One result of Herschel’s discoveries among the stars and nebulÆ is that his studies of the Sun and planets, with the exception of the discovery of Uranus, have been completely thrown into the shade. Nevertheless, his work in solar and planetary astronomy alone would have gained for him a higher position in astronomy than his contemporaries. The planets, satellites, and comets were all attentively studied by the great astronomer; indeed, the scientific investigation of the surfaces of Mars and Saturn began with Herschel. “His attention to the Sun,” Miss Clerke truly remarks, “might have been exclusive, so diligent was his scrutiny of its shining surface.” Sunspots were specially investigated by Herschel, who closely studied their peculiarities, regarding them as depressions in the solar atmosphere. He also paid much attention to the faculÆ, but Herschel’s solar observations were very valuable, and did much for our knowledge of the orb of day. His theory of the Sun’s constitution—a development of the hypothesis put forward by Alexander Wilson (1714-1786), Professor of Astronomy in Glasgow—was, however, very far from the truth. This was almost the only instance in which Herschel was mistaken. He regarded the Sun as a cool, dark globe, “a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system.” In his opinion an extensive atmosphere surrounded the Sun, the upper stratum forming what SchrÖter named the “photosphere.” This atmosphere, estimated as two or three thousand miles in depth, was regarded as giving out light and heat. Below this shining atmosphere there existed, Herschel believed, a region of clouds protecting the globe of the Sun from the Herschel closely scrutinised the surfaces of the planets. Mercury alone was neglected by him. From 1777 to 1793 he observed Venus, with the object of determining the rotation period, but he was unable to observe any markings on the surface of the planet. He did not place reliance on SchrÖter’s value of the rotation period (about twenty-three hours). Meanwhile, SchrÖter announced the existence on Venus of mountains which rose to five or six times the height of Chimborazo. As to these, said Herschel, “I may venture to say that no eye which is not considerably better than mine, or assisted by much better instruments, will ever get a sight of them.” Herschel demonstrated the existence of an extensive atmosphere round Venus. “The analogy between Mars and the Earth,” Herschel wrote in 1783, “is perhaps by far the greatest in the whole Solar System.” In 1777 he began, in his house at Bath, a series of observations on the red planet, which yielded results of the utmost importance. Fixing his attention on the white spots at the north and south poles,—discovered by Maraldi, nephew of Cassini,—he soon ascertained the fact that they During Herschel’s lifetime the four small planets, Ceres, Pallas, Juno, and Vesta, were discovered by Piazzi, Olbers, and Harding. The great astronomer was much interested in these small worlds. He commenced a search through the Zodiacal constellations for new planets, but failed. He was of opinion that many minor planets would be discovered. Accepting Olbers’ theory of the disruption of a primitive planet, Herschel calculated that Mercury might be broken up into 35,000 globes equal to Pallas. Meanwhile Herschel named the four new planets “Asteroids,” owing to their minute size. He estimated the diameter of Ceres at 162 miles and Pallas at 147 miles, but Professor Barnard’s measures have shown them to be larger. In connection with the discovery of the Asteroids, Herschel showed a very fine spirit. In Herschel’s time astronomers were acquainted with three of the outer planets,—Jupiter, Saturn, and Uranus,—all of which were closely studied by the great astronomer. The belts of Jupiter were supposed by him to be analogous to the “trade-winds” in the atmosphere of the Earth; while the drifting-spots on Jupiter’s disc and their irregular movements were carefully noted. His observations on the four satellites of Jupiter led him to believe that, like our Moon, they rotated on their axes in a period equal to that of their revolution round their primary—an opinion shared by Laplace, and by many modern astronomers. Herschel’s researches regarding Saturn were, however, much more important than those on On completing his famous 40-foot reflector, Herschel, on August 28, 1789, turned it on Saturn and its five known satellites. Near the planet, and in the plane of the ring, was seen another object, which Herschel believed to be a sixth satellite. To settle the question, he watched the planet for several hours to see if the object would partake in the planet’s motion. Finding that it did, he announced it as a new satellite, which he found to revolve round Saturn in 1 day 8 hours. About three weeks later, on September 17, Herschel discovered another satellite yet closer to Saturn, revolving The eighth satellite, Japetus, was shown by Herschel to rotate on its axis in a period equal to that of its revolution, and his observations were confirmed by modern observers. “I cannot,” Herschel said, “help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondaries of our Solar System; and we may conjecture that probably most of the satellites are governed by the same law.” In April 1805 Herschel observed the globe of Saturn to present not a spherical but a “square-shouldered” aspect. It was for long believed that this was an optical illusion; but Proctor and others have shown that it is quite possible for storms in Saturn’s atmosphere to cause the planet’s apparent distortion in shape. Herschel paid much attention to the planet Uranus, which he discovered on March 13, 1781. The discovery of Uranus, which was mentioned in a previous chapter, was in a sense the most striking of Herschel’s achievements. Uranus was the first planet discovered within the memory In January 1787 Herschel discovered two satellites to Uranus, with the aid of his 20-foot telescope. These satellites he believed to revolve round Uranus in 8 days and 13 days respectively, and accordingly he made a drawing of what their positions should be on February 10. On that day he found them in their predicted places. In 1797 he announced that the satellites revolved round Uranus in orbits at right angles to the ecliptic, and in a retrograde direction. In subsequent years Herschel believed that he had discovered other four satellites to Uranus, but he was unable Although Herschel made several important observations on the Moon, and measured the heights of the lunar mountains, he was not a devoted student of our satellite. Caroline Herschel remarks in her memoirs that if it had not been for clouds or moonlight, neither her brother nor herself would have got any sleep; adding that Herschel on the moonlight nights prepared his papers or made visits to London. However, he did make some investigations, and in 1783 and 1787 believed himself to have witnessed the eruption of three lunar volcanoes. He afterwards concluded, however, that what he believed to be eruptions was really the reflexion of earth-shine from the white peaks of the lunar mountains. Herschel never discovered a comet, leaving that branch of astronomy to his sister, who discovered eight of these objects. He was, however, much interested We have now completed our sketch of Herschel’s important labours regarding our Solar System. As Miss Clerke says, “A whole cycle of discoveries and successful investigations began and ended with him.” But through observing the stars he made a further discovery in connection with the Solar System; indeed, one of the greatest discoveries in the history of astronomy—the movement through space of the Sun, carrying with it planets and comets. “If the proper motion of the stars be admitted,” said Herschel, “who can deny that of our Sun?” Of course it was plain that the motion of the Sun could only be detected through the resulting apparent motion of the stars. Thus, if the Sun is moving in a certain direction, the stars in front will appear to open out, while those behind will close up. But the problem is by no means so easy as this. The stars are also in motion, and, before the solar Herschel was far in advance of his time regarding the solar motion. The two greatest astronomers of the next generation, Bessel and Sir John Herschel, rejected the results reached by Sir William Herschel. But in 1837 Argelander, after a profound mathematical discussion, confirmed Herschel’s views, and proved the solar motion to be a reality. Since that date the problem has been attacked by various methods by Otto Struve, Gauss, MÄdler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn, Campbell, and others, with the result that the reality of the solar motion and of the direction fixed by Herschel has been proved beyond a doubt. As Sir Robert Ball well remarks, mathematicians have exhausted every refinement, “but only to confirm the truth of that splendid theory which seems to have been one of the flashes of Herschel’s genius.” In his volume ‘Herschel and his Work,’ Mr James Sime writes: “To Herschel belongs the credit not merely of having suspected the revolution of sun around sun in the far-distant realms of space, but also of actually detecting that this Herschel, on January 10, 1782, submitted to the Royal Society a catalogue of 269 double Herschel did not merely prove the revolution of the binary stars; he assigned periods to those which he had particularly studied. He believed the period of Castor to be 342 years; In 1782 the French astronomer, Charles Messier (1730-1817), published a list of 103 nebulÆ. In the following year Herschel commenced his famous sweeps of the heavens with his large reflectors, and during these he made many remarkable discoveries. In 1786 he published in the ‘Philosophical Transactions’ of the Royal Society a catalogue of a thousand new nebulÆ and star-clusters, in which he gave the position of each object with a short description of its appearance, written by Caroline Herschel while her brother actually had the object before his eyes. In 1786 Herschel published a catalogue of another thousand clusters and nebulÆ, followed in 1802 by a list of 500; making a total of 2500 clusters and nebulÆ discovered by the great astronomer. This alone would have gained a great name for William Herschel in this branch of astronomy. In the space of only twenty years 2500 nebulÆ and clusters had been discovered. The various nebulÆ and clusters were divided into eight classes, as follows: the At first Herschel believed all nebulÆ to be clusters of stars, the irresolvable nebulÆ being supposed to be farther from our system than the resolvable nebulÆ. As many of the nebulÆ which Messier could not resolve had yielded to Herschel’s instruments, Herschel believed that increase of telescopic power would resolve the hazy spots of light which remained nebulous. In the paper of 1785, in which Herschel dealt with the construction of the heavens, he stated his belief that many of the nebulÆ were external galaxies—universes beyond the Milky Way; and in 1786 he remarked that he had discovered fifteen hundred universes! Arago, Mitchel, Nichol, Chambers, and other writers quite misinterpreted Herschel’s views on the nebulÆ when they said that he believed them to be all external galaxies. In 1785 Herschel was now gradually giving up his theory of external galaxies and his “disc-theory” of the Universe; but he still believed even the nebulous objects to be irresolvable only through immensity of distance. In 1791, however, he drew attention to a remarkable star in Taurus, surrounded by a nebulous atmosphere, regarding which he wrote, “View, for instance, the nineteenth cluster of my sixth class, and afterwards cast your eye on this cloudy star. Our judgment, I will venture to say, will be that the nebulosity about the star is not of a starry This was written in 1791, five years before Laplace propounded his nebular theory. Meanwhile Herschel, believing that “these nebulous stars may serve as a clue to unravel other mysterious phenomena,” found that the theory of a “shining fluid” would suit the appearance of the irresolvable planetary nebulÆ and the great nebula in Orion much better than the extravagant idea of “external universes.” Herschel now considered the Orion nebula to be much nearer to the Solar System than he formerly did, and ceased to regard it as external to the Galaxy. For twenty years Herschel patiently observed the nebulÆ, and it was not until 1811 that he propounded his nebular hypothesis of the evolution of the Sun and stars. He found the gaseous matter in all stages of condensation, from the diffused cloudy nebulÆ like that in Orion, through the planetary nebula and the regular nebula, to the perfect stars, like Sirius and the Sun. Herschel’s nebular theory Sir Robert Ball says: “Not from abstract speculation like Kant, nor from mathematical suggestion like Laplace, but from accurate and laborious study of the heavens, was the great William Herschel led to the conception of the nebular theory of evolution.” Herschel’s nebular theory was wider and less rigorous than that of Laplace. Laplace reached his theory by reasoning backwards; Herschel by observing the nebulÆ in process of condensation. Consequently, while Laplace’s theory has required modification, Herschel’s, from its width, is universally accepted, because there is nothing mathematically rigorous in it. The great German did not go into details like his French contemporary. He sketched the evolution of the stars in a wider sense. The astronomer’s “1500 universes,” Miss Clerke remarks, “had now logically ceased to exist.” Herschel had gathered much evidence about nebular distribution which shattered his belief in external universes, although he still thought in 1818 that some galaxies were included among the non-gaseous nebulÆ. In 1784 Herschel pointed out that the clusters and nebulÆ “are arranged to run in strata”; and some time later “A knowledge of the construction of the heavens has always been the ultimate object of my observations.” So Herschel wrote in 1811. All his investigations were secondary to the problem which was constantly before his mind—the extent and structure of the Universe. He aspired to be the Copernicus of the Sidereal System. Although Bruno, Kepler, Wright, Kant, and Lambert had speculated regarding the construction of the heavens, they had not the slightest evidence on which to base their ideas. There was no science of sidereal astronomy. The stars were observed only to assist navigation, and the primary object of star-catalogues was to further knowledge of the motions of the planets. In Herschel’s day, also, the distances of the stars had not been measured, and he had to base his views on the distribution of the stars. In 1784, therefore, he commenced a survey of the heavens, in order to ascertain the number of stars in various parts of the sky. This method, which Thus Herschel concluded that the Universe extended in the direction of the Galaxy to 850 times the mean distance of stars of the first magnitude. In the direction of the galactic poles the thickness was only 155 times the distance of stars of the same magnitude. Herschel was thus enabled to sketch the probable form of the Universe, which he regarded as cloven at one of its extremities, the cleft being represented Strange to say, Herschel’s original ideas regarding the Universe were accepted for many years by astronomical writers. Arago accepted Herschel’s original theory, unaware that he had in reality abandoned it, and he was followed by a host of French and English writers who did not take the trouble to read each of Herschel’s papers, merely quoting that of 1785, and believing that it represented his final ideas on the subject. Even Sir John Herschel seems to have been unaware that his father gave up the disc theory of the Universe. The famous German astronomer, Wilhelm Struve, after an exhaustive study of Herschel’s papers, was enabled to prove in 1847 that the theory had been abandoned The observations made by Herschel himself eventually proved fatal to the disc theory—a hypothesis which he had all along held very lightly. His ideas about subordinate clusters within the Milky Way were soon confirmed, and though in 1799 he still adhered to the disc theory, he wrote in 1802, “I am now convinced, by a long inspection and continued examination of it, that the Milky Way itself consists of stars In 1811 Herschel wrote as follows: “I must freely confess that by continuing my sweeps of the heavens, my opinion of the arrangement of the stars, and their magnitudes, and some other particulars, has undergone a gradual change; and, indeed, when the novelty of the subject is considered we cannot be surprised that many things formerly taken for granted should on examination prove to be different from what they were generally but incautiously supposed to be. For instance, an equal scattering of the stars may be admitted in certain calculations; but when we examine the Milky Way, or the closely compressed clusters of stars, of which my catalogues have recorded so many instances, this supposed equality of scattering must be given up.” This was the virtual abandonment of the disc theory. Six years later Herschel announced that in six cases he had failed to resolve the In 1814 Herschel was “still engaged in a series of observations for ascertaining a scale whereby the extent of the Universe, as far as it is possible for us to penetrate into space, may be fathomed.” In 1817 he described another method of star-gauging, which Arago and other writers have confused with that which he devised in 1785. The two methods, however, were quite Herschel, at the time of his death, left unsolved the problem of the construction of the heavens. It is still unsolved, and will doubtless remain so until astronomers know more about the distances and motions of the stars. His last observation of the Galaxy showed that even Herschel’s star-gauges, and those of his son, still remain of immense value to astronomers in any discussion of the construction of the heavens. Thus, although they failed to reveal to Herschel the structure of the Universe, they have been of much use to his successors. Herschel’s discussion of the supreme problem—the ultimate object of his observations—constitutes one of the most interesting chapters in the history of science, and marks a new era in human thought. In the words of Miss Clerke: “One cannot reflect without amazement that the special life-task set himself by this struggling musician—originally a penniless deserter from the Hanoverian Guard—was nothing less than to search out the ‘construction of the heavens.’ He did not accomplish it, for that was impossible; but he never relinquished, and, in grappling with it, laid deep and sure the foundations of sidereal science.” |