Telescopic Work for Starlight Evenings

by William F. Denning

On November 17, 1866, Schmidt announced that the lunar crater Linné, about 5½ miles in diameter, and situated in the Mare Serenitatis, had disappeared! He averred that he had been familiar with the object as a deep crater since 1841, but in October 1866 he found its place occupied by a whitish cloud. This cloud was always visible, but the crater itself appeared to have become filled up, and was certainly invisible under its former aspect. Such a definite statement, emanating as it did from a diligent and experienced student of selenography, naturally aroused keen interest, and Linné at once became the object of wide-spread observation. But a reference to Schröter’s results, obtained in the latter part of the last century, threw some doubt upon the alleged change. This observer had figured Linné on November 5, 1788, as a round white spot, and there is nothing in his drawing indicating a crateriform aspect. His description of Linné was:—“A flat, somewhat doubtful crater, which appears as a round white spot.” Mr. Huggins regarded Schröter’s observations as correctly expressing the appearance of this object in 1867 under the same conditions of illumination. On the other hand, Lohrmann (1823) and Mädler (1831) referred to Linné as a deep crater, and in terms inconsistent both with Schröter’s drawing and with the present aspect of the object. The outcome of the many fresh observations that were collected was that Linné appeared as a white cloud, with a small black crater within a large shallow-ringed depression. But as usual in such cases, the observers were far from being unanimous as to the details of the formation; and certainly in regard to a lunar object this need occasion no surprise, for slight differences in the angle of illumination produce marked changes in the aspect of lunar features. The fact of actual change could not be demonstrated, and the negative view appears to have subsequently gained weight.

Another instance of alleged activity on the Moon was notified by Dr. Klein in the spring of 1877. He saw a deep black crater about 18 miles to the W.N.W. of Hyginus, and in a particular place where he had previously recognized no such object, though he had frequently examined the region and was perfectly familiar with it. Forthwith every telescope was directed to this part of the Moon. The maps of earlier observers were eagerly consulted, and lunar photographs scanned for traces of the new object. Many drawings were made of the district near Hyginus and of the remarkable rill or cleft connected with it; but amongst both old and new records some puzzling discordances were detected. Many of the observers, instead of finding Dr. Klein’s new formation a sharply-cut, deep crater, saw it rather in the character of a saucer-like depression; and I drew it under this aspect on several occasions with a 10-inch reflector. The fact, therefore, of its being a new feature admitted of no valid and convincing proofs, and thus the same uncertainty remains attached to this object as to Linné, nothing being absolutely proved[16]. The problem as to whether the Moon is still the seat of physical activity has yet to be solved.

Many circumstances are antagonistic to the discovery of changes on the Moon. As the Sun’s altitude is constantly varying with reference to lunar objects, they assume different aspects from hour to hour. In a short interval the same formations become very dissimilar. When the Sun is rising above the more minute craters they are often distinguished in their true characters; but near the period of full Moon they are visible as bright spots, and it is impossible to tell whether they represent craters or conical hills. With a vertical Sun, as at the full, all the shadows have disappeared—in fact, the entire configuration has been transformed, and many of the interesting lineaments displayed at the crescent phase are no longer seen. The Moon’s libration also introduces slight differences in the appearance of objects. And these are not the only drawbacks; for observations, in themselves, are seldom accordant, and it is found that drawings and descriptions are not always to be reconciled, though referring to identical and invariable features. The lunar landscape must be studied under the same conditions of illumination and libration, with the same instrument and power, and in a similar state of atmosphere, if results are to be strictly comparable. But it is very rarely that observations can be effected under precisely equal conditions; hence discordances are found amongst the records.

The whole of the Moon’s visible sphere exhibits striking imprints of convulsions and volcanic action in past times, though no such forces appear to operate now. The surface seems to have become quiescent, and to have assumed a rigidity inconsistent with the idea of present energy. But we cannot be absolutely certain that minute changes are not taking place, and, being minute, the prospect of their detection is somewhat remote. Students of lunar scenery will probably have to watch details with scrupulous care and for long periods before an instance of real activity can be demonstrated.

Lunar Formations.—The Moon abounds in objects of very diversified character, and they have been classified according to peculiarities of structure. The names of eminent astronomers have been applied to many of the more definite features—a plan of nomenclature which originated with Riccioli, who published a lunar map at the middle of the seventeenth century. The following brief summary comprises many of the principal formations:—

Mare. A name applied by Hevelius to denote the large and relatively level plains on the Moon, which present some similarity to terrestrial seas. They are visible to the naked eye as dusky spots, and in a telescope show many craters, hills, and mounds, and some extensive undulations of surface.

Palus (Marsh) and Lacus (Lake) were titles given by Riccioli to minor areas of a dark colour, and exhibiting greater variety of detail and tint than the Maria.

Sinus (Bay) has been applied to objects like deep bays on the borders of the Maria.

Walled Plains extend from 40 to 150 miles in diameter, and are commonly surrounded by a terraced wall or mountain-ranges. The interiors are tolerably level, though often marked with crater-pits, mounds, and ridges.

Mountain-Rings. These represent rings of mountains and hills, enclosing irregularities, possibly furnished by the debris of the crumbling exterior walls, which, in certain instances, appear to have fallen inwards.