It has become customary, within the last few years, for popular writers upon astronomy to distinguish two branches of that science by the terms “old” and “new.” By the old astronomy they understand the investigation of the comparative structure and of the relative places and movements of the bodies composing the universe, so far as this inquiry can be conducted by direct observation of them, or by inference from the facts thus observed. These were the only means of astronomical research available to the ancients, and until a time within the memory of the present generation they still continued to be almost exclusively in use. The principal exception to this rule was formed by the study of variable stars, which had been seriously undertaken by a few observers more than a century ago. In this work, the facts to be observed are the variations in the quantities of light emitted at different times by the stars under examination; and such researches, conducted with no special apparatus except ordinary telescopes, form a connecting link between the old astronomy and the new. But since the ancient astronomers so far entered upon this field as to classify the stars according to their brightness, and also because the observation of variable stars, as above described, does not require special instrumental appliances, it is presumably to be regarded as a part of the old astronomy. The new astronomy is distinguished by its use of new apparatus designed for the measurement or analysis of the light of the stars, and for the discovery of stellar radiations imperceptible by the eye. Photometers, spectroscopes, and, still more recently, the art of stellar photography have been employed in these novel inquiries, with results in the enlargement of our knowledge perhaps as marked as those which were derived, more than two centuries ago, from the invention of the telescope.
Recent writers, in the elation which naturally attends the sudden opening of a path into a new region of discovery, have occasionally expressed themselves as if the old astronomy had accomplished all its work, and had lost its originally attractive and interesting character. But the account here to be given of the discovery of a new stellar system will probably make it appear that those phenomena of the universe which are open to direct observation are not likely as yet to have been fully explored, and especially that a sufficiently acute mind can still extract from them an indefinite series of brilliant conclusions.
A very familiar and yet a highly inspiring experience to the scientific mind is the continual suggestion of new inquiries by the facts developed during the progress of one previously undertaken. Nothing more forcibly exhibits the unbounded extent of the field open to investigation, and the connection of all its parts, however dissimilar they at first appear. From this point of view, some notice of the research which gave rise to that forming our principal subject will be desirable on account of its own interest, as well as because it has led to one still more interesting.
It is a question which for many years has concerned geologists no less than astronomers whether the axis of the earth maintains an invariable position in the earth itself, or shifts from one position to another. This question, which originated in the last century, if not earlier, has no direct reference to the varying direction of the terrestrial axis in space, which has long been observed and understood. But, independent of this admitted change of direction, another may occur as a purely geographical phenomenon, in consequence of which the latitudes of particular places may be changed. If we suppose such a change to proceed indefinitely, the poles might be transported to what is now the torrid zone, and the arctic regions might thus acquire an equatorial position and climate. If the movement were limited to a small amount, it would result in slight variations of latitude, which, without becoming noticeable to mankind in general, might attract the attention of astronomers and geographers.
It has long been suspected that the discrepancies between the results of different determinations of the latitudes of certain observatories were not wholly to be ascribed to errors of observation, but were due to real changes of latitude. However, until within a few years the question remained an open one. More definite knowledge of the subject was then acquired by special series of observations undertaken by German astronomers, from which it seems to be a well-established fact that the latitude of a place is not a fixed quantity, but is subject to perceptible, although small, variations in the course of a few months. To test this conclusion still more thoroughly an expedition was sent to the Hawaiian Islands, where observations upon the latitude were conducted simultaneously with others in Germany. If the north pole at any time actually moves away from Germany, it must move towards those islands, where the latitude must accordingly increase, while it diminishes at German stations. The result of the observations confirmed the belief previously founded upon the European observations alone, and it is now an accepted theory that certain changes of latitude do occur in short periods of time. Whether there is also a slower and more progressive change in the place of the pole still remains to be decided.
The discovery of the new phenomenon naturally increased the interest of older records of observations, in which the nature and progress of former changes in latitude might now be studied. Among those who undertook researches of this kind was the well-known astronomer, Dr. S. C. Chandler, of Cambridge, Massachusetts. He reached the very interesting conclusion that the places of the poles describe approximate circles, which at the present day are about fifty feet in diameter, in a period having recently had the length of four hundred and twenty-seven days, but that for the last hundred and fifty years the magnitude of the disturbance has on the whole tended to decrease, while its period has increased. Each successive revolution of the pole now seems to occupy two days more than that preceding it. About 1730 the period was only a year in length, while the circles described by the poles were perhaps a hundred feet in diameter. Previous to 1730 the development of practical astronomy was insufficient to furnish material for the determination of such minute changes, and it is only since 1840 that the course of the variation could be continuously studied.
The ordinary methods of determining the latitude of a place depend upon the observation of the apparent altitudes of stars, and those stars which pass near the zenith of any observer are, best adapted for his use in such an investigation. One of the stars which passes near the zeniths of places in southern Europe and in a large part of the United States is that known as Algol, which is remarkable for its periodical variations of light. Partly on account of its convenience as a point of reference in observations of latitude and in other work of a similar kind, and partly on account of the attention drawn to it by its variability, this star has been observed frequently; and the records of these observations, accordingly, formed part of the material examined by Dr. Chandler in his researches into the variation of terrestrial latitudes. His examination suggested to him that the recorded observations of Algol exhibited variations due to a change of place in the star itself as well as to changes of terrestrial latitude, and from this suggestion he proceeded to the remarkable conclusions some account of which will here be attempted. His previous discovery with regard to latitudes well deserves a much fuller statement in these pages than has just been allowed to it, but it is necessary to select from so much interesting material those portions which seem most decidedly adapted to popular explanation.
Algol is a star in the constellation Perseus, and when above the horizon, during a clear night, is at all times easily visible without a telescope. It was observed, nearly two centuries ago, to be subject to some variation in brightness, and the general course of this variation has been known for more than a hundred years. At intervals somewhat less than three days the star gradually loses, and again recovers, more than half its customary brightness, the process being completed within about nine hours. A plausible explanation of this phenomenon suggested itself as soon as the facts just mentioned had been ascertained. According to this explanation, which has been decidedly confirmed by recent spectroscopic observations in Germany, a large dark body, at a comparatively small distance from Algol, revolves about it in an orbit the plane of which is presented nearly edgewise to a terrestrial spectator. Hence at every revolution of the dark body it passes between Algol and the observer, so as to cut off for the time a portion of the light ordinarily received from the star.
Attentive observation of these changes during the last hundred years has disclosed a variation in the length of the period occupied by the assumed revolution. This variation is too small to exhibit itself in any short series of observations, but its accumulated effects become manifest in time. Such an accumulation may be likened to that of the errors of a timepiece. Suppose that a watch is compared once a month with a clock kept free from error, and that it is found at the beginning of the experiment to be one minute fast, while the three following comparisons show it to be fifty seconds fast, thirty seconds fast, and forty seconds fast. Its rate must obviously have been varying during the period of observation, and yet on any day in the course of that period it might have been carefully compared for a single hour with the standard clock without the detection of any gain or loss whatever. The lapse of three months has shown, however, not merely that the watch does not keep perfect time, but that it is not uniformly gaining or losing. In like manner, now that Algol has been observed for a century, we are able to assert with confidence not merely that the length of its period changes, but that this change is not uniform. The actual difference in the length of the period at different times amounts to only a few seconds; but if the time at which the star should appear faint is computed for an interval of fifty years by means of the average length of one period of its variation in brightness, the error might amount to four hours, although, as will shortly be explained, this would not always be the case. Similar alterations have been noticed in the length of the periods in which some other variable stars go through their changes of brightness, and these alterations have long offered an interesting problem, with no obvious explanation.
From his study of the recorded observations made to determine the apparent place of Algol among the other stars, Dr. Chandler was led to conclude, as has been said, that this place had been subject to gradual changes. On further examination, he found that these changes were apparently related to those which were known to occur in the period of the star’s variation in brightness. This circumstance suggested a highly plausible and interesting explanation of both sets of phenomena, — the alterations in the star’s period of variation, and the alterations of its apparent place in the sky.
According to this explanation, Algol is moving in an orbit approximately comparable in its dimensions with that of Uranus about the sun, and not very greatly inclined to the line along which the terrestrial observer looks at it. The time which the star occupies in completely traversing this orbit is about one hundred and thirty years. Its average apparent place in the sky approximately marks the corresponding place of the centre of its orbit; and, since the orbit is seen almost edgewise, the star seems nearly to occupy this central place when it is on the side of the orbit towards the terrestrial spectator, or on the opposite side, about sixty-five years later. At the intermediate points, which it reaches thirty-two and a half years earlier or later, it will accordingly seem as far as may be from its average place. Now, at these points it is obviously moving nearly towards us or away from us. When moving towards us, at each return of its temporary loss of brightness, it is a little nearer to us than on the last previous return of this phenomenon. Hence the light which the star emits reaches us a little sooner than it would have done if the source of light had remained stationary during the interval. The period, therefore, is shortest when Algol is most directly approaching us, and longest when it is receding. from us most rapidly. When it is traversing the side of its orbit nearest us, or the opposite side, the period has its average length; and at the same time, as we have seen, the star is nearly in its average place in the sky.
The success of a prediction founded on the average length of the period, and attempting to state the times when the star will appear faintest about sixty-five years later, will depend upon the place of the star in its orbit. If such a prediction is made when Algol is most rapidly approaching us, it will be tolerably correct; for during the first half of the sixty-five years the star will have continued to approach, and its period, consequently, will have been of less than average length; during the second half of the sixty-five years the star will have been receding, and its period will have been of more than average length. Upon the whole, therefore, the period will have been of nearly its average length, so that the assumption on which the prediction was founded will have proved to be correct. But if a similar prediction is made at one of the times when the star is nearest to us or most remote, the period during the ensuing sixty-five years will have been constantly greater or less than it is upon the average, and the prediction will be largely in error. We are, accordingly, to expect the errors of computation from the average length of the period to be greatest when the star holds nearly its average place in the sky, and this was shown by Dr. Chandler actually to occur.
The cause of the movement of Algol in the orbit attributed to it by the new hypothesis remains to be explained, but the explanation is comparatively simple. Algol must be one of a system of revolving bodies, and the only one in that system which is bright enough to be perceptible to us. In the absence of any similar cases, we might be inclined to reject this supposition, on the ground that the sun, which is the brightest body in our own system, is also so much larger than any of the others that it is scarcely affected by any of them in its movements. But the bright stars Sirius and Procyon have long been known to shift their apparent places in a manner which makes it evident that their movements are affected by companions, which, although much fainter, cannot be much less massive. In the case of Sirius, one such companion has actually been discovered in the position and with the relative movement required by theory; but the companion or companions of Procyon are still known only by inference. Accordingly, the presumption that Algol belongs to the same class of stars, and is attended by massive companions, cannot appear unreasonable.
It may now naturally be asked whether the companion of Algol which is assumed to pass between it arid us, and thus to produce its periodical decline of brightness, may not itself be the body which disturbs its motion. A moment’s consideration, however, will show the great improbability of this supposition. The period of the mutual revolution of Algol and this close companion is less than three days, instead of one hundred and thirty years; nor can any form of action well be imagined which can cause the second period to arise from the first. But the evidence now before us for the existence of at least one more body belonging to the system tends to strengthen the previous belief that the close companion really exists, and periodically eclipses the bright star.
Perhaps the most interesting feature in the theory proposed by Dr. Chandler is the fact that, if it is correct, it supplies means of determining the distance between us and Algol. The differences in the period of the star’s variation in brightness, as it approaches the observer or recedes from him, combined with the known velocity of light, enable us to determine the speed with which Algol moves in its orbit. Knowing also the number of years in which the entire orbit is traversed, we may next find the length of that orbit in miles, or in any more convenient unit of length which we may prefer. We also know the apparent displacement of the star in the sky occasioned by its movement. Combining the apparent and the real dimensions of the orbit, we may find the distance of the star by methods familiar to surveyors as well as to astronomers; but it must not be supposed that this distance can be very accurately calculated from the material as yet at hand. The provisional result reached by Dr. Chandler places the star at such a distance from us that light which occupies eight minutes in reaching us from the sun needs more than forty-six years to come to us from Algol. Hence, when the star appears faint, it is not because the occulting body is now between us and it, but because it did pass before it almost half a century ago.
It will easily be understood that the calculations by which the theoretical orbit and distance of Algol have been deduced are by no means of the simple character which has been aimed at in the explanations of the new theory given above. It must also be remembered that every such theory is intended, not as a finality, but rather as a stimulus to further observation. Algol will hence-forward be more carefully observed than ever. It remains to be seen whether the period of the star’s variation, which has been diminishing, will begin to increase, and when it will again begin to diminish; whether the apparent place of Algol will continue to change in conformity with the theory; and especially whether the distance of the entire system can be determined by the comparison of the place of Algol with that of any faint star in its apparent vicinity during a single year. This is the only method ordinarily available for finding the distance of a fixed star. The observations are often pursued for more than one year, but the change of place to be observed runs through its whole course in that time.
Moreover, the hypothetical distant companion of Algol may not be so absolutely destitute of light as to be beyond the reach of vision. It will be an interesting problem for observers having at their command the most powerful telescopes to search for it in the direction from Algol assigned to it by theory.