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The Solar Eclipse of 1870.

The total Solar Eclipse, which is to render famous the month of December in the present year—famous, that is, in astronomical annals—deserves, we think, some special notice in our pages. In 1860, the Himalaya was fitted out by Government for the use of the savants desirous of observing the Solar Eclipse visible that year in Spain, and the results of the expedition were so important as fully to warrant the liberality of the Government. The eclipse of the present year will also be visible in Spain, but the path of the sun's shadow will pass as far north as Cape Spartel, and, crossing Algeria, will go northwards, via Sicily, and in the direction of Constantinople. The totality of the eclipse will not last so long as that of the Indian eclipse in 1868. The sun will be hidden from sight for no longer period than about two minutes twelve seconds, and whatever observations our astronomers are anxious to make must be made in that brief interval. It may not unreasonably be suggested that, in so short a time, no data can be ascertained worth the cost and trouble the proposed expedition will necessitate; but the reader requires to be informed that the most valuable acquisitions lately won in the region of solar physics have been the result of observations which may almost be described as momentary.

What, then, is the important point on which our astronomers hope to gain information from a close examination of the approaching eclipse?

This question has been ably answered by a well-informed writer in the Daily News. The great problem to be solved is that of the strange appearance of the solar corona; of that glory of light which rings round the great luminary when totally eclipsed, to it, as some astronomers assert, a purely optical phenomenon, or, as others more reasonably declare, "one of the most imposing of all the features of the solar system." It is true that the former opinion is held by such men as Faye, Lockyer, and Professor Airy; but if it be founded on fact, the phenomenon loses all its importance, and nearly all its interest. If the other opinion prove correct, and the corona is discovered to be in reality an appendage of the orb of day, then the mind must at once acknowledge its unsurpassed splendour, its magnificent proportions. As its glow and ethereal radiancy often extends several degrees from the eclipsed sun, its diameter cannot be less than 2,800,000 miles.

Assume, then, that it is shaped like a globe, that it is the globular envelope (so to speak) of the sun, and we may conclude that its outer boundary would at most enclose a volume twenty-seven times as large as the sun's, or, in other words, twenty-seven million times larger than our terrestrial world.

It is in reference to this magnificent phenomenon that we hope to obtain some satisfactory information in December 1870. The results ascertained from the eclipse of 1868 proved to be almost directly contradictory to those obtained last year in the United States, and the problem now is, to discover why these results were contradictory.

In the meantime,—observes the writer already quoted,—some astronomers say that the observations already made suffice to show the real nature of the solar corona. "It has frequently happened that, with the means of solving a problem ready at their hands, astronomers have been content rather to wait till new observations removed their doubts, than to undertake the work of carefully analysing the facts already discovered. It is urged that the atmospheric explanation (Faye's and Lockyer's) is opposed by simple optical considerations; that the blackness of the moon's disc, in the very heart of the corona, affords the most unmistakable evidence that the moon lies nearer to us than the corona." The solar glare which, according to Faye's theory, illuminates our atmosphere, ought, according to this view—and ordinary reasoners will think this argument irrefragable—to cover the lunar disc as well as the surrounding heavens.

It is also argued, very forcibly, that the active atmosphere above the horizon of the observer is partially obscured during total eclipse, while all that part lying in the direction in which the corona is seen is wholly shielded from the direct rays of the sun, and cannot, therefore, furnish the "atmospheric glare" on which M. Faye relies.

M. Faye's theory is powerfully combated in a paper read by Mr Proctor before the Royal Astronomical Society, and a summary of which appeared in "Nature."[93]

He remarked that if we in reality possess sufficient evidence to determine whether the corona is or is not a solar appendage, it would be unfortunate for, and in some sense a discredit to science, if the precious seconds available for observers in December next were wasted upon observations directed to such a point determinable beforehand.

He proceeded to express his belief that the corona was no terrestrial phenomenon, arguing that the very blackness of the moon as compared with the corona (to which we have already referred), shows that the coronal splendour is behind the moon. In fact, the moon is projected on the corona as on a background; whereas, if the light be due to atmospheric glare, the corona ought to be a foreground.

This argument, however, may fail on the ground of its very simplicity. Mr Proctor, therefore, proceeded to inquire whether air which lies between the observer and the corona is really illuminated. He pointed out that all round the sun, for many degrees, perfect darkness should prevail if the illumination of the atmosphere by direct solar light were in question. As to the atmospheric glare caused by the coloured flames and prominences of the sun's disc, it must be comparatively small, bearing no higher proportion to the actual light of the atmosphere than ordinary atmospheric glare bears to actual sunlight,—a very small proportion indeed.

But a fatal objection to the theory that the corona is due either to the glare from the prominences or to light reflected from the surrounding air, consisted in the fact that the so-called glare ought to cover the moon's disc.

Mr Proctor next referred to a number of observations in support of the view that the coronal light is not terrestrial; such as the appearance of glare during partial eclipses, the glare always trenching on the lunar disc; the relatively greater darkness of the central part of the lunar disc in annular eclipses; the visibility of that part of the lunar disc which lies beyond the sun in partial eclipses, the limb being seen dark on the background of the sky; and the visibility of the corona in partial eclipses;—were its most distinctive peculiarities, having been recognised when the sun's face is not wholly covered.

What, then, is the actual nature of the corona?

May it not consist of the denser parts of meteoric systems travelling round the sun?

Leverrier has shown, and Mr Buxendell has helped to show, that the motion of Mercury's perihelion indicates the presence of a ring of bodies in the vicinity of the sun; but we have good reason for believing that for each meteor system our earth encounters, there must be millions on millions whose perihelia lie within the earth's orbit. Since the earth meets with fifty-six such systems, it will be seen how enormous probably is the total number.

Is, then, the corona simply the projection of these systems, illuminated as they must be by an intensity of solar light exceeding many hundred-fold that which illumines our summer days, nay, in certain places, were raised by his heat to a state of incandescence?

Such are some of the questions which, it is hoped, the eclipse of December 1870 will help to solve; and we think our readers will admit that their importance justifies us in looking forward to the approaching phenomenon with anxious curiosity. The bounds of human knowledge are gradually enlarging; and as they enlarge, they do but more vividly illustrate the infinite wisdom and perfectness of the scheme of creation.

Coloured Stars.

Before bringing these pages to a close, and appending to our description of Everyday Objects (everyday, but not commonplace) the word "Finis," we wish to direct the reader's attention to the subject of coloured stars; not because these beautiful spheres can be fairly classed under our general title, but because they serve to show the peculiar attractiveness and novelty of scientific inquiry, and the wonderful results that spring from the persevering study of God's universe.

To the eye of the ordinary observer, one star seems much like another; these "lamps of night" differ, indeed, in brilliancy, in their distance from the earth, in their relative magnitudes; but to all appearance they exhibit a complete identity in physical constitution, and their white light has furnished poetry with one of its commonest images.

But the astronomer, armed with his wonder-revealing glass, has discovered that innumerable stars are exquisitely coloured; that some are blue, and others green, and others red. If on a clear night you examine Perseus with your telescope, you will find that the star Eta is of a glowing red. In the system of Ophiuchus you will encounter a blue star; in that of Draco, a deep red star; Taurus has a large red and a small bluish star; and in Argo, a blue sun is attended by a dark-red satellite.

The stars occur in double, triple, or multiple groups or clusters; and the systems so constituted differ from what we presumptuously call the Solar System in their colouring; and in this variety or diversity, yet another variety is visible. The binary coloured systems are not all composed of blue and red suns. In that of Gamma AndromedÆ the great central orb is of an orange hue, the moon revolving round it green as emerald. What results from the marriage of these two colours—from this union of orange and emerald? Do we not see in it, says a French astronomer,[94] a combination, if the phrase is allowable, full of youth (assortiment plein de jeunesse)—a grand, a magnificent orange-coloured sun in the midst of the firmament—next to it, a resplendent emerald, which interweaves with the gold its greenish gleams?

The following lively picture is borrowed from the graphic pages of M. Flammarion.

There is more variety, he says, in the innumerable star-systems which crowd the fields of space, than in all the changes which the most skilful opticians can project on the screen of a magic lantern. Some of the planetary universes lighted up by two suns display the entire gamut of colours above blue, but are without the sparkling tints of gold and purple which contribute so greatly to the luminosity of our world.

In this category may be placed certain systems situated in the constellations of AndromedÆ, the Serpent, Ophiuchus, and Coma Berenices.

Others are illuminated by red suns, as, for example, a double star in Leo.

Others, again, are wholly dominated by blue and yellow, or, at least, are kindled by a blue sun and a yellow sun, which afford but a limited series of the tints or shades comprised in the combinations of these primitive colours; such are the systems of Ceta, Eridanus (where one is straw-coloured, the other blue), Cameleopardalis, Orion, Rhinoceros, Gemini, BoÖtes (where the greater one is yellow, and the lesser a greenish blue), and Cygnus (where the smaller is "blue as a sapphire"). On the other hand, combinations of red and green may be found in Cassiopeia, Coma Berenices, and Hercules.

There are other stellar systems which, according to Flammarion, more nearly approximate to our own, in the sense that one of the suns illuminating them has, like ours, a white light,—the source of all colours,—while its neighbouring luminary casts a permanent reflection upon everything. Such, for instance, are the worlds revolving round the great sun of Alpha, in Aries; this great sun is white, but a smaller sun is constantly visible in the sky, whose azure reflections cover as with "a diaphanous veil" the various objects exposed to its rays. Number 26 in Ceta presents the same conditions, which are those, too, of a great number of the most brilliant stars. In the case of the worlds gravitating round the principal world of these binary systems, the originative white lustre would appear to create the infinite varieties noticeable upon the earth, with the distinction, of course, that an azure gleam constantly issues from the other sun; but for the planets gravitating round the latter, the predominant colouring will be blue, tempered by the action of the remoter white sun.

And just as white suns have blue suns for their companions, so have red suns yellow, or vice versÂ. What a strange and curious radiance—like that of "a dome of many-coloured glass"—must be shed upon a planet by its red and green or yellow and blue suns! What striking contrasts, what fairylike changes, must be produced by a red day and a green day succeeding alternately to a white day, or a day of darkness! Why do not our poets take up such a theme as this? Can they find aught more remarkable or more eËry in the dreams of their imagination?

Supposing that these wonderful spheres are surrounded, like Jupiter or Saturn, by a ring of satellites, what can be their aspect when lit up by several suns, each sun a source of different-coloured light?

This one, we may imagine, will be divided into hemispheres, red and blue, that will suspend in the firmament a crescent of gold; another will seem to drop from the azure circle like a gorgeous cluster of emerald fruit. Ruby moons, opaline moons, moons of jasper and amethyst,—all shining with a mystic and indescribable radiance,—surely the heavens are decked with jewels like an Eastern queen!

Fig. 82.—"Bright with the beauty of the silver moon."