Jupiter, the greatest planet of the Solar System, has perhaps been more persistently studied by astronomers than any other. In the early nineteenth century the prevalent idea was that Jupiter was a world similar to the Earth, only much larger,—a view held by Herschel and other famous astronomers, and put forward by Brewster in ‘More Worlds than One.’ This view prevailed for many years, although Buffon in 1778, and Kant in 1785, had stated their belief in the idea that Jupiter was still in a state of great heat—in fact, that the great planet was a semi-sun. This idea, however, was long in being adopted by astronomers, and very little attention was paid to Nasmyth’s expression of the same opinion in 1853. The older view still held the field—namely, that the belts of Jupiter represented trade-winds, and that a world similar to the terrestrial lay below the Jovian clouds. In 1860 George Philip Bond (1826-1865), director of the Harvard Observatory, found from experiments that Jupiter seemed to give out more light than it received, but he did not dare to suggest that Jupiter was self-luminous, considering that the inherent light might result from Jovian auroras.

In 1865 ZÖllner showed that the rapid motions of the cloud-belts on both Jupiter and Saturn indicated a high internal temperature. At the distance of Jupiter sun-heat is only one twenty-seventh as great as on the Earth, and would be quite incapable of forming clouds many times denser than those on the Earth. In 1871 ZÖllner drew attention to the equatorial acceleration of Jupiter, analogous to the same phenomenon on the Sun. In 1870 these opinions of ZÖllner’s were adopted and supported by Proctor in his ‘Other Worlds than Ours.’ In his subsequent volumes Proctor did much to popularise the idea, which is now accepted all over the astronomical world.

During the century many valuable observations on Jupiter were made by numerous observers, among them Airy, MÄdler, Webb, Schmidt, and others. Much time was devoted to the accurate determination of the rotation period, which was fixed at 9 hours 55 minutes 36·56 seconds by Denning in observations from 1880 to 1903. No really important discovery was made till 1878, when Niesten at Brussels discovered the “great red spot,” a ruddy object 25,000 miles long by 7000 broad, attached to a white zone beneath the southern equatorial belt. This remarkable object has been observed ever since. In 1879 its colour was brick-red and very conspicuous, but it soon began to fade, and RiccÓ’s observation at Palermo in 1883 was thought to be the last. After some months, however, it brightened up, and, notwithstanding changes of form and colour, it is still visible, a permanent feature of the Jovian disc. In 1879 a group of “faculÆ,” similar to those on the Sun, was observed at Moscow by Theodor Alexandrovitch BrÉdikhine (1831-1904), and at Potsdam by Wilhelm Oswald Lohse (born 1845). It was soon observed that the rotation period, as determined from the great red spot, was not constant, but continually increasing. A white spot in the vicinity completed its rotation in 5½ minutes less, indicating the differences of rotation on Jupiter.

The great red spot has been observed since its discovery by Denning at Bristol and George Hough (born 1836) at Chicago. Twenty-eight years of observation have not solved the mystery of its nature. The researches made on it, in the words of Miss Clerke, “afforded grounds only for negative conclusions as to its nature. It certainly did not represent the outpourings of a Jovian volcano; it was in no sense attached to the Jovian soil—if the phrase have any application to the planet; it was not a mere disclosure of a glowing mass elsewhere seethed over by rolling vapours.”

In 1870 Arthur Cowper Ranyard (1845-1894), the well-known English astronomer, began to collect records of unusual phenomena on the Jovian disc to see if any period regulated their appearance. He came to the conclusion that, on the whole, there was harmony between the markings on Jupiter and the eleven-year period on the Sun. The theory of inherent light in Jupiter, however, has not been confirmed. The great planet was examined spectroscopically by Huggins from 1862 to 1864, and by Vogel from 1871 to 1873. The spectrum showed, in addition to the lines of reflected sunlight, some lines indicating aqueous vapour, and others which have not been identified with any terrestrial substance. A photographic study of the spectrum of Jupiter was made at the Lowell Observatory by Slipher in 1904, probably the most exhaustive investigation on the subject. The spectroscope has, however, given little support to the theory of inherent light, and “we are driven to conclude that native emissions from Jupiter’s visible surface are local and fitful, not permanent and general.”

Herschel’s idea, that the rotations of the four satellites of Jupiter were coincident with their revolutions, has on the whole been confirmed by recent researches, although in the case of the two near satellites (Io and Europa) W. H. Pickering’s observations in 1893 indicated shorter rotation periods. There is much to learn regarding the geography of the satellites, although in 1891 Schaeberle and Campbell at the Lick Observatory observed belts on the surface of Ganymede, the third satellite analogous to those on Jupiter. Surface-markings on the satellites have also been seen by Barnard at the Lick Observatory, and by Douglass at Flagstaff.

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 “Phoebe.” 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.

In 1841 a student at the University of Cambridge resolved to grapple with the problem. John Couch Adams, born at Lidcot in Cornwall in 1819, entered in 1839 the University of Cambridge, where he graduated in 1843. From 1858 Professor of Astronomy at Cambridge, and from 1861 director of the Observatory, he died on January 21, 1892, after a life spent in devotion to mathematical astronomy. In 1843, on taking his degree, he commenced the investigation of the orbit of Uranus. For two years he worked at the difficult question, and by September 1845 came to the conclusion that a planet revolving at a certain distance beyond Uranus would produce the observed irregularities. He handed to James Challis (1803-1882), the director of the Cambridge Observatory, a paper containing the elements of what was named by Adams “the new planet.” On October 21 of the same year he visited Greenwich Observatory, and left a paper containing the elements of the planet, and approximately fixing its position in the heavens. But the Astronomer-Royal of England, Sir George Biddell Airy (1801-1892), had little faith in the calculations of the young mathematician. He always considered the correctness of a distant mathematical result to be a subject rather of moral than of mathematical evidence: in fact, regarding Uranus, the Astronomer-Royal almost called in question the correctness of the law of gravitation. Besides, the novelty of the investigations aroused scepticism, and the fact that Adams was a young man, and inexperienced, went against Airy’s acceptance of the theory. However, he wrote to Adams questioning him on the soundness of his idea. Adams thought the matter trivial, and did not reply. Airy, therefore, took no interest in the investigations, and no steps were taken to search for the unseen planet. Meanwhile the Rev. W. R. Dawes happened to see Adams’ papers lying at Greenwich, and wrote to his friend, the well-known astronomer Lassell, who was in possession of a very fine reflector, erected at his residence near Liverpool, asking him to search for the planet. But Lassell was suffering from a sprained ankle, and Dawes’ letter was accidentally destroyed by a housemaid. So Adams’ theory remained in obscurity.

The question now came under the notice of FranÇois Jean Dominique Arago (1786-1853), the director of the Paris Observatory. He recognised in a young friend of his a rising genius, who was competent to solve the problem. Urban Jean Joseph Le Verrier, born at Saint Lo, in Normandy, in 1811, became in 1837 astronomical teacher in the École Polytechnique, and in 1853 director of the Paris Observatory. In consequence of differences with his staff he was obliged, in 1870, to resign from this position, but two years later was restored to the post, which he held till his death on September 23, 1877.

In 1845, ignorant of the fact that Adams had already solved the problem, Le Verrier began his investigations of the irregular motions of Uranus. In a memoir communicated to the Academy of Sciences in November of that year, he demonstrated that no known causes could produce these disturbances. In a second memoir, dated June 1, 1846, he announced that an exterior planet alone could produce these effects. But Le Verrier had now before him the difficult task of assigning an approximate position to the unseen body, so that it might be telescopically discovered. After much calculation Le Verrier, in his third memoir (August 31, 1846), assigned to the planet a position in the constellation Aquarius.

Meanwhile one of Le Verrier’s papers happened to reach Airy. Seeing its resemblance to Adams’ papers, which had been lying on his desk for months, his scepticism vanished, and he suggested to Challis that the planet should be searched for with the Cambridge equatorial. In July 1846 the search was commenced. The planet was actually observed on August 4 and 12, but, owing to the absence of star maps, it was not recognised. “After four days of observing,” he wrote to Airy, “the planet was in my grasp if I had only examined or mapped the observations.”

Le Verrier wrote to Encke, the illustrious director of the Berlin Observatory, desiring him to make a telescopic search for a planetary object situated in the constellation Aquarius, as bright as a star of the eighth magnitude and possessed of a visible disc. “Look where I tell you,” wrote the French astronomer, “and you will see an object such as I describe.” Encke ordered his two assistants, Galle and D’Arrest, to make a search on the night of September 23, 1846. In a few hours Galle observed an object not marked in the star-maps of the Berlin Observatory, which had been recently published. The following night sufficed to show that the object was in motion, and was therefore a new planet. On September 29 Challis found the planet at Cambridge, but he was too late, as the priority of the discovery was now lost to Adams. The planet received the name of “Neptune.”

For some time, indeed, it appeared as if the French astronomer alone was to receive the honour of the discovery. But on October 3, 1846, a letter from Sir John Herschel appeared in the ‘AthenÆum’ in which he referred to the discovery made by Adams. The French scientists were extremely jealous. Indeed, Arago actually declared that, when Neptune was under discussion, the entire honour should go to Le Verrier, and the name of Adams should not even be mentioned,—Arago’s line of reasoning being that it was not the man who first made a discovery who should receive the credit, but he who first made it public. However, the credit of the discovery is now given equally to Adams and Le Verrier, both of whom are regarded as among the greatest of astronomers.

Only a fortnight after the discovery of Neptune, the astronomer Lassell observed a satellite to the distant planet on October 10, 1846. This discovery was confirmed in July 1847 by the discoverer himself, and shortly afterwards by Bond and Otto Struve. Regarding the globe of Neptune, we know practically nothing. No markings of any kind have been observed on its surface. However, in 1883 and 1884, Maxwell Hall, an astronomer in Jamaica, noticed certain variations of brilliance which suggested a rotation-period of eight hours, but this was not confirmed by any other astronomer. The spectrum of Neptune has been investigated by various observers, who have found it to be similar to that of Uranus.

The existence of a trans-Neptunian planet has been suspected by many astronomers. In November 1879 the first idea of its existence was thrown out by Flammarion in his ‘Popular Astronomy.’ Flammarion noticed that all the periodical comets in the Solar System have their aphelion near the orbit of a planet. Thus Jupiter owns about eighteen comets; Saturn owns one, and probably two; Uranus two or three; and Neptune six. The third comet of 1862, however, along with the August meteors, goes farther out than the orbit of Neptune. Accordingly, Flammarion suggested the existence of a great planet, assigning it a period of 330 years and a distance of 4000 millions of miles.