Since the time of Galileo no addition had been made to the system of satellites revolving round Jupiter. Profound surprise was created, therefore, by the announcement of the discovery of a fifth satellite by Barnard at the Lick Observatory, on September 9, 1892. The satellite, one of the faintest of telescopic objects, was discovered with the great 36-inch telescope, and its existence was soon confirmed by Andrew Anslie Common (1841-1903), with his great 5-foot reflector at Ealing, near London. The new satellite was found by Barnard to revolve round Jupiter in 11 hours 57 minutes at a mean distance of 112,000 miles.
Although the existence of other satellites of Jupiter was predicted by Sir Robert Stawell Ball (born 1840) soon after the discovery of the fifth, much surprise was created by the announcement, in January 1905, that a sixth satellite had been discovered by Perrine, who, in the following month, announced the discovery of a seventh. These discoveries were made by photography, the objects being very faint. The periods of revolution were found to be 242 days and 200 days for the sixth and seventh satellites respectively, the mean distances being 6,968,000 and 6,136,000 miles. It is possible that they may belong to a zone of asteroidal satellites. In fact, the fifth moon may belong to a similar zone, so that Jupiter may have two asteroidal zones; but this is anticipating future discovery.
A particular charm has always attached itself to the study of Saturn, the ringed planet. The magnificent system of rings has for two and a half centuries been the object of wonder and admiration in the Solar System, and accordingly they have been exhaustively studied by many eminent observers. While observing the two bright rings of Saturn on June 10, 1838, Galle noticed what Miss Clerke calls “a veil-like extension of the lucid ring across half the dark space separating it from the planet.” No attention, however, was paid to Galle’s observation. On November 15, 1850, William Cranch Bond (1789-1859), of the Harvard Observatory in Massachusetts, discovered the same phenomenon under its true form—that of a dusky ring interior to the more brilliant one. A fortnight later, before the news of Bond’s observation, Dawes made the same discovery independently at Wateringbury in England. This ring is known as the dusky or “crape” ring.
The discovery of the dusky ring brought to the front the problem of the composition of the ring-system. Laplace and Herschel considered the rings to be solid, but this was denied in 1848 by Edouard Roche (1820-1880), who believed them to consist of small particles, and in 1851 by G. P. Bond, who asserted that the variations in the appearance of the system were sufficient to negative the idea of their solidity; but he suggested that the rings were fluid. In 1857 the question was taken up by the Scottish physicist, James Clerk-Maxwell (1831-1879), who proved by mathematical calculation that the rings could be neither solid nor fluid, but were due to an aggregation of small particles, so closely crowded together as to present the appearance of a continuous whole. Clerk-Maxwell’s explanation—which had been suggested by the younger Cassini in 1715, and by Thomas Wright in 1750—was at once adopted, and has since been proved by observation. In 1888 Hugo Seeliger (born 1849), director of the Munich Observatory, showed from photometric observations the correctness of the satellite-theory; while Barnard in 1889 witnessed an eclipse of the satellite Japetus by the dusky ring. The satellite did not disappear, but was seen with perfect distinctness. The final demonstration of the meteoric nature of the rings was made by Keeler at the Alleghany Observatory in 1895, with the aid of the spectroscope. By means of Doppler’s principle, he found that the inner edge of the ring revolved in a much shorter time than the outer, proving conclusively that they could not be solid. This was confirmed by the observations of Campbell at Mount Hamilton, Henri Deslandres at Meudon, and Bélopolsky at Pulkowa.
In 1851 a startling theory regarding Saturn’s rings was put forward by the famous Otto Wilhelm von Struve (1819-1905). Comparing his measurements on the rings made at Pulkowa in 1850 and 1851 with those of other astronomers for the past two hundred years, he reached the conclusion that the inner diameter of the ring was decreasing at the rate of sixty miles a-year, and that the bodies composing the rings were being drawn closer to the planet. Accordingly, Struve calculated that only three centuries would be required to bring about the precipitation of the ring-system on to the globe of Saturn. In 1881 and 1882 Struve, expecting a further decrease, made another series of measures, but these did not confirm his theory, which was accordingly abandoned.
The study of the globe of Saturn has made less progress than that of the rings. The surface of the planet had been known since before the time of Herschel to be covered with belts, but as spots seldom appear on Saturn, only one determination of the rotation period had been made, that by Herschel. Much interest was aroused, therefore, by the discovery, by Hall, at Washington, on December 7, 1876, of a bright equatorial spot. Hall studied this spot during sixty rotations of the planet, determining the period as 10 hours 14 minutes 24 seconds. This was confirmed by Denning in 1891, and by Stanley Williams, an English observer, in the same year. On June 16, 1903, Barnard, at the Yerkes Observatory, discovered a bright spot, from which he deduced a rotation period of 10 hours 39 minutes,—a period considerably longer than that found by Hall. In the same year various spots on Saturn were observed by Denning, who found a period of 10 hours 37 minutes 56·4 seconds, and at Barcelona by José Comas Sola, now director of the Observatory there, who may be considered Spain’s leading astronomer. The result of these observations has been to show that the spots on Saturn have probably a proper motion of their own, apart from the rotation of the planet. As to the spectrum of Saturn, little has been learned. It closely resembles that of Jupiter. In 1867 Janssen, observing from the summit of Mount Etna, found traces of aqueous vapour in the planet’s atmosphere.
In the chapters on Herschel we have seen that he discovered the sixth and seventh satellites of Saturn. The next discovery was made on September 19, 1848, by W. C. Bond, at Harvard, Massachusetts, and independently by William Lassell (1799-1880), at Starfield, near Liverpool. The new satellite received the name of Hyperion, and was found to be situated at a distance of about 946,000 miles from Saturn. Its small size led Sir John Herschel to the idea that it might be an asteroidal satellite. Fifty years elapsed before another satellite of Saturn was discovered. In 1888 W. H. Pickering commenced a photographic search for new satellites of the planet. At last, on developing some photographs of Saturn, taken on August 16, 17, and 18, 1898, he found traces of a new satellite which he named “Phœbe.” But, as the satellite was not seen or photographed again for some years, many astronomers were sceptical as to its existence. However, photographs taken in 1900, 1901, and 1902 revealed the satellite, which was again photographed in 1904, and seen visually by Barnard in the same year with the 40-inch Yerkes telescope. At that time the discoverer brought out the amazing fact that the motion of the satellite is retrograde—a fact which he attempts to explain by a new theory of the former rotation of Saturn. He likewise demonstrated that its distance from Saturn varied from 6,120,000 to 9,740,000 miles. Early in 1905 Pickering announced the discovery of a tenth satellite of Saturn, which received the name of Themis, with a period and mean distance nearly similar to Hyperion, so that Sir John Herschel’s idea of Hyperion being an asteroidal satellite is being confirmed after a lapse of half a century.
If little is known of the globe of Saturn, still less is known regarding Uranus. Dusky bands resembling those of Jupiter were observed by Young at Princeton in 1883. In the following year Paul and Prosper Henry discerned at Paris two grey parallel lines on the disc of the planet. This was confirmed by the observations of Perrotin at Nice, which also indicated rotation in a period of ten hours. In 1890 Perrotin again took up the study and re-observed the dark bands. On the other hand, no definite results regarding the planet were obtained by the Lick observers in 1889 and 1890. Measurements of the planet by Young, Schiaparelli, Perrotin, and others indicate a considerable polar compression. The spectrum of the planet has been studied by Secchi, Huggins, Vogel, Keeler, Slipher, and others. The spectrum shows six bands of original absorption, a line of hydrogen, which, says Miss Clerke, “implies accordingly the presence of free hydrogen in the Uranian atmosphere, where a temperature must thus prevail sufficiently high to reduce water to its constituent elements.” From a photographic study of the spectrum at the Lowell Observatory in 1904, Slipher observed a line corresponding to that of helium, indicating the presence of that element in the planet’s atmosphere.
Herschel left our knowledge of the Uranian satellites in a very uncertain state. The two outer satellites, Titania and Oberon, were rediscovered in 1828 by his son, but the other four, which he was believed to have discovered, were never seen again. In 1847 two inner satellites, Ariel and Umbriel, were discovered by Lassell and Otto Struve respectively, their existence being finally confirmed by Lassell’s observations in 1851.
After the discovery of Uranus by Herschel, mathematical astronomers determined its orbit and calculated its position in the future. Alexis Bouvard, the calculating partner of Laplace, published tables of the planet’s motions, founded on observations made by various astronomers who had considered it a star before its discovery by Herschel; but as the planet was not in the exact position which Bouvard predicted, he rejected the earlier observations altogether. For a few years the planet conformed to the Frenchman’s predictions, but shortly afterwards it was again observed to move in an irregular manner, and the discrepancy between observation and the calculations of mathematicians became intolerable. Did the law of gravitation not hold good for the frontiers of the Solar System? Gradually astronomers arrived at the conclusion that Uranus was being attracted off its course by the influence of an unseen body, an exterior planet. Bouvard himself was one of the first to make the suggestion, but died before the planet was discovered. An English amateur, the Rev. T. J. Hussey, resolved to make, in 1834, a determination of the place of the unseen body, but found his powers inadequate; and in 1840 Bessel laid his plans for an investigation of the problem, but failing health prevented him carrying out his design.