The following is a list of lunar objects published in the Selenographical Journal, classed in accordance with this scale:—
0 deg. Black shadows. 1 deg. Darkest portions of the floors of Grimaldi and Riccioli. 1 1/2 deg. Interiors of Boscovich, Billy, and Zupus. 2 deg. Floors of Endymion, Le Monnier, Julius Caesar, Cruger, and Fourier a. 2 1/2 deg. Interiors of Azout, Vitruvius, Pitatus, Hippalus, and Marius. 3 deg. Interiors of Taruntius, Plinius, Theophilus, Parrot, Flamsteed, and Mercator. 3 1/2 deg. Interiors of Hansen, Archimedes, and Mersenius. 4 deg. Interiors of Manilius, Ptolemaeus, and Guerike. 4 1/2 deg. Surface round Aristillus, Sinus Medii. 5 deg. Walls of Arago, Landsberg, and Bullialdus. Surface round Kepler and Archimedes. 5 1/2 deg. Walls of Picard and Timocharis. Rays from Copernicus. 6 deg. Walls of Macrobius, Kant, Bessel, Mosting, and Flamsteed. 6 1/2 deg. Walls of Langrenus, Theaetetus, and Lahire. 7 deg. Theon, Ariadaeus, Bode B, Wichmann, and Kepler. 7 1/2 deg. Ukert, Hortensius, Euclides. 8 deg. Walls of Godin, Bode, and Copernicus. 8 1/2 deg. Walls of Proclus, Bode A, and Hipparchus c. 9 deg. Censorinus, Dionysius, Mosting A, and Mersenius B and c. 9 1/2 deg. Interior of Aristarchus, La Peyrouse DELTA. 10 deg. Central peak of Aristarchus.
TEMPERATURE OF THE MOON'S SURFACE.—Till the subject was undertaken some years ago by Lord Rosse, no approach was made to a satisfactory determination of the surface temperature of the moon. From his experiments he inferred that the maximum temperature attained, at or near the equator, about three days after full moon, does not exceed 200 deg. C., while the minimum is not much under zero C. Subsequent experiments, however, both by himself and Professor Langley, render these results more than doubtful, without it is admitted that the moon has an atmospheric covering. Langley's results make it probable that the temperature never rises above the freezing-point of water, and that at the end of the prolonged lunar night of fourteen days it must sink to at least 200 deg. below zero. Mr. F.W. Verey of the Alleghany Observatory has recently conducted, by means of the bolometer, similar researches as to the distribution of the moon's heat and its variation with the phase, by which he has deduced the varying radiation from the surface in different localities of the moon under various solar altitudes.
LUNAR OBSERVATION.—In observing the moon, we enjoy an advantage of which we cannot boast when most other planetary bodies are scrutinised; for we see the actual surface of another world undimmed by palpable clouds or exhalations, except such as exist in the air above us; and can gaze on the marvellous variety of objects it presents much as we contemplate a relief map of our own globe. But inasmuch as the manifold details of the relief map require to be placed in a certain light to be seen to the best advantage, so the ring-mountains, rugged highlands, and wide-extending plains of our satellite, as they pass in review under the sun, must be observed when suitable conditions of illumination prevail, if we wish to appreciate their true character and significance.
As a general rule, lunar objects are best seen when they are at no great distance from "the terminator," or the line dividing the illumined from the unillumined portion of the spherical surface. This line is constantly changing its position with the sun, advancing slowly onwards towards the east at a rate which, roughly speaking, amounts to about 30.5 min. in an hour, or passing over 10 deg. of lunar longitude in about 19 hrs. 40 mins. When an object is situated on this line, the sun is either rising or setting on the neighbouring region, and every inequality of the surface is rendered prominent by its shadow; so that trifling variations in level and minor asperities assume for the time being an importance to which they have no claim. If we are observing an object at lunar sunrise, a very short time, often only a few minutes, elapses before the confusion caused by the presence of the shadows of these generally unimportant features ceases to interfere with the observation, and we can distinguish between those details which are really noteworthy and others which are trivial and evanescent. Every formation we are studying should be observed, and drawn if possible, under many different conditions of illumination. It ought, in fact, to be examined from the time when its loftiest heights are first illumined by the rising sun till they disappear at sunset. This is, of course, practically impossible in the course of one lunation, but by utilising available opportunities, a number of observations may be obtained under various phases which will be more or less exhaustive. It cannot be said that much is known about any object until an attempt has been made to carry out this plan. Features which assume a certain appearance at one phase frequently turn out to be altogether different when viewed under another; important details obscured by shadows, craters masked by those of neighbouring objects, or by the shadows of their own rims, are often only revealed when the sun has attained an altitude of ten degrees or more. In short, there is scarcely a formation on the moon which does not exemplify the necessity of noting its aspect from sunrise to sunset. Regard must also be had to libration, which affects to a greater or less degree every object; carrying out of the range of observation regions near the limb at one time, and at another bringing into view others beyond the limits of the maps, which represent the moon in the mean state of libration. The area, in fact, thus brought into view, or taken out of it, is between 1/12th and 1/13th of the entire area of the moon, or about the 1/6th part of the hemisphere turned away from the earth. It is convenient to bear in mind that we see an object under nearly the same conditions every 59 d. 1 h. 28 m., or still more accurately, after the lapse of fifteen lunations, or 442 d. 23 h. Many observers avoid the observation of objects under a high light. This, however, should never be neglected when practicable, though in some cases it is not easy to carry out, owing to the difficulty in tracing details under these circumstances.
Although to observe successfully the minuter features, such as the rills and the smaller craterlets, requires instruments of large aperture located in favourable situations, yet work of permanent value may be accomplished with comparatively humble telescopic means. A 4 inch achromatic, or a silver-on-glass reflector of 6 or 6 1/2 inches aperture, will reveal on a good night many details which have not yet been recorded, and the possessor of instruments of this size will not be long in discovering that the moon, despite of what is often said, has not been so exhaustively surveyed that nothing remains for him to do.
Only experience and actual trial will teach the observer to choose the particular eyepiece suitable for a given night or a given object. It will be found that it is only on very rare occasions that he can accomplish much with powers which, perhaps only on two or three nights in a year in this climate, tell to great advantage; though it sometimes happens that the employment of an eyepiece, otherwise unsuitable for the night, will, during a short spell of good definition, afford a fleeting glimpse of some difficult feature, and thus solve a doubtful point. It has often been said that the efficiency of a telescope depends to a great extent on "the man at the eye end." This is as true in the case of the moon as it is in other branches of observational astronomy.
Observers, especially beginners, frequently fall into great error in failing to appreciate the true character of what they see. In this way a shallow surface depression, possibly only a few feet below the general level of the neighbouring country, is often described as a "vast gorge," because, under very oblique light, it is filled with black shadow; or an insignificant hillock is magnified into a mountain when similarly viewed. Hence the importance, just insisted on, of studying lunar features under as many conditions as possible before finally attempting to describe them.
However indifferent a draughtsman an observer may be, if he endeavours to portray what he sees to the best of his ability, he will ultimately attain sufficient skill to make his work useful for future reference: in any case, it will be of more value than a mere verbal description without a sketch. Doubt and uncertainty invariably attend to a greater or less extent written notes unaccompanied by drawings, as some recent controversies, respecting changes in Linne and elsewhere, testify. Now that photographs are generally available to form the basis of a more complete sketch, much of the difficulty formerly attending the correct representation of the outline and grosser features of a formation has been removed, and the observer can devote his time and attention to the insertion and description of less obvious objects.
PROGRESS OF SELENOGRAPHY.—Till within recent years, the systematic study of the lunar surface may be said to have been confined, in this country at any rate, to a very limited number of observers, and, except in rare instances, those who possessed astronomical telescopes only directed them to the moon as a show object to excite the wonder of casual visitors. The publication of Webb's "Celestial Objects" in 1859, the supposed physical change in the crater Linne, announced in 1866, and the appearance of an unrecorded black spot near Hyginus some ten years later, had the effect of awakening a more lively interest in selenography, and undoubtedly combined to bring about a change in this respect, which ultimately resulted in the number of amateurs devoting much of their time to this branch of observational astronomy being notably increased. Still, large telescopes, as a rule, held aloof for some unexplained reason, or were only employed in a desultory and spasmodic fashion, without any very definite object. When the Council of the British Association for the Advancement of Science, stimulated by the Linne controversy, deemed the moon to be worthy of passing attention, observations, directed to objects suspected of change (the phenomena on the floor of Plato) were left to three or four observers, under the able direction of Mr. Birt, the largest instruments available being an 8 1/4 inch reflector and the Crossley refractor of 9 inches aperture! During the last decade, however, all this has been changed, and we not only have societies, such as the British Astronomical Association, setting apart a distinct section for the systematic investigation of lunar detail, but some of the largest and most perfect instruments in the world, among them the noble refractor on Mount Hamilton, employed in photographing the moon or in scrutinising her manifold features by direct observation. Hence, it may be said that selenography has taken a new and more promising departure, which, among other results, must lead to a more accurate knowledge of lunar topography, and settle possibly, ere long, the vexed question of change, without any residuum of doubt.