Of all the planets Mars has been most studied during the nineteenth century. Many illustrious astronomers have devoted years to the study of the red planet, with the result that more is known of the surface of Mars than of any other celestial body, with the exception of the Moon. After the time of Herschel, the leading students of Mars were Beer and Mädler, who carefully studied the planet from 1828 to 1839. They identified at each opposition the same dark spots, frequently obscured by mists, and they also made the most accurate determination of the rotation period, which they fixed at 24 hours 37 minutes 23 seconds. This estimate was confirmed in 1862 byFriedrich Kaiser(1808-1872) of Leyden, in 1869 byRichard Anthony Proctor(1837-1888), and in 1892 byHenricius Gerardus van de Sande Bakhuyzen(born 1838), director of the Leyden Observatory. In 1862 Lockyer identified the various markings seen by Beer and Madler in 1830. The other great names in Martian study prior to 1877 are Angelo Secchi andWilliam Rutter Dawes(1799-1868), who studied Mars from 1852 to 1865 and secured a very valuable series of drawings. These drawings were used by Proctor for the construction of the first reliable map of Mars, which was published in 1870 in his work, ‘Other Worlds than Ours.’Proctor gave names to the various Martian features, the reddish-ochre portions of the disc being named continents and the bluish-green portions seas; and Proctor’s views on Mars found favour for many years. In 1877, however, Schiaparelli opened a new era in the study of Mars. In September of that year, during the very favourable opposition of the planet, Schiaparelli, while executing a trigonometrical survey of the disc, discovered that the continents were cut up by numerous long dark streaks, which he calledcanali. In 1879, to his surprise, he found that some of the canals had become double; and he confirmed this in 1881 and at subsequent oppositions. Meanwhile, as Schiaparelli was the only observer who had hitherto seen the canals, there was much scepticism as to their reality. In 1886, however, they were seen at the Nice Observatory byHenri Perrotin(1845-1904), who also observed their duplication. Since 1886 they have been observed by many astronomers, including Camille Flammarion in France,William Frederick Denning(born 1848) in England,Vincenzo Cerulli(born 1859) in Italy, Percival Lowell and W. H. Pickering in the United States. In 1892 W. H. Pickering successfully observed the canals, and discovered at the junctions of two or more canals roundblack spots, to which he gave the name of “lakes,” in keeping with the view that the dark regions of the planet were seas.
In 1894 Percival Lowell erected at Flagstaff, Arizona, an observatory for the specific purpose of observing Mars and its canals in good and steady air. He was assisted by W. H. Pickering and byAndrew Ellicott Douglass(born 1867). During a year’s study Douglass measured the Martian atmosphere and discovered canals crossing the dark regions of the planet, finally disproving the idea of their aqueous character. Lowell recognised all Schiaparelli’s canals, and discovered many more. He also attentively studied the south polar cap of Mars, which disappeared entirely on October 12, 1894. Lowell noticed, also, that as the cap melted the canals became darker, as if water was being conveyed down; and accordingly he adopted the view put forward by Schiaparelli, that the canals are waterways lined on either side by banks of vegetation. His observations were published in the end of 1895 in his work ‘Mars.’ He is of opinion that the reddish-ochre regions or “continents” are deserts, and the greenish areas marshy tracts of vegetation. The lakes are named by him “oases,” and, as Miss Clerke observes, he “does not shrink from the fullimplication of the term.” He regards the canals as strips of vegetation fertilised by a small canal, much too small to be seen, an idea which originated with W. H. Pickering. The canals are believed by Lowell to be waterways down which the water from the melting polar cap is conveyed to the various oases. He considers, in fact, that the canals are constructed by intelligent beings with the express purpose of fertilising the oases, regarded by him as centres of population. He remarks that water is scarce on the planet, owing to its small size, and as a consequence the inhabitants are forced to utilise every drop. The canal system is the result.
Lowell’s theory has not been cordially received—although it is now gradually gaining popularity,—and several other hypotheses have been propounded to explain the canals. Proctor, who died some years before Lowell’s theory was given to the world, regarded them as rivers, but this view may now be looked upon as abandoned. It was suggested that the canals might be cracks in the surface of Mars or meteors ploughing tracks above it: and ProfessorJohn Martin Schaeberle(born 1853) of the Lick Observatory put forward the view that the canals were chains of mountains running over the light and dark regions. None of these theories, however, gainedpopularity, and had to give way to a more popular theory, the “illusion” hypothesis, put forward by the Italian astronomer Cerulli, and supported by Newcomb and Maunder. On the basis of the illusion theory, Newcomb explains that the “canaliform” appearance “is not to be regarded as a pure illusion on the one hand or an exact representation of objects on the other. It grows out of the spontaneous action of the eye in shaping slight and irregular combinations of light and shade, too minute to be separately made out into regular forms.” Experiments were made by Maunder in 1902, and the results pointed to the truth of the theory that the canals were really illusions. But the studies of Lowell at the oppositions of 1903 and 1905 have seriously weakened the hypothesis of Cerulli and Maunder, and strongly confirm the theory of the artificial origin of the canals. In 1903 Lowell was enabled, from a study of the development of the canals, to show the probability of their artificial nature, and his study of the double canals showed a distinct plan in their distribution. Finally, on May 11, 1905, several photographs of Mars were secured at the Lowell Observatory, on which the canals appeared, not as dots of light and shade, as on the illusion theory, but as straight dark lines. This goes far to prove thereality of the canals,—in spite of the ridicule cast on them and their observers,—and consequently the truth of the theory of intelligent life in Mars.
Meanwhile the old-fashioned Martian observations have been continued in less favourable climates than Arizona and Italy by various astronomers, among them the famous Camille Flammarion, the American astronomersJames Edward Keeler(1857-1900),Edward Emerson Barnard(born 1857), the English astronomer W. F. Denning, and others. These conscientious and painstaking observers have done much for Martian study in increasing the number of accurate delineations of the Martian surface.
The spectrum of Mars was first examined by Huggins in 1867. He found distinct traces of water-vapour, and this was confirmed by Vogel in 1872, and by Maunder some years later. In 1894, however,William Wallace Campbell(born 1862), the American astronomer, observing from the Lick Observatory, California, was unable to detect the slightest difference between the spectra of Mars and the Moon, indicating that Mars had no appreciable atmosphere; and from this he deduced that the Martian polar caps could not be composed of snow and ice, but of frozen carbonic acid gas. In 1895, however, Vogel confirmed hisprevious observations, and reaffirmed the presence of water-vapour in the Martian atmosphere.
During the opposition of 1830, Mädler undertook an extensive search for a Martian satellite, but was unsuccessful. In 1862 the search was resumed byHeinrich Louis D’Arrest(1822-1875), the famous German observer, who was also unsuccessful. Accordingly the red planet was referred to by Tennyson as the “moonless Mars.” In 1877 the search was taken up byAsaph Hall, the self-made American astronomer, born at Goshen, Connecticut, in 1829, and employed from 1862 to 1891 at the Naval Observatory, Washington. During the famous opposition of August 1877, favoured by the great 26-inch refractor, he succeeded in discovering two very small satellites of Mars, to which he gave the names of Phobos and Deimos. He determined the time of revolution of Phobos at 7 hours 39 minutes, and that of Deimos at 30 hours 17 minutes,—Phobos revolving round Mars more than three times for one rotation of the planet on its axis. These two satellites are very small, not more than thirty miles in diameter. After Hall’s successful search, photographs were exposed at the Paris Observatory for other Martian satellites, but none was discovered. No further moons have been found belonging to the redplanet, nor is it likely that any further satellites of Mars are in existence.
The discovery of a zone of small planets in the space between Mars and Jupiter belongs completely to the nineteenth century, although the existence of a planet in the vacant space was suspected three centuries ago. In 1772 the subject was taken up byJohann Elert Bode(1747-1826), afterwards director of the Berlin Observatory, who investigated a curious numerical relationship, since known as Bode’s Law, connecting the distances of the planets. If four is added to each of the numbers—0, 3, 6, 12, 24, 48, 96, and 192, the resulting series represents pretty accurately the distances of the planets from the Sun, thus—4 (Mercury), 7 (Venus), 10 (The Earth), 16 (Mars), 28, 52, (Jupiter), and 100 (Saturn). After the discovery of Uranus, in 1781, it was found that it filled up the number 196. Bode, however, saw that the number 28, between Mars and Jupiter, was vacant, and predicted the discovery of the planet. Aided byFranz Xavier von Zach(1754-1832), he called a congress of astronomers, which assembled in 1800 at Schröter’s observatory at Lilienthal, when, for the purpose of searching for the missing planet, the zodiac was divided into twenty-four zones, each of which was givento a separate astronomer. One of them was reserved forGiuseppe Piazzi(1746-1826), director of the Observatory of Palermo.
Born in 1746 at Ponte, in Lombardy, Giuseppe Piazzi, after entering the Theatine Order of monks, became in 1780 Professor of Mathematics at Palermo, where an observatory was erected in 1791; and at that observatory Piazzi worked till his death in 1826. In 1792 he commenced a great star-catalogue, and while making his nightly observations he discovered, on January 1, 1801—the first night of the nineteenth century,—what he took to be a tailless comet, but which proved to be a small planet revolving round the sun in the vacant space. The discovery was hailed by Bode and Von Zach with much enthusiasm, and Piazzi named the planet Ceres. The little planet was, however, soon lost in the rays of the sun before sufficient observations had been made; but the great mathematician,Friedrich Gauss(1777-1855), came to the rescue, and pointed out the spot where the planet was to be rediscovered. In that spot it was found on December 31, 1801, by Von Zach at Gotha, and on the following evening byHeinrich Olbers(1758-1840) at Bremen.
On March 28, 1802, while observing Ceresfrom his house at Bremen, Olbers was struck by the presence of a strange object near the path of the planet. At first he supposed it to be a variable star at maximum brilliance, but a few hours showed him that it was in motion, and was therefore another planet. He named it Pallas, and propounded the theory that the two “Asteroids”—so named by Herschel—were fragments of a trans-Martian planet, which, through some accident, had been shattered to pieces in the remote past. Olbers urged the necessity of searching for more small planets. His advice was taken. In 1804Karl Ludwig Harding(1765-1834), Schröter’s assistant, discovered Juno, and Olbers himself detected Vesta, March 29, 1807.
After 1816 the search was relinquished, as no more planets were discovered. In 1830, however, a German amateur,Karl Ludwig Hencke(1793-1866), ex-postmaster of Driessen, commenced a search for new planets, which was rewarded, after fifteen years, by the discovery of Astræa, December 8, 1845. On July 1, 1847, he made another discovery, that of Hebe. A few weeks later,John Russell Hind(1823-1895), the English astronomer, discovered Iris. Since 1847 not a year has passed without one or more planetsbeing found, sometimes as many as twenty being discovered in a single year. Some astronomers have made the search for asteroids their chief business. The principal asteroid discoverers have beenChristian H. F. Peters(1813-1890), Henri Perrotin,Paul Henry(1848-1905),Prosper Henry(1849-1903), James Watson,Robert Luther(1822-1900),Johann Palisa(born 1848), andMax Wolf(born 1863).
In 1891 a new impulse was given to asteroid study by the application of photography by Max Wolf to the discovery of the minor planets. It occurred to Wolf that the asteroid would be represented on the plate by a trail, caused by its motion during the time of exposure; and assisted byArnold Schwassmann(born 1870),Luigi Carnera(born 1875), and others, Wolf has discovered over a hundred asteroids, and he has the whole field of asteroid hunting to himself. Few minor planets are now discovered by the older method. In 1901 Wolf invented his new instrument of research, the stereo-comparator, which, on the principle of the old-fashioned stereoscope, represents the planetary bodies as suspended in space far in front of the stars. In this way this ingenious astronomer has been enabled to discover asteroids at thefirst glance: year by year fresh discoveries are announced from the Heidelberg Observatory, until more than five hundred asteroids are now known.
Waning interest in the ever-increasing family of asteroids was revived in 1898 by the discovery byKarl Gustav Witt(born 1866) of a small planet, to which he gave the name of Eros, which comes nearer to the Earth than Mars, and which is of great assistance to astronomers in the determination of the solar parallax. For some time prior to 1898 astronomers had considered it a waste of time to search for new asteroids; but this idea is not now so popular, in view of the benefit conferred on astronomy by the discovery of Eros.
Of the physical nature of the asteroids astronomers know nothing. Only the four largest have been measured. For many years it was supposed that Vesta, the brightest of the asteroids, was also the largest. The measures of Barnard with the great Lick refractor in 1895, however, showed that Ceres is the largest, with a diameter of 477 miles. Pallas comes next, with a diameter of 304 miles; while the diameters of Vesta and Juno are respectively 239 and 120 miles. Barnard saw no traces of atmosphereround any of the asteroids. It should be stated that in 1872 Vogel thought he could detect an “air-line” in the spectrum of Vesta: he admitted that the observation required confirmation, but it has not been corroborated either by himself or any other observer.
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 1860George 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. Noreally 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 byTheodor Alexandrovitch Brédikhine(1831-1904), and at Potsdam byWilhelm 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 andGeorge 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 onlyfor 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 1870Arthur 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 concludethat 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 byAndrew Anslie Common(1841-1903), with his great 5-foot reflector at Ealing, near London. The newsatellite 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 SirRobert 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, Gallenoticed 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 byEdouard 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 1888Hugo 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 Deslandresat Meudon, and Bélopolsky at Pulkowa.
In 1851 a startling theory regarding Saturn’s rings was put forward by the famousOtto Wilhelm von Struve(1819-1905). Comparing his measurements on the rings made at Pulkowa in 1850 and 1851 with those of other astronomersfor 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 byStanley Williams, an English observer, in the same year. On June 16, 1903, Barnard, at the Yerkes Observatory, discovered a bright spot, fromwhich 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 byJosé 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 byWilliam 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 bandsresembling 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 twoouter 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 exteriorplanet. 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 toJames Challis(1803-1882), the director of the Cambridge Observatory, a paper containing the elements of what was named by Adams “the new planet.” OnOctober 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, SirGeorge 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 ofFrançois 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 telescopicallydiscovered. 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 notmarked 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. Thisdiscovery 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.
Two independent investigators,David Peck Todd(born 1855) in America andGeorge Forbesin Scotland, have since undertaken to find the planet. Todd, utilising the “residual perturbations” of Uranus, assigned a period of 375 years for his planet. Forbes, on the other hand, working from the comet theory, stated his belief in the existence of two planets with periods of 1000 and 5000 years respectively. In October 1901 he computed the position of the new planet on the celestial sphere, fixing its position in the constellation Libra, and computing its size to be greater than Jupiter. A search was made by means of photography, in 1902, but without success. Nevertheless, astronomers are pretty confident of the existence of one or more trans-Neptunian planets. Lowell is very definite on this subject when he says in regard to meteor groups, “The Perseids and the Lyrids go out to meet the unknown planet, which circles at a distance of about forty-five astronomical units from the Sun. It may seem strange to speak thus confidently of what no mortal eye has seen, but the finger of the sign-board of phenomena points so clearly as to justify the definite article. The eye of analysis has already suspected the invisible.”
At the time of Herschel the ancient superstitions in regard to comets had to a great extent vanished, thanks mainly to the return of Halley’s comet in 1758. Yet, although comets had ceased to be objects of terror, no explanation or rational theory of their nature was put forward until the appearance of the great comet of 1811. This comet was visible from March 26, 1811, to August 17, 1812, a period of 510 days. It was one of the most magnificent comets ever seen, its tail being 100 millions of miles in length and its head 127,000 miles in diameter. This wonderful phenomenon was the subject of much investigation, particularly by Olbers, the great German astronomer.
Heinrich Wilhelm Matthias Olbers was born at Arbergen, a village near Bremen, October 11, 1758. His father was a clergyman who, inaddition to considerable mathematical powers, was an enthusiastic lover of astronomy. At the age of thirteen young Olbers became deeply interested in that science. While taking an evening walk in the month of August, he observed the Pleiades, and determined to find out to which constellation they belonged. He therefore bought some books on astronomy, along with a few charts of the sky, and he began to study the science with much enthusiasm. He read every book he could lay his hands on, and a few months sufficed to make him acquainted with all the constellations.
In 1777, when in his nineteenth year, Olbers entered the University of Göttingen to study medicine, and at the same time he learned much regarding mathematics and astronomy from the mathematician Kaestner. When twenty-one years of age he observed the stars at Göttingen, and devised a method of calculating the orbits of comets, the idea coming to him while he was attending at the bedside of a fellow-student who had taken ill. “Although not made public until 1797,” writes Miss Clerke, “‘Olbers’ method’ was then universally adopted, and is still regarded as the most expeditious and convenient in cases where absolute rigour is not required. By its introduction,not only many a toilsome and thankless hour was spared, but workers were multiplied and encouraged in the pursuit of labours more useful than attractive.”
Towards the end of 1781 he returned to Bremen, settled as a medical doctor, and continued in practice for about forty-one years. But although he had adopted perhaps the most toilsome profession, his love of science prevailed, and night after night he explored the heavens with untiring zeal. He never slept more than four hours, and the upper part of his house in the Sandgasse, in Bremen, was fitted up with astronomical instruments. The largest telescope which he possessed was a refractor 3¾ inches in aperture. He remained in active practice till 1823, when he retired, and was enabled to devote more attention to his beloved science. He died on March 2, 1840, at the advanced age of eighty-one.
Miss Clerke says of Olbers, “Night after night, during half a century and upwards, he discovered, calculated, or observed the cometary visitants of northern skies.” He was the discoverer of the comet of 1815, known as Olbers’ comet. It moves round the Sun in a period of over seventy years, and returned to perihelion in 1887, forty-seven years after the death of itsdiscoverer. The great comet of 1811 was the subject of a memoir which Olbers published the following year, and in which he originated the “electrical repulsion” theory of comets’ tails. Even after the fulfilment of Halley’s great prediction, comets were still looked upon with profound awe, and the popular fear regarding them was still prevalent. Olbers, however, showed that the tails of comets resulted from purely natural causes. He regarded the Sun as possessed of a repulsive as well as an attractive force, and considered the tails to be vapours repelled from the nucleus of the comet by the Sun. He calculated that in the comet of 1811 the particles of matter expelled from the head reached the tail in eleven minutes, with a velocity comparable to that of light. The theory of electrical repulsion, since elaborated by other observers, is now generally accepted among astronomers. No other hypothesis represents in such a complete manner the formation and growth of the luminous appendages of the celestial bodies so picturesquely called “pale-winged messengers” as that put forward by the physician of Bremen.
Some years after Olbers’ famous theory was given to the world, a great advance was madein cometary astronomy by another great German astronomer, his friend and pupil Encke. The son of a Hamburg clergyman, Johann Franz Encke was born in that city in 1791, and died in 1865 at Spandau. After taking part in the war against Napoleon, he was in 1822 appointed director of the Gotha Observatory, being called to Berlin in 1825. In early life he was the pupil of Olbers and Gauss, and his investigations and discoveries formed an epoch in astronomy. His most famous discovery related to the little comet which bears his name. The comet was discovered byJ. L. Pons(1761-1831) at Marseilles, although it had previously been seen by Méchain and Caroline Herschel. In 1819 Encke computed the orbit of the comet, and boldly announced that it would reappear in 1822, its period being about 3¼ years, or 1208 days. In 1822 the comet, true to Encke’s prediction, returned to perihelion, and was observed at Paramatta in Australia, the perihelion passage taking place within three hours of the time predicted by Encke. As Miss Clerke remarks, “The importance of this event will be better understood when it is remembered that it was only the second instance of the recognised return of a comet; and that it, moreover, established the existence of a newclass of celestial bodies, distinguished as comets of short period.”
In 1825 the comet was again observed by Valz, passing perihelion on September 16, and in 1828 it was seen by Struve. Encke now made a very remarkable discovery. Determining its period with great accuracy, in 1832 he found that his comet returned to perihelion two and a half hours before the predicted time. As this repeatedly happened, Encke put forward the theory that the acceleration was due to the existence of a resisting medium in the neighbourhood of the Sun, too rarefied to retard the planetary motions, but quite dense enough to make the comet’s path smaller, and to eventually precipitate it on the Sun. The theory was widely accepted, but after 1868 the acceleration began to decrease, diminishing by one-half; besides, no other comet is thus accelerated, and the hypothesis has accordingly been abandoned.
The second comet recognised as periodic was that discovered on February 27, 1826, by an Austrian officer,Wilhelm von Biela(1782-1856), and ten days later by the French observer,Gambart(1800-1836), both of whom, in computing its orbit, noticed a remarkable similarity to the orbits of comets which appeared in 1772 and 1805. Accordingly, they concluded it tobe periodic, with a period of between six and seven years. The comet returned in 1832. In 1828 Olbers had published certain calculations showing that portions of the comet would sweep over the part of the Earth’s orbit a month later than the Earth itself. This gave rise to a panic that the comet would destroy the Earth, which did not subside till it was announced by Arago that the Earth and the comet would at no time approach to within fifty million miles of each other. The comet returned again in the end of 1845. It was kept well in view by astronomers in Europe and America. On December 19, 1846, Hind noticed that the comet was pear-shaped, and ten days later it had divided in two. The two comets returned again in 1852 and were well observed; but they were never seen again, at least as comets. Their subsequent history belongs to meteoric astronomy.
A comet discovered by Faye at Paris in 1843 was found to have a period of seven and a half years. It has returned regularly since its discovery, true to astronomical prediction. Its motion was particularly investigated for traces of a resisting medium, byDidrik Magnus Axel Möller(1830-1896), director of the Lund Observatory, who reached a negative conclusion.