The Story of the Solar System

by George F. Chambers

In the year 553 A.D., and again in the year 626 A.D. the Sun remained obscured for several months; but these facts (if facts they are) besides being ill-observed, and clothed, no doubt, in extremely exaggerated language, are brought to our notice as having occurred at epochs which are quite independent of one another, whilst the variations in the markings on the Sun, which we have just been talking about, present an almost mathematical regularity of sequence.

We must now institute some inquiries as to the causes of the periodicity of the spots. A periodicity so well established would naturally invite astronomers to seek the causes which produced it. The presence of spots only in the Zodiacal regions led Galileo to suspect the existence of some relation between the spots and the position of the planets; but there is in this a mere surmise, which, when it was made, had nothing to justify it, and it is still impossible for us to say anything for certain on the point. The determining cause of the periodicity may exist in the interior of the Sun, and may depend on circumstances which will for ever remain unknown to us. Or it may be something external: it may be due after all to the influence of the planets. It remains for us, therefore, to search and see if any such influence can be traced.

According to Wolf, the attraction of the planets, or of some of them, is the real cause of the periodicity which we are dealing with; that attraction producing on the surface of the solar globe true tides, which give birth to the spots, these tides themselves experiencing periodic variations owing to the periodic changes of position of the celestial bodies which cause them. It has even been thought safe to assert that the fact of the principal period coinciding with the revolution of Jupiter is of momentous significance; but this coincidence seems purely accidental, and no certain conclusion can be drawn as to this matter. The influence of Mercury and Venus would perhaps be much more potent, for their distance from the Sun is not very great, and this should render their influence more sensible. On the other hand, their masses appear to be too small to be capable of producing any sufficient effect.

De La Rue, Balfour Stewart, and Löwy most perseveringly studied this point of solar physics. They seem to have arrived at the conclusion that the conjunctions of Venus and Jupiter do exercise a certain amount of influence on the number of the spots and on their latitude; and that this influence is less considerable when Venus is situated in the plane of the solar equator. At any rate it is a fact, that a great number of the visible inequalities in a duly plotted curve of the spots do really correspond to special positions of these two planets.

In order to determine with more precision these coincidences and the importance which attaches to them, De La Rue extended his inquiries. He separately analysed many different groups of spots, selecting for his purpose more particularly those of which the observations happened to have been specially continuous and complete, giving a preference moreover to those which had been observed in the central portions of the Sun’s disc. From an investigation of 794 groups De La Rue arrived at the following conclusions:—(1) If we take a meridian passing through the middle of the disc and represented by a diameter perpendicular to the equator, we find that the mean size of the spots is not the same with regard to that meridian. It appears certain that the correction required for perspective does not suffice to explain this difference; and that another element must be introduced in order to secure that the apparent dimensions of the spots may be the same on both sides. We do not yet possess a very clear explanation of this fact; but the most probable is this:—the spots are surrounded by a projecting bank, which seems to disappear in part during their transit across the Sun. This bank is more elevated on the preceding than on the following side; accordingly, the spots ought to seem smaller when they are in the eastern half of the disc; larger when they are in the western half; for in the first position the observer’s eye meets an elevated obstacle, which hides a portion of the spot itself. (2) De La Rue specially studied the spots observed at the times when the planets Venus and Mars were at a heliocentric distance from the Earth equal to 0, 90, 180, and 270 degrees, and arrived at this result; the spots are larger in the part of the Sun which is away from Venus and Mars, and they are smaller on the side on which these planets happen to be. The same result was obtained, whether Carrington’s figures or the Kew photographs were employed. (3) Meanwhile it does not appear that Jupiter emits any similar influence. This influence should be easily perceived, for if we calculate the action of the planets in the way that we calculate the tides, treating it as directly proportional to the masses and inversely proportional to the cubes of the distances, the influence of Jupiter should greatly outweigh that of Venus.

Wolf thought that he had noticed traces of some influence being exerted by Saturn; but this remains altogether without confirmation.

De La Rue noticed that large spots are generally situated at extremities of the same diameter. This law also often applies to the development of large prominences. The coincidence agrees well with the theory that there exists on the Sun some action resembling that of our tides.

Whatever may be the amount of probability which attaches to these explanations we ought not to forget that we are still far off from possessing the power of giving a vigorous demonstration of them. If we consider with attention the periodical variations of the spots we shall not be long in coming to the conclusion that it is impossible to connect them directly with any one astronomical function in particular, for the spots appear in a sudden and irregular manner which contrasts in a striking degree with the continuous and progressive action of the ordinary perturbations which we meet with in the study of Celestial Mechanics. There is but one reply possible to this objection. The spots and their changes must be visible manifestations of the periodical activity of the Sun—an activity which itself depends (as assumed) on the action of the planets and on their relative positions. The cause, thus defined, of the Sun’s activity may be very regular; the activity itself may vary in a continuous manner without the resulting phenomena possessing the same continuity and the same regularity. We see this in the periodical succession of the Seasons on the Earth. The position of the Sun, and consequently its manner of acting upon our globe, varies with a remarkable uniformity, but nevertheless the meteorological phenomena which result are irregular and capricious. Thus it comes about that physicists are more and more inclined to believe that the spots are only secondary effects produced by causes more important and more fundamental.

Whatever may be our ignorance as to the causes which produce variations in the Sun’s activity we may at least draw one conclusion from the preceding remarks: it is, that the Sun is a very long way from having arrived at a state of tranquillity and freedom from internal commotion. On the contrary, it is the seat of great movements. Its activity is subject to numberless periodical changes which ought in their turn to influence the intensity of the heat and light given out by the Sun; and so re-act on the planets which receive their heat, light, and life from the Sun.

No account of the periodicity of the spots on the Sun can be deemed complete which does not include information respecting certain other periodical phenomena which have been found to exhibit features of alternation closely resembling in their sequence and character the periodical changes which take place in regard to the spots on the Sun. There is evidently a deep mystery lying hid under the curious fact (which is clearly established) that the 11-year period of the spots coincides in a manner as unexpected as it is certain with the period of the variation of terrestrial magnetism. The magnetic needle is subject to a diurnal variation which reaches its extreme amount every 11 years, and not only so, but the epoch of maximum variation corresponds with the epoch of the maximum prevalence of Sun spots. And similarly years in which the needle is least disturbed are also years in which the Sun spots are fewest. Two other very curious discoveries have also been made which are in evident close connection with the foregoing. The manifestation of the Aurora Borealis and of those strange currents of electricity known as magnetic earth currents (which travel below the Earth’s surface and frequently interfere with telegraphic operations), likewise exhibit periodical changes which take 11 years to go through all their stages. This fact alone would be sufficiently curious, but when we come to find that the curve which exhibits the changes these two manifestations of force go through, also shows that their maxima and minima are contemporaneous with the maxima and minima of the Sun spots and magnetic needle variations, we cannot doubt that (to use Balfour Stewart’s words) “a bond of union exists between these four phenomena. The question next arises, what is the nature of this bond? Now, with respect to that which connects Sun spots with magnetic disturbances we can as yet form no conjecture.” To cut a long story short, it may be said generally that whilst without doubt electricity is the common basis of the three last-named of the four phenomena just mentioned, it seems scarcely too great a stretch of the imagination to go one step further and suggest that electricity has in some or other occult manner something to do with all these things and therefore with the spots on the Sun.