Four years after the death of Herschel, an apothecary in the little German town of Dessau procured a small telescope, with which he began to observe the Sun. The name of this apothecary wasSamuel Heinrich Schwabe(1789-1875). In 1826 he commenced to observe the spots on the Sun’s disc, counting them from day to day, more for self-amusement than from any hope of discovery; for previous astronomers had agreed that no law regulated the number of the sun-spots. Every clear day Schwabe pointed his telescope at the Sun and took his record of the spots; this he continued for forty-three years, until within a few years of his death on April 11, 1875. As early as 1843 Schwabe hinted that a possible period of ten years regulated the distribution of the spots on the Sun, but no attention was given to his idea. In 1851, however, the result of his twenty-sixyears of observation was published in Humboldt’s ‘Cosmos,’ and Schwabe was able to show that the spots increased and decreased in a period of about ten years. Astronomers at once recognised the importance of Schwabe’s work, and in 1857 he was rewarded by the Gold Medal of the Royal Astronomical Society of London.
Rudolf Wolf(1813-1892) of the Zürich Observatory now undertook to search through the records of sun-spot observation, from the days of Galileo and Scheiner, to find traces of the solar cycle discovered by Schwabe. He was successful, and was enabled to correct Schwabe’s estimate of the length of the period, fixing it as on the average 11·11 years. Additional interest, however, was given to Schwabe’s and Wolf’s investigations by the remarkable discoveries which followed. In September 1851John Lamont(1805-1879), a Scottish astronomer,—born at Braemar in Aberdeenshire, but employed as director of the Munich Observatory,—after searching through the magnetic records collected at Göttingen and Munich, discovered that the magnetic variations indicated a period of 10⅓ years. Soon after this SirEdward Sabine(1788-1883), the English physicist, from a discussion of an entirely different set of observations, independently demonstrated the samething, proving conclusively that once in about ten years magnetic disturbances reached their height of violence; and Sabine was not slow to notice the correspondence between the magnetic period and the sun-spot period. In the same year (1852) Wolf andAlfred Gautier(1793-1881) independently made the same discovery, which had thus been made by four separate investigators.
In the same year an English amateur astronomer,Richard Christopher Carrington(1826-1875), commenced a series of solar observations which led to some remarkable discoveries. From observations on the spots, Carrington discovered that while the Sun’s rotation was performed in 25 days at the equator, it was protracted to 27½ days midway between the equator and the poles. In 1858 Carrington demonstrated the fact that spots are scarce in the vicinity of the solar equator, but are confined to two zones on either side, becoming scarce again at thirty-five degrees north or south of the equator. Contemporary with Carrington wasFriedrich Wilhelm Gustav Spörer(1822-1895), who was born in Berlin in 1822 and died at Giessen, July 7, 1895. He commenced his solar observations about the same time as Carrington, and independently discovered the Sun’s equatorialacceleration. From observations at his little private observatory at Anclam in Pomerania, continued at the Astrophysical Observatory in Potsdam, Spörer demonstrated a remarkable law regarding sun-spots. This law is thus described by a well-known astronomer: “The disturbance which produces the spots of a given sun-spot period first manifests itself in two belts about thirty degrees north and south of the Sun’s equator. These belts then draw in toward the equator, and the sun-spot maximum occurs when their latitude is about sixteen degrees; while the disturbance gradually and finally dies out at a latitude of eight or ten degrees. Two or three years before this disappearance, however, two new zones of disturbance show themselves. Thus, at the sun-spot minimum there are four well-marked spot-belts,—two near the equator, due to the expiring disturbance, and two in high latitudes, due to the newly beginning outbreak.” These remarkable discoveries, which resulted from the investigations of Schwabe, Carrington, and Sporer, are a brilliant example of what may be done by amateurs in astronomy.
At the time when Carrington and Spörer were pursuing these researches, the spectroscope came into use as an astronomical instrument, and since 1859 solar astronomy has been almost entirelyspectroscopic. Before we can rightly understand the principles of spectroscopic astronomy, we must go back to the life and work of its founder—Joseph von Fraunhofer.
The son of a poor glazier,Joseph von Fraunhoferwas born on March 6, 1787, at Straubing, in Bavaria. His father and mother having died when their son was quite young, the boy, on account of his poverty, was apprenticed to a looking-glass manufacturer in Munich named Weichselberger, who acted tyrannically, keeping him all day at hard work. Still the lad borrowed some old books, and spent his nights in study. Young Fraunhofer lodged in an old tenement in Munich, which on July 21, 1801, collapsed, burying in its ruins its occupants. All were killed but Fraunhofer, who, though seriously injured, was dug out from the ruins four hours later. The distressing accident was witnessed by Prince Maximilian Joseph, Elector of Bavaria. He became interested in Fraunhofer, and presented him with a sum of money. Of this he made good use. He was already interested in optics, and he bought some books on that subject, as well as a glass-polishing machine. The remainder of the money served to procure his release from his tyrannical master, Weichselberger.
Fraunhofer became acquainted with prominentscientists at Munich, who provided him with books on optics and mathematics. Meanwhile the young optician occupied his time in shaping and finishing lenses. In 1806 he entered the optical department of the Optical and Physical Institute of Munich, and the following year, when only twenty years of age, was appointed to the chief post in that department. In 1814 he commenced his investigations with the prism, which have made his name famous.
Newton had found that, in passing through a prism, white light is dispersed into its primary colours, making up the band of coloured light known as the solar spectrum. But he failed to recognise the existence of dark lines in the spectrum. Casually seen in 1802 byWilliam Hyde Wollaston(1786-1828), an English physicist, these lines were first thoroughly examined by Fraunhofer. Allowing light from the Sun to pass through a prism attached to the telescope, he was amazed to find several dark lines in the spectrum. By the year 1814 he had detected no less than 300 or 400 of these lines. Fraunhofer named the more prominent lines by the letters of the alphabet, from A in the red to H in the violet. They are now known as the Fraunhofer lines. At first he was much perplexed regarding the nature of the dark lines.He suspected that they might be an optical effect, depending on the quality of the glass used, and he tried different prisms, but the lines were still to be seen. Then he turned his prism to bright clouds to see if they were visible in reflected sunlight, and he found that they were. He examined the Moon and again perceived them, as moonlight is merely reflected sunlight; and they were also conspicuous in the spectra of the planets. It was thus proved that these lines were characteristic of sunlight, whether direct or reflected. It was, however, still possible that they might be caused by the passage of the rays of light from the celestial bodies through the Earth’s atmosphere. In order to test this theory, Fraunhofer examined the spectra of the brighter stars. He found that the lines visible in the solar spectrum were not to be seen in the spectra of the stars, thus proving that the lines were not merely an atmospheric effect. Each star, Fraunhofer observed, had a different spectrum from both the Sun and from other stars. These spectra were also characterised by numerous dark lines, much fainter than those in the solar spectrum.
Although he ascertained the existence of the dark lines in the Sun’s spectrum, Fraunhofer never really found out what they represented.As Miss Giberne expresses it, “Although he now saw the lines he could not understand them: he could not read what they said. They spoke to him indeed about the Sun, but they spoke to him in a foreign language, the key to which he did not possess.” However, he expressed the belief that the pair of lines in the solar spectrum, which he marked D, coincided with the pair of bright lines emitted by incandescent sodium. Although he doubtless suspected that the lines conveyed intelligence regarding the elements in the Sun, he never was able properly to decipher their meaning. Had he lived, he would probably have made the great discovery; but these investigations were cut short by his sudden and untimely death on June 7, 1826.
After the death of Fraunhofer, very little was done to forward the study of spectrum analysis. Investigations in this branch of research were made, however, by SirJohn Herschel(1792-1871),William Allen Miller(1817-1870), SirDavid Brewster(1781-1868), and others. Two famous men of science had partly discovered the secret. These were SirGeorge Stokes(1819-1903), of Cambridge, andAnders John Angström(1812-1872) of Upsala. Of Angström’s work, published in 1853, it has been said that it would “have obtained a highcelebrity if it had appeared in French, English, or German, instead of Swedish.”
It was not until 1859 that the principles of spectrum analysis were fully enunciated byGustav Robert Kirchhoff(1824-1887), and his colleague in the University of Heidelberg,Robert Wilhelm Bunsen(1811-1899). Kirchhoff demonstrated that a luminous solid or liquid gives a continuous spectrum, and a gaseous substance a spectrum of bright lines. In the words of Miss Clerke, “Substances of every kind are opaque to the precise rays which they emit at the same temperature. That is to say, they stop the kinds of light or heat which they are then actually in a condition to radiate.... This principle is fundamental to solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer lines. The identical characters which are written bright in terrestrial spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff made several determinations of the substances in the Sun, proving the existence of sodium, iron, calcium, magnesium, nickel, barium, copper, and zinc. His great map of the solar spectrum was published by the Berlin Academy in 1860, and represented an enormous amount of labour. It was succeeded by another map by Angström, published in 1868. But both ofthese maps have been recently superseded by the investigations of SirJoseph Norman Lockyer(born 1836), and of the American physicist,Henry Augustus Rowland(1848-1901). Rowland largely increased our knowledge of the elements in the solar atmosphere.
The spectroscope had become, by 1868, a recognised instrument of astronomical research, and in that year it was applied during the famous total eclipse, visible in India. There were many eclipse problems, arising from the observations made by the eclipse expeditions of 1842, 1851, and 1860. The eclipse of 1851 had finally proved that the red flames seen surrounding the Sun during total eclipses belonged to the Sun, and not to the Moon, as many astronomers had believed. At the eclipse of 1860, visible in Spain, the Italian astronomer,Angelo Secchi(1818-1878), and the Englishman,Warren De la Rue(1815-1889), secured photographs of the solar prominences. The problem of 1868 was the constitution of these prominences.
Pierre Jules César Janssen, born in Paris in 1824, was stationed at Guntoor, in India, to observe the eclipse. He succeeded in observing the spectrum of the prominences during the progress of totality, and found it to be one of bright lines, proving the gaseous nature of thesun-flames. During the progress of the eclipse, Janssen was specially struck by the brilliancy of the bright lines, and it occurred to him that the prominence-spectrum could be observed in full daylight, if sufficient dispersive power was used to enfeeble the ordinary continuous spectrum. At ten o’clock on the following morning, August 19, 1868, Janssen applied his spectroscope to the sun, and observed the prominence-spectrum. After a month’s observation in India, he sent to the French Academy an account of his success. A short time, however, before his report arrived, the Academy had received a similar one from Lockyer, who had independently made the same discovery. Two years previously, in 1866, the new method had occurred to him, but his spectroscope was not powerful enough; and although he ordered a more powerful one at once, it was not until October 16, 1868, that he had the instrument in his hands. Four days later he observed the prominence-spectrum in full daylight.
The next advance in the study of the prominences was announced in 1869. Janssen and Lockyer had shown astronomers how to observe the spectrum of the prominences; but the researches of other two famous astronomers enabled observers to see the forms of the prominences.These two men wereWilliam Huggins(born 1824) andJohann Carl Friedrich Zöllner. The latter astronomer, born in Leipzig in 1834, was one of the most successful students of the solar prominences. He was Professor of Astrophysics in the University of Leipzig, a position which he filled with success until his untimely death on April 25, 1882. Independently of Huggins, he found that by opening the slit of the spectroscope wider, the forms of the prominences themselves could be seen. The study of the prominences was at once taken up by the most famous solar observers: these were Huggins and Lockyer in England, Spörer and Zöllner in Germany, Janssen in France, Secchi, Respighi, and Tacchini in Italy, Young in America. ToCharles Augustus Young(born 1834) we owe the careful study of individual prominences. On September 7, 1871, he observed the most gigantic outburst on the sun ever witnessed, fragments of an exploded prominence reaching a height of 100,000 miles: Young, also, made the first attempt to photograph the prominences.
To the Italian school of astronomers, however, we owe the persistent and systematic study of the prominences. Among them the three greatest names areAngelo Secchi(1818-1878),Lorenzo Respighi(1824-1889), andPietro Tacchini(1838-1905). After the death of Secchi, the recognised head of spectroscopy in Italy was Pietro Tacchini. Born at Modena in 1838, he was appointed director at Modena in 1859, assistant at Palermo in 1863, and director at Rome in 1879. In 1870 he commenced to take daily observations of the prominences, noting their sizes, forms, and distribution, and these observations were continued for thirty-one years, until within four years of Tacchini’s death, which took place on March 24, 1905. Tacchini did for the study of the prominences what Schwabe did for the spots. The Italian spectroscopists found that the prominences increased and decreased every eleven years in harmony with the spots. Tacchini demonstrated that the streamers of the solar corona originate in regions where the prominences are most numerous, and that the shape of the corona, on the whole, varies in sympathy with the prominences.
The researches of Lockyer indicated that the prominences originated in a shallow gaseous atmosphere which he termed the chromosphere. Formerly astronomers had to observe only isolated prominences, but in 1892 an American astronomer,George Ellery Hale(born 1868), formerly director of the Yerkes Observatory,and now director of the Solar Observatory in California, succeeded in photographing, by an ingenious process, the whole of the chromosphere, prominences, and faculæ visible on the solar surface.
Another solar envelope was discovered in 1870 by Dr Charles Augustus Young, who from 1866 to 1877 directed the Observatory at Dartmouth, New Hampshire, and from 1877 to 1905, that at Princeton, New Jersey. During the eclipse of December 22, 1870, Young was stationed at Tenez de Frontena, Spain. As the solar crescent grew apparently thinner before the disc of the Moon, “the dark lines of the spectrum,” he says, “and the spectrum itself gradually faded away, until all at once, as suddenly as a bursting rocket shoots out its stars, the whole field of view was filled with bright lines, more numerous than one could count. The phenomenon was so sudden, so unexpected, and so wonderfully beautiful, as to force an involuntary exclamation.” The phenomenon was observed for two seconds, and the impression was left on the astronomer that a bright line had taken the place of every dark one in the solar spectrum, the spectrum being completely reversed. Hence the name which was given to the hypothetical envelope—“the reversing layer.” For long the existenceof the reversing layer was disputed by numerous astronomers. In 1896 photographs taken during the solar eclipse of that year finally demonstrated the existence of the “flash spectrum” as seen by Young.
The last of the solar appendages, the corona, can only be seen during total eclipses. The researches of Young and Janssen indicate that it is partly gaseous and partly meteoric in its constitution; and various photographs, taken at the eclipses since 1870, have demonstrated its variation in shape, which is in harmony with the eleven-year period. Several attempts have been made to observe the corona without an eclipse. In 1882 Huggins made the attempt, but failed, and Hale, with his photographic process, had no better success. More recently, in 1904, a Russian astronomer,Alexis Hansky, observing from the top of Mont Blanc, secured some photographs on which he believes the corona is represented, but so far his observations have not been confirmed by other astronomers.
The application of the spectroscope to the motions on the solar surface is perhaps one of the most wonderful triumphs in astronomical science. In 1842Christian Doppler(1803-1853), Professor of Mathematics at Prague, had expressedthe view that the colour of a luminous body must be changed by its motion of approach or recession. It was obvious to Doppler that if the body was approaching, a larger number of light waves must be entering the eye of the observer than if it were retreating. Miss Clerke thus illustrates Doppler’s principle: “Suppose shots to be fired at a target at fixed intervals of time. If the marksman advances, say, twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals.” It occurred to various astronomers that it would be possible to measure cyclones and hurricanes in the Sun, not by change of colour in the spectrum, but by the shifting of the lines; and in 1870 this was successfully done by Lockyer. In the next few years efforts to measure the solar rotation were made by Young, Zöllner, and others, who succeeded in measuring the displacement of the lines, but not the time of rotation. This was reserved for the famous Swedish astronomer, Dunér.
Nils Christopher Dunér, born in 1839 in Scania, was employed as an assistant at LundObservatory from 1858 to 1888, when he was appointed director of the Observatory at Upsala. In that year he commenced a study of the solar rotation, measuring it by means of Doppler’s principle. He confirmed the telescopic work of Carrington and Spörer on the equatorial acceleration, and measured the displacement up to within fifteen degrees of the poles. He brought out the surprising fact that the rotation period of the Sun is there protracted to 38½ days. These remarkable researches were published in 1891.
In 1873 the Astronomer-Royal of England commenced at Greenwich Observatory to photograph the Sun daily. This work has been carried on there byEdward Walter Maunder(born 1851), and Greenwich Observatory possesses a photographic record of sun-spots. At the Meudon Astrophysical Observatory, near Paris, Janssen has, since 1876, secured photographs of the solar surface, which were comprised in a great atlas, published by him in January 1904. These photographs have revealed a remarkable phenomenon—the “réseau photospherique,” the distribution over the solar surface of blurred patches of light, which Janssen considers are inherent in the Sun. The Greenwich records of sun-spots and ofmagnetic disturbances have been made use of by Maunder in his remarkable studies, promulgated in 1904, of the connection between sun-spots and terrestrial magnetism. Maunder finds that on the average magnetic storms are dependent on the presence of sun-spots, and on the size of the spot. The magnetic action, he finds, does not radiate equally in all directions from the sun-spots, but along definite and restricted lines.
Herschel’s hypothesis of a dark and cool globe beneath the solar photosphere was seen to be untenable after the introduction of the spectroscope. The first important theory as to the solar constitution was that advanced in 1865 by the French astronomer,Hervé Faye(1814-1902). Numerous other theories were afterwards advanced by Secchi, Zöllner, Young, and others, but a complete description of the various developments in solar theorising cannot be given here. There is no complete “theory” of the exact constitution of every part of the Sun, but the unpretentious “Views of Professor Young on the Constitution of the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent the latest ideas of the foremost solar investigator. Professor Young regards the reversing layer and the chromosphere as “simplythe uncondensed vapours and gases which form the atmosphere in which the clouds of the photosphere are suspended.” He says that the contraction theory of Helmholtz,—explained in another chapter,—advanced to explain the maintenance of the Sun’s heat, is true so far as it goes; but that it is all the truth is now made doubtful by the discovery of radium, which “suggests that other powerful sources of energy may co-operate with the mechanical in maintaining the Sun’s heat.”
The important question of the distance of the Sun was thoroughly investigated in 1824 byJohann Franz Encke(1791-1865), then of Seeberg, near Gotha, who, from a discussion of the transits of Venus in 1761 and 1769, found a parallax of 8″·571, corresponding to a mean distance of 95,000,000 miles. This value was accepted for thirty years, untilPeter Andreas Hansen(1795-1874), in 1854, andUrban Jean Joseph Le Verrier(1811-1877), in 1858, found from mathematical investigations that the distance indicated was too great. Preparations were accordingly made for the observation of the transits of Venus, which took place respectively on December 8, 1874, and December 6, 1882. On the first occasion many expeditions were sent to view the transit, consisting ofFrench, German, American, English, Scottish, Italian, Russian, and Dutch astronomers, and it was hoped that the solar parallax would be accurately measured once for all. However, the transit, although favoured with good weather, was not successful, owing to the difficulty of making exact measurements, by reason of the illumination and refraction in the atmosphere of Venus. Accordingly the values deduced for the parallax were far from unanimous. The transit of 1882 was not observed so extensively, as astronomers had found the transit of Venus to be by no means the best method. In 1877 SirDavid Gill(born 1843), the great Scottish astronomer, determined the solar parallax successfully from measures of the parallax of Mars in opposition. His value was 8″·78, corresponding to 93,080,000 miles. Some years previous to thisJohann Gottfried Galle(born 1812), the German astronomer, had, from measurements of the parallax of the asteroid Flora, deduced a solar parallax of 8″·87. Gill’s work at the Cape in 1888, on the Asteroids, was successful in giving a more accurate value than the transit of Venus: in 1900 and 1901 measures of the parallax of the asteroid Eros, the nearest minor planet, were made by many different observatories, and agree with the other results. Thevalues which have been derived from the velocity of light, and from the constant of aberration, are fairly in agreement with those derived from direct measurement. On the whole, the most probable value of the parallax is about 8″·8, indicating a mean distance of about 92,700,000 miles, with a “probable error” of about 150,000 miles.
What a different picture the sun presents to us at the beginning of the twentieth century from that which it presented to Herschel and his contemporaries at the beginning of the nineteenth! To Herschel, the Sun was a cool dark globe, surrounded by a luminous atmosphere. As the outcome of the researches and discoveries outlined in this chapter, the Sun is now seen to be a vast central world, which is over a million times larger than the Earth. In the words of an able writer, “It is most probably a world of gases, where most of the metals and metallic gases that we know exist only as vapours, even at the Sun’s surface, hotter than any furnace on earth, and getting a still fiercer heat for every mile of descent lower. Of that heat in the Sun’s interior we can form no conception. The pressure within the Sun is equally inconceivable. A cannon-ball weighing 100 lb. on earth would weigh 2700on the Sun. Thus a mighty conflict goes on unceasingly between imprisoned and expanding gases and vapours struggling to burst out, and massive pressures holding them down. For reasons we cannot fully understand, no equilibrium is reached. For millions of years up-rushes and down-rushes of the white-hot materials have been proceeding on that bright photosphere which gives us light, and looks a picture of calm and quiescence. Above that is a comparatively thin rose-coloured layer, the chromosphere, agitated with fiery ‘prominences,’ and outside all these the coronal glory—all alike pointing to immeasurable activities.”
The following remark of Professor Newcomb shows our inability to realise the solar activity. “Suppose,” he says, “every foot of space in a whole country covered with 13-inch cannon, all pointed upward, and all discharged at once. The result would compare with what is going on inside the photosphere about as much as a boy’s popgun compares with the cannon.”
It is somewhat remarkable that the one celestial body which Herschel neglected was our satellite, the Moon; and it is also remarkable that the Moon was for many years the chief object of study of his contemporary astronomer,Johann Hieronymus Schröter(1745-1816). Born at Erfurt, near Hanover, on August 30, 1745, Johann Hieronymus Schröter was originally intended for the study of law, for which he was sent to the University of Göttingen. At the same time he studied mathematics, and particularly astronomy, under the mathematician, Kaestner of Göttingen. Deeply interested in music, he became acquainted with the Herschel family, and, inspired by William Herschel’s example, determined to study the heavens. In 1779 he became the possessor of a small achromatic refractor, and commenced to observe the Sun and Moon. In 1778 he entered the legalprofession at Hanover, and four years later he was appointed “oberamtmann” or Chief Magistrate of Lilienthal—“the Vale of Lilies”—in the Duchy of Bremen. At Lilienthal Schröter erected a small observatory, and acquired in 1785 one of Herschel’s 7-foot reflectors. In 1792 the astronomer superintended the construction of a 13-foot reflector, made by Schrader of Kiel, who transferred his workshop to Lilienthal. With these instruments the great work of Schröter was accomplished.
Schröter directed his powers of observation to the study of the Moon. He originated the study of the surface of the Moon, and founded the branch of astronomy known as selenography, or the study of the Moon’s surface. The foundations of this branch were laid in 1791 with the publication of Schröter’s ‘Seleno-topographische Fragmente’. The astronomer determined to make a comparative study of the surface of our satellite, and before 1801 discovered eleven “rills” or clefts on the Moon’s surface, and recognised a large number of craters. He likewise believed that he had seen a lunar atmosphere, an observation of which was made by him in February 1792. Schröter seems never to have doubted what Herschel and his contemporaries believed—thatthe Moon was a living world with volcanoes in active eruption, surrounded by an atmosphere, and inhabited by beings like ourselves. Unfortunately, Schröter was not good at making drawings of what he saw; nevertheless, he accomplished a vast amount of work. In the little observatory at Lilienthal the foundations were laid of the comparative study of the surface of the Moon.
But these observations were destined to be rudely interrupted. In 1810 Hanover was occupied by the invading troops of Napoleon, and Schröter lost his appointment as Chief Magistrate of Lilienthal, and also his income. But there was worse to follow. On April 20, 1813, three years after, the French, under Vandamme, with that cruelty which seems to belong to warfare, occupied Lilienthal, and set fire to the little village. A few days later the French soldiers entered the observatory and burned it to the ground. All Schröter’s precious observations, accumulated after thirty-four years’ labour, were destroyed with a few exceptions, the observations on Mars narrowly escaping the conflagration. Unable to forget the destruction of his observatory, and without the means to repair the loss, he lived only three years after the disaster. He died on August 29, 1816, “leavingbehind him,” says Mr Arthur Mee, “an imperishable record, and a noble example to observers of all time.”
Wilhelm Gotthelf Lohrmann, a land-surveyor of Dresden, continued the observations of Schröter, and in 1824 published four of the twenty-five proposed sections of a large lunar chart. In 1827, however, his sight began to fail, and he was obliged to abandon his intention. But a successor had already appeared on the scene.Johann Heinrich von Mädler(1794-1874) was born in Berlin in 1794, and, after a severe struggle to earn a living, entered the University of Berlin in 1817. In 1824 he became acquainted withWilhelm Beer(1797-1850), a wealthy banker, who had come to him for instruction in astronomy, and who erected in 1829 an observatory near his villa in Berlin, where pupil and tutor pursued their studies.
In 1830 Mädler, with Beer’s assistance, commenced a great trigonometrical survey of the surface of the Moon. The observations of Beer and Mädler were made with no larger instrument than a 3¾-inch refractor. They ascertained the positions of 919 lunar spots, and measured the height of 1095 mountains. Their great chart of the Moon—which was afterwards followed by a smaller one—was issued in fourparts during 1834-36. “The amount of detail,” wrote Proctor, “is remarkable, and the labour actually bestowed upon the work will appear incredible.” The chart has neither been revised nor superseded, and it remains to this day one of the standard works on the subject.
The chart was succeeded in 1837 by a descriptive volume entitled ‘Der Mond.’ In this work Beer and Mädler did much for the progress of lunar astronomy. Their observations led to a change of opinion regarding our satellite’s physical condition. Herschel, Schröter, Olbers, and other astronomers seem to have considered the Moon a living world. Mädler declared that it was a dead world. He believed it to be destitute of life of any kind, and the changes observed by Schröter and other observers were put down as illusions. ‘Der Mond’ was the end of Mädler’s work in lunar astronomy, for, receiving an appointment at Dorpat, he went there in 1846, and retained his post until within a few years of his death, which took place at Hanover on March 14, 1874.
Mädler’s successor in the field of lunar astronomy wasJohann Friedrich Julius Schmidt(1825-1884), who was born at Eutin in Lübeck in 1825. At a very early age he gave indications of a taste for astronomy. Fortunately hisfather possessed a small hand telescope, with which young Schmidt commenced his lunar studies. Appointed assistant at Bonn and Olmütz and director at Athens successively, he kept up his persistent study of the surface of the Moon for over forty years. In 1839, when fourteen years of age, he began the valuable series of observations which were destined to form the basis of his great chart of the surface of the Moon. Between 1853 and 1858, when employed at Olmütz, Schmidt made and calculated no fewer than 4000 micrometrical measures of the altitudes of lunar mountains. Before 1866 Schmidt had found no fewer than 278 “rills,” and his discoveries were the means of augmenting the number of these curious objects to nearly a thousand.
In a word, it may be said that Schmidt drew out a lunar geography, and the result of his labours, together with those of Schröter and Mädler, is that in a sense we now know the features of the Moon better than those of the Earth. For instance, astronomers see the whole surface of the Moon spread before their eyes, while geographers can never have a similar view of the terrestrial features: we have never seen the poles of the Earth, while the lunar poles are well known to astronomers. Fortwenty years after his appointment at Athens, Schmidt worked at fixing the positions of lunar objects, measuring the heights of mountains and the depths of craters. An idea of his enthusiasm in constructing his great chart may be gained from the fact that he made almost a thousand original sketches.
Mädler’s dogmatic assertion that the Moon was entirely a dead world was generally believed until Schmidt made observations to the contrary. From 1837 to 1866 the popular opinion was that our satellite was an absolutely dead world. Consequently there was little progress in lunar astronomy during those thirty years. Although Mädler’s view was much nearer the truth than the opinions of his predecessors, it was also too positive. His confident assertion, which was received without hesitation, was never questioned until Schmidt came upon the scene. To Schmidt the Moon was not entirely dead, and it was he who brought forward indisputable evidence as to the existence of changes on its surface. In October 1866 he announced that the crater Linné had lost all appearance of such, and that it had become entirely effaced. Lohrmann and Mädler had observed it under a totally different aspect, as also had Schmidt himselffrom 1840 to 1843. There was great excitement in the astronomical world on Schmidt’s announcement, and many astronomers denied the change, although Schmidt’s observation was confirmed by Secchi and Webb. The evidence in favour of it preponderated, and very few observers now consider the Moon’s surface to be absolutely changeless.
In 1865 Schmidt had begun to arrange his observations on the Moon into the form of a chart. At first he decided to have a chart of six feet diameter, divided, like that of Mädler, into four sections. But in April 1868, on making an estimate of the value of such a chart, he was dissatisfied, and determined to construct a map of the same size divided into twenty-five sections instead of four. He began the work in 1868, and after six years the great map was completed. After some delay the German Government undertook to issue the chart at their expense, and it was published in 1879, after fourteen years of preparation. It contained no fewer than 30,000 objects, and its completed diameter was six feet three inches—more than double the size of any previous map of the Moon. Indeed, it was probably the greatest contribution ever made to lunar astronomy. Schmidt lived only a few yearsafter the publication of his great chart. He died at Athens, in his fifty-ninth year, February 8, 1884.
Schmidt’s announcement of the change in the appearance of Linné was followed in 1878 by a statement byHermann Joseph Klein(born 1842) of Cologne, to the effect that a new crater had been formed to the north of the well-known lunar crater, Hyginus. The change in this case, however, is by no means so certain as in that of Linné. It will be observed that the majority of the students of the Moon were Germans. In England the study was not taken up until 1864, when a Lunar Committee of the British Association was appointed. Some good lunar work was done by the well-known astronomer,Thomas William Webb(1807-1885), while the study was popularised byJames Nasmyth(1808-1890), the famous engineer, who published, in 1874, in conjunction withJames Carpenterof Greenwich Observatory, a beautifully-illustrated volume entitled ‘The Moon.’ This was succeeded, in 1876, by the larger work ofEdmund Neison(now Nevill), Government Astronomer of Natal. About this time several English astronomers, devoted to the study of the Moon, formed themselves into the Selenographical Society. Aftera few years, however, the society came to an end, and the enthusiasts formed themselves into the lunar section of the British Astronomical Association, on the foundation of that society in 1890. Chief among those English selenographers wasThomas Gwyn Elger(1837-1897), whose observations of the Moon and drawings of the various craters were of the utmost value. Two years before his death, in 1895, Elger published his important work, ‘The Moon,’ along with an exhaustive chart of the visible face of our satellite.
Herschel and Schröter firmly believed in the existence of a lunar atmosphere, the latter believing that he had actually observed the Moon’s atmospheric envelope. Early in the nineteenth century it was soon observed, however, that on the Moon passing over and occulting stars, these stars disappeared suddenly behind the Moon’s limb, instead of gradually, as they should have done, had an atmosphere of any density existed. Accordingly astronomers gave up believing in a lunar atmosphere. On January 4, 1865, Huggins observed with his spectroscope the occultation of a small star in Pisces. There was not the slightest sign of absorption in a lunar atmosphere; the entire spectrum vanished at once.
Lunar photography was introduced as long ago as 1858 byLewis Morris Rutherfurd(1816-1892), the well-known American astronomer; but for years very little was done in this matter, although Rutherfurd secured fairly good photographs. Rutherfurd, De la Rue, and the older astronomical photographers took photographs of the entire Moon, but this plan was abandoned in favour of what Miss Clerke calls “bit by bit photography.” About 1890 this method was introduced, and has been followed with success byMaurice Loewy(born 1833), and his assistant, Pusiex, at the Paris Observatory; byLadislas Weinekat Prague; by the astronomers of the Lick Observatory; and byWilliam Henry Pickering(born 1858), the distinguished astronomer of Harvard, whose discoveries and investigations have created quite a new interest in lunar astronomy. These investigations were commenced in 1891 at Arequipa, on the slope of the Andes, in Peru. An occultation of Jupiter, witnessed by W. H. Pickering on October 12, 1892, gave support to the view that a very tenuous lunar atmosphere does exist. In 1900 he established, near Mandeville, Jamaica, a temporary astronomical station, where he obtained many excellent photographs. Totally he secured eighty plates. These appeared, as the first completephotographic lunar atlas ever published, in his work ‘The Moon’ (1903), in which he sums up all his observations since 1891, and concludes that “the evidence in favour of the idea that volcanic activity upon the Moon has not yet ceased is pretty strong, if not fairly conclusive.”
Pickering points out that the density of the lunar atmosphere is not greater than one ten-thousandth of that at the Earth’s surface, and, under these circumstances, water cannot exist above freezing-point, which of course brings us to the subject of snow. He considers that snow is observed on the mountain peaks and near the poles of the Moon, and he believes his conclusion to be verified by observations on the well-known crater, Linné. He brings forward evidence of the probable existence on the Moon of organic life, pointing out that the difference between the conditions of the Earth and the Moon is not so great as that above and below the ocean on our own planet. He has collected evidence of the existence of something resembling vegetation on the Moon “coming up, flourishing, and dying, just as vegetation springs and withers on the Earth.”
The first successful attempt to measure the heating power of moonlight was made in 1846 on Mount Vesuvius byMelloni, an Italian physicist,whose results were confirmed four years later byZantedeschi, another Italian. The most important work in this direction was accomplished by the presentEarl of Rosse(born in 1840), who in the years 1869-72 believed himself to have measured the lunar heat; but these conclusions were not altogether confirmed by the observations of DrOtto Boeddicker(Lord Rosse’s astronomer), during the total lunar eclipse of October 4, 1884. Further investigations on this subject were afterwards made bySamuel Pierpont Langley(1834-1906), of Alleghany, and by his assistant,Frank Very.
The motion of the Moon and its perturbations were made the subject of deep study by the famousPierre Simon Laplace(1749-1827), the contemporary of Herschel, and the worthy successor of Newton. He devoted much attention to the secular acceleration of the Moon’s mean motion, a problem which had baffled the greatest mathematicians. After a profound discussion he found, in 1787, that the average distance of the Earth and Moon from the Sun had been slowly increasing for several centuries, the result being an increase in the Moon’s velocity. In the third volume of the ‘Mécanique Céleste’ Laplace worked out the lunar theory in great detail, although he calculated no lunar tables. Afterhis death the subject was taken up byCharles Theodore Damoiseau(1768-1846), and the most important advance was made byGiovanni Antonio Amadeo Plana(1781-1864), the director of the Turin Observatory, who published in 1832 a very complete lunar theory. The work of Plana was followed by that ofPeter Andreas Hansen(1795-1874), whose lunar tables were used for the Nautical Almanac, and whom Professor Simon Newcomb considers to be the greatest master of celestial mechanics since Laplace. The theory of the Moon’s motion was worked out in detail by the famous astronomerCharles Eugene Delaunay(1816-1872), who from 1870 till 1872 occupied the post of director of the Paris Observatory. Delaunay was about to work out the lunar tables when, in 1872, he was accidentally drowned by the capsizing of a pleasure-boat at Cherbourg. The work accomplished in this direction bySimon Newcomb(born 1835) is of great importance, particularly in his correction of Hansen’s tables.John Couch Adams(1819-1892), one of the discoverers of Neptune, while at work on the lunar theory, had occasion to correct Laplace’s supposed solution of the acceleration of the lunar motion. On going over the calculation Adams found that several quantities, omitted by Laplaceas unimportant, showed that the Moon has a minute increase of speed for which the theory of gravitation will not account,—a conclusion opposed by Plana, Hansen, and Pontécoulant, but fully confirmed by Delaunay. Delaunay suggested in 1865 that the minute apparent increase was due to the retardation of the Earth’s rotation by tidal friction. This brings us to the subject of celestial evolution, which is discussed in another chapter.
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 intransit 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, andLewis 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 andCharles 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 in1800 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 byFriedrich 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 Schiaparelliwas 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 determinationof 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 thatWilliam 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 byHermann 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 astronomerFrancesco 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 observationson 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,CamilleFlammarion(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 byFranz von Gruithuisen, and in 1878 by the French astronomerTrouvelot(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 Schröterby 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 ofSeth Carlo Chandler(born 1846) at Cambridge, Mass., with his newly-invented instrument, the “almucantar,” which indicated an appreciable variation of latitude. This was confirmed byFriedrich Küstner(born 1856), now director of the Observatory at Bonn. The idea now occurred to Chandler to search through the older recordsto 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,hemust 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.