Part 9

M. Schwabe, of Dessau, has discovered that the abundance or paucity of spots displayed by the sun’s surface is subject to a law of periodicity. This has been confirmed by M. Wolf, of Berne, who shows that the period of these changes, from minimum to minimum, is 11 years and 11-hundredths of a year, being exactly at the rate of nine periods per century, the last year of each century being a year of minimum. It is strongly corroborative of the correctness both of M. Wolf’s period and also of the periodicity itself, that of all the instances of the appearance of spots on the sun recorded in history, even before the invention of the telescope, or of remarkable deficiencies in the sun’s light, of which there are great numbers, only two are found to deviate as much as two years from M. Wolf’s epochs. Sir William Herschel observed that the presence or absence of spots had an influence on the temperature of the seasons; his observations have been fully confirmed by M. Wolf. And, from an examination of the chronicles of Zurich from A.D. 1000 to A.D. 1800, he has come to the conclusion “that years rich in solar spots are in general drier and more fruitful than those of an opposite character; while the latter are wetter and more stormy than the former.”

The most extraordinary fact, however, in connection with the spots on the sun’s surface, is the singular coincidence of their periods with those great disturbances in the magnetic system of the earth to which the epithet of “magnetic storms” has been affixed.

These disturbances, during which the magnetic needle is greatly and universally agitated (not in a particular limited locality, but at one and the same instant of time over whole continents, or even over the whole earth), are found, so far as observation has hitherto extended, to maintain a parallel, both in respect of their frequency of occurrence and intensity in successive years, with the abundance and magnitude of the spots in the same years, too close to be regarded as fortuitous. The coincidence of the epochs of maxima and minima in the two series of phenomena amounts, indeed, to identity; a fact evidently of most important significance, but which neither astronomical nor magnetic science is yet sufficiently advanced to interpret.--_Herschel’s Outlines._

The signification and connection of the above varying phenomena (Humboldt maintains) can never be manifested in their entire importance until an uninterrupted series of representations of the sun’s spots can be obtained by the aid of mechanical clock-work and photographic apparatus, as the result of prolonged observations during the many months of serene weather enjoyed in a tropical climate.

M. Schwabe has thus distinguished himself as an indefatigable observer of the sun’s spots, for his researches received the Royal Astronomical Society’s Medal in 1857. “For thirty years,” said the President at the presentation, “never has the sun exhibited his disc above the horizon of Dessau without being confronted by Schwabe’s imperturbable telescope; and that appears to have happened on an average about 300 days a-year. So, supposing that he had observed but once a-day, he has made 9000 observations, in the course of which he discovered about 4700 groups. This is, I believe, an instance of devoted persistence unsurpassed in the annals of astronomy. The energy of one man has revealed a phenomenon that had eluded the suspicion of astronomers for 200 years.”

HAS THE MOON AN ATMOSPHERE?

The Moon possesses neither Sea nor Atmosphere of appreciable extent. Still, as a negative, in such case, is relative only to the capabilities of the instruments employed, the search for the indications of a lunar atmosphere has been renewed with fresh augmentation of telescopic power. Of such indications, the most delicate, perhaps, are those afforded by the occultation of a planet by the moon. The occultation of Jupiter, which took place on January 2, 1857, was observed with this reference, and is said to have exhibited no _hesitation_, or change of form or brightness, such as would be produced by the refraction or absorption of an atmosphere. As respects the sea, if water existed on the moon’s surface, the sun’s light reflected from it should be completely polarised at a certain elongation of the moon from the sun; and no traces of such light have been observed.

MM. Baer and Maedler conclude that the moon is not entirely without an atmosphere, but, owing to the smallness of her mass, she is incapacitated from holding an extensive covering of gas; and they add, “it is possible that this weak envelope may sometimes, through local causes, in some measure dim or condense itself.” But if any atmosphere exists on our satellite, it must be, as Laplace says, more attenuated than what is termed a vacuum in an air-pump.

Mr. Hopkins thinks that if there be any lunar atmosphere, it must be very rare in comparison with the terrestrial atmosphere, and inappreciable to the kind of observation by which it has been tested; yet the absence of any refraction of the light of the stars during occultation is a very refined test. Mr. Nasmyth observes that “the sudden disappearance of the stars behind the moon, without any change or diminution of her brilliancy, is one of the most beautiful phenomena that can be witnessed.”

Sir John Herschel observes: The fact of the moon turning always the same face towards the earth is, in all probability, the result of an elongation of its figure in the direction of a line joining the centres of both the bodies, acting conjointly with a non-coincidence of its centre of gravity with its centre of symmetry.

If to this we add the supposition that the substance of the moon is not homogeneous, and that some considerable preponderance of weight is placed excentrically in it, it will be easily apprehended that the portion of its surface nearer to that heavier portion of its solid content, under all the circumstances of the moon’s rotation, will permanently occupy the situation most remote from the earth.

In what regards its assumption of a definite level, air obeys precisely the same hydrostatical laws as water. The lunar atmosphere would rest upon the lunar ocean, and form in its basin a lake of air, whose upper portions at an altitude such as we are now contemplating would be of excessive tenuity, especially should the provision of air be less abundant in proportion than our own. It by no means follows, then, from the absence of visible indications of water or air on this side of the moon, that the other is equally destitute of them, and equally unfitted for maintaining animal or vegetable life. Some slight approach to such a state of things actually obtains on the earth itself. Nearly all the land is collected in one of its hemispheres, and much the larger portion of the sea in the opposite. There is evidently an excess of heavy material vertically beneath the middle of the Pacific; while not very remote from the point of the globe diametrically opposite rises the great table-land of India and the Himalaya chain, on the summits of which the air has not more than a third of the density it has on the sea-level, and from which animated existence is for ever excluded.--_Herschel’s Outlines_, 5th edit.

LIGHT OF THE MOON.

The actual illumination of the lunar surface is not much superior to that of weathered sandstone-rock in full sunshine. Sir John Herschel has frequently compared the moon setting behind the gray perpendicular façade of the Table Mountain at the Cape of Good Hope, illuminated by the sun just risen from the opposite quarter of the horizon, when it has been scarcely distinguishable in brightness from the rock in contact with it. The sun and moon being nearly at equal altitudes, and the atmosphere perfectly free from cloud or vapour, its effect is alike on both luminaries.

HEAT OF MOONLIGHT.

M. Zantedeschi has proved, by a long series of experiments in the Botanic Gardens at Venice, Florence, and Padua, that, contrary to the general opinion, the diffused rays of moonlight have an influence upon the organs of plants, as the Sensitive Plant and the _Desmodium gyrans_. The influence was feeble compared with that of the sun; but the action is left beyond further question.

Melloni has proved that the rays of the Moon give out a slight degree of Heat (see _Things not generally Known_, p. 7); and Professor Piazzi Smyth, from a point of the Peak of Teneriffe 8840 feet above the sea-level, has found distinctly perceptible the heat radiated from the moon, which has been so often sought for in vain in a lower region.

SCENERY OF THE MOON.

By means of the telescope, mountain-peaks are distinguished in the ash-gray light of the larger spots and isolated brightly-shining points of the moon, even when the disc is already more than half illuminated. Lambert and Schroter have shown that the extremely variable intensity of the ash-gray light of the moon depends upon the greater or less degree of reflection of the sunlight which falls upon the earth, according as it is reflected from continuous continental masses, full of sandy deserts, grassy steppes, tropical forests, and barren rocky ground, or from large ocean surfaces. Lambert made the remarkable observation (14th of February 1774) of a change of the ash-coloured moonlight into an olive-green colour bordering upon yellow. “The moon, which then stood vertically over the Atlantic Ocean, received upon its right side the green terrestrial light which is reflected towards her when the sky is clear by the forest districts of South America.”

Plutarch says distinctly, in his remarkable work _On the Face in the Moon_, that we may suppose the _spots_ to be partly deep chasms and valleys, partly mountain-peaks, which cast long shadows, like Mount Athos, whose shadow reaches Lemnos. The spots cover about two-fifths of the whole disc. In a clear atmosphere, and under favourable circumstances in the position of the moon, some of the spots are visible to the naked eye; as the edge of the Apennines, the dark elevated plain Grimaldus, the enclosed _Mare Crisium_, and Tycho, crowded round with numerous mountain ridges and craters.

Professor Alexander remarks, that a map of the eastern hemisphere, taken with the Bay of Bengal in the centre, would bear a striking resemblance to the face of the moon presented to us. The dark portions of the moon he considers to be continental elevations, as shown by measuring the average height of mountains above the dark and the light portions of the moon.

The surface of the moon can be as distinctly seen by a good telescope magnifying 1000 times, as it would be if not more than 250 miles distant.

LIFE IN THE MOON.

A circle of one second in diameter, as seen from the earth, on the surface of the moon contains about a square mile. Telescopes, therefore, must be greatly improved before we could expect to see signs of inhabitants, as manifested by edifices or changes on the surface of the soil. It should, however, be observed, that owing to the small density of the materials of the moon, and the comparatively feeble gravitation of bodies on her surface, muscular force would there go six times as far in overcoming the weight of materials as on the earth. Owing to the want of air, however, it seems impossible that any form of life analogous to those on earth can subsist there. No appearance indicating vegetation, or the slightest variation of surface which can in our opinion fairly be ascribed to change of season, can any where be discerned.--_Sir John Herschel’s Outlines._

THE MOON SEEN THROUGH LORD ROSSE’S TELESCOPE.

In 1846, the Rev. Dr. Scoresby had the gratification of observing the Moon through the stupendous telescope constructed by Lord Rosse at Parsonstown. It appeared like a globe of molten silver, and every object to the extent of 100 yards was quite visible. Edifices, therefore, of the size of York Minster, or even of the ruins of Whitby Abbey, might be easily perceived, if they had existed. But there was no appearance of any thing of that nature; neither was there any indication of the existence of water, or of an atmosphere. There were a great number of extinct volcanoes, several miles in breadth; through one of them there was a line of continuance about 150 miles in length, which ran in a straight direction, like a railway. The general appearance, however, was like one vast ruin of nature; and many of the pieces of rock driven out of the volcanoes appeared to lie at various distances.

MOUNTAINS IN THE MOON.

By the aid of telescopes, we discern irregularities in the surface of the moon which can be no other than mountains and valleys,--for this plain reason, that we see the shadows cast by the former in the exact proportion as to length which they ought to have when we take into account the inclinations of the sun’s rays to that part of the moon’s surface on which they stand. From micrometrical measurements of the lengths of the shadows of the more conspicuous mountains, Messrs. Baer and Maedler have given a list of heights for no less than 1095 lunar mountains, among which occur all degrees of elevation up to 22,823 British feet, or about 1400 feet higher than Chimborazo in the Andes.

If Chimborazo were as high in proportion to the earth’s diameter as a mountain in the moon known by the name of Newton is to the moon’s diameter, its peak would be more than sixteen miles high.

Arago calls to mind, that with a 6000-fold magnifying power, which nevertheless could not be applied to the moon with proportionate results, the mountains upon the moon would appear to us just as Mont Blanc does to the naked eye when seen from the Lake of Geneva.

We sometimes observe more than half the surface of the moon, the eastern and northern edges being more visible at one time, and the western or southern at another. By means of this libration we are enabled to see the annular mountain Malapert (which occasionally conceals the moon’s south pole), the arctic landscape round the crater of Gioja, and the large gray plane near Endymion, which conceals in superficial extent the _mare vaporum_.

Three-sevenths of the moon are entirely concealed from our observation; and must always remain so, unless some new and unexpected disturbing causes come into play.--_Humboldt._

The first object to which Galileo directed his telescope was the mountainous parts of the moon, when he showed how their summits might be measured: he found in the moon some circular districts surrounded on all sides by mountains similar to the form of Bohemia. The measurements of the mountains were made by the method of the tangents of the solar ray. Galileo, as Helvetius did still later, measured the distance of the summit of the mountains from the boundary of the illuminated portion at the moment when the mountain summit was first struck by the solar ray. Humboldt found no observations of the lengths of the shadows of the mountains: the summits were “much higher than the mountains on our earth.” The comparison is remarkable, since, according to Riccioli, very exaggerated ideas of the height of our mountains were then entertained. Galileo like all other observers up to the close of the eighteenth century, believed in the existence of many seas and of a lunar atmosphere.

THE MOON AND THE WEATHER.

The only influence of the Moon on the Weather of which we have any decisive evidence is the tendency to disappearance of clouds under the full moon, which Sir John Herschel refers to its heat being much more readily absorbed in traversing transparent media than direct solar heat, and being extinguished in the upper regions of our atmosphere, never reaches the surface of the atmosphere at all.

THE MOON’S ATTRACTION.

Mr. G. P. Bond of Cambridge, by some investigations to ascertain whether the Attraction of the Moon has any effect upon the motion of a pendulum, and consequently upon the rate of a clock, has found the last to be changed to the amount of 9/1000 of a second daily. At the equator the moon’s attraction changes the weight of a body only 1/7000000 of the whole; yet this force is sufficient to produce the vast phenomena of the tides!

It is no slight evidence of the importance of analysis, that Laplace’s perfect theory of tides has enabled us in our astronomical ephemerides to predict the height of spring-tides at the periods of new and full moon, and thus put the inhabitants of the sea on their guard against the increased danger attending the lunar revolutions.

MEASURING THE EARTH BY THE MOON.

As the form of the Earth exerts a powerful influence on the motion of other cosmical bodies, and especially on that of its neighbouring satellite, a more perfect knowledge of the motion of the latter will enable us reciprocally to draw an inference regarding the figure of the earth. Thus, as Laplace ably remarks: “an astronomer, without leaving his observatory, may, by a comparison of lunar theory with true observations, not only be enabled to determine the form and size of the earth, but also its distance from the sun and moon; results that otherwise could only be arrived at by long and arduous expeditions to the most remote parts of both hemispheres.” The compression which may be inferred from lunar inequalities affords an advantage not yielded by individual measurements of degrees or experiments with the pendulum, since it gives a mean amount which is referable to the whole planet.--_Humboldt’s Cosmos_, vol. i.

The distance of the moon from the earth is about 240,000 miles; and if a railway-carriage were to travel at the rate of 1000 miles a-day, it would be eight months in reaching the moon. But that is nothing compared with the length of time it would occupy a locomotive to reach the sun from the earth: if travelling at the rate of 1000 miles a-day, it would require 260 years to reach it.

CAUSE OF ECLIPSES.

As the Moon is at a very moderate distance from us (astronomically speaking), and is in fact our nearest neighbour, while the sun and stars are in comparison immensely beyond it, it must of necessity happen that at one time or other it must _pass over_, and _occult_ or _eclipse_, every star or planet within its zone, and, as seen from the _surface_ of the earth, even somewhat beyond it. Nor is the sun itself exempt from being thus hidden whenever any part of the moon’s disc, in this her tortuous course, comes to _overlap_ any part of the space occupied in the heavens by that luminary. On these occasions is exhibited the most striking and impressive of all the occasional phenomena of astronomy, an _Eclipse of the Sun_, in which a greater or less portion, or even in some conjunctures the whole of its disc, is obscured, and, as it were, obliterated, by the superposition of that of the moon, which appears upon it as a circularly-terminated black spot, producing a temporary diminution of daylight, or even nocturnal darkness, so that the stars appear as if at midnight.--_Sir John Herschel’s Outlines._

VAST NUMBERS IN THE UNIVERSE.

The number of telescopic stars in the Milky Way uninterrupted by any nebulæ is estimated at 18,000,000. To compare this number with something analogous, Humboldt calls attention to the fact, that there are not in the whole heavens more than about 8000 stars, between the first and the sixth magnitudes, visible to the naked eye. The barren astonishment excited by numbers and dimensions in space when not considered with reference to applications engaging the mental and perceptive powers of man, is awakened in both extremes of the universe--in the celestial bodies as in the minutest animalcules. A cubic inch of the polishing slate of Bilin contains, according to Ehrenberg, 40,000 millions of the siliceous shells of Galionellæ.

FOR WHAT PURPOSE WERE THE STARS CREATED?

Surely not (says Sir John Herschel) to illuminate _our_ nights, which an additional moon of the thousandth part of the size of our own would do much better; nor to sparkle as a pageant void of meaning and reality, and bewilder us among vain conjectures. Useful, it is true, they are to man as points of exact and permanent reference; but he must have studied astronomy to little purpose, who can suppose man to be the only object of his Creator’s care, or who does not see in the vast and wonderful apparatus around us provision for other races of animated beings. The planets derive their light from the sun; but that cannot be the case with the stars. These doubtless, then, are themselves suns; and may perhaps, each in its sphere, be the presiding centre round which other planets, or bodies of which we can form no conception from any analogy offered by our own system, are circulating.[19]

NUMBER OF STARS.

Various estimates have been hazarded on the Number of Stars throughout the whole heavens visible to us by the aid of our colossal telescopes. Struve assumes for Herschel’s 20-feet reflector, that a magnifying power of 180 would give 5,800,000 for the number of stars lying within the zones extending 30° on either side of the equator, and 20,374,000 for the whole heavens. Sir William Herschel conjectured that 18,000,000 of stars in the Milky Way might be seen by his still more powerful 40-feet reflecting telescope.--_Humboldt’s Cosmos_, vol. iii.

The assumption that the extent of the starry firmament is literally infinite has been made by Dr. Olbers the basis of a conclusion that the celestial spaces are in some slight degree deficient in _transparency_; so that all beyond a certain distance is and must remain for ever unseen, the geometrical progression of the extinction of light far outrunning the effect of any conceivable increase in the power of our telescopes. Were it not so, it is argued that every part of the celestial concave ought to shine with the brightness of the solar disc, since no visual ray could be so directed as not, in some point or other of its infinite length, to encounter such a disc.--_Edinburgh Review_, Jan. 1848.

STARS THAT HAVE DISAPPEARED.

Notwithstanding the great accuracy of the catalogued positions of telescopic fixed stars and of modern star-maps, the certainty of conviction that a star in the heavens has actually disappeared since a certain epoch can only be arrived at with great caution. Errors of actual observation, of reduction, and of the press, often disfigure the very best catalogues. The disappearance of a heavenly body from the place in which it had been before distinctly seen, may be the result of its own motion as much as of any such diminution of its photometric process as would render the waves of light too weak to excite our organs of sight. What we no longer see, is not necessarily annihilated. The idea of destruction or combustion, as applied to disappearing stars, belongs to the age of Tycho Brahe. Even Pliny makes it a question. The apparent eternal cosmical alternation of existence and destruction is not annihilation; it is merely the transition of matter into new forms, into combinations which are subject to new processes. Dark cosmical bodies may by a renewed process of light again become luminous.--_Humboldt’s Cosmos_, vol. iii.

THE POLE-STAR FOUR THOUSAND YEARS AGO.

Sir John Herschel, in his _Outlines of Astronomy_, thus shows the changes in the celestial pole in 4000 years: