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.