Galileo and His Judges

by F. R. Wegg-Prosser

The effect of all this on the Copernican system and the evidence on which it rested, was to raise that system from a simple though strong probability, a question on which at any rate something might be said for and against it, to a probability of almost overwhelming force; for it not only showed how the heavenly bodies moved, but it explained the cause of their motions, and in a word furnished the key that unlocked the arcana of Nature. When you came to know not only how the Moon and the planets moved, but the law which regulated their movements, and when you found that all fitted into one harmonious whole (at least with some minor exceptions), it was not easy to refuse assent to a theory supported by such powerful evidence.

Yet in saying this we are perhaps rather viewing the question from our present standpoint, than as a contemporary would have done. As a matter of fact, Newton’s hypothesis, though eagerly received in England, met with a long opposition on the Continent, and particularly in France, where Descartes’ theory of vortices reigned supreme for many years. It must not be supposed that these Cartesian philosophers were anti-Copernicans; far otherwise, only they accounted for the celestial motions in a different way from Newton, and, as every one now admits, in a wrong way.

I have already remarked that there were some apparent difficulties in the application of the law of universal gravitation to all the heavenly bodies, and that these have been removed by subsequent calculation. One of these difficulties, if indeed it could be so called (for it hardly amounted to that), has been solved within living memory. It was noticed that the planet Uranus showed signs of perturbation from some unknown reason; and even the work I have just quoted, “Whewell’s History of the Inductive Sciences,” published in 1847, contains the following sentence: “Uranus still deviates from his tabular place, and the cause remains yet to be discovered.” Two astronomers, one French and one English, Le Verrier and Adams, found out the cause by discovering the existence, each independently of the other, of an exterior planet revolving in an orbit more distant by far than that of Uranus; to this planet the name of Neptune has been given, and his existence is one more confirmatory proof of the theory of gravitation.

The Copernican system had been built up and consolidated by Newton’s great discovery; but another piece of evidence, of a most important character, was added by the investigations of Bradley, Professor of Astronomy at Oxford, and afterwards Astronomer Royal; this careful observer, while engaged in endeavouring to detect such an apparent motion of the fixed stars (so called) as would indicate an annual parallax, noticed that another motion existed different from that which the annual parallax would produce, and for which he could not account; the apparent orbits described by the stars observed depended on the distance of the stars from the pole of the ecliptic; the phenomenon was different from anything hitherto discovered, and one or two modes of explanation were tried in vain. Accident, however, turned Bradley’s thoughts in the right direction; he was one day in a boat on the Thames, and observed that the vane on the mast gave a different apparent direction to the wind, according as the boat sailed in different courses. Here, then, was the solution of the difficulty: it was already known from Römer’s investigations that light moved with a finite velocity, and if so it would naturally produce the same effect as that observed in the boat, or to take an illustration very commonly given, like that which any one finds when moving along rapidly in a shower of rain, in which latter case the rain seems to fall not in the direction it has when one is at rest, but in a direction compounded of that and the one opposite to the person’s line of motion.

Bradley soon drew the correct conclusion, that light acted in precisely the same way upon the Earth as it moved in its orbit, and that the apparent annual displacement of the stars, as detected by him, arose from this sole cause. All the great astronomers who followed him have agreed with his conclusions, and the phenomenon in question, which is called the aberration of light, has conferred a lasting fame on its discoverer. And the remarkable point about it is this, that not only does it give a fresh illustration to the Copernican theory, but it is one of the very few scientific facts that cannot (so far as our knowledge of the subject goes) be explained in any other way. It is, therefore, generally considered as a critical test of the truth of the system.

There are two other phenomena, on which however I do not propose to dwell at any length, known as precession and nutation, which it is not easy to explain otherwise than by the modern theory of astronomy and the principle of gravitation; the latter of these two owed its discovery to Bradley, and the former to Hipparchus, who could not have been aware of its real cause, though he had observed the fact of its occurrence.

But passing on from these, I may call attention to one most remarkable result of modern scientific research, connected with the stars. In Galileo’s day, it was a drawback to the Copernican theory that none of the stars showed the smallest annual parallax; in popular language, none of them seemed to undergo any change of place, however small, when observed at opposite points of the Earth’s orbit, or as the opponents would have said, the Earth’s imagined orbit. A displacement of this kind, I need hardly repeat, must not be confounded with that other motion which Bradley observed and explained. This was one of Tycho Brahé’s reasons for rejecting the Copernican system, and it was one of the best arguments used by the opponents of Galileo. As the enormous distance of the stars from the Earth was, as we have already seen, at that time unknown, the celestial distances generally being under-estimated even by the best astronomers, the argument had an apparent force, which no one now would attribute to it. Galileo himself had some hope of overcoming the difficulty by discovering some annual displacement in certain stars, but it is needless to add that his instruments were unequal to such a task. Subsequent observers tried various methods, but without any real success until the present century, when Bessel and other observers found that a star called 61 Cygni had a certain annual parallax; and not long afterwards, Henderson, making his observations at the Cape of Good Hope on a conspicuous star in the constellation of the Centaur, a constellation belonging to the southern hemisphere, found at length that this star, which in fact is a double star, and known as α Centauri, had a parallax of nearly 1″; subsequent calculations show it to be probably rather less, that is to say about 0″·91. This means that it is more than twenty billions of miles distant, and that light takes more than three years to travel from α Centauri to the earth. It is, however, believed to be much the nearest of all the stars, no other coming within double of the distance.

Now it is difficult to evade the conclusion which naturally follows from these results, that the Earth really does move in an annual orbit round the Sun. It is no part of my present task to give a list of the stars of which the parallax has been found, but I may say there are several others besides the two I have named; and I know of no method of accounting for the fact in any way but by the annual motion of the Earth, unless we suppose some instrumental error to have occurred. There have been so many of these in times past that it may seem rash to exclude such a possibility, but, considering the perfection of modern scientific instruments, it is in the highest degree improbable; and we may fairly reckon the parallaxes of the stars as a strong confirmation of the already strong evidence in favour of the Copernican theory—a theory which, as we have seen, was, from a purely scientific point of view, very probable in the days of Galileo, overwhelmingly probable after the great discovery of Newton, and at the present time, with all the light that subsequent research and observation have thrown on it, scarcely short of a moral certainty.

I may repeat once more that it has not, indeed, that absolute physical certainty, arising from direct experiment, which has been obtained in other scientific investigations; but, allowing for this faint element of instability, we may fairly say that no truth of natural philosophy stands on a firmer basis.

And for Galileo, who lived before the day when, as Whewell says, “Astronomy passed from boyhood to mature manhood,” we may fairly say that, after we have censured his faults and his errors, after we have ascertained that he was not a hero or a “martyr of science,” we must still recognise the fact that he was one of the greatest natural philosophers of his day, pre-eminent in astronomy, in mechanics, in mathematics. To his honour also be it added, that his religious faith, and his respect for the Church and her authority, so far as we can judge, never failed. Whatever his defects may have been—want of prudence, want of candour, want of consideration for others—we can easily perceive that he would never have been willingly drawn into any controversy intended to provoke antagonism between Religion and Science.