Plutarch records that Plato, who had always taught that the sun turned round the earth, had changed his opinion towards the end of his life, regretting that he had not placed the sun in the centre of the universe, which was the only place, he then thought, that was suitable for that star.
Three centuries before Jesus Christ, Aristarchus of Samos is said by Aristotle to have composed a special work to defend the motion of the earth against the contrary opinions of philosophers. In this work, which is now lost, he laid down in the most positive manner that "the sun remains immovable, and that the Earth moves round it in a circular curve, of which that star is the centre." It would be impossible to state this in clearer terms; and what makes his meaning more clear, if possible, is that he was persecuted for it, being accused of irreligion and of troubling the repose of Vesta—"because," says Plutarch, "in order to explain the phenomena, he taught that the heavens were immovable, and that the earth accomplished a motion of translation in an oblique line, at the same time that it turned round its own axis." This is exactly the opinion that Copernicus took up, after an interval of eighteen centuries—and he too was accused of irreligion.
In passing from the Greeks to the Romans, and from them to the middle ages, the doctrine of Aristarchus underwent a curious modification, assimilating it to the system of Tycho Brahe, which we shall hereafter consider, rather than to that of Copernicus. This consisted in making the planets move round the sun, while the sun itself revolved round the earth, and carried them with him, and the heavens revolved round all. Vitruvius and Macrobius both taught this doctrine. Although Cicero and Seneca, with Aristotle and the Stoics, taught the immobility of the earth in the centre of the universe, the question seemed undecided, to Seneca at least, who writes:—"It would be well to examine whether it is the universe that turns about the immovable earth, or the earth that moves, while the universe remains at rest. Indeed some men have taught that the earth is carried along, unknown to ourselves, that it is not the motion of the heavens that produces the rising and setting of the stars, but that it is we who rise and set relatively to them. It is a matter worthy of contemplation, to know in what state we are—whether we are assigned an immovable or rapidly-moving home—whether God makes all things revolve round us, or we round them."
The double motion of the earth, then, is an idea revived from the Grecian philosophers. The theory was known indeed to Ptolemy, who devotes a whole chapter in his celebrated Almagesta to combat it. From his point of view it seemed very absurd, and he did not hesitate to call it so; and it was in reality only when fresh discoveries had altered the method of examining the question that the absurdities disappeared, and were transferred to the other side. Not until it was discovered that the earth was no larger and no heavier than the other planets could the idea of its revolution and translation have appeared anything else than absurd. We are apt to laugh at the errors of former great men, while we forget the scantiness of the knowledge they then possessed. So it will be instructive to draw attention to Ptolemy's arguments, that we may see where it is that new knowledge and ideas have led us, as they would doubtless have led him, had he possessed them, to a different conclusion.
His argument depends essentially on the observed effects of weight. "Light bodies," he says, "are carried towards the circumference, they appear to us to go up; because we so speak of the space that is over our heads, as far as the surface which appears to surround us. Heavy bodies tend, on the contrary, towards the middle, as towards a centre, and they appear to us to fall down, because we so speak of whatever is under our feet, in the direction of the centre of the earth. These bodies are piled up round the centre by the opposed forces of their impetus and friction. We can easily see that the whole mass of the earth, being so large compared with the bodies that fall upon it, can receive them without their weight or their velocity communicating to it any perceptible oscillation. Now if the earth had a motion in common with all the other heavy bodies, it would not be long, on account of its weight, in leaving the animals and other bodies behind it, and without support, and it would soon itself fall out of heaven. Such would be the consequences of its motion, which are most ridiculous even to imagine."
Against the idea of the earth's diurnal rotation he argued as follows:—"There are some who pretend that nothing prevents us from supposing that the heaven remains immovable, and the earth turns round upon its axis from west to east, accomplishing the rotation each day. It is true that, as far as the stars are concerned, there is nothing against our supposing this, if guided only by appearances, and for greater simplicity; but those who do so forget how thoroughly ridiculous it is when we consider what happens near us and in the air. For even if we admit, which is not the case, that the lighter bodies have no motion, or only move as bodies of a contrary nature, although we see that aërial bodies move with greater velocity than terrestrial—if we admit that very dense and heavy bodies have a rapid and constant motion of their own, whereas in reality they obey but with difficulty the impulses communicated to them—we should then be obliged to assert that the earth, by its rotation, has a more rapid motion than any of the bodies that are round it, as it makes so large a circuit in so short a time. In this case the bodies which are not supported by it would appear to have a motion contrary to it, and no cloud or any flying bird could ever appear to go to the east, since the earth would always move faster than it in that direction."
The Almagesta was for a long time the gospel of astronomers; to believe in the motion of the earth was to them more than an innovation, it was simply folly. Copernicus himself well expresses the state of opinion in which he found the question, and the process of his own change, in the following words:—"And I too, taking occasion by these testimonies, commenced to cogitate on the motion of the earth, and although that opinion appeared absurd, I thought that as others before me had invented an assemblage of circles to explain the motion of the stars, I might also try if, by supposing the earth to move, I could not find a better account of the motions of the heavenly bodies than that with which we are at present contented. After long researches, I am at last convinced that if we assign to the circulation of the earth the motions of the other planets, calculation and observation will agree better together. And I have no doubt that mathematicians will be of my opinion, if they will take the trouble to consider carefully and not superficially the demonstrations I shall give in this work." Although the opinions of Copernicus had been held before, it is very just that his should be the name by which they are known; for during the time that elapsed before he wrote, the adherents of such views became fewer and fewer, until at last the very remembrance of them was almost forgotten, and it required research to know who had held them and taught them. It took him thirty years' work to establish them on a firm basis. We shall make no excuse for quoting further from his book, that we may know exactly the circumstances, as far as he tells us, of his giving this system to the world.
"I hesitated for a long time whether I should publish my commentaries on the motions of the heavenly bodies, or whether it would not be better to follow the example of certain Pythagoreans, who left no writings, but communicated the mysteries of their philosophy orally from man to man among their adepts and friends, as is proved by the letter of Lysidas to Hipparchus. They did not do this, as some suppose, from a spirit of jealousy, but in order that weighty questions, studied with great care by illustrious men, might not be disparaged by the idle, who do not care to undertake serious study, unless it be lucrative, or by shallow-minded men, who, though devoting themselves to science, are of so indolent a spirit that they only intrude among philosophers, like drones among bees.
"When I hesitated and held back, my friends pressed me on. The first was Nicolas Schonberg, Cardinal of Capua, a man of great learning. The other was my best friend, Tideman Gysius, Bishop of Culm, who was as well versed in the Holy Scriptures as in the sciences. The latter pressed me so much that he decided me at last to give to the public the work I had kept for more than twenty-seven years. Many illustrious men urged me, in the interest of mathematics, to overcome my repugnance and to let the fruit of my labours see the light. They assured me that the more my theory of the motion of the earth appeared absurd, the more it would be admired when the publication of my work had dissipated doubts by the clearest demonstrations. Yielding to these entreaties, and buoying myself with the same hope, I consented to the printing of my work."
He tried to guard himself against the attacks of dogmatists by saying, "If any evil-advised person should quote against me any texts of Scripture, I deprecate such a rash attempt. Mathematical truths can only be judged by mathematicians."