Part 18

The zetetic faithful reason about this matter as though the impressions of the senses were trustworthy under all conditions, familiar or otherwise; whereas, in point of fact, we know that the senses often deceive, even under familiar conditions, and almost always deceive under conditions, which are not familiar. A person, for example, accustomed to the mist and haze of our British air, is told by the sense of sight, when he is travelling where a clearer atmosphere prevails, that a mountain forty miles from him is a hill a few miles away. On the other hand, an Italian travelling through the Highlands is impressed with the belief that all the features of the scenery are much larger (because he supposes them much more remote) than they really are. A hundred such instances of deception might easily be cited. The conditions under which the aeronaut observes the earth are certainly less familiar than those under which the Briton views the Alps and Apennines, or the Italian views Ben Lomond or Ben Lawers. It would be rash, therefore, even if no other evidence were available, to reject the faith that the earth is a globe because, as seen from a balloon, it looks like a basin. Indeed, to be strictly logical, the followers of Parallax ought on this account to adopt the faith that the earth is not flat, but basin-shaped, which hitherto they have not been ready to do.

We have seen that Parallax describes a certain experiment on the Bedford Level, which, if made as he states, would have shown certainly that something was wrong in the accepted system--for a six-mile straight-edge along water would be as severe a blow to the belief in a round earth, as a straight line on the sea-surface from Queenstown to New York. Another curious experiment adorns his little book, which, if it could be repeated successfully before a dozen trustworthy witnesses, would rather astonish men of science. Having, he says, by certain reasoning--altogether erroneous, but that is a detail--convinced himself that, on the accepted theory, a bullet fired vertically upwards ought to fall far to the west of the place whence it was fired, he carefully fixed an air-gun in a vertical position, and fired forty bullets vertically upwards. All these fell close to the gun--which is not surprising, though it must have made such an experiment rather dangerous; but two fell back into the barrel itself--which certainly was very surprising indeed. One might fairly challenge the most experienced gunner in the world to achieve one such vertical shot in a thousand trials; two in forty bordered on the miraculous.

The earth-flatteners I have been speaking of claim, as one of their objects, the defence of Scripture. But some of the earth-flatteners of the last generation (or a little farther back) took quite another view of the matter. For instance, Sir Richard Phillips, a more vehement earth-flattener than Parallax, was so little interested in defending the Scriptures, that in 1793 he was sentenced to a year's imprisonment for selling a book regarded as atheistic. In 1836 he attempted the conversion of Professor De Morgan, opening the correspondence with the remark that he had 'an inveterate abhorrence of all the pretended wisdom of philosophy derived from the monks and doctors of the Middle Ages, and not less those of higher name who merely sought to make the monkish philosophy more plausible, or so to disguise it as to mystify the mob of small thinkers.' He seems himself to have succeeded in mystifying many of those whom he intended to convert. Admiral Smyth gives the following account of an interview he had with Phillips: 'This pseudo-mathematical knight once called upon me at Bedford, without any previous acquaintance, to discuss "those errors of Newton, which he almost blushed to name," and which were inserted in the "Principia" to "puzzle the vulgar." He sneered with sovereign contempt at the "Trinity of Gravitating Force, Projectile Force, and Void Space," and proved that all change of place is accounted for by motion.' [Startling hypothesis!] 'He then exemplified the conditions by placing some pieces of paper on a table, and slapping his hand down close to them, thus making them fly off, which he termed applying the momentum. All motion, he said, is in the direction of the forces; and atoms seek the centre by "terrestrial centripetation"--a property which causes universal pressure; but in what these attributes of pushing and pulling differ from gravitation and attraction was not expounded. Many of his "truths" were as mystified as the conundrums of Rabelais; so nothing was made of the motion.'

A favourite subject of paradoxical ideas has been the moon's motion of rotation. Strangely enough, De Morgan, who knew more about past paradoxists than any man of his time, seems not to have heard of the dispute between Keill and Bentley over this matter in 1690. He says, 'there was a dispute on the subject, in 1748, between James Ferguson and an anonymous opponent; and I think there have been others;' but the older and more interesting dispute he does not mention. Bentley, who was no mathematician, pointed out in a lecture certain reasons for believing that the moon does not turn on her axis, or has no axis on which she turns. Keill, then only nineteen years old, pointed out that the arguments used by Bentley proved that the moon does rotate instead of showing that she does not. (Twenty years later Keill was appointed Savilian Professor of Astronomy at Oxford. He was the first holder of that office to teach the Newtonian astronomy.)

In recent times, as most of my readers know, the paradox that the moon does not rotate has been revived more than once. In 1855 it was sustained by Mr. Jellinger Symons, one of whose staunchest supporters, Mr. H. Perigal, had commenced the attack a few years earlier. Of course, the gist of the argument against the moon's rotation lies in the fact that the moon always keeps the same face turned towards the earth, or very nearly so. If she did so exactly, and if her distance from the earth were constantly the same, then her motion would be exactly the same as though she were rigidly connected with the earth, and turned round an axis at the earth. The case may be thus illustrated: Through the middle of a large orange thrust one short rod vertically, and another long rod horizontally; thrust the further end of the latter through a small apple, and now turn the whole affair round the short vertical rod as an axis. Then the apple will move with respect to the orange as the moon would move with respect to the earth on the suppositions just made. No one in this case would say that the apple was turning round on its axis, since its motion would be one of rotation round the upright axis through the orange. Therefore, say the opponents of the moon's rotation, no one should say that the moon turns round on her axis.

Of course, the answer would be obvious even if the moon's motions were as supposed. The moon is not connected with the earth as the apple is with the orange in the illustrative case. If the apple, without rigid connection with the orange, were carried round the orange so as to move precisely as if it were so connected, it would unquestionably have to rotate on its axis, as any one will find who may try the experiment. Thus for the straight rod thrust through the apple substitute a straight horizontal bar carrying a small basin of water in which the apple floats. Sway the bar steadily and slowly round, and it will be found (if a mark is placed on the apple) that the apple no longer keeps the same face towards the centre of motion; but that, to cause it to do so, a slow motion of rotation must be communicated to the apple in the same direction and at the same rate (neglecting the effects of the friction of the water against the sides of the basin) as the bar is rotating. In my 'Treatise on the Moon' I have described and pictured a simple apparatus by which this experiment may easily be made.

But, of course, such experiments are not essential to the argument by which the paradox is overthrown. This argument simply is, that the moon as she travels on her orbit round the sun--the real centre of her motion--turns every part of her equator in succession towards him once in a lunar month. At the time of new moon the sun illuminates the face of the moon turned from us; at the time of full moon he illuminates the face which has been gradually brought round to him as the moon has passed through her first two quarters. As she passes onwards to new moon again, the face we see is gradually turned from him until he shines full upon the other face. And so on during successive lunations. This could not happen unless the moon rotated. Again, if we lived on the moon we should find the heaven of the fixed stars turning round from east to west once in rather more than twenty-seven days; and unless we supposed, as we should probably do for a long time, that our small world was the centre of the universe, and that the stars turned round it, we should be compelled to admit that it was turning on its own axis from west to east once in the time just named. There would be no escape. The mere fact that all the time the stars thus seemed to be turning round the moon, the earth would not so seem to move, but would lie always in the same direction, would in no sort help to remove the difficulty. Lunarian paradoxists would probably argue that she was in some way rigidly connected with the moon; but even they would never think of arguing that their world did not turn on its axis, unless they maintained that it was the centre of the universe. This, I think, they would very probably do; but as yet terrestrial paradoxists have not, I believe, maintained this hypothesis. I once asked Mr. Perigal whether that was the true theory of the universe--the moon central, the earth, sun, and heavens carried round her. He admitted that his objections to accepted views were by no means limited to the moon's rotation; and, if I remember rightly, he said that the idea I had thrown out in jest was nearer the truth than I thought, or used words to that effect. But as yet the theory has not been definitely enunciated that the moon is the boss of the universe.

Comets, as already mentioned, have been the subjects of paradoxes innumerable; but as yet comets have been so little understood, even by astronomers, that paradoxes respecting them cannot be so readily dealt with as those relating to well-established facts. Among thoroughly paradoxical ideas respecting comets, however, may be mentioned one whose author is a mathematician of well-deserved repute--Professor Tait's 'Sea-Bird Theory' of Comets' Tails. According to this theory, the rapid formation of long tails and the rapid changes of their position may be explained on the same principle that we explain the rapid change of appearance of a flight of sea-birds, when, from having been in a position where the eye looks athwart it, the flight assumes a position where the eye looks at it edgewise. In the former position it is 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 Comètes,' 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.

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, 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 planets revolving round the sun, and a contest arose for the possession of these mythical planets. Tardé maintained that they should be called Astra Borbonia, in honour of the royal family of France; but C. Malapert insisted that they should be called Sidera Austriaca. Meantime the outside world laughed at the spots, and their names, and the astronomers who were thought to have invented both. 'Fabritius puts only three spots,' wrote Burton in his 'Anatomy of Melancholy,' 'and those in the sun; Apelles 15, and those without the sun, floating like the Cyanean Isles in the Euxine Sea. Tardé the Frenchman hath observed 33, and those neither spots nor clouds as Galileus supposed, but planets concentric with the sun, and not far from him, with regular motions. Christopher Schemer' [a significant way of spelling Scheiner's name], 'a German Suisser Jesuit, divides them in maculas et faculas, and will have them to be fixed in solis superficie and to absolve their periodical and regular motions in 27 or 28 dayes; holding withall the rotation of the sun upon his centre, and are all so confident that they have made schemes and tables of their motions. The Hollander censures all; and thus they disagree among themselves, old and new, irreconcilable in their opinions; thus Aristarchus, thus Hipparchus, thus Ptolomæus, thus Albategnius, etc., with their followers, vary and determine of these celestial orbs and bodies; and so whilst these men contend about the sun and moon, like the philosophers in Lucian, it is to be feared the sun and moon will hide themselves, and be as much offended as she was with those, and send another message to Jupiter, by some new-fangled Icaromenippus, to make an end of all these curious controversies, and scatter them abroad.'

It is well to notice how in this, as in many other instances, the very circumstance which makes scientific research trustworthy caused the unscientific to entertain doubt. If men of science were to arrange beforehand with each other what observations they should publish, how their accounts should be ended, what theories they would endeavour to establish, their results would seem far more trustworthy, their theories far more probable, than according to the method actually adopted. Science, which should be exact, seems altogether inexact, because one observer seems to obtain one result, another a different result. Scientific theories seem unworthy of reliance because scientific men entertain for a long time rival doctrines. But in another and a worthier sense than as the words are used in the 'Critic,' when men of science do agree their agreement is wonderful. It is wonderful, worthy of all admiration, because before it has been attained errors long entertained have had to be honestly admitted; because the taunt of inconsistency is not more pleasant to the student of science than to others, and the man who having a long time held one doctrine adopts and enforces another (one perhaps which he had long resisted), is sure to be accused by the many of inconsistency, the truly scientific nature of his procedure being only recognised by the few. The agreement of men of science ought to be regarded also as most significant in another sense. So long as there is room for refusing to admit an important theory advanced by a student of science, it is natural that other students of science should refuse to do so; for in admitting the new theory they are awarding the palm to a rival. In strict principle, of course, this consideration ought to have no influence whatever; as a matter of fact, however, men of science, being always men and not necessarily strengthened by scientific labours against the faults of humanity, the consideration has and must always have influence. Therefore, when the fellow-writers and rivals of Newton or of his followers gave in their adhesion to the Newtonian theory; when in our own time--but let us leave our own time alone, in this respect--when, speaking generally, a novel doctrine, or some new generalisation, or some great and startling discovery, is admitted by rival students of the branch of astronomy to which it belongs, the probability is great that the weight of evidence has been found altogether overwhelming.

Let us now, however, turn to cases in which, while many observations seem to point to some result, it has appeared that, after all, those observations must have been illusory.

A striking instance in point is found in the perplexing history of the supposed satellite of Venus.

On January 25, 1672, the celebrated astronomer, J.D. Cassini saw a crescent shaped and posited like Venus, but smaller, on the western side of the planet. More than fourteen years later, he saw a crescent east of the planet. The object continued visible in the latter case for half an hour, when the approach of daylight obliterated the planet and this phantom moon from view. The apparent distance of the moon from Venus was in both cases small, viz., only one diameter of the planet in the former case, and only three-fifths of that diameter in the latter.

Next, on October 23, 1740, old style, the optician Short, who had had considerable experience in observation, saw a small star perfectly defined but less luminous than Venus, at a distance from the planet equal to about one-third of the apparent diameter of our moon. This is a long distance, and would correspond to a distance from Venus certainly not less than the moon's distance from the earth. Short was aware of the risk of optical illusion in such matters, and therefore observed Venus with a second telescope; he also used four eye-pieces of different magnifying power. He says that Venus was very distinct, the air very pure, insomuch that he was able to use a power of 240. The seeming moon had a diameter less than a third of Venus's, and showed the same phase as the planet. Its disc was exceedingly well defined. He observed it several times during a period of about one hour.

Still more convincing, to all appearance, is the account of the observations made by M. Montaigne, as presented to the Academy of Sciences at Paris by M. Baudouin in 1761. The transit of Venus which was to take place on June 6 in that year led to some inquiry as to the satellite supposed to have been seen by Cassini and Short, for of course a transit would be a favourable occasion for observing the satellite. M. Montaigne, who had no faith in the existence of such an attendant, was persuaded to look for it early in 1761. On May 3 he saw a little crescent moon about twenty minutes of arc (nearly two-thirds the apparent diameter of our moon) from the planet. He repeated his observation several times that night, always seeing the small body, but not quite certain, despite its crescent shape, whether it might not be a small star. On the next evening, and again on May 7 and 10, he saw the small companion apparently somewhat farther from Venus and in a different position. He found that it could be seen when Venus was not in the field of view. The following remarks were made respecting these observations in a French work, 'Dictionnaire de Physique,' published in 1789:--'The year 1761 will be celebrated in astronomy in consequence of the discovery that was made on May 3 of a satellite circulating round Venus. We owe it to M. Montaigne, member of the Society of Limoges. M. Baudouin read before the Academy of Sciences at Paris a very interesting memoir, in which he gave a determination of the revolution and distance of the satellite. From the calculations of this expert astronomer we learn that the new star has a diameter about one-fourth that of Venus, is distant from Venus almost as far as the moon from our earth, has a period of nine days seven hours' [much too short, by the way, to be true, expert though M. Baudouin is said to have been], 'and its ascending node'--but we need not trouble ourselves about its ascending node.

Three years later Rödkier, at Copenhagen, March 3 and 4, 1764, saw the satellite of Venus with a refracting telescope 38 feet long, which should have been effective if longitude has any virtue. He could not see the satellite with another telescope which he tried. But several of his friends saw it with the long telescope. Amongst others, Horrebow, Professor of Astronomy, saw the satellite on March 10 and 11, after taking several precautions to prevent optical illusion. A few days later Montbaron, at Auxerre, who had heard nothing of these observations, saw a satellite, and again on March 28 and 29 it appeared, always in a different position.