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 59d 1h 28m, when the terminator is resting on nearly identical parts of the surface. In periods of 442d 23h (= 15 lunations) there is another repetition of similar phase; also in periods of 502d 0h 28m (= 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 59d 1h 28m. 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 5h 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 5h 20m 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.”