In the High Heavens/Chapter 1

by Robert Stawell Ball

CHAPTER I.

 

THE MOVEMENTS OF THE SOLAR SYSTEM.

 

HAVE often speculated as to the appearance which the heavens must have presented at very remote epochs. I do not now merely refer to such epochs as those to which human history extends. There can be no great difference between the aspect of the skies now and the aspect which they presented when Ptolemy or Hipparchus observed them. No doubt we must admit that some changes have taken place, for change is the law of nature. In a thousand years, or in a hundred years, or ten years, or even in one year, a number of alterations take place in the positions of the fixed stars which are quite perceptible to the refined measurements of the modern observatory, though they would not suffice to produce a derangement of the heavens large enough to be discernible by unassisted observation. But in the present chapter I specially want to consider the variations in the aspect of the heavens, which would be presented not merely after the lapse of a few centuries or a few tens of centuries, but after stretches of time much longer still. We know from the testimony of the rocks that our earth has been the abode of living creatures for periods which it seems impossible to express in less than thousands of thousands of years. It therefore seems an interesting question to investigate the possible amount of transformation which the heavens have undergone in, let us say, a million of years.

The line of reasoning we shall employ suffices at all events to show how mighty is the transformation which has occurred within geological periods. The changes in the heavens are as profound as the changes in the earth. Let us consider the case of a star, or other celestial body, which moves through space at the constant rate of 20 miles a second. I have adopted this particular velocity as fairly typical of sidereal motions generally. It is rather larger than the speed with which the earth moves in its orbit. The velocities of many of the stars are, however, known to be quite as great as that which we have assumed. Indeed, in the case of many stars the speed is greatly in excess of 20 miles a second. There are several stars certainly known to be moving twice as fast; nor are speeds considerably higher than even this unobserved. We may, for instance, mention the famous star known to astronomers as Groombridge No. 1830, which hurries along at the rate of at least 200 miles a second. Indeed, in some cases stellar velocities are attained which appear to be even greater than that just mentioned. We do not therefore make any extravagant supposition in adopting a speed of 20 miles per second, as the basis of our calculation. This being granted it is now a simple problem to discover with sufficient approximation the change in apparent visibility which such a body would undergo after the lapse of a stated period of time. Twenty miles a second means 1,200 miles a minute. That is of course 72,000 miles per hour, 1,728,000 miles per day, or 630,720,000 miles per annum. It therefore follows that in a million years the distance through which a star will move, on the assumption we have made, cannot be less than 600,000,000,000,000 miles.

I do not think that the effect of these considerations on the continuance of the visibility of stars throughout vast periods of time has always been quite fully appreciated. The figures given will provide a demonstration that there must have been a vast change in the appearance of the heavens within the lapse of the last million years. To carry the inquiry much farther, we ought, however, to be acquainted with the distance of each star under consideration, and this is an element of which we are ignorant in the great majority of cases. A single instance will, however, suffice for an illustration. The nearest star as far as we yet know in the northern hemisphere is 61 Cygni. There have been, it is true, some discrepancies between the various determinations of its distance which different astronomers have obtained. I think, however, that we cannot be far wrong in adopting a value of 50,000,000,000,000 miles. I shall therefore take this magnitude as a typical distance to which we may apply the arguments of the present chapter. It appears that in the course of a million years a star with the average speed of 20 miles a second would move over a distance which was about a dozen times as great as the distance between 61 Cygni and the solar system. It will be noted that in expressing the speed of the star it was assumed that the solar system remained at rest. If, indeed, the solar system had a motion equal and parallel to that of the star, it would have been impossible to determine that of the latter, unless the motion of the solar system were itself known. It must, therefore, be borne in mind that the velocities with which we are at present concerned must be regarded as relative measurements conducted on the supposition that the solar system itself remained fixed.

If a star were displaced as much as a dozen times its original distance from the sun, it is obvious that a tremendous change in the apparent lustre of the star would be the consequence. If the body had moved directly away from the sun, the distance between the two objects would be increased in the proportion of thirteen to one. Had the star travelled in about the opposite direction, it would have passed comparatively near to the earth and its distance from the solar system, in the course of a million years, would be increased in the proportion of about eleven to one. On the other hand, if the star had moved in a direction perpendicular to the line from the star to the solar system, then the distance between the sun and the star would be rather more than twelve times as great as it was originally.

It is thus evident that whatever be the direction in which the star may happen to have moved, the distance between the solar system and the star will have increased during the lapse of a million years to nearly, if not quite, a dozen times as much as it was at the commencement of the same period. This leads to a remarkable conclusion with respect to the permanence of the visible heavens.

According to the laws of optics, the apparent lustre of a star varies inversely as the square of its distance. If the distance of a star be doubled, then its brightness is decreased to one-fourth. If the star be removed to a distance three times its original amount, then the apparent brightness declines to one-ninth. We may therefore infer that if a star be withdrawn to a distance which is twelve times that which it has at present, then the lustre of the star will be reduced to one hundred and forty-fourth part of its primitive value. This reasoning shows that any star which is now situated at the distance and maintains the proper motion we have supposed, will in the course of a million years have its apparent lustre reduced to the one hundred and forty-fourth part of that which it now displays. It need hardly be said that such a reduction of brightness would be certainly sufficient to render most of the stars wholly invisible. Indeed, it is only stars which possess exceptional lustre at their present distance which would continue to be visible, even as telescopic objects, when they had suffered so serious a loss.

These considerations lead to consequences of a remarkable character. We have adopted an average value of the proper motions, and it appears that on such an assumption it is highly improbable that any two stars not physically connected as a binary pair, should remain in comparative proximity for a period so long as a million of years. The case may perhaps be conveniently likened to that of a number of ships which are each bent on their different courses. At any particular time each of these vessels will generally have several other ships in view, but as each moves on its way the distances gradually alter, so that in the course of an hour or two the vessels have become so far dispersed that not one of those which was first seen is now above the horizon, while other ships, not at first to be discerned, have come within sight.

These considerations illustrate the transient nature of the appearance of the starry heavens when we contemplate periods of time comparable with those which the facts of geology demand for the requirements of earth -history. It is quite true that for the convenience of the argument I have been obliged to take specific numbers and to assume certain conditions, but there can be no doubt that the illustration is sufficient for demonstrating that in the course of the next million years the disposition of the sidereal heavens must present a totally different appearance from what it now shows. By the same reasoning we feel assured that if a view of the heavens had been obtained from this earth a million years ago, it must have been totally different from that offered by our present skies. Conceive that a man were transported back to the time when those great forests were flourishing whose remains have been preserved in the form of coal. It is, I believe, practically certain that few, if any, of the stars that now adorn our skies would be then discernible by him. I do not mean to say that there were no stars visible at the time of the coal forests. It would be much more reasonable to suppose that the firmament was as richly spangled with gems then as it is now, but those heavens would not be the heavens which we know. The estimates of geological chronology generally received would place the date of the great coal forests at an epoch far more remote than a million years back. Indeed, if anyone were to maintain that the remoteness of the period had to be expressed in tens of millions of years, I do not know of any facts by which he could be contradicted. The longer the time the more complete would have been the transformation in the visible objects on the sky.

Except that the stars of this remote antiquity must have been totally different from our present stars, we know but little of them. Indeed, I might almost say we can know nothing. Possibly some of our telescopic nebulae and clusters, whose distances are believed in many cases to be greatly in excess of the average distance of the stars, might not have been so totally transformed by their proper motion even in millions of years as to be unrecognisable by an eye familiar with the appearance they bear at present. There can, however, be no doubt that greatly as the stars may have changed the aspect they present to the terrestrial inhabitants, the sun, to which in the same time those inhabitants owe so much can have undergone but little appreciable alteration. The luxuriant vegetation of the coal measures demonstrates that the great luminary must have dispensed light as well as heat in those ancient days. The same fact is strikingly exemplified by the presence of eyes in extinct animals. Indeed, in some cases the eyes of creatures now only known to us by their fossil remains, seem to have been of the most elaborate character. Who that has ever visited any of our geological museums has not been interested in examining the great eye of the ichthyosaurus? In that unique organ of vision there is a remarkable apparatus of bony plates, apparently intended for adaptation of the organ to varying conditions. It is obvious that for some reason or other, as to which we can only speculate, a visual organ of excessive power and adaptability was required for this wondrous fish-reptile. I have often, indeed, longed to know what must have been the aspect which the heavens presented to that strange creature provided with such a marvellous optical instrument. No doubt his eye was generally employed for a much more practical purpose than that of astronomical contemplation. If, however, an ichthyosaurus ever did spare a glance at the heavens, what would have been the sight that would have met his gaze?

The sun would have shone on his earth as on ours. The luminary was certainly larger then to some trifling extent than it is at present. It was, however, in all probability nearly as bright as it is now, though it is just possible that photometric measurements would have shown it to be not quite so lustrous as the orb we know. For, paradoxical as it may seem, there are grounds for believing that the sun, though on the whole losing heat, may nevertheless be waxing somewhat brighter and hotter, and radiating more fiercely than then. But this is a matter which at present we need not further pursue; suffice it now to say that there is not the least reason to think there could have been any very considerable change in the physical characteristics of the sun, as the ichthyosaurus saw it arid as we see it now.

The moon, too, at that remote epoch must have run through just the same phases as it does at present. No doubt our satellite was somewhat nearer to the earth in those days than it is now. Its orb would, therefore, have seemed larger, and its periodic time would have been somewhat less. New moon must then have succeeded new moon at a somewhat briefer interval than at present. It is quite possible that the lunar craters which are now so completely extinct may not have exhausted their pristine energy before the days in which the ichthyosaurus flourished. This circumstance would perhaps have made the telescopic picture of the moon of that period vastly more interesting than any views of our satellite which are now to be obtained- No very great difference would have been noticeable between the planets of this remote antiquity and the planets of our skies. Venus would then, as now, have gone through that beautiful series of changes from the evening star to the morning star, and the intervals would have been much the same as they are at present. The moons of Jupiter, his belts and his orbit, would offer no striking variation from the Jupiter as now disclosed to astronomers. The rings of Saturn would probably have been much the same then as now, though it may be admitted that certain changes in the details of the Saturnian system have been thought to be in progress. As to what may have been the condition of the planet Mars, some million of years ago, we really have no idea. It is not improbable that the face of that globe would then have been very different from the globe which we now see. But the orbit in which Mars revolved even a million years ago would not have differed widely from that which it now traverses.

Speaking generally, we may say that the appearance of the planets, at all events to the unassisted eye, and also the movements of these bodies, would have been not unlike the corresponding phenomena which they now exhibit. Doubtless then, as now, comets must from time to time have flashed across the heavens; doubtless also meteors and showers of shooting stars must have rained down, perhaps in even greater abundance than they do at present. Probably solid meteorites may have landed on the earth even with greater frequency than in these latter days. But it must be admitted that in other respects the appearance of the heavens would have been totally different in the days of the ichthyosaurus from that which we now know. The Great Bear would not then have been discernible as the most striking group in the northern sky. Orion and the other well-known groups would not have yet come into vision. The pole would not then have been indicated by the pole star, and whatever may have been the brightest star in the heavens it is almost certain that it cannot have been Sirius. A zodiac there was, no doubt, but the signs by which it was to be marked were not the Ram and the Bull and the Heavenly Twins, or the other groups which have discharged that duty throughout the ages of human history. It may have been that the Milky Way was a luminous girdle around the heavens in the time of the ichthyosaurus, as it is at present. But it is certain that the general features of the heavens must have been profoundly modified between the long distant past and the present.

It may, therefore, be noted as a curious circumstance that the only permanent feature of our heavens in regard to such periods as those we are considering are not the fixed stars but the wandering planets. As they wandered then so they wander still, ever remaining members of that system over which the sun presides. It is no doubt impossible for us to form any conception as to what those stars or groups of stars may have been which adorned the skies of ancient geological times in the same way as our own constellations brighten our present skies. Calculations so instructive elsewhere will not suffice here. No methods known to us, or conceivable by us, can ever reproduce what the heavens must have been like at periods of millions of years ago. There could be no more interesting sight than a glimpse at the starry heavens in the time of the ichthyosaurus. We have read somewhere of a fable to the effect that the last object on which an eye rested ere it closed imprinted its picture permanently on the retina. Would that such a notion were founded on fact, and that the impression of the last celestial picture on which the eye of the ichthyosaurus gazed before he breathed his last were treasured up in the fossilized organism.

In the consideration of the gradual transformation of the heavens, I have found it convenient to speak as if the earth, or rather the solar system to which the earth belongs, occupied a fixed position in space. But when we have learned that some or all of the stars are in movement, it seems right to inquire whether the sun might not also participate in the motion. May not the sun be engaged in some mighty voyage through the celestial spaces, taking in its company the earth and the other planetary bodies by which it is attended? Here is, indeed, a grand problem; I propose to enter into its discussion with the assistance of certain recent investigations.

In the first place, it must be remembered that for the sun to be actually devoid of movement could be little short of miraculous. There are, of course, an infinite number of different movements possible, for there is every degree of velocity and difference of direction to be considered. On the other hand there is only one type of rest, or absolute quiescence, and it would be just as likely that a body should possess any stated velocity, say ten miles, ten yards, and three quarters of an inch per second, as that it should possess that particular characteristic implied in the absence of all movement whatsoever. It thus appears that, even in the absence of direct testimony on the subject, there is only one chance that the sun should be at rest, while the chances that it is not at rest are absolutely infinite. Under these circumstances rational beings will conclude that the sun is not at rest, and once we have admitted that the system is in motion, our next duty will be to discover the characteristics of that movement.

There are several different methods by which this problem has been investigated. They all lead to results which are in such substantial accordance that there can be no reasonable doubt that the difficult problem of the motion of the sun has been solved with considerable approximation.

It will be noted that in this inquiry there are two different problems which have to be considered. The first of these relates to the direction of the sun's movement, and the second to its velocity. These investigations have hitherto generally been conducted simultaneously. It is, however, now apparent that the most satisfactory solution of the problem is to be obtained by employing one of the methods for the determination of the direction of the sun's motion, and a quite different method for the determination of its velocity. I shall deal with the two branches of the subject consecutively.

The only method of learning the actual displacement of the solar system in space must be founded on the observation of bodies external to that system; at all events to the extent of not participating in the motion with which the system is animated. Of course the only such bodies available are the stars. Here at once we are met by the difficulty that the stars are themselves in movement. These movements affect the value of the stars, as points of reference, so seriously, that if there were only one or two stars available for the inquiry it would be utterly impossible for us ever to discover the movements of the solar system. But there are, of course, hundreds, or rather thousands, of stars, which can be made to render assistance. No doubt these stars are themselves endowed with movements, but their journeys are so varied that the effects they produce tend to neutralise each other, so far as our present purpose is concerned. We are thus enabled to investigate the problem as if the stars were at rest, when a sufficiently large number of them are considered together.

Supposing the solar system to be bound on a journey through the celestial spaces, it is obvious that in the course of time apparent displacements would be thereby produced in the relative positions of the stars. The nature of the effects produced may be seen from the following illustration, which has often been given before, but may serve us once again. Let us think of a harbour, the entrance of which is marked by two lights, one on either hand. As the ship approaches the harbour the two lights, which, while the vessel was still a long way off, seemed close together, begin to open out. As the vessel approaches still nearer the lights spread wider and wider until at last, just as the ship enters the port, the two lights have opened so completely that one is on the right hand and the other on the left. In this manner we become familiar with the conception that the lights seem to spread away from the point towards which the motion is directed. In like manner it is easy to see that when a vessel is sailing away from the port, the beacons seem gradually to draw in together. This consideration provides us with the means of discovering the point in the sky towards which the motion of the sun is directed. We expect that point to be indicated by the circumstance that the stars appear to be spreading away from it.

The correctness of the inference that this is the spot towards which the system is moving will be confirmed, if at the same time it be noticed that the stars are drawing in towards that point in the celestial sphere which lies diametrically opposite. This investigation has been conducted with extreme care. Many astronomers, beginning with William Herschel, have applied themselves to its solution. The most complete investigation is that recently undertaken by Herr Stumpe. He has employed a very large number of stars, and he has adopted every precaution to insure accuracy in the results. Means are provided to enable the precision of his determinations to be properly tested. Stumpe has divided the stars used in his inquiry into four different groups, and he has obtained an independent determination from each of these groups. It is naturally of much interest to inquire where the point in the heavens is situated towards which, at the present time, the solar system appears to be wending its way. Each of the four groups which Stumpe employed has given him a distinct determination. The several investigations agree in locating the point within the limits of the constellation Lyra, adjoining that region of Hercules in which the earlier and less complete investigation of the same problem had located the apex of the sun's way. One of the four points lies actually at the wonderful double-double Epsilon
Figure 1: The Constellation Lyra
The star δ marks the point in the heavens towards
which the sun's motion is directed.

Lyræ, quite close to the brilliant star Vega. It is therefore interesting to notice that this gem of the Northern skies is also in the vicinity of the actual spot to which the system must be tending. Each of the other three groups of stars employed in Stumpe's researches places the apex at some little distance, though still within the boundaries of the same constellation. We shall certainly not be far wrong if we adopt the average position indicated by these four sets of stars as the final result of this most elaborate inquiry. This point lies close to the star Delta Lyræ, as shown in the above figure (Fig. 1). Perhaps it may be of interest to know how this particular point may be identified. It can be speedily picked up in the heavens, as follows: Every owner of a telescope is acquainted with Beta Cygni, the most glorious coloured double star that the northern heavens have to offer. A line from Vega to Beta Cygni shows at about one-fourth of the way a bright star, which is Delta Lyrae. It is towards this particular spot of the heavens that the sun, bearing the earth and all the other planets with it, is hurrying at this moment.

The sweep of the solar system through space is represented in the adjoining figure (Fig. 2). The sketch may serve to illustrate the principles on which the determination of the solar velocity is based, but of course it is out of the question that the proper proportions could be observed in such a diagram. In the course of a century the advance of the system towards Lyra will make the stars appear to move in the manner represented by the arrows affixed thereto. Two of the stars are thus seen to spread away from Lyra, while the positions of the other stars are such that they seem to draw in towards the opposite point of the celestial sphere, which is sufficiently indicated by the star Pi Puppis.

If the region to which the motion of our system is directed be adorned by the splendour of Lyra, it is noteworthy that the opposite part of the sphere from which we seem to be flying is also remarkable for its stellar glories. It lies almost midway between Sirius and Canopus.

We shall now turn to the investigation of the allied problem as to the velocity with which the solar system wends its way. A different method of studying the subject which has lately come into practical application
Fig. 2.—The Voyage of the Solar System.

is here to be introduced. It may be remarked that the determination of the speed of the system requires quite a different class of information from that which suffices to give the situation of the apex of the sun's way. No doubt if we knew the distance of the stars we could then deduce the rate of the solar motion from observation of the apparent displacements of the stars. Unfortunately we are so ignorant of the distances of the great majority of the stars that the process indicated is almost wholly inapplicable with any satisfactory results. It is therefore fortunate that we have a method of investigating the problem which does not require any knowledge as to the stellar distances.

In other parts of this volume we shall dwell upon the important information which the spectroscope has been made to yield with respect to the movement of celestial bodies along the line of vision. It is of the essence of this method of research that the determination of the velocity of the body is obtained quite independently of the star's distance from the observer. The indications of the spectroscope assure us that a star is moving with a velocity of, say, five miles, or ten miles, or some other pace, per second towards the observer or from the observer, and this information has no connection whatever with the remoteness of the star. On this account it becomes specially applicable to the problem we have now to consider.

Of course it will be understood that the stars are themselves endowed with absolute movements. It is therefore not always correct to assume that when the solar system and a star are lessening their distances this is to be attributed entirely or even largely to the movement of the solar system. Both the solar system and the star are moving, and the motion observed is the resultant of the two. If, therefore, we had only a few stars wherewith to control the inquiry, the spectroscope method could add no information with regard to the movement of the solar system. But when there is so large a number of stars distributed in such a way that we are entitled to assume that on the whole there is about as much motion in one direction as there is in exactly the opposite direction, then it becomes possible to eliminate the disturbing effects introduced by the fact that the stars are not themselves at rest. When we employ stars enough, we may proceed in the inquiry exactly as if the stars were individually at rest, and as if all the motion perceived could be attributed to the movement of the solar system. No doubt there would be a fallacy in this proceeding if it happened that there was a general drift of the stellar motions in one direction. If this was the case, then the methods now employed would attribute, and would falsely attribute, a movement to the solar system equal and opposite to that with which the sidereal system was animated. It is, however, found that by a judicious selection of the stars, we are able to preclude the possibility of such a consentaneous sidereal movement as is here contemplated.

It is obvious that a movement of the solar system towards one region in the heavens must be accompanied by a general diminution of the distances between the stars in that region and our solar system. On the other hand, the distances from the solar system to the stars at the opposite region of the celestial sphere will be generally increased by such a movement. If the stars were at rest, or if a sufficient number of stars were taken to eliminate the consequences of their individual motions, then the spectroscopic search for the movements of the solar system becomes very simple. It is only necessary to measure the rate at which the solar system alters its distance from the various points around. That point on the celestial sphere where the star shows the velocity of approach to be greatest, can only be the point towards which the solar system is directing its motion. The opposite point of the heavens is that from which the movement of retreat is greatest. At intermediate points there will, throughout one hemisphere, be indications of approach, and throughout the other indications of retreat.

The stars employed in these researches are those which have been investigated spectroscopically by Professor Vogel in his memorable Potsdam observations. The number of the stars made use of was 51. It must be observed that though the method under consideration is admirably adapted to discover the speed of the solar system, it is not equally fitted to indicate the position of the point on the heavens towards which the sun's motion is directed. It can be shown that the conditions are such as to render the detection of that position by the use of the spectroscope much inferior to the determinations afforded by the former process employed by Stumpe. I therefore only use the spectroscopic method for the determination of the velocity deduced on the supposition that the apex of the sun's way is indistinguishable from the position of Delta Lyræ. The result of Vogel's investigation is to show that the velocity of the solar system is about eight miles a second. The accuracy possessed by this result is as usual best indicated by its probable error. It can be shown that the probable error of the statement, that the solar velocity is eight miles per second, is about two, that is to say, it is equally probable that the error of the result lies below two miles a second as that it lies above.

In conclusion it would seem that the sun and the whole solar system are bound on a voyage to that part of the sky which is marked by the star Delta Lyræ. It also appears that the speed with which this motion is urged is such as to bring us every day about 700,000 miles nearer to this part of the sky. In one year the solar system accomplishes a journey of no less than 250,000,000,000 miles. As you look at Delta Lyræ to-night you may reflect that within the last twenty-four hours you have travelled towards it through a distance of nearly three-quarters of a million of miles. So great are the stellar distances, that a period of not less than 180,000 years would be required before our system, even moving at this impetuous speed, could traverse a distance equal to that by which we are separated from the nearest of the stars.