Another difficulty arose as regards falling bodies. If the earth is continually rotating from west to east, a body dropped from a height ought not to fall to a point vertically below its starting-point, but to a point somewhat further west, since the earth will have slipped away a certain distance during the time of the fall. To this difficulty the answer was found by Galileo’s law of inertia, but in the time of Copernicus no answer was forthcoming.
There is an interesting book by E. A. Burtt, called The Metaphisical Foundations of Modern Physical Science ( 1925), which sets forth with much force the many unwarrantable assumptions made by the men who founded modern science. He points out quite truly that there were in the time of Copernicus no known facts which compelled the adoption of his system, and several which militated against it. “Contemporary empiricists, had they lived in the sixteenth century, would have been the first to scoff out of court the new philosophy of the universe.” The general purpose of the book is to discredit modern science by suggesting that its discoveries were lucky accidents springing by chance from superstitions as gross as those of the Middle Ages. I think this shows a misconception of the scientific attitude: it is not what the man of science believes that distinguishes him, but how and why he believes it. His beliefs are tentative, not dogmatic; they are based on evidence, not on authority or intuition. Copernicus was right to call his theory a hypothesis; his opponents were wrong in thinking new hypotheses undesirable.
The men who founded modern science had two merits which are
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not necessarily found together: immense patience in observation, and great boldness in framing hypotheses. The second of these merits had belonged to the earliest Greek philosophers; the first existed, to a considerable degree, in the later astronomers of antiquity. But no one among the ancients, except perhaps Aristarchus, possessed both merits, and no one in the Middle Ages possessed either. Copernicus, like his great successors, possessed both. He knew all that could be known, with the instruments existing in his day, about the apparent motions of the heavenly bodies on the celestial sphere, and he perceived that the diurnal rotation of the earth was a more economical hypothesis than the revolution of all the celestial spheres. According to modern views, which regard all motion as relative, simplicity is the only gain resulting from his hypothesis, but this was not his view or that of his contemporaries. As regards the earth’s annual revolution, there was again a simplification, but not so notable a one as in the case of the diurnal rotation. Copernicus still needed epicycles, though fewer than were needed in the Ptolemaic system. It was not until Kepler discovered his laws that the new theory acquired its full simplicity.
Apart from the revolutionary effect on cosmic imagination, the great merits of the new astronomy were two: first, the recognition that what had been believed since ancient times might be false; second, that the test of scientific truth is patient collection of facts, combined with bold guessing as to laws binding the facts together. Neither merit is so fully developed in Copernicus as in his successors, but both are already present in a high degree in his work.
Some of the men to whom Copernicus communicated his theory were German Lutherans, but when Luther came to know of it, he was profoundly shocked. “People give ear,” he said, “to an upstart astrologer who strove to show that the earth revolves, not the heavens or the firmament, the sun and the moon. Whoever wishes to appear clever must devise some new system, which of all systems is of course the very best. This fool wishes to reverse the entire science of astronomy; but sacred Scripture tells us that Joshua commanded the sun to stand still, and not the earth.” Calvin, similarly, demolished Copernicus with the text: “The world also is stablished, that it cannot be moved” (Ps. XCIII, 1), and exclaimed: “Who will venture to place the authority of Copernicus above that of the Holy Spirit?” Protestant clergy were at least as bigoted as Catholic ecclesiastics;
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nevertheless there soon came to be much more liberty of speculation in Protestant than in Catholic countries, because in Protestant countries the clergy had less power. The important aspect of Protestantism was schism, not heresy, for schism led to national Churches, and national Churches were not strong enough to control the lay government. This was wholly a gain, for the Churches, everywhere, opposed as long as they could practically every innovation that made for an increase of happiness or knowledge here on earth.
Copernicus was not in a position to give any conclusive evidence in favour of his hypothesis, and for a long time astronomers rejected it. The next astronomer of importance was Tycho Brahe
( 1546-1601), who adopted an intermediate position: he held that the sun and moon go round the earth, but the planets go round the sun. As regards theory he was not very original. He gave, however, two good reasons against Aristotle’s view that everything above the moon is unchanging. One of these was the appearance of a new star in 1572, which was found to have no daily parallax, and must therefore be more distant than the moon. The other reason was derived from observation of comets, which were also found to be distant. The reader will remember Aristotle’s doctrine that change and decay are confined to the sublunary sphere; this, like everything else that Aristotle said on scientific subjects, proved an obstacle to progress.
The importance of Tycho Brahe was not as a theorist, but as an observer, first under the patronage of the king of Denmark, then under the Emperor Rudolf II. He made a star catalogue, and noted the positions of the planets throughout many years. Towards the end of his life Kepler, then a young man, became his assistant. To Kepler his observations were invaluable.
Kepler ( 1571-1630) is one of the most notable examples of what can be achieved by patience without much in the way of genius. He was the first important astronomer after Copernicus to adopt the heliocentric theory, but Tycho Brahe’s data showed that it could not be quite right in the form given to it by Copernicus. He was influenced by Pythagoreanism, and more or less fancifully inclined to sun-worship, though a good Protestant. These motives no doubt gave him a bias in favour of the heliocentric hypothesis. His Pythagoreanism also inclined him to follow Plato Timaeus in supposing that cosmic significance must attach to the five regular solids. He used
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them to suggest hypotheses to his mind; at last, by good luck, one of these worked.
Kepler’s great achievement was the discovery of his three laws of Planetary motion. Two of these he published in 1609, and the third in 1619. His first law states: The planets describe elliptic orbits, of which the sun occupies one focus. His second law states: The line joining a planet to the sun sweeps out equal areas in equal times. His third law states: The square of the period of revolution of a planet is proportioned to the cube of its average distance from the sun.
Something must be said in explanation of the importance of these laws.
The first two laws, in Kepler’s time, could only be proved in the case of Mars; as regards the other planets, the observations were compatible with them, but not such as to establish them definitely. It was not long, however, before decisive confirmation was found.
The discovery of the first law, that the planets move in ellipses, required a greater effort of emancipation from tradition than a modern man can easily realize. The one thing upon which all astronomers, without exception, had been agreed, was that all celestial motions are circular, or compounded of circular motions. Where circles were found inadequate to explain planetary motions, epicycles were used. An epicycle is the curve traced by a point on a circle which rolls on another circle. For example: take a big wheel and fasten it flat on the ground; take a smaller wheel which has a nail through it, and roll the smaller wheel (also flat on the ground) round the big wheel, with the point of the nail touching the ground. Then the mark of the nail in the ground will trace out an epicycle. The orbit of the moon, in relation to the sun, is roughly of this kind: approximately, the earth describes a circle round the sun, and the moon meanwhile describes a circle round the earth. But this is only an approximation. As observation grew more exact, it was found that no system of epicycles would exactly fit the facts. Kepler’s hypothesis, he found, was far more closely in accord with the recorded positions of Mars than was that of Ptolemy, or even that of Copernicus.
The substitution of ellipses for circles involved the abandonment of the æsthetic bias which had governed astronomy ever since Pythagoras. The circle was a perfect figure, and the celestial orbs were perfect bodies–originally gods, and even in Plato and Aristotle closely
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related
