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By Root 3377 0

sun. The fact that the sun is not exactly in the centre marred the simplicity of his theory. Though he had heard of the Pythagorean doctrines, he does not seem to have known of Aristarchus's heliocentric theory, but there is nothing in his speculations that could not have occurred to a Greek astronomer. What was important in his work was the dethronement of the earth from its geometrical pre-eminence. In the long run, this made it difficult to give to man the cosmic importance assigned to him in the Christian theology, but such consequences of his theory would not have been accepted by Copernicus, whose orthodoxy was sincere, and who protested against the view that his theory contradicted the Bible.

There were genuine difficulties in the Copernican theory. The greatest of these was the absence of stellar parallax. If the earth at any one point of its orbit is 186,000,000 miles from the point at which it will be in six months, this ought to cause a shift in the apparent positions of the stars, just as a ship at sea which is due north from one point of the coast will not be due north from another. No parallax was observed, and Copernicus rightly inferred that the fixed stars must be very much more remote than the sun. It was not till the nineteenth century that the technique of measurement became sufficiently precise for stellar parallax to be observed, and then only in the case of a few of the nearest stars.

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 Metaphysical 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 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