1 In passing, however, I would venture to touch on this question of central suns, or of central but opaque orbs round which the stars may revolve, in order to remove a very prevalent misconception. It seems to be commonly supposed that we cannot imagine such orbs to lie far enough away to account for their not being discernible either as orbs of light or by hiding more distant stars, without depriving them of the attractive influence necessary to sway the motions of the stars. This, however, is not the case. An orb looking as bright as Sirius, but ten times as far away, if of equal density and inherent brightness, would be a thousand times more massive, while the effect of distance would only be to reduce its attraction one hundred times. It would, therefore, attract our sun ten times as strongly as Sirius actually does. In like manner, an orb one hundred times as far away as Sirius, but so large as to appear as bright, would attract our sun one hundred times as strongly, and so on. So that it cannot be positively asserted that among the stars visible to us there may not be the central sun of the sidereal scheme-inordinately large and massive compared with the rest, but reduced by distance to the same order of brightness.
Thus we see that the sun cannot be regarded as an orb moving within the scheme of stars, and by his own movement causing the chief apparent motions of the surrounding orbs. His motion is but part of a grand scheme of motions, whose laws are as yet unknown to us. We may recognise in the method of research which has now been so successfully applied, the sole means of determining what those laws may be. We can now tell the very rate, in miles per annum, at which the suns are approaching or receding from us; and though we have no reason for believing that our sun occupies in any sense a central position - so that we have yet to learn at what rate and in what way the stars move around the true centre of their system, - yet it is far from unlikely that if we can but ascertain the motions of a sufficient number of stars, we shall have the means of judging where the centre lies round which these motions are taking place.
The astronomer may well look with doubt, however, on the efforts which are being made to solve this stupendous problem. If we may judge from the analogy of our own solar system, we can see that in the far more complicated scheme of the stars there must exist innumerable features to perplex the observer. If we imagine a being placed in the midst of the solar system, and enabled to study the various apparent motions visible from his stand-point, and if we further suppose him gifted with the power of measuring the rate at which the various orbs are approaching him or receding from him, then we know that if his scrutiny were but continued long enough, he could not fail to recognise the laws which exist within that system and regulate all those motions. Where at first all had seemed confusion, our imaginary observer would recognise in the course of time a beautiful harmony; motions which had appeared discordant would be found to be in reality subordinated into one grand scheme. But if we suppose our observer to occupy his imaginary stand-point for a few hours, or even for a few days only, how imperfect would be his ideas of the harmony of the celestial motions! He would see the primary planets moving apparently in diverse directions and at inconsistent rates; the secondary planets apparently travelling with non-accordant motions and on different paths; the asteroids would perplex him by their wide range of apparent distribution; meteoric systems would appear to conform to no recognisable law; and the movements of comets would seem altogether inexplicable.
Yet the terrestrial observer of the infinitely more complicated sidereal system is in reality even less favourably circumstanced than our imaginary observer of the planetary scheme. The motions which come within his ken are more minute, compared with the real dimensions of the stellar paths, than the motion of Saturn or Jupiter in a single second compared with the wide orbits traversed by these planets. We cannot tell whether the observed motion of a star is that by which it is carried on some vast independent orbit; or is its motion within some subordinate scheme; or, lastly, is for the most part due to the sun's own motion within the sidereal system. When we see the stars of the same constellation carried in different directions, we cannot tell whether the real motions are diverse in character, or whether the diversity is but apparent, like the apparent advance and retrogression of planets which, nevertheless, are travelling in a common direction around a common centre.
But precisely because the difficulties which surround the problem of the stellar motions are so stupendous, we must so much the more carefully examine every feature which observation may reveal to us. To do otherwise were to abandon the problem as altogether hopeless.
Now it cannot but be recognised that in this respect the new method of research is peculiarly promising. For whereas all former methods have dealt only with apparent motion, this method tells us of the real rate of stellar displacements. We have seen how it has disposed of the inferences which had been formed as to the sun's velocity, and the average velocities of stellar motion; let us inquire what has been its bearing on the views of astronomers respecting the stellar universe regarded as a scheme or system.
Other methods of dealing with the motions of the stars had related chiefly to the question of the sun's journey through space, until Madler was led to inquire whether the motions of the stars might not afford the means of determining where the centre of the stellar system may lie. Limiting his range of inquiry, in the first instance, by certain preliminary considerations, he proceeded to examine the direction of the apparent stellar motions in a particular region of the heavens. It seemed likely to him that the centre of the universe would be near the Milky Way, and probably on that band of conspicuous stars which extends over the Greater Dog, Orion, the Bull, Perseus, and Cassiopeia. Still further, he reasoned that if the sun is circling around the central orb, this body must lie on a line square to the sun's path; so that if we imagine a line extending from the point in the heavens from which the sun is travelling to the point towards which he is travelling, then the central orb must lie somewhere on or near to a plane through the sun and square to that line. Now such a plane would cut the Milky Way in two places, one in the northern heavens in Perseus, the other in the southern heavens between the Altar and the Centaur. Madler further indicates reasons for believing that the centre of the sidereal universe lies towards the northern region of the Milky Way. Lastly, seeing that not far from the northern region there is a remarkable star cluster, the Pleiades, he was led to examine the region around the Pleiades for those signs which he thought likely to exist towards that part of the heavens where lies the centre of the sidereal universe. We do not enter here into a consideration of the reasoning which led Madler to conclude that in that part of the heavens the stars would all appear to be moving in the same general direction, for they are rather recondite. That, however, was his anticipation; and as he found that the stars in the constellation Taurus are nearly all moving southwards, he was satisfied that he had not been mistaken in setting the Pleiades as the central region of the universe, and the star Alcyone, the brightest of the Pleiades, as the central orb around which all the stars revolve.