A Century's Progress in Astronomy :: CHAPTER V. THE INNER PLANETS.

CHAPTER V. THE INNER PLANETS.

Much progress has been made during the last hundred years in our knowledge of the planets. In fact, the study of Mercury only dates from the commencement of the nineteenth century. Our knowledge of the vicinity of the Sun is very limited, and Mercury is difficult of observation. So limited, in fact, is our knowledge of the Sun’s surroundings, that it is not yet known for certain whether there is a planet, or planets, between Mercury and the Sun. Perturbations in the motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to the belief that a planet of about the size of Mercury, or else a zone of asteroids, existed between Mercury and the Sun. It was, however, obvious that such a planet could only be seen when in transit across the Sun’s disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault, informed Le Verrier that he had seen a round object in transit over the Sun’s disc. Le Verrier, certain that this was the missing planet, named it “Vulcan,” and calculated its orbit, assigning it a revolution period of twenty days. But it was never seen again. Transits of “Vulcan” were fixed for 1877 and 1882, but nothing was seen on these dates. During the total eclipse of July 29, 1878, two observers—James Watson (1838-1880), the well-known astronomer, and Lewis Swift (born 1820)—believed themselves to have discovered two separate planets, and ultimately claimed two planets each, which were never heard of again. During the total eclipse of 1883 an active watch for “suspicious objects” was kept, but with no result. At the eclipses of 1900 and 1901 respectively, photographs were exposed by the American astronomers, W. H. Pickering and Charles Dillon Perrine (born 1867), but on none of these plates could any trace of “Vulcan” be found. At the total eclipse of August 30, 1905, plates were again exposed, but no announcement has been made of an intra-Mercurial planet; and the prevalent opinion among astronomers is that no planet comparable with Mercury in size exists between that planet and the Sun.

The study of the physical appearance of Mercury was inaugurated by SchrÖter, who in 1800 noticed that the southern horn of the crescent presented a blunted appearance, which he attributed to the existence of a mountain eleven miles in height. From observations of this mountain he came to the conclusion that the planet rotated in 24 hours 4 minutes. This was afterwards reduced by Friedrich Wilhelm Bessel (1784-1846) to 24 hours 53 seconds.

After the time of SchrÖter there was no astronomer who paid much attention to either Mercury or Venus until the arrival on the scene of the most persistent planetary observer and one of the foremost astronomers of the nineteenth century. Giovanni Virginio Schiaparelli was born at Savigliano, in Piedmont, in 1835, and graduated at Turin in 1854. Called to Milan as assistant in the Brera Observatory in 1860, he became director in 1862, and there for thirty-eight years he studied astronomy in all its aspects, making a great name for himself in various branches of the science. In 1900 he retired from the post of director, and pursues his astronomical researches in his retirement.

In 1882 Schiaparelli took up the study of Mercury in the clear air of Milan. Instead of observing the planet through the evening haze, like SchrÖter and others, he examined it by day, and was enabled to follow it hourly instead of looking at it for a short period when near the horizon. At length, after seven years’ observation, he announced, on December 8, 1889, that Mercury performs only one rotation during its revolution round the Sun—in fact, that its day and year coincide. As a consequence, the planet keeps the same face towards the Sun, one side having everlasting day and the other perpetual night; but owing to the libratory movement of Mercury—the result of uniform motion on its axis and irregular motion in its orbit—the Sun rises and sets on a small zone of the planet’s surface. Schiaparelli’s observations indicated that Mercury is a much spotted globe, with a moderately dense atmosphere, and he was enabled to form a chart of its surface-markings.

Schiaparelli’s conclusions remained until 1896 unconfirmed and yet not denied, although most astronomers were sceptical on the subject. In 1896 the subject was taken up by the American astronomer, Percival Lowell (born 1855), who, in the clear air of Arizona, confirmed Schiaparelli’s conclusions, fixing 88 days as the period of rotation. He remarked, however, that no signs of an atmosphere or clouds were visible to him. The surface of Mercury, he says, is colourless,—“a geography in black and white.” The determination of the rotation period by Schiaparelli and Lowell is now generally accepted, and is confirmed by the theory of tidal friction. It is only right to add that William Frederick Denning (born 1848) in 1881 suspected a rotation period of 25 hours, but this remains unconfirmed. In April 1871 the spectrum of Mercury was examined by Hermann Carl Vogel (born 1842) at Bothkamp. He suspected traces of an atmosphere similar to ours, but was not certain. Of more interest are the photometric observations of ZÖllner in 1874. These observations indicated that the surface of Mercury is rugged and mountainous, and comparable with the Moon,—a conclusion supported by Lowell’s observations in 1896.

Venus, the nearest planet to the Earth, has been attentively studied for three centuries, and still comparatively little is known regarding it. This is due to its remarkable brilliancy, combined with its proximity to the Sun. The great problem at the beginning of the nineteenth century was the rotation of the planet. In 1779 the subject was taken up by SchrÖter at Lilienthal. Nine years later, from a faint streak visible on the disc, he concluded that rotation was performed in 23 hours 28 minutes, and in 1811 this was reduced by seven minutes; but as Herschel was unable to observe the markings seen by SchrÖter, many astronomers were inclined to be sceptical regarding the accuracy of the Lilienthal observers results. SchrÖter also observed the southern horn of Venus when in the crescent form to be blunted, and he ascribed this to the existence of a great mountain, five or six times the elevation of Chimborazo; while he observed irregularities along the terminator, which he considered to be more strongly marked than those on the Moon. SchrÖter’s opinion on this point, although rejected by Herschel, was confirmed by MÄdler, Zenger, Ertborn, Denning, and by the Italian astronomer Francesco Di Vico (1805-1848), director of the Observatory of the Collegio Romano. In 1839 Di Vico attacked the problem of the rotation, and his results were confirmatory of those of SchrÖter. He estimated that the axis of Venus was inclined at an angle of 53° to the plane of its orbit. Meanwhile a series of important observations had been made on Venus by the Scottish astronomer and theologian, Thomas Dick (1772-1857), who suggested daylight observations on Venus to solve the problem of the rotation.

In 1877 the question was attacked by Schiaparelli, who commenced a series of observations on Venus at Milan in that year. The results of his studies were summed up in 1890 in five papers contributed to the Milan Academy. He came to the conclusion that the markings observed by SchrÖter, Di Vico, and others were not really permanent, and concentrated his attention on round white spots, which remained fixed in position. Instead of observing Venus in the evening, Schiaparelli followed it by day, watching it continuously on one occasion for eight hours. But the markings remained fixed. Schiaparelli accordingly concluded that the planet’s rotation was performed in probably 225 days, equal to the time of revolution. One face is turned towards the Sun continually, while the other is perpetually in darkness.

The announcement was so startling that, as Miss Clerke says, “a clamour of contradiction was immediately raised, and a large amount of evidence on both sides of the question has since been collected.” Perrotin at Nice, Tacchini at Rome, Cerulli at Teramo, Mascari at Catania and Mount Etna, and Lowell in Arizona, all in favourable climates, confirmed Schiaparelli’s results, as also did a second series of observations by the Milan astronomer himself in 1895. On the other hand, Neisten, Trouvelot, Camille Flammarion (born 1842), and others, under less favourable climatic conditions, arrived at a period of 24 hours. Aristarch BÉlopolsky (born 1854), from spectroscopic observations at Pulkowa, by means of Doppler’s principle, found a period of 12 hours. Lowell, by the same principle, found, in 1901-03, a period of 225 days, in agreement with Schiaparelli’s results. This is the last word on the subject. Schiaparelli’s rotation period, confirmed by the theory of tidal friction, is generally accepted.

That Venus has an atmosphere was one of the conclusions reached by SchrÖter in 1792; and in this at least he was correct, as the atmosphere of Venus, illuminated by the solar rays, has been seen extending round the entire disc of the planet. Spectroscopic observations by Tacchini, Ricco, and Young, during the transits of 1874 and 1882, indicated the existence of water-vapour in the planet’s atmosphere. Very little has been discovered regarding the “geography” of Venus. White patches at the supposed “poles” of the planet were observed in 1813 by Franz von Gruithuisen, and in 1878 by the French astronomer Trouvelot (1827-1895). The secondary light of Venus, similar to the “old Moon in the new Moon’s arms,” was repeatedly observed since the time of SchrÖter by Vogel, Lohse, Zenger, and others. Vogel attributed it to twilight, and Lamp, a German observer, to electrical processes analogous to our aurorÆ. In 1887 a Belgian astronomer, Paul Stroobant, submitted to a searching examination all the supposed observations of a satellite of Venus, and was enabled to explain nearly all the supposed satellites as small stars which happened to lie near the planet’s path in the sky at the time of observation.

The study of our own planet can hardly be said to belong to the realm of astronomy. Nevertheless, it is through astronomical observation that the motion of the North Pole has been discovered. For many years it has been a problem whether there is a variation of latitude resulting from the motion of the pole. Euler had declared, from theoretical investigation, that, were there such a motion, the period must be 10 months. The question was revived in 1885 by the observations of Seth Carlo Chandler (born 1846) at Cambridge, Mass., with his newly-invented instrument, the “almucantar,” which indicated an appreciable variation of latitude. This was confirmed by Friedrich KÜstner (born 1856), now director of the Observatory at Bonn. The idea now occurred to Chandler to search through the older records to discover if there was any trace of the variation of latitude, with the result that he brought out a period of 14 months instead of 10. This aroused much interest, and many prominent astronomers denied Chandler’s results, which were announced in 1891. As a well-known astronomer has expressed it, “Euler’s work had shown what period the motion must have, and any appearance of another period must be due to some error in the observations. Chandler replied to the effect that he did not care for Euler’s mathematics: the observations plainly showed 14 months, and if Euler said 10, he must have made the mistake. I do not exaggerate the situation in the least; it was a deadlock: Chandler and observation against the whole weight of observation and theory.” It was now shown by Newcomb that Euler had assumed the Earth to be an absolutely rigid body, while modern investigations show that it is not so. Chandler’s discovery is now accepted, and proves that the North Pole is not fixed in position, but has a small periodic motion, though never twelve yards from its mean position. That the small resulting variation in the position of the stars has been noticed at all is a striking illustration of the accuracy of astronomical observation.

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 by Friedrich Kaiser (1808-1872) of Leyden, in 1869 by Richard Anthony Proctor (1837-1888), and in 1892 by Henricius 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 and William 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 called canali. 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 by Henri 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 round black 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 by Andrew 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 full implication 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 Professor John 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, gained popularity, 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 the reality 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 astronomers James 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 his previous 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 by Heinrich 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 by Asaph 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 red planet, 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 by Johann 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 by Franz 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 given to a separate astronomer. One of them was reserved for Giuseppe 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 by Heinrich Olbers (1758-1840) at Bremen.

On March 28, 1802, while observing Ceres from 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 1804 Karl 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 planets being 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 been Christian 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), and Max 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 by Arnold 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 the first 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 by Karl 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 atmosphere round 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.

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