Astronomy.

The difficulty of understanding these marvellous truths has been glanced at by an old divine (seeThings not generally Known, p. 1); but the rarity of their full comprehension by those unskilled in mathematical science is more powerfully urged by Lord Brougham in these cogent terms:

Satisfying himself of the laws which regulate the mutual actions of the planetary bodies, the mathematician can convince himself of a truth yet more sublime than Newton’s discovery of gravitation, though flowing from it; and must yield his assent to the marvellous position, that all the irregularities occasioned in the system of the universe by the mutual attraction of its members are periodical, and subject to an eternal law, which prevents them from ever exceeding a stated amount, and secures through all time the balanced structure of a universe composed of bodies whose mighty bulk and prodigious swiftness of motion mock the utmost efforts of the human imagination. All these truths are to the skilful mathematician as thoroughly known, and their evidence is as clear, as the simplest proposition of arithmetic to common understandings. But how few are those who thus know and comprehend them! Of all the millions that thoroughly believe these truths, certainly not a thousand individuals are capable of following even any considerable portion of the demonstrations upon which they rest; and probably not a hundred now living have ever gone through the whole steps of these demonstrations.—Dissertations on Subjects of Science connected with Natural Theology, vol. ii.

Sir David Brewster thus impressively illustrates the same subject:

Minds fitted and prepared for this species of inquiry are capable of appreciating the great variety of evidence by which the truths of the planetary system are established; but thousands of individuals, and many who are highly distinguished in other branches of knowledge, are incapable of understanding such researches, and view with a sceptical eye the great and irrefragable truths of astronomy.That the sun is stationary in the centre of our system; that the earth moves round the sun, and round its own axis; that the diameter of the earth is 8000 miles, and that of the sunone hundred and ten times as great; that the earth’s orbit is 190,000,000 of miles in breadth; and that if this immense space were filled with light, it would appear only like a luminous point at the nearest fixed star,—are positions absolutely unintelligible and incredible to all who have not carefully studied the subject. To millions of our species, then, the great Book of Nature is absolutely sealed; though it is in the power of all to unfold its pages, and to peruse those glowing passages which proclaim the power and wisdom of its Author.

That the sun is stationary in the centre of our system; that the earth moves round the sun, and round its own axis; that the diameter of the earth is 8000 miles, and that of the sunone hundred and ten times as great; that the earth’s orbit is 190,000,000 of miles in breadth; and that if this immense space were filled with light, it would appear only like a luminous point at the nearest fixed star,—are positions absolutely unintelligible and incredible to all who have not carefully studied the subject. To millions of our species, then, the great Book of Nature is absolutely sealed; though it is in the power of all to unfold its pages, and to peruse those glowing passages which proclaim the power and wisdom of its Author.

Astronomy is a useful aid in discovering the Dates of ancient Monuments. Thus, on the ceiling of a portico among the ruins of Tentyris are the twelve signs of the Zodiac, placed according to the apparent motion of the sun. According to this Zodiac, the summer solstice is in Leo; from which it is easy to compute, by the precession of the equinoxes of 50″·1 annually, that the Zodiac of Tentyris must have been made 4000 years ago.

Mrs. Somerville relates that she once witnessed the ascertainment of the date of a Papyrus by means of astronomy. The manuscript was found in Egypt, in a mummy-case; and its antiquity was determined by the configuration of the heavens at the time of its construction. It proved to be a horoscope of the time of Ptolemy.

This poetic designation dates back as far as the early period of Anaximenes; but the first clearly defined signification of the idea on which the term is based occurs in Empedocles. This philosopher regarded the heaven of the fixed stars as a solid mass, formed from the ether which had been rendered crystalline by the action of fire.

In the Middle Ages, the fathers of the Church believed the firmament to consist of from seven to ten glassy strata, incasing each other like the different coatings of an onion. This supposition still keeps its ground in some of the monasteries of southern Europe, where Humboldt was greatly surprised to hear a venerable prelate express an opinion in reference to the fall of aerolites at Aigle, that the bodies we called meteoric stones with vitrified crusts were not portions of the fallen stone itself, but simply fragments of the crystal vault shattered by it in its fall.

Empedocles maintained that the fixed stars were riveted to the crystal heavens; but that the planets were free and unconstrained. It is difficult to conceive how, according to Plato in theTimæus, the fixed stars, riveted as they are to solid spheres, could rotate independently.

Among the ancient views, it may be mentioned that the equal distance at which the stars remained, while the whole vault of heaven seemed to move from east to west, had led to the idea of a firmament and a solid crystal sphere, in which Anaximenes (who was probably not much later than Pythagoras) had conjectured that the stars were riveted like nails.

The Pythagoreans, in applying their theory of numbers tothe geometrical consideration of the five regular bodies, to the musical intervals of tone which determine a word and form different kinds of sounds, extended it even to the system of the universe itself; supposing that the moving, and, as it were, vibrating planets, exciting sound-waves, must produce aspheral music, according to the harmonic relations of their intervals of space. “This music,” they add, “would be perceived by the human ear, if it was not rendered insensible by extreme familiarity, as it is perpetual, and men are accustomed to it from childhood.”

The Pythagoreans affirm, in order to justify the reality of the tones produced by the revolution of the spheres, that hearing takes place only where there is an alternation of sound and silence. The inaudibility of the spheral music is also accounted for by its overpowering the senses. Aristotle himself calls the Pythagorean tone-myth pleasing and ingenious, but untrue.

Plato attempted to illustrate the tones of the universe in an agreeable picture, by attributing to each of the planetary spheres a syren, who, supported by the stern daughters of Necessity, the three Fates, maintain the eternal revolution of the world’s axis. Mention is constantly made of the harmony of the spheres, though generally reproachfully, throughout the writings of Christian antiquity and the Middle Ages, from Basil the Great to Thomas Aquinas and Petrus Alliacus.

At the close of the sixteenth century, Kepler revived these musical ideas, and sought to trace out the analogies between the relations of tone and the distances of the planets; and Tycho Brahe was of opinion that the revolving conical bodies were capable of vibrating the celestial air (what we now call “resisting medium”) so as to produce tones. Yet Kepler, although he had talked of Venus and the Earth sounding sharp in aphelion and flat in perihelion, and the highest tone of Jupiter and that of Venus coinciding in flat accord, positively declared there to be “no such things as sounds among the heavenly bodies, nor is their motion so turbulent as to elicit noise from the attrition of the celestial air.” (SeeThings not generally Known, p. 44.)

Although this opinion was maintained incidentally by various writers both on astronomy16and natural religion, yet M.Fontenelle was the first individual who wrote a treatise on thePlurality of Worlds, which appeared in 1685, the year before the publication of Newton’sPrincipia. Fontenelle’s work consists of five chapters: 1. The earth is a planet which turns round its axis, and also round the sun. 2. The moon is a habitable world. 3. Particulars concerning the world in the moon, and that the other planets are also inhabited. 4. Particulars of the worlds of Venus, Mercury, Mars, Jupiter, and Saturn. 5. The fixed stars are as many suns, each of which illuminates a world. In a future edition, 1719, Fontenelle added, 6. New thoughts which confirm those in the preceding conversations, and the latest discoveries which have been made in the heavens. The next work on the subject was theTheory of the Universe, or Conjectures concerning the Celestial Bodies and their Inhabitants, 1698, by Christian Huygens, the contemporary of Newton.

The doctrine is maintained by almost all the distinguished astronomers and writers who have flourished since the true figure of the earth was determined. Giordano Bruna of Nola, Kepler, and Tycho Brahe, believed in it; and Cardinal Cusa and Bruno, before the discovery of binary systems among the stars, believed also that the stars were inhabited. Sir Isaac Newton likewise adopted the belief; and Dr. Bentley, Master of Trinity College, Cambridge, in his eighth sermon on the Confutation of Atheism from the origin and frame of the world, has ably maintained the same doctrine. In our own day we may number among its supporters the distinguished names of the Marquis de la Place, Sir William and Sir John Herschel, Dr. Chalmers, Isaac Taylor, and M. Arago. Dr. Chalmers maintains the doctrine in hisAstronomical Discourses, which one Alexander Maxwell (who did not believe in the grand truths of astronomy) attempted to controvert, in 1820, in a chapter of a volume entitledPlurality of Worlds.

Next appearedOf a Plurality of Worlds, attributed to the Rev. Dr. Whewell, Master of Trinity College, Cambridge; urging the theological not less than the scientific reasons for believing in the old tradition of a single world, and maintaining that “the earth is really the largest planetary body in the solar system,—its domestic hearth, and the only world in the universe.” “I do not pretend,” says Dr. Whewell, “to disprove the plurality of worlds; but I ask in vain for any argument which makes the doctrine probable.” “It is too remote from knowledge to be either proved or disproved.” Sir David Brewster has replied to Dr. Whewell’s Essay, inMore Worlds than One, the Creed of the Philosopher and the Hope of the Christian, emphatically maintaining that analogy strongly countenances the idea of all the solar planets, if not all worlds in the universe, being peopled with creatures not dissimilar in being and nature to theinhabitants of the earth. This view is supported inScientific Certainties of Planetary Life, by T. C. Simon, who well treats one point of the argument—that mere distance of the planets from the central sun does not determine the condition as to light and heat, but that the density of the ethereal medium enters largely into the calculation. Mr. Simon’s general conclusion is, that “neither on account of deficient or excessive heat, nor with regard to the density of the materials, nor with regard to the force of gravity on the surface, is there the slightest pretext for supposing that all the planets of our system are not inhabited by intellectual creatures with animal bodies like ourselves,—moral beings, who know and love their great Maker, and who wait, like the rest of His creation, upon His providence and upon His care.” One of the leading points of Dr. Whewell’s Essay is, that we should not elevate the conjectures of analogy into the rank of scientific certainties; and that “the force of all the presumptions drawn from physical reasoning for the opinion of planets and stars being either inhabited or uninhabited is so small, that the belief of all thoughtful persons on this subject will be determined by moral, metaphysical, and theological considerations.”

Sir David Brewster, in his eloquent advocacy of the doctrine of “more worlds than one,” thus argues for their peopling:

Man, in his future state of existence, is to consist, as at present, of a spiritual nature residing in a corporeal frame. He must live, therefore, upon a material planet, subject to all the laws of matter, and performing functions for which a material body is indispensable. We must consequently find for the race of Adam, if not races that may have preceded him, a material home upon which they may reside, or by which they may travel, by means unknown to us, to other localities in the universe. At the present hour, the inhabitants of the earth are nearlya thousand millions; and by whatever process we may compute the numbers that have existed before the present generation, and estimate those that are yet to inherit the earth, we shall obtain a population which the habitable parts of our globe could not possibly accommodate. If there is not room, then, on our earth for the millions of millions of beings who have lived and died upon its surface, and who may yet live and die during the period fixed for its occupation by man, we can scarcely doubt that their future abode must be on some of the primary or secondary planets of the solar system, whose inhabitants have ceased to exist like those on the earth, or upon planets in our own or in other systems which have been in a state of preparation, as our earth was, for the advent of intellectual life.

Sir William Herschel, in 1785, conceived the happy idea of counting the number of stars which passed at differentheights and in various directions over the field of view, of fifteen minutes in diameter, of his twenty-feet reflecting telescope. The field of view each time embraced only 1/833000th of the whole heavens; and it would therefore require, according to Struve, eighty-three years to gauge the whole sphere by a similar process.

M. F. W. G. Struve gives as the splendid result of the united studies of MM. Argelander, O. Struve, and Peters, grounded on observations made at the three Russian observatories of Dorpat, Abo, and Pulkowa, “that the velocity of the motion of the solar system in space is such that the sun, with all the bodies which depend upon it, advances annually towards the constellation Hercules171·623 times the radius of the earth’s orbit, or 33,550,000 geographical miles. The possible error of this last number amounts to 1,733,000 geographical miles, or to aseventhof the whole value. We may, then, wager 400,000 to 1 that the sun has a proper progressive motion, and 1 to 1 that it is comprised between the limits of thirty-eight and twenty-nine millions of geographical miles.”

That is, taking 95,000,000 of English miles as the mean radius of the Earth’s orbit, we have 95 × 1·623 = 154·185 millions of miles; and consequently,English Miles.The velocity of the Solar System154,185,000in the year.””422,424in a day.””17,601in an hour.””293in a minute.””57in a second.The Sun and all his planets, primary and secondary, are therefore now in rapid motion round an invisible focus. To that now dark and mysterious centre, from which no ray, however feeble, shines, we may in another age point our telescopes, detecting perchance the great luminary which controls our system and bounds its path: into that vast orbit man, during the whole cycle of his race, may never be allowed to round.—North-British Review, No. 16.

That is, taking 95,000,000 of English miles as the mean radius of the Earth’s orbit, we have 95 × 1·623 = 154·185 millions of miles; and consequently,

The Sun and all his planets, primary and secondary, are therefore now in rapid motion round an invisible focus. To that now dark and mysterious centre, from which no ray, however feeble, shines, we may in another age point our telescopes, detecting perchance the great luminary which controls our system and bounds its path: into that vast orbit man, during the whole cycle of his race, may never be allowed to round.—North-British Review, No. 16.

M. Arago has found, by experiments with the polariscope, that the light of gaseous bodies is natural light when it issues from the burning surface; although this circumstance does not prevent its subsequent complete polarisation, if subjected tosuitable reflections or refractions. Hence we obtaina most simple method of discovering the nature of the sunat a distance of forty millions of leagues. For if the light emanating from the margin of the sun, and radiating from the solar substanceat an acute angle, reach us without having experienced any sensible reflections or refractions in its passage to the earth, and if it offer traces of polarisation, the sun must bea solid or a liquid body. But if, on the contrary, the light emanating from the sun’s margin give no indications of polarisation, theincandescentportion of the sun must begaseous. It is by means of such a methodical sequence of observations that we may acquire exact ideas regarding the physical constitution of the sun.—Note to Humboldt’s Cosmos, vol. iii.

The extraordinary structure of thefully luminousDisc of the Sun, as seen through Sir James South’s great achromatic, in a drawing made by Mr. Gwilt, resembles compressed curd, or white almond-soap, or a mass of asbestos fibres, lying in aquaquaversusdirection, and compressed into a solid mass. There can be no illusion in this phenomenon; it is seen by every person with good vision, and on every part of the sun’s luminous surface or envelope, which is thus shown to be not aflame, but a soft solid or thick fluid, maintained in an incandescent state by subjacent heat, capable of being disturbed by differences of temperature, and broken up as we see it when the sun is covered with spots or openings in the luminous matter.—North-British Review, No. 16.

Copernicus named the sun the lantern of the world (lucerna mundi); and Theon of Smyrna called it the heart of the universe. The mass of the sun is, according to Encke’s calculation of Sabine’s pendulum formula, 359,551 times that of the earth, or 355,499 times that of the earth and moon together; whence the density of the sun is only about ¼ (or more accurately 0·252) that of the earth. The volume of the sun is 600 times greater, and its mass, according to Galle, 738 times greater, than that of all the planets combined. It may assist the mind in conceiving a sensuous image of the magnitude of the sun, if we remember that if the solar sphere were entirely hollowed out, and the earth placed in its centre, there would still be room enough for the moon to describe its orbit, even if the radius of the latter were increased 160,000 geographical miles. A railway-engine, moving at the rate of thirty miles an hour, would require 360 years to travel from the earth to the sun. The diameter of the sun is rather more than one hundred and eleven times the diameter of the earth. Therefore the volume or bulk of the sun must be nearlyone million four hundred thousandtimes that of the earth. Lastly, if all the bodies composing the solar system were formed into one globe, it would be only about the five-hundredth part of the size of the sun.

The dilated size (generally) of the Sun or Moon, when seen near the horizon, beyond what they appear to have when high up in the sky, has nothing to do with refraction. It is an illusion of the judgment, arising from the terrestrial objects interposed, or placed in close comparison with them. In that situation we view and judge of them as we do of terrestrial objects—in detail, and with an acquired attention to parts. Aloft we have no association to guide us, and their insulation in the expanse of the sky leads us rather to undervalue than to over-rate their apparent magnitudes. Actual measurement with a proper instrument corrects our error, without, however, dispelling our illusion. By this we learn that the sun, when just on the horizon, subtends at our eyes almost exactly the same, and the moon a materiallyless, angle than when seen at a greater altitude in the sky, owing to its greater distance from us in the former situation as compared with the latter.—Sir John Herschel’s Outlines.

This phenomenon is the progressive motion of the centre of gravity of the whole solar system in universal space. Its velocity, according to Bessel, is probably four millions of miles daily, in arelativevelocity to that of 61 Cygni of at least 3,336,000 miles, or more than double the velocity of the revolution of the earth in her orbit round the sun. This change of the entire solar system would remain unknown to us, if the admirable exactness of our astronomical instruments of measurement, and the advancement recently made in the art of observing, did not cause our progress towards remote stars to be perceptible, like an approximation to the objects of a distant shore in apparent motion. The proper motion of the star 61 Cygni, for instance, is so considerable, that it has amounted to a whole degree in the course of 700 years.—Humboldt’s Cosmos, vol. i.

Mr. Ponton has by means of a simple monochromatic photometer ascertained that a small surface, illuminated by mean solar light, is 444 times brighter than when it is illuminated by a moderator lamp, and 1560 times brighter than when it is illuminated by a wax-candle (short six in the lb.)—the artificial light being in both instances placed at two inches’ distance from the illuminated surface. And three electric lights, eachequal to 520 wax-candles, will render a small surface as bright as when it is illuminated by mean sunshine.

It is thence inferred, that a stratum occupying the entire surface of the sphere of which the earth’s distance from the sun is the radius, and consisting of three layers of flame, each 1/1000th of an inch in thickness, each possessing a brightness equal to that of such an electric light, and all three embraced within a thickness of 1/40th of an inch, would give an amount of illumination equal in quantity and intensity to that of the sun at the distance of 95 millions of miles from his centre.

And were such a stratum transferred to the surface of the sun, where it would occupy 46,275 times less area, its thickness would be increased to 94 feet, and it would embrace 138,825 layers of flame, equal in brightness to the electric light; but the same effect might be produced by a stratum about nine miles in thickness, embracing 72 millions of layers, each having only a brightness equal to that of a wax-candle.18

Mr. J. J. Waterston, in 1857, made at Bombay some experiments on the photographic power of the sun’s direct light, to obtain data in an inquiry as to the possibility of measuring the diameter of the sun to a very minute fraction of a second, by combining photography with the principle of the electric telegraph; the first to measure the element space, the latter the element time. The result is that about 1/20000th of a second is sufficient exposure to the direct light of the sun to obtain a distinct mark on a sensitive collodion plate, when developed by the usual processes; and the duration of the sun’s full action on any one point is about 1/9000th of a second.

M. Schatt, a young painter of Berlin, after 1500 experiments, succeeded in establishing a scale of all the shades of black which the action of the sun produces on photographic paper; so that by comparing the shade obtained at any given moment on a certain paper with that indicated on the scale, the exact force of the sun’s light may be determined.

All moving power has its origin in the rays of the sun. While Stephenson’s iron tubular railway-bridge over the Menai Straits, 400 feet long, bends but half an inch under the heaviest pressure of a train, it will bend up an inch and a half from its usual horizontal line when the sun shines on it forsome hours. The Bunker-Hill monument, near Boston, U.S., is higher in the evening than in the morning of a sunny day; the little sunbeams enter the pores of the stone like so many wedges, lifting it up.

In winter, the Earth is nearer the Sun by about 1/30 than in summer; but the rays strike the northern hemisphere more obliquely in winter than the other half year.

M. Pouillet has estimated, with singular ingenuity, from a series of observations made by himself, that the whole quantity of heat which the Earth receives annually from the Sun is such as would be sufficient to melt a stratum of ice covering the entire globe forty-six feet deep.

By the action of the sun’s rays upon the earth, vegetables, animals, and man, are in their turn supported; the rays become likewise, as it were, a store of heat, and “the sources of those great deposits of dynamical efficiency which are laid up for human use in our coal strata” (Herschel).

A remarkable instance of the power of the sun’s rays is recorded at Stonehouse Point, Devon, in the year 1828. To lay the foundation of a sea-wall the workmen had to descend in a diving-bell, which was fitted with convex glasses in the upper part, by which, on several occasions in clear weather, the sun’s rays were so concentrated as to burn the labourers’ clothes when opposed to the focal point, and this when the bell was twenty-five feet under the surface of the water!

Darkness of complexion has been attributed to the sun’s power from the age of Solomon to this day,—“Look not upon me, because I am black, because the sun hath looked upon me:” and there cannot be a doubt that, to a certain degree, the opinion is well founded. The invisible rays in the solar beams, which change vegetable colour, and have been employed with such remarkable effect in the daguerreotype, act upon every substance on which they fall, producing mysterious and wonderful changes in their molecular state, man not excepted.—Mrs. Somerville.

The fluctuation in the sun’s direct heating power amounts to 1/15th, which is too considerable a fraction of the whole intensity not to aggravate in a serious degree the sufferings of those who are exposed to it in thirsty deserts without shelter. The amount of these sufferings, in the interior of Australia for instance, are of the most frightful kind, and would seem far to exceed what have ever been undergone by travellers in thenorthern deserts of Africa. Thus Captain Sturt, in his account of his Australian exploration, says: “The ground was almost a molten surface; and if a match accidentally fell upon it, it immediately ignited.” Sir John Herschel has observed the temperature of the surface soil in South Africa as high as 159° Fahrenheit. An ordinary lucifer-match does not ignite when simply pressed upon a smooth surface at 212°; butin the act of withdrawing itit takes fire, and the slightest friction upon such a surface of course ignites it.

In order to compare the Light of the Sun with that of a Star, Dr. Wollaston took as an intermediate object of comparison the light of a candle reflected from a bulb about a quarter of an inch in diameter, filled with quicksilver; and seen by one eye through a lens of two inches focus, at the same time that the star on the sun’s image,placed at a proper distance, was viewed by the other eye through a telescope. The mean of various trials seemed to show that the light of Sirius is equal to that of the sun seen in a glass bulb 1/10th of an inch in diameter, at the distance of 210 feet; or that they are in the proportion of one to ten thousand millions: but as nearly one half of this light is lost by reflection, the real proportion between the light from Sirius and the sun is not greater than that of one to twenty thousand millions.

Humboldt selects the following example from historical records as to the occurrence of a sudden decrease in the light of the Sun:

A.D.33, the year of the Crucifixion. “Now from the sixth hour there was darkness over all the land till the ninth hour” (St. Matthewxxvii. 45). According toSt. Luke(xxiii. 45), “the sun was darkened.” In order to explain and corroborate these narrations, Eusebius brings forward an eclipse of the sun in the 202d Olympiad, which had been noticed by the chronicler Phlegon of Tralles (Ideler,Handbuch der Mathem. Chronologie, Bd. ii. p. 417). Wurn, however, has shown that the eclipse which occurred during this Olympiad, and was visible over the whole of Asia Minor, must have happened as early as the 24th of November 29A.D.The day of the Crucifixion corresponded with the Jewish Passover (Ideler, Bd. i. pp. 515–520), on the 14th of the month Nisan, and the Passover was always celebrated at the time of thefull moon. The sun cannot therefore have been darkened for three hours by the moon. The Jesuit Scheiner thinks the decrease in the light might be ascribed to the occurrence of large sun-spots.

The important influence exerted by the Sun’s body, as amass, upon Terrestrial Magnetism, is confirmed by Sabine in the ingenious observation, that the period at which the intensity of the magnetic force is greatest, and the direction of the needle most near to the vertical line, falls in both hemispheres between the months of October and February; that is to say, precisely at the time when the earth is nearest to the sun, and moves in its orbit with the greatest velocity.

The Heat of the Sun is dissipated and lost by radiation, and must be progressively diminished unless its thermal energy be supplied. According to the measurements of M. Pouillet, the quantity of heat given out by the sun in a year is equal to that which would be produced by the combustion of a stratum of coal seventeen miles in thickness; and if the sun’s capacity for heat be assumed equal to that of water, and the heat be supposed drawn uniformly from its entire mass, its temperature would thereby undergo a diminution of 20·4° Fahr. annually. On the other hand, there is a vast store of force in our system capable of conversion into heat. If, as is indicated by the small density of the sun, and by other circumstances, that body has not yet reached the condition of incompressibility, we have in the future approximation of its parts a fund of heat, probably quite large enough to supply the wants of the human family to the end of its sojourn here. It has been calculated that an amount of condensation which would diminish the diameter of the sun by only the ten-thousandth part, would suffice to restore the heat emitted in 2000 years.

Mr. Sharp, of Dublin, exhibited to the British Association in 1849 a Dial, consisting of a cylinder set to the day of the month, and then elevated to the latitude. A thin plane of metal, in the direction of its axis, is then turned by a milled head below it till the shadow is a minimum, when a dial on the top shows the hours by one hand, and the minutes by another, to the precision of about three minutes.

During the summer, in the northern hemisphere, places near the North Pole are incontinual sunlight—the sun never sets to them; while during that time places near the South Pole never see the sun. When it is summer in the southern hemisphere, and the sun shines on the South Pole without setting, the North Pole is entirely deprived of his light. Indeed, at the Poles there is butone day and one night; for thesun shines for six months together on one Pole, and the other six months on the other Pole.

Professor Airy, in hisSix Lectures on Astronomy, gives a masterly analysis of a problem of considerable intricacy, viz. the determination of the parallax of the sun, and consequently of his distance, by observations of the transit of Venus, the connecting link between measures upon the earth’s surface and the dimensions of our system. The further step of investigating the parallax, and consequently the distance of the fixed stars (where that is practicable), is also elucidated; and the author, with evident satisfaction, thus sums up the several steps:

By means of a yard-measure, a base-line in a survey was measured; from this, by the triangulations and computations of a survey, an arc of meridian on the earth was measured; from this, with proper observations with the zenith sector, the surveys being also repeated on different parts of the earth, the earth’s form and dimensions were ascertained; from these, and a previous independent knowledge of the proportions of the distances of the earth and other planets from the sun, with observations of the transit of Venus, the sun’s distance is determined; and from this, with observations leading to the parallax of the stars, the distance of the stars is determined. Andevery step in the process can be distinctly referred to its basis, that is, the yard-measure.

Each of these bodies excites, by its attraction upon the waters of the sea, two gigantic waves, which flow in the same direction round the world as the attracting bodies themselves apparently do. The two waves of the moon, on account of her greater nearness, are about 3½ times as large as those excited by the sun. One of these waves has its crest on the quarter of the earth’s surface which is turned towards the moon; the other is at the opposite side. Both these quarters possess the flow of the tide, while the regions which lie between have the ebb. Although in the open sea the height of the tide amounts to only about three feet, and only in certain narrow channels, where the moving water is squeezed together, rises to thirty feet, the might of the phenomenon is nevertheless manifest from the calculation of Bessel, according to which a quarter of the earth covered by the sea possesses during the flow of the tide about 25,000 cubic miles of water more than during the ebb; and that, therefore, such a mass of water must in 6¼ hours flow from one quarter of the earth to the other.—Professor Helmholtz.

Sir John Herschel describes these phenomena, when watched from day to day, or even from hour to hour, as appearing to enlarge or contract, to change their forms, and at length disappear altogether, or to break out anew in parts of the surface where none were before. Occasionally they break up or divide into two or more. The scale on which their movements takes place is immense. A single second of angular measure, as seen from the earth, corresponds on the sun’s disc to 461 miles; and a circle of this diameter (containing therefore nearly 167,000 square miles) is the least space which can be distinctly discerned on the sun as avisible area. Spots have been observed, however, whose linear diameter has been upwards of 45,000 miles; and even, if some records are to be trusted, of very much greater extent. That such a spot should close up in six weeks time (for they seldom last much longer), its borders must approach at the rate of more than 1000 miles a-day.

The same astronomer saw at the Cape of Good Hope, on the 29th March 1837, a solar spot occupying an area of nearfive square minutes, equal to 3,780,000,000 square miles. “The black centre of the spot of May 25th, 1837 (not the tenth part of the preceding one), would have allowed the globe of our earth to drop through it, leaving a thousand miles clear of contact on all sides of that tremendous gulf.” For such an amount of disturbance on the sun’s atmosphere, what reason can be assigned?

The Rev. Mr. Dawes has invented a peculiar contrivance, by means of which he has been enabled to scrutinise, under high magnifying power, minute portions of the solar disc. He places a metallic screen, pierced with a very small hole, in the focus of the telescope, where the image of the sun is formed. A small portion only of the image is thus allowed to pass through, so that it may be examined by the eye-piece without inconveniencing the observer by heat or glare. By this arrangement, Mr. Dawes has observed peculiarities in the constitution of the sun’s surface which are discernible in no other way.

Before these observations, the dark spots seen on the sun’s surface were supposed to be portions of the solid body of the sun, laid bare to our view by those immense fluctuations in the luminous regions of its atmosphere to which it appears to be subject. It now appears that these dark portions are only an additional and inferior stratum of a very feebly luminous or illuminated portion of the sun’s atmosphere. This again in its turn Mr. Dawes has frequently seen pierced with a smaller and usually much more rounded aperture, which would seemat last to afford a view of the real solar surface of most intense blackness.

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 fromA.D.1000 toA.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 AstronomicalSociety’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.”

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 nohesitation, 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 conjointlywith 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.

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.

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 theDesmodium 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 aslight degree of Heat (seeThings 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.

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 workOn the Face in the Moon, that we may suppose thespotsto 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 enclosedMare 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.

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.

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.

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 ofNewton 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 themare 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 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.

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 amountof 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.

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.

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 mustpass over, andoccultoreclipse, every star or planet within its zone, and, as seen from thesurfaceof 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 tooverlapany part of the space occupiedin the heavens by that luminary. On these occasions is exhibited the most striking and impressive of all the occasional phenomena of astronomy, anEclipse 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.

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.

Surely not (says Sir John Herschel) to illuminateournights, 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

Various estimates have been hazarded on the Number of Stars throughout the whole heavens visible to us by the aid ofour 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 intransparency; 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.

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.

Sir John Herschel, in hisOutlines of Astronomy, thus shows the changes in the celestial pole in 4000 years:

At the date of the erection of the Pyramid of Gizeh, which precedes the present epoch by nearly 4000 years, the longitudes of all the starswere less by 55° 45′ than at present. Calculating from this datum the place of the pole of the heavens among the stars, it will be found to fall near α Draconis; its distance from that star being 3° 44′ 25″. This being the most conspicuous star in the immediate neighbourhood, was therefore the Pole Star of that epoch. The latitude of Gizeh being just 30° north, and consequently the altitude of the North Pole there also 30°, it follows that the star in question must have had at its lowest culmination at Gizeh an altitude of 25° 15′ 35″. Now it is a remarkable fact, that of the nine pyramids still existing at Gizeh, six (including all the largest) have the narrow passages by which alone they can be entered (all which open out on the northern faces of their respective pyramids) inclined to the horizon downwards at angles the mean of which is 26° 47′. At the bottom of every one of these passages, therefore, the Pole Star must have been visible at its lower culmination; a circumstance which can hardly be supposed to have been unintentional, and was doubtless connected (perhaps superstitiously) with the astronomical observations of that star, of whose proximity to the pole at the epoch of the erection of these wonderful structures we are thus furnished with a monumental record of the most imperishable nature.

The Pleiades prove that, several thousand years ago even as now, stars of the seventh magnitude were invisible to the naked eye of average visual power. The group consists of seven stars, of which six only, of the third, fourth, and fifth magnitudes, could be readily distinguished. Of these Ovid says (Fast.iv. 170):

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