Attractive aspect of the Moon.—Absence of air and water.—Only one Hemisphere visible.—Earthshine.—Telescopic observations of the lunar surface.—Eclipses.—Lunar changes.—Formations.—Plato and other objects described.—Table of Moon’s age and formations near terminator.—Occultations of stars.—Visibility of the new Moon.

Early in autumn, when the evenings are frequently clear, many persons are led with more force than usual to evince an interest in our satellite, and to desire information which may not be conveniently obtained at the time. The aspect of the Moon at her rising, near the time of the full, during the months of August, September, and October, is more conspicuously noticeable than at any other season of the year, on account of the position she then assumes on successive nights, enabling her to rise at closely identical times for several evenings together. The appearance of her large, ruddy globe at near the same hour, and her increasing brilliancy as her horizontal rays give way under a more vertical position, originated the title of “Harvest Moon,” to commemorate the facility afforded by her light for the ingathering of the corn preceding the time of the autumnal equinox.

It will be universally admitted that the Moon possesses special attractions for us, as being situated nearer than any other celestial body, and forming the inseparable companion or tributary world to the Earth. The many important influences she exercises have led to her becoming the object of close investigation; so that her motions and physical appearances have been ascertained with a remarkable degree of exactness and amplitude. Her movements regulate the tides; her positions are of the utmost moment to the mariner; her light is the welcome beacon of the wayfarer, and its picturesque serenity has ever formed the theme of poets. To the practical astronomer she constitutes an orb perfectly unique as regards extent and variety of detail; and questions relating to the physical condition of her surface, now and in past ages, supply a fund of endless speculation to the theorist.

The mean apparent diameter of the Moon is 31' 5, and it varies from 29' 21 at perigee to 33' 31 at apogee. Her real diameter is 2160 miles, and her mean distance slightly exceeds 237,000 miles. Her revolution round the Earth (= sidereal period) is performed in 27^d 7^h 43^m 11^s·46, but the time from one new moon to another (= synodical period) is 29^d 12^h 44^m 3^s. The Moon’s motion through the firmament is at the rate of 13° 10' 35 per day and 32' 56 per hour. Thus she travels over a space slightly exceeding her own diameter in one hour. The linear value of 1 at the distance of our satellite is 1·16 mile, and of 1' 69½ miles.

When we critically survey the face of the Moon with a good telescope, we see at once that her surface is broken up into a series of craters of various sizes, and that some very irregular formations are scattered here and there, which present a similar appearance to elevated mountain-ranges. The crateriform aspect of the Moon is perhaps the more striking feature, from its greater extent; and we recognize in the individual forms a simile to the circular cavities formed in slag or some other hard substances under the action of intense heat. In certain regions of the Moon, especially those near the south pole, the disk is one mass of abutting craters, and were it not for the obvious want of symmetry in form and uniformity of size, the appearance would be analogous to that of a gigantic honeycomb. These craters are commonly surrounded by high walls or ramparts, and often include conical hills rising from their centres to great heights. While the eye examines these singular structures, and lingers amongst the mass of intricate detail in which the whole surface abounds, we cannot but feel impressed at the marvellous sharpness of definition with which the different features are presented to our view. It matters not to what district we direct our gaze, there is the same perfect serenity and clearness of outline. Not the slightest indication can be discerned anywhere of mist or other obscuring vapours hanging over the lunar landscape.

Absence of Air and Water.—Now it is palpable from this that the Moon has no atmosphere of sufficient density to render itself appreciable; for such an appendage, if it existed in any visible form, would at once obtrude upon the attention, and we should probably recognize some of the characteristics common to the behaviour of our terrestrial clouds. But nothing of the kind is apparent on the Moon: there is an unbroken transparency spread over the whole extent of the Moon’s scenery; whence we conclude that if any air exists on the surface it is of extremely attenuated nature, and possibly confined to the bottom of the craters and low-lying formations, which are arranged in such prolific manner on our satellite.

Nor is there any perceptible intimation of water upon the Moon. It is true that several dark grey patches have been given names, leading one directly to the inference that lakes and seas comprise part of the surface phenomena. Thus there is the Mare Serenitatis (“the sea of serenity”) and many other designations of similar import, which we cannot but insist are wrongly applied and calculated to lead to misapprehension. Before the invention of the telescope furnished us with the means of accurately determining the character of the lunar features, such apellations may have been considered eligible; but now that the non-existence of water in any extensive form is admitted, the titles are rendered obsolete. Still their retention is in some respects advisable, for any sweeping change in a recognized system of nomenclature must cause confusion, and the names alluded to serve a useful end in facilitating reference; so that, under the circumstances, it would perhaps be unwise to attempt reform, or to introduce an innovation which must occasion many difficulties.

Only one Hemisphere visible.—In discussing the nature and appearances of the lunar formations, it must be distinctly understood that our remarks apply to those visible on the side invariably turned towards the Earth. For, in point of fact, there is a considerable expanse of the lunar disk never perceptible from the Earth at all. This is occasioned by the circumstance that the Moon rotates upon her axis in precisely the same time as she revolves around the Earth, and is therefore enabled to present the same side towards us on all occasions. A slight tilting (called libration) takes place, so that we are allowed a glimpse of fragments of the side normally invisible, and its analogous aspect leads us to suppose that there is no great distinction between the features of the Moon’s visible and invisible hemispheres. From exact computations it appears that we are enabled to see a proportion of 59/100 of the surface, and that the remaining 41/100 are permanently beyond our reach.

Earthshine.—A few mornings before new moon, and on a few evenings after it, the whole outline of the dark portion of the lunar globe may be distinctly perceived. A feeble illumination like twilight pervades the opaque part, and this is really earthlight thrown upon our satellite, for near the times of new moon the Earth appears at her brightest (her disk being fully illuminated) as seen from the Moon. The French term for this light is la lumiÈre cendrÉe, or “the ashy light.” The appearance is often popularly referred to in our own country as “the old Moon in the new Moon’s arms.” Some of the old observers remarked that the waning Moon showed this earthlight more strongly than the new Moon.

Telescopic Observations of the Lunar Surface.—Our telescopes give by far the most pleasing view of the Moon when she is in a crescent shape. At such a period the craters and mountains, with their dark shadows, are splendidly displayed. A good view is also obtainable with the Moon at first or last quarter, or when the disk is gibbous. But the full Moon is decidedly less attractive; for the shadows have all disappeared, and the various formations have quite lost their distinctive character. The disk is enveloped in a flood of light, causing glare, and though there is a large amount of detail, including systems of bright rays, many differences of tint, and bright spots, yet the effect is altogether less satisfactory than at the time of a crescent phase.

The nature of the work undertaken by the amateur must largely depend upon his opportunities and the capacity of his appliances. It is evident that in the investigation of lunar details it is essential to be very particular in recording observations; for unless the conditions of illumination are nearly the same, lunar objects will present little resemblance. He should therefore examine the formations at intervals of 59^d 1^h 28^m, when the terminator is resting on nearly identical parts of the surface. In periods of 442^d 23^h (= 15 lunations) there is another repetition of similar phase; also in periods of 502^d 0^h 28^m (= 17 lunations).

The observer, in entering results into his note-book, should state the Moon’s age to the nearest minute, and give aperture and power of telescope and state of sky. Those objects which he has recorded at one lunation should be re-observed after an intervening lunation, or at intervals of 59^d 1^h 28^m. He will then find his notes and drawings are comparable. By the persistent scrutiny of special structures he will discern more and more of their details; in other words, he will find his eye soon acquires power with experience and familiarity with the object. Comparisons of his own work with the charts and records of previous observers will be sure to interest him greatly, and the differences which he will almost certainly detect may exert a useful influence in inciting him to ascertain the source of them. He must not be premature in attributing such discordances to actual changes on the Moon; for he must remember that perfect harmony is rarely to be found in the experiences of different observers. But whenever his own results are inconsistent with those of others, the fact should be carefully noted and the observations repeated and rediscussed with a view to reconcile them. The charts and descriptions of former selenographers are excellent in their way, and the outcome of much zealous labour; but they contain omissions and inaccuracies which it has been impracticable to avoid. The amateur who discovers a mountain, craterlet, or rill not depicted on his lunar maps must therefore neither regard it as a new formation or as a new discovery; for it may have been overlooked by some of the previous observers, and is possibly drawn or described in a work which he does not happen to have consulted. Such differences should, however, always be announced, as they clear the way for others working in the same field.

A small instrument, with an object-glass of about 2½ inches, will reveal a large amount of intricate detail on the surface of our satellite, and will afford the young student many evenings of interesting recreation. But for a more advanced survey of the formations, with the view to discover unknown objects or traces of physical change in known features, a telescope of at least 8 or 10 inches aperture is probably necessary, and powers of 300, 350, and more.

Eclipses of the Moon.—These phenomena comprise a variety of interesting aspects. They are less numerous, in actual occurrence, than solar eclipses in the proportion of about 2 to 3; but they are more frequently visible, because they may be witnessed from any part of an entire hemisphere, whereas eclipses of the Sun are only observable from a tract of the Earth’s surface not exceeding 180 miles in breadth. The Moon may remain totally eclipsed for a period of 2 hours 4 minutes, and the whole duration, including the penumbral obscuration from its first to its last projection, is about 6 hours. Sometimes the Moon suffers total eclipse twice in the same year, and both may be visible, as in 1844, 1877, 1964, &c. It is possible for three such eclipses to occur within a single year, as in 1544. In 1917 there will be three total lunar eclipses, but not all visible in England. In the latter year there will be no less than seven eclipses, as in 1935.

On the last two occasions—Oct. 4, 1884, and Jan. 28, 1888—when the Moon was totally immersed in the Earth’s shadow, the atmosphere was very clear; and it is hoped equally favourable conditions will attend the similar phenomena of Nov. 15, 1891, Sept. 4, 1895, and Dec. 27, 1898. One of the most interesting features during these temporary obscurations of our satellite is the occultation of small stars. Prof. Struve compiled a list of no less than 116 of these objects that would pass behind the Moon’s shadowed limb during the eclipse of Oct. 4, 1884.

Another important effect is the variable colouring on the Moon. This differs considerably in relative intensity as seen during successive eclipses, and the cause is not perhaps fully accounted for. Kepler thought it due to differences in humidity of those parts of the Earth’s atmosphere through which the solar rays pass and are refracted to the eclipsed Moon. The intense red hue which envelopes the lunar surface on such occasions is due to the absorption of the blue rays of light by our atmosphere. The sky at sunset is often observed to be similarly coloured, and from the operation of similar causes. Sometimes the Moon entirely disappears when eclipsed, but on other occasions remains distinctly obvious, like a bright red ball suspended in the firmament. On May 5, 1110, Dec. 9, 1620, May 18, 1761, and June 10, 1816, our satellite is said to have become absolutely imperceptible during eclipse. Wargentin, who described the appearance in 1761, remarks:—“The Moon’s body disappeared so completely that not the slightest trace of any portion of the lunar disk could be discerned, either with the naked eye or with the telescope.” On Oct. 4, 1884, I noticed that the opacity was much greater than usual; at the middle period of the eclipse the Moon’s diameter was apparently so much reduced that she looked like a dull, faint, nebulous mass, without sharply determinate outlines. The effect was similar to that of a star or planet struggling through dense haze. Yet, on March 19, 1848, the Moon “presented a luminosity quite unusual. The light and dark places on the face of our satellite could be almost as well made out as on an ordinary dull moonlight night.” On July 12, 1870, Feb. 27 and Aug. 23, 1877, and Jan. 28, 1888, the Moon, as observed at Bristol, was also fairly bright when totally immersed in the Earth’s shadow. In explanation of these singular differences, Dr. Burder has suggested that Kepler’s views seem inadequate, and that the solar corona is probably implicated in producing light and dark eclipses. He concludes that, as the corona sometimes extends to considerable distances from the Sun, and is very variable in brightness, it may have sufficient influence to occasion the effects alluded to.

Lunar Changes.—The question as to whether physical changes are occurring in the surface-formations of our satellite is one which offers attractive inducements to telescopic observers. Though the Moon appears to have passed the active state, it is very possible that trivial alterations continue to affect some of her features. In April 1787 Sir W. Herschel wrote:—“I perceive three volcanoes in different places of the dark part of the new Moon. Two of them are already nearly extinct, or otherwise in a state of going to break out; the third shows an eruption of fire or luminous matter.” SchrÖter, however, was correctly of opinion that these appearances were due to reflected light from the Earth falling upon elevated spots of the Moon having unusual capacity to return it. SchrÖter himself thought he detected sudden changes in 1791. He says that, on the 30th of December, at 5^h P.M., with a 7-foot reflector magnifying 161 times, he perceived the commencement of a small crater on the S.W. declivity of the volcanic mountain in the Mare Crisium, having a shadow of at least 2' 5. On the 11th of January, 1792, at 5^h 20^m P.M., on looking at the place again he could see neither the new crater nor its shadow. In this case the disappearance was doubtless an apparent one, merely due to the reversed illumination under which the object was examined in the interval of 12 days.

Many other observers besides Herschel have been struck with the brightness of certain spots situated in the opaque region of the lunar disk; but there is no doubt the cause has been uniformly one and the same, viz. the highly reflective properties of some of the mountains (notably of one named Aristarchus), which are distinctly visible as luminous spots amid the relatively dark regions surrounding them. They afford no certain evidence of existing volcanic energy, and in the light of modern researches such an idea cannot be entertained.

On June 10, 1866, Temple noticed a remarkable light appearance, agreeing with the position of Aristarchus, upon the dark side of the Moon, faintly illuminated by earthshine. The object did not exhibit a faint white light analogous to that of other craters in the dark side, but it was star-like, diffused, in colour reddish yellow, and evidently dissimilar to other bright spots. He wrote, in reference to this matter:—“Of course I am far from surmising a still active chemical outbreak, as such an outbreak supposes water and an atmosphere, both of which are universally allowed not to exist on the Moon, so that the crater-forming process can only be thought of as a dry, chemical, although warm one.”

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 proved16. 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.

Ring-Plains are more circular and regular in type than the walled plains, and consist of a moderately flat surface surrounded by a single wall. Crater-Plains are somewhat similar, and seldom exceed 20 miles in diameter. They “rise steeply from the mass of debris around the foot of their walls to a considerable height, and then fall precipitously to the interior in a rough curved slope, whilst on their walls, especially on the exterior, craterlets and crater-cones often exist in considerable numbers.”

Craters, Craterlets, and Crater-Pits. Usually circular in form, and severally offering distinctions as to dimensions and shape. The craters are surrounded by walls, rising abruptly to tolerable heights, and pretty regular in their contour. When the Sun is rising the shadow of the walls falls upon the interior of the craters, and many of these dark conspicuous objects are to be seen near the Moon’s terminator. With a high Sun some of the craters are extremely bright. In proof of the large number of these objects, it may be noted here that in MÄdler’s lunar map (1837) 7735 craters are figured, while in Schmidt’s (1878) there are no less than 32,856!

Crater-Cones. Conical hills or mountains, visible as small luminous spots about the period of full Moon. They are from 1/2 to 3 miles in diameter, and show deep central depressions. It is somewhat difficult to distinguish them from the ordinary mountain-peaks and white spots, and they are not unlike the cones of terrestrial volcanoes.

Rills or Clefts. These are very curious objects. They were first discovered by SchrÖter in 1787, and some of them are to be traced over a considerable extent of the lunar surface, their entire length being 200 or 300 miles. They have the appearance of cuttings or canals, and are sometimes straight, sometimes bent, and not unfrequently develop branches which intersect each other. They apparently run without interruption through many varieties of lunar objects. The bottoms of these rills are nearly flat, and look not unlike dried riverbeds. Some observers have regarded them as fractures or cracks in the Moon’s surface; but their appearance and circumstances of arrangement are opposed to such a view. Our present knowledge includes more than 1000 of these rills.

Mountain-Ranges are chains of lofty peaks and highlands, sometimes divided by rills and numerous ravines and cross valleys. Some of these ranges are of vast magnitude, and the summits of the mountains reach altitudes between 15,000 and 20,000 feet, and sometimes even more.

Mountain-Ridges are to be found scattered in the greatest abundance in the most disturbed localities of the lunar surface. They sometimes connect several formations, or surmount ravines or depressions of large extent. Peaks attaining altitudes of more than 5000 feet rise from them, and they range in several cases over 100 miles.

Ray-Centres. Systems of radiating light-streaks, having a mountain-ring as the centre of divergence, and stretching to distances of some hundreds of miles round. Tycho, Copernicus, Kepler, Anaxagoras, Aristarchus, and Olbers are pronounced examples of this class.

In Beer and MÄdler’s chart of the Moon the names are attached to the various formations, as they are also in Neison’s maps and in some other works. One of these will be absolutely necessary to the student in prosecuting his studies. He will then have a ready means of acquainting himself with the various formations, and making comparisons between his new results and the drawings of earlier selenographers. I would refer the reader to Neison’s and Webb’s books for many references in detail to lunar features, and must be content here with a brief description of a few leading objects:—

Plato is an extensive walled plain, 60 miles in diameter, and situated on the N.E. boundary of the Mare Imbrium. Nasmyth and Carpenter describe the wall as “serrated with noble peaks, which cast their black shadows across the plateau in a most picturesque manner, like the towers and spires of a great cathedral.” It has received a large amount of attention, with a view to trace whether changes are occurring in the numerous white spots and streaks lying in its interior. In 1869-71 Mr. Birt collected many observations, and on discussing them was led to believe that “there is strong probability that activity, of a character sufficient to render its effects visible, had been manifested.” The inquiry was renewed by Stanley Williams in 1882-84, and he concluded that the results were strongly confirmatory of actual change having occurred since 1869-71. The relative visibility of several of the bright spots had altered in the interim, and the curious intermingling bright streaks also exhibited traces of variation. At sunrise the interior of Plato is pure grey; but with the sun at a considerable height above it, the plain becomes a dark steel-grey. The change is an abnormal one, and difficult to explain. South of Plato there is a fine example of an isolated peak, named Pico, which is about 8000 feet high.

Great Alpine Valley. This object, supposed to have been discovered by Bianchini in 1727, and having a length, according to MÄdler, of 83 miles and a breadth varying from 3½ to 6 miles, is a very conspicuous depression situated near Plato, and running from the Mare Frigoris to the Mare Imbrium. It exhibits at its southern extremity an oval formation, and a narrow gorge issues from it to the northward, opening out further on, and imparting to the whole appearance a shape which Webb likened to a Florence oil-flask. Elger has fully described this singular structure. “It is only when far removed from the terminator that its V-shaped outlet to the Mare Imbrium flanked on either side by the lofty Alps can be traced to advantage, or the flask-like expansion with the constricted gorge leading up to it from the N.W. satisfactorily observed. At other times these features are always more or less concealed by the shadows of neighbouring heights. The details of the upper or more attenuated end of the valley are, however, best seen under a setting sun, when many striking objects come to light, of which few traces appear at other times.”

Archimedes. One of the most definite and regular of the walled plains. It is 60 miles in diameter, with a wall rising about 4200 feet above the surface. Some small craters and various streaks diversify its centre.

Tycho. A grand ring-plain, 54 miles in diameter and about 17,000 feet (= nearly 3 miles) deep, and forming the centre of the chief ray-system of the Moon. The light-radiations stretch over one fourth of the visible hemisphere at the full, but they are imperceptible with the Sun’s altitude below 20°. These remarkable radiations from Tycho form a striking aspect of lunar scenery, and any small telescope reveals them. Webb has termed Tycho “the metropolitan formation of the Moon;” and the idea embodied in the expression must strike observers as very apposite. This object is visible to the naked eye at the time of full. A fine hill rises from its centre to a height of 5500 feet.

Copernicus. A magnificent ring-plain, 56 miles in diameter, and surrounded by a wall (in which there are terraces and lofty peaks, separated by ravines) attaining an elevation of 11,000 feet. There is a central hill of nearly 2500 feet. From Copernicus light-streaks are plentifully extended on all sides, and apparently connect this object with the many others of similar character which are situated in this region. Neison says that near Copernicus the light-streaks unite and form a kind of nimbus or light-cloud about it. The streaks are most conspicuous towards the N., where they are from 5 to 14 miles in width. To the N.W. of Copernicus, about halfway in the direction of the neighbouring ring-plain Eratosthenes (and N. of Stadius), there is a considerable number of crater-pits. MÄdler figured sixty-one of these, and regarded that number as certainly less than half the total number visible. They appear to be ranged in rows or streams, and are so close together in places as to nearly form crater-rills. Schmidt saw the ground hereabout pierced like a honeycomb, and managed to count about 300 little craters; but they are so thickly strewn in this district that exact numbers or places cannot be assigned. They are best observable when the Sun is rising on the E. wall of Copernicus. The interior of this fine object shows six or seven peaks, which are often capped with sunshine, and very brilliant amid the black shadow thrown from the surrounding wall.

Theophilus. Another ring-plain, and one of the deepest visible. Its terraced lofty wall, 64 miles in diameter, rises in a series of peaks to heights varying between 14,000 and 18,000 feet. There is a central mountain, broken by ravines; but from one of the masses a peak ascends to a height of about 6000 feet.

Petavius. A large walled plain, surrounded by a double wall or rampart, which rises to 11,000 feet on its E. side. There are hills and ridges in the interior, and a central peak, A, reaching to 5500 feet above the E. part of the floor, which is convex in form. A smaller peak, of nearly 4000 feet, lies W. of A. Several small craterlets have been seen in the interior.

Newton. The deepest walled plain known upon the Moon’s surface. In form it is elliptical; its length is 143 miles, while its breadth is only 69 miles. The walls show the terracing so common in these objects, and one lofty peak reaches the unusual height of 24,000 feet above the floor. The interior includes some small craters, mountain protuberances, and other irregularities. Neison says that, owing to “the immense height of the wall, a great part of the floor is entirely lost in shadow, neither Earth nor Sun being ever visible from it.”

Grimaldi. An immense walled plain, extending over 148 miles from N. to S. and about 130 miles from E. to W. Its interior is very dark. Clavius is another grand example of this class of object, and is rather larger than Grimaldi, but unfavourably placed near the S. pole. Schickard may also be mentioned as a large formation of similar type, and situated near the S.E. limb of the Moon.

Rill or Cleft of Hyginus. A conspicuous example of the lunar rills, and one which yields to very moderate instruments. Neison notes that it is readily visible in a 2-inch telescope; while Webb remarks that a power of only 40, in a good instrument, is enough to show it under any illumination. The rill is about 150 miles long. It cuts through a number of crater-pits, and MÄdler found so many widenings in it that it appeared like a confluent train of craters. The rill traverses the large crater-pit Hyginus, which is 3-3/4 miles in diameter and moderately deep. Other fine examples of rill-systems will be found between Rheita and Metius and near Triesnecker and Ramsden.

Straight Wall. A singular structure on the E. side of the ring-plain Thebit. It is a ridge or wall, which looks regular enough for a work of art, according to Webb. Its average height is 450 feet (SchrÖter), 1004 feet (MÄdler), or 880 feet (Schmidt). These several determinations are given to show the discordances sometimes found in the measures of good observers. This object is about 60 miles long; at one extremity lies a small crater, at the other there is a branching mountain nearly 2000 feet high. Elger has drawn this object, under both a rising and a setting sun, in the Liverpool Astronomical Society’s ‘Journal,’ vol. v. p. 156, and remarks that it may be well observed at from 20 to 30 hours after the Moon’s first quarter.

Valley near Rheita. South of the ring-plain Rheita, on the S.W. limb, there is an enormous valley, which extends in its entire length over 187 miles, with a width ranging from 10 to 25 miles. There are several fine valleys in this particular region.

Leibnitz Mountains. These are really situated on the further hemisphere of the Moon, but libration brings them into view, and they are sometimes grandly seen in profile on the S. margin. Four of the peaks ascend to elevations of 26,000 or 27,000 feet, and one mass, towering far above the others, is fully 30,000 feet in height, and is unquestionably the most lofty mountain on the Moon.

DÖrfel Mountains. Visible on the Moon’s S.S.E. limb. They exhibit three peaks, which, on the authority of SchrÖter, rise to more than 26,000 feet above the average level of the limb. The loftiest mountains on the Earth are in the Himalayas—a range of immense extent to the N. of India. The three highest peaks are Mount Everest (29,002 feet), Kunchinjinga (28,156 feet), and Dhawalagiri (28,000 feet). The only lunar mountain more elevated than these is that of the Leibnitz range, which, as we have already stated, ascends to fully 30,000 feet.

Apennines. A vast chain of mountains, extending over more than 450 miles of the lunar surface. Huygens is the most elevated peak, rising to more than 18,000 feet, and on its summit it shows a small crater. There are several other very lofty peaks in this range. The Sun rises upon the westerly region of these mountains at the time of first quarter, and the peaks and ridges, with their contrasting shadows, create a gorgeous effect just within, and projecting into the darkness beyond, the terminator. There is an immense amount of detail to be studied here, and much of it is within the reach of small instruments.

As the lunar mountains and craters are best seen near the terminator, it may be useful to give a table of objects thus favourably placed between the times of new and full Moon. The summary may assist the student, though it does not aim at exactness, only even days being given.

Objects near the Terminator.

Occultations of Stars.—Among the various phenomena to which the lunar motions give rise none are more pleasing to the possessors of small telescopes than occultations of stars. Several of these occurrences are visible every month. If the amateur has the means of obtaining accurate time, he will engage himself usefully in noting the moments of disappearance and reappearance of the stars occulted. This work is efficiently done, it is true, at some of our observatories, and therefore little real necessity exists for amateurs to embark in routine work which can be conveniently undertaken at establishments where they have better appliances and trained observers to use them. The mere watching of an occultation, apart from the registry of exact results, is interesting; and there are features connected with it which have proved exceedingly difficult to account for. The stars do not always disappear instantaneously. On coming up to the edge of the Moon they have not been suddenly blotted out, but have appeared to hang on the Moon’s limb for several seconds. This must arise from an optical illusion, from the action of a lunar atmosphere, or the stars must be observed through fissures on the Moon’s edge. The former explanation is probably correct; for it has happened that two observers at the same place have received different impressions of the phenomenon. One has seen the star apparently projected on the Moon’s limb for about 5 seconds, while the other has witnessed its sudden extinction, in the usual manner, as it met the Moon’s edge. New observations, made with good instruments and reliable eyes, and fully described, will doubtless throw more light on the peculiar effects sometimes recorded.

Visibility of the new and old Moon.—It is an interesting feature of observation to note how soon after conjunction the Moon’s thin crescent is observable with the naked eye. A case has been mentioned in which the old Moon was seen one morning before sunrise and the new Moon just after sunset on the next day. At Bristol, on the evening of March 30, 1881, I saw the new Moon at 7^h 10^m, the horizon being very clear in the west. She was then only 20^h 38^m old. On June 4, 1875, I observed the Moon’s crescent at 9^h 10^m, or 22^h 49^m after new Moon. Dr. Degroupet, of Belgium, saw the old Moon on the morning of Nov. 22, 1889, between 6^h 47^m and 7^h 22^m G.M.T., or within 18^h 22^m of the time of new Moon.