The Story of the Heavens

of sketching, and of measuring. If he be one of those useful astronomers[Pg 292] who devote their energies not to actual telescopic work, but to forming calculations based on the observations of others, then the beautiful system of Saturn provides copious material. He has to foretell the different phases of the ring, to announce to astronomers when each feature can be best seen, and at what hour each element can be best determined. He has also to predict the times of the movements of Saturn's satellites, and the other phenomena of a system more elaborate than that of Jupiter. Lastly, if the astronomer be one of that class—perhaps, from some points of view, the highest class of all—who employ the most profound researches of the human intellect to unravel the dynamical problems of astronomy, he, too, finds in Saturn problems which test to the utmost, even if they do not utterly transcend, the loftiest flights of analysis. He discovers in Saturn's ring an object so utterly unlike anything else, that new mathematical weapons have to be forged for the encounter. He finds in the system so many extraordinary features, and such delicacy of adjustment, that he is constrained to admit that if he did not actually see Saturn's rings before him, he would not have thought that such a system was possible. The mathematician's labours on this wondrous system are at present only in their infancy. Not alone are the researches of so abstruse a character as to demand the highest genius for this branch of science, but even yet the materials for the inquiry have not been accumulated. In a discussion of this character, observation must precede calculation. The scanty observations hitherto obtained, however they may illustrate the beauty of the system, are still utterly insufficient to form the basis of that great mathematical theory of Saturn which must eventually be written. But Saturn possesses an interest for a far more numerous class of persons than those who are specially devoted to astronomy. It is of interest, it must be of interest, to every cultivated person who has the slightest love for nature. A lover of the picturesque cannot behold Saturn in a telescope without feelings of the liveliest emotion; while, if his reading and reflection have previously rendered him aware of the[Pg 293] colossal magnitude of the object at which he is looking, he will be constrained to admit that no more remarkable spectacle is presented in the whole of nature. We have pondered so long over the fascinations of Saturn's ring that we can only give a very brief account of that system of satellites by which the planet is attended. We have already had occasion to allude more than once to these bodies; it only remains now to enumerate a few further particulars. It was on the 25th of March, 1655, that the first satellite of Saturn was detected by Huyghens, to whose penetration we owe the discovery of the true form of the ring. On the evening of the day referred to, Huyghens was examining Saturn with a telescope constructed with his own hands, when he observed a small star-like object near the planet. The next night he repeated his observations, and it was found that the star was accompanying the planet in its progress through the heavens. This showed that the little object was really a satellite to Saturn, and further observations revealed the fact that it was revolving around him in a period of 15 days, 22 hours, 41 minutes. Such was the commencement of that numerous series of discoveries of satellites which accompany Saturn. One by one they were detected, so that at the present time no fewer than nine are known to attend the great planet through his wanderings. The subsequent discoveries were, however, in no case made by Huyghens, for he abandoned the search for any further satellites on grounds which sound strange to modern ears, but which were quite in keeping with the ideas of his time. It appears that from some principle of symmetry, Huyghens thought that it would accord with the fitness of things that the number of satellites, or secondary planets, should be equal in number to the primary planets themselves. The primary planets, including the earth, numbered six; and Huyghens' discovery now brought the total number of satellites to be also six. The earth had one, Jupiter had four, Saturn had one, and the system was complete. Nature, however, knows no such arithmetical doctrines as those which Huyghens attributed to her. Had he been less[Pg 294] influenced by such prejudices, he might, perhaps, have anticipated the labours of Cassini, who, by discovering other satellites of Saturn, demonstrated the absurdity of the doctrine of numerical equality between planets and satellites. As further discoveries were made, the number of satellites was at first raised above the number of planets; but in recent times, when the swarm of minor planets came to be discovered, the number of planets speedily reached and speedily passed the number of their attendant satellites. It was in 1671, about sixteen years after the discovery of the first satellite of Saturn, that a second was discovered by Cassini. This is the outermost of the older satellites; it takes 79 days to travel round Saturn. In the following year he discovered another; and twelve years later, in 1684, still two more; thus making a total of five satellites to this planet. [Pg 295] Fig. 68.—Transit of Titan and its Shadow, by F. Terby Louvain, 12th April, 1892. The complexity of the Saturnian system had now no rival in the heavens. Saturn had five satellites, and Jupiter had but four, while at least one of the satellites of Saturn, named Titan, was larger than any satellite of Jupiter.[28] Some of the discoveries of Cassini had been made with telescopes of quite monstrous dimensions. The length of the instrument, or rather the distance at which the object-glass was placed, was one hundred feet or more from the eye of the observer. It seemed hardly possible to push telescopic research farther with instruments of this cumbrous type. At length, however, the great reformation in the construction of astronomical instruments began to dawn. In the hands of Herschel, it was found possible to construct reflecting telescopes of manageable dimensions, which were both more powerful and more accurate than the long-focussed lenses of Cassini. A great instrument of this kind, forty feet long, just completed by Herschel, was directed to Saturn on the 28th of August, 1789. Never before had the wondrous planet been submitted to a scrutiny so minute. Herschel was familiar with the labours of his predecessors. He had often looked at Saturn and his five moons in inferior telescopes; now again he saw the five moons and a star-like object so near the plane of the ring that he conjectured this to be a sixth satellite. A speedy method of testing this conjecture was at hand. Saturn was then moving rapidly over the heavens. If this new object were in truth a satellite, then it must be carried on by Saturn. Herschel watched with anxiety to see whether this would be the case. A short time sufficed to answer the question; in two hours and a half the planet had moved to a distance quite appreciable, and had carried with him not only the five satellites already known, but also this sixth object. Had this been a star it would have been left behind; it was not left behind, and hence it, too, was a satellite. Thus, after the long lapse of a century, the telescopic discovery of satellites to Saturn recommenced. Herschel, as was his wont, observed this object with unremitting ardour, and discovered that it was much nearer to Saturn than any of the previously known satellites. In accordance with the general law, that the nearer the satellite the shorter the period of revolution, Herschel found that this little moon completed a revolution in about 1 day, 8 hours, 53 minutes. The same great telescope, used with the same unrivalled skill, soon led Herschel to a still more interesting discovery.[Pg 296] An object so small as only to appear like a very minute point in the great forty-foot reflector was also detected by Herschel, and was by him proved to be a satellite, so close to the planet that it completed a revolution in the very brief period of 22 hours and 37 minutes. This is an extremely delicate object, only to be seen by the best telescopes in the brief intervals when it is not entirely screened from view by the ring. Again another long interval elapsed, and for almost fifty years the Saturnian system was regarded as consisting of the series of rings and of the seven satellites. The next discovery has a singular historical interest. It was made simultaneously by two observers—Professor Bond, of Cambridge, Mass., and Mr. Lassell, of Liverpool—for on the 19th September, 1848, both of these astronomers verified that a small point which they had each seen on previous nights was really a satellite. This object is, however, at a considerable distance from the planet, and requires 21 days, 7 hours, 28 minutes for each revolution; it is the seventh in order from the planet. Yet one more extremely faint outer satellite was discerned by photography on the 16th, 17th, and 18th August, 1898, by Professor W.H. Pickering. This object is much more distant from the planet than the larger and older satellites. Its motion has not yet been fully determined, but probably it requires not less than 490 days to perform a single revolution. From observations of the satellites it has been found that 3,500 globes as heavy as Saturn would weigh as much as the sun. A law has been observed by Professor Kirkwood, which connects together the movements of the four interior satellites of Saturn. This law is fulfilled in such a manner as leads to the supposition that it arises from the mutual attraction of the satellites. We have already described a similar law relative to three of the satellites of Jupiter. The problem relating to Saturn, involving as it does no fewer than four satellites, is one of no ordinary complexity. It involves the theory of Perturbations to a greater degree than that to[Pg 297] which mathematicians are accustomed in their investigation of the more ordinary features of our system. To express this law it is necessary to have recourse to the daily movements of the satellites; these are respectively— Satellite.Daily Movement. I.382°·2. II.262°·74. III.190°·7. IV.131°·4. The law states that if to five times the movement of the first satellite we add that of the third and four times that of the fourth, the whole will equal ten times the movement of the second satellite. The calculation stands thus:— 5 times I. equals 1911°·0 III. equals 190°·7 II. 262°·74 4 times IV. equals 525°·6 10 ———— ———— 2627°·3 equal 2627°·4 nearly. Nothing can be simpler than the verification of this law; but the task of showing the physical reason why it should be fulfilled has not yet been accomplished. Saturn was the most distant planet known to the ancients. It revolves in an orbit far outside the other ancient planets, and, until the discovery of Uranus in the year 1781, the orbit of Saturn might well be regarded as the frontier of the solar system. The ringed planet was indeed a worthy object to occupy a position so distinguished. But we now know that the mighty orbit of Saturn does not extend to the frontiers of the solar system; a splendid discovery, leading to one still more splendid, has vastly extended the boundary, by revealing two mighty planets, revolving in dim telescopic distance, far outside the path of Saturn. These objects have not the beauty of Saturn; they are, indeed, in no sense effective telescopic pictures. Yet these outer planets awaken an interest of a most special kind. The discovery of each is a classical event in the history of astronomy, and the opinion has been maintained, and perhaps with reason, that the discovery of Neptune, the more remote of the two, is the greatest achievement in astronomy made