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.