CHAPTER II.
HERSCHEL THE DISCOVERER.
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 could not observe them to the north and south of the Sun, thus proving their connection with the spots which are confined to the regions north and south of the equator. “There is all over the Sun a great unevenness,” said Herschel, “which has the appearance of a mixture of small points of an unequal light; but they are evidently a roughness of high and low parts.”
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 glowing atmosphere, and reflecting much of the light intercepted by them. The spots were believed to be openings in these atmospheres, caused by the action of winds, the umbra or dark portion of the spot thus representing the globe of the Sun, which Herschel believed to be “richly stored with inhabitants.” This theory held its ground for many years. Newton, it is true, believed the Sun to be gaseous, but he propounded no hypothesis of its constitution. Herschel’s theory, on the other hand, was fully developed, plausible, and attractive. It was held by eminent men of science until 1860, when the revelations of the spectroscope showed it to be quite untenable. The theory was supported for many years by Sir John Herschel, who, however, abandoned it in 1864. Herschel made several attempts to ascertain whether any connection existed between the state of the Sun and the condition of the Earth. In 1801 he was inclined to believe that “some temporary defect of vegetation” resulted from the absence of sun-spots, which, he thought, “may lead us to expect a copious emission of heat, and, therefore, mild seasons.” Herschel believed, in fact, that food became dear at the times of spot-minima. It may be remarked that Herschel never noted the spot-period of eleven years, the discovery of which was afterwards made by Schwabe.
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 waxed and waned in size, the north polar cap shrinking during the summer of the northern hemisphere, increasing in winter, and vice versa in the southern hemisphere. He regarded the caps as masses of snow and ice deposited from “a considerable, though moderate, atmosphere,” a theory now generally accepted. Herschel gave an immense impetus to the study of Mars. He carefully examined the planet’s surface, and the dark markings were regarded by him as oceans.
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 ‘The Edinburgh Review’ Brougham declared that Herschel had devised the word “asteroid,” so that the discoveries of Piazzi and Olbers might be kept on a lower level than his own discovery of Uranus. Many scientists would have been much offended at this contemptible insult, but Herschel merely remarked that he had incurred “the illiberal criticism of ‘The Edinburgh Review,’” and that the discovery of the Asteroids “added more to the ornament of our system than the discovery of another planet could have done.”
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 Jupiter. The globe of the planet, the rings and the satellites, were favourite objects of study at Bath and Slough. In 1794 he perceived a spot on the surface of Saturn, and made the first determination of the rotation of the planet, which he fixed as 10 hours 16 minutes,—a result confirmed by modern astronomers. The rings were subjected to the closest scrutiny. Herschel believed them to be solid, and he also considered them to revolve round Saturn in about 10 hours. It appears that he observed the famous “dusky ring,” but supposed it to be a belt on the surface of the planet. He also studied Cassini’s division in the ring, ascertaining its reality.