Myths and Marvels of Astronomy

scarcely visible (when at a distance), in the latter it is seen as a well-defined streak; and as a very slight change of position of each bird may often suffice to render an extensive flight thus visible throughout its entire length, which but a few moments before had been invisible, so the entire length of a comet's tail may be brought into view, and apparently be formed in a few hours, through some comparatively slight displacement of the individual meteorites composing it. This paradox--for paradox it unquestionably is--affords a curious illustration of the influence which mathematical power has on the minds of men. Every one knows that Professor Tait has potential mathematical energy competent to dispose, in a very short time, of all the difficulties involved in his theory; therefore few seem to inquire whether this potential energy has ever been called into action. It is singular, too, that other mathematicians of great eminence have been content to take the theory on trust. Thus Sir W. Thomson, at the meeting of the British Association at Edinburgh, described the theory as disposing easily of the difficulties presented by Newton's comet in 1680. Glashier, in his translation of Guillemin's 'Les Cometes,' speaks of the theory as one not improbably correct, though only to be established by rigid investigation of the mathematical problems involved. In reality, not five minutes' inquiry is needed to show any one acquainted with the history of long-tailed comets that Tait's theory is quite untenable. Take Newton's comet. It had a tail ninety millions of miles long, extending directly from the sun as the comet approached him, and seen, four days later, extending to the same distance, and still directly from the sun, as the comet receded from him in an entirely different direction. According to Tait's sea-bird theory, the earth was at both these epochs in the plane of a sheet of meteorites forming the tail; but on each occasion the sun also was in the same plane, for the edge of the sheet of meteorites was seen to be directly in a line with the sun. The comet's head, of course, was in the same plane; but three points, not in a straight line, determine a plane. Hence we have, as the definite result of the sea-bird theory, that the layer or stratum of meteorites, forming the tail of Newton's comet, lay in the same plane which contained the sun, the earth, and the comet. But the comet crossed the ecliptic (the plane in which the earth travels round the sun) between the epochs named, crossing it at a great angle. When crossing it, then, the great layer of meteorites was in the plane of the ecliptic; before crossing it the layer was greatly inclined to that plane one way, and after crossing it the layer was greatly inclined to that plane another way. So that we have in no way escaped the difficulty which the sea-bird theory was intended to remove. If it was a startling and, indeed, incredible thing that the particles along a comet's tail should have got round in four days from the first to the second position of the tail considered above, it is as startling and incredible that a mighty layer of meteorites should have shifted bodily in the way required by the sea-bird theory. Nay, there is an element in our result which is still more startling than any of the difficulties yet mentioned; and that is, the singular care which the great layer of meteorites would seem to have shown to keep its plane always passing through the earth, with which it was in no way connected. Why should this preference have been shown by the meteor flock for our earth above all the other members of the solar system?--seeing that the sea-bird theory _requires_ that this comet, and not Newton's comet alone but all others having tails, should not only be thus complaisant with respect to our little earth, but should behave in a totally different way with respect to every other member of the sun's family. We can understand that, while several have been found who have applauded the sea-bird paradox for what it _might_ do in explaining comets' tails, its advocates have as yet not done much to reconcile it with cometic observation. The latest astronomical paradox published is perhaps still more startling. It relates to the planet Venus, and is intended to explain the appearance presented by this planet when crossing the sun's face, or, technically, when in transit. At this time she is surrounded by a ring of light, which appears somewhat brighter than the disc of the sun itself. Before fully entering on the sun's face, also, the part of Venus's globe as yet outside the sun's disc is seen to be girt round by a ring of exceedingly bright light--so bright, indeed, that it has left its record in photographs where the exposure was only for the small fraction of a second allowable in the case of so intensely brilliant a body as the sun. Astronomers have not found it difficult to explain either peculiarity. It has been proved clearly in other ways that Venus has an atmosphere like our own, but probably denser. As the sun is raised into view above the horizon (after he has really passed below the horizon plane) by the bending power of our air upon his rays, so the bending power of Venus's air brings the sun into our view round the dark body of the planet. But the new paradox advances a much bolder theory. Instead of an atmosphere such as ours, Venus has a glass envelope; and instead of a surface of earth and water, in some cases covered with clouds, Venus has a surface shining with metallic lustre.[53] The author of this theory, Mr. Jos. Brett, startled astronomers by announcing, a few years ago, that with an ordinary telescope he could see the light of the sun's corona without the aid of an eclipse, though astronomers had observed that the delicate light of the corona fades out of view with the first returning rays of the sun after total eclipse. The latest paradoxist, misled by the incorrect term 'centrifugal force,' proposes to 'modify, if not banish,' the old-fashioned astronomy. What is called centrifugal force is in truth only inertia. In the familiar instance of a body whirled round by a string, the breaking of the string no more implies that an active force has pulled away the body, than the breaking of a rope by which a weight is pulled implies that the weight has exerted an active resistance. Of course, here again the text-books are chiefly in fault. Such are a few among the paradoxes of various orders by which astronomers, like the students of other sciences, have been from time to time amused. It is not altogether, as it may seem at first sight, 'a sin against the twenty-four hours' to consider such matters; for much may be learned not only from the study of the right road in science, but from observing where and how men may go astray. I know, indeed, few more useful exercises for the learner than to examine a few paradoxes, when leisure serves, and to consider how, if left to his own guidance, he would confute them. XI. _ON SOME ASTRONOMICAL MYTHS._ The expression 'astronomical myth' has recently been used, on the title-page of a translation from the French, as synonymous with false systems of astronomy. It is not, however, in that sense that I here use it. The history of astronomy presents the records of some rather perplexing observations, not confirmed by later researches, but yet not easily to be explained away or accounted for. Such observations Humboldt described as belonging to the myths of an uncritical period; and it is in that sense that I employ the term 'astronomical myth' in this essay. I propose briefly to describe and comment on some of the more interesting of these observations, which, in whatever sense they are to be interpreted, will be found to afford a useful lesson. It is hardly necessary, perhaps, to point out that the cases which I include here I regard as really cases in which astronomers have been deceived by illusory observations. Other students of astronomy may differ from me as respects some of these instances. I do not wish to dogmatise, but simply to describe the facts as I see them, and the impressions which I draw from them. Those who view the facts differently will not, I think, have to complain that I have incorrectly described them. At the outset, let me point out that some observations which were for a long time regarded as mythical have proved to be exact. For instance, when as yet very few telescopes existed, and those very feeble, Galileo's discovery of moons travelling round Jupiter was rejected as an illusion for which Satan received the chief share of credit. There is an amusing and yet in one aspect almost pathetic reference to this in his account of his earlier observations of Saturn. He had seen the planet apparently attended on either side by two smaller planets, as if helping old Saturn along. But on December 4, 1612,[54] turning his telescope on the planet, he found to his infinite amazement not a trace of the companion planets could be seen; there in the field of view of his telescope was the golden-tinted disc of the planet as smoothly rounded as the disc of Mars or Jupiter. 'What,' he wrote, 'is to be said concerning so strange a metamorphosis? Are the two lesser stars consumed after the manner of the solar spots? Have they vanished or suddenly fled? Has Saturn, perhaps, devoured his children? Or were the appearances, indeed, illusion or fraud with which the glasses have so long deceived me as well as many others to whom I have shown them? Now, perhaps, is the time come to revive the well-nigh withered hopes of those who, guided by more profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter impossibility of the existence of those things which the telescope appears to show. I do not know what to say in a case so surprising, so unlooked for, and so novel. The shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me.' We now know that these observations, as well as those made soon after by Hevelius, though wrongly interpreted, were correct enough. Nay, we know that if either Galileo or Hevelius had been at the pains to reason out the meaning of the alternate visibility and disappearance of objects looking like attendant planets, they must have anticipated the discovery made in 1656 by Huyghens, that Saturn's globe is girdled about by a thin flat ring so vast that, if a score of globes like our earth were set side by side, the range of that row of worlds would be less than the span of the Saturnian ring system. There is a reference in Galileo's letter to the solar spots; 'Are the two lesser stars,' he says, 'consumed after the manner of the solar spots?' When he thus wrote the spots were among the myths or fables of astronomy, and an explanation was offered, by those who did not reject them utterly, which has taken its place among forsaken doctrines, those broken toys of astronomers. It is said that when Scheiner, himself a Jesuit, communicated to the Provincial of the Jesuits his discovery of the spots on the sun, the latter, a staunch Aristotelian, cautioned him not to see these things. 'I have read Aristotle's writings from beginning to end many times,' he said, 'and I can assure you I have nowhere found in them anything similar to what you mention' [amazing circumstances!] 'Go, therefore, my son, tranquillise yourself; be assured that what you take for spots on the sun are the faults of your glasses or your eyes.' As the idea was obviously inadmissible that a celestial body could be marked by spots, the theory was started that the dark objects apparently seen on the sun's body were in reality small