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. For twenty 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 himself from 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 years after 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 by Hermann 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 by James Nasmyth (1808-1890), the famous engineer, who published, in 1874, in conjunction with James Carpenter of Greenwich Observatory, a beautifully-illustrated volume entitled ‘The Moon.’ This was succeeded, in 1876, by the larger work of Edmund 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. After a 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 was Thomas 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 by Lewis 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 by Maurice Loewy (born 1833), and his assistant, Pusiex, at the Paris Observatory; by Ladislas Weinek at Prague; by the astronomers of the Lick Observatory; and by William 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 complete photographic 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 by Melloni, an Italian physicist, whose results were confirmed four years later by Zantedeschi, another Italian. The most important work in this direction was accomplished by the present Earl 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 Dr Otto Boeddicker (Lord Rosse’s astronomer), during the total lunar eclipse of October 4, 1884. Further investigations on this subject were afterwards made by Samuel 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 famous Pierre 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. After his death the subject was taken up by Charles Theodore Damoiseau (1768-1846), and the most important advance was made by Giovanni 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 of Peter 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 astronomer Charles 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 by Simon 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 Laplace as 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.