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.
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.