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 2700 on 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.”
CHAPTER IV.
THE MOON.
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 legal profession 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—that the 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, “leaving behind 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 with Wilhelm 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 four parts 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 was Johann 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 his father 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.