuance. It was in vain that Copernicus replied, that gravity was, probably, nothing else besides a tendency in the different parts of the same Planet, to unite themselves to one another; that this tendency took place, probably, in the parts of the other Planets, as well as in those of the Earth; that it could very well be united with a circular motion; that it might be equally natural to the whole body of the Planet, and to every part of it; that his adversaries themselves allowed, that a circular motion was natural to the heavens, whose diurnal revolution was infinitely more rapid than even that motion which he had bestowed upon the Earth; that though a like motion was natural to the Earth, it would still appear to be at rest to its inhabitants, and all the parts of it to tend in a straight line to the centre, in the same manner as at present. But this answer, how satisfactory soever it may appear to be now, neither did nor could appear to be satisfactory then. By admitting the distinction betwixt natural and violent motions, it was founded upon the same ignorance of mechanical principles with the objection. The systems of Aristotle and Hipparchus supposed, indeed, the diurnal motion of the heavenly bodies to be infinitely more rapid than even that dreadful movement which Copernicus bestowed upon the Earth. But they supposed, at the same time, that those bodies were objects of a quite different species, from any we are acquainted with, near the surface of the Earth, and to which, therefore, it was less difficult to conceive that any sort of motion might be natural. Those objects, besides, had never presented themselves to the senses, as moving otherwise, or with less rapidity, than these systems represented them. The imagination, therefore, could feel no difficulty in following a representation which the senses had rendered quite familiar to it. But when the Planets came to be regarded as so many Earths, the case was quite altered. The imagination had been accustomed to conceive such objects as tending rather to rest than motion; and this idea of their natural 363 inertness, encumbered, if one may say so, and clogged its flight whenever it endeavoured to pursue them in their periodical courses, and to conceive them as continually rushing through the celestial spaces, with such violent and unremitting rapidity.

Nor were the first followers of Copernicus more fortunate in their answers to some other objections, which were founded indeed in the same ignorance of the laws of motion, but which, at the same time, were necessarily connected with that way of conceiving things, which then prevailed universally in the learned world.