Part 6

The first point to be noticed, as showing that the aurora depends partly on extra-terrestrial circumstances, is the fact that the frequency of its appearance varies greatly from time to time. It is said that the aurora was hardly ever seen in England during the seventeenth century, though the northern magnetic pole was then much nearer to England than it is at present. Halley states that before the great aurora of 1716 none had been seen (or at least recorded) in England for more than eighty years, and no remarkable aurora since 1574. In the records of the Paris Academy of Sciences no aurora is mentioned between 1666 and 1716. At Berlin one was recorded in 1707 as a very unusual phenomenon; and the one seen at Bologna in 1723 was described as the first which had ever been seen there. Celsius, who described in 1733 no less than three hundred and sixteen observations of the aurora in Sweden between 1706 and 1732, states that the oldest inhabitants of Upsala considered the phenomenon as a great rarity before 1716. Anderson, of Hamburg, states that in Iceland the frequent occurrence of auroras between 1716 and 1732 was regarded with great astonishment. In the sixteenth century, however, they had been frequent.

Here, then, we seem to find the evidence of some cause external to the earth, as producing auroras, or at least as tending to make their occurrence more or less frequent. The earth has remained to all appearance unchanged in general respects during the last three centuries, yet in the sixteenth her magnetic poles have been frequently surrounded by auroral streamers; during the seventeenth these streamers have been seldom seen; during the last two-thirds of the seventeenth century auroras have again been frequent; and during the present century they have occurred sometimes frequently during several years in succession, at others very seldom.

Let us inquire a little more closely into the circumstances attending auroral displays, in order to ascertain what external cause it is which thus influences their occurrence.

Connected as auroras are with the phenomena of terrestrial magnetism, we may expect to find some help in our inquiry from the study of these phenomena.

Now it appears certain that magnetic phenomena are partly influenced by changes in the sun's condition. We may well believe that they are in the main due to the sun's ordinary action, but the peculiarities which affect them seem to depend on _changes_ in the sun's action. It is found that the daily oscillation of the magnetic needle corresponds with the diurnal change in the position of the sun owing to the earth's rotation. An annual change affecting that oscillation depends on the varying distance of the sun as the year proceeds. The daily change is not only greater than the annual, but is characterized by irregularities, when the face of the sun shows the greatest number of spots. It was found by General Sabine, says Mr. Balfour Stewart, "that the aggregate value of magnetic disturbances at Toronto attained a maximum in 1848, nor was he slow to remark that this was also Schwabe's period of maximum sun-spots. It was afterwards found, by observations made at Kew, that 1859 (another of Schwabe's years) was also a year of maximum magnetic disturbance.... There is also some reason to believe that on one occasion our luminary was caught in the very act. On the first of September, 1869, two astronomers, Carrington and Hodgson, were independently observing the sun's disc, which exhibited at that time a very large spot, when, about a quarter past eleven, they noticed a very bright star of light suddenly break out over the spot and move with great velocity across the sun's surface. On Mr. Carrington sending afterwards to Kew Observatory, at which place the position of the magnet is recorded continuously by photography, it was found that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed." The dip of the magnetic needle, its deflection from the north, the inferiority of its directive force, were all three simultaneously and abruptly altered, and continued so for many hours.

Nor are we left in any doubt as to the connection between such well-marked disturbances of the magnetic needle. While the needle was thus violently displaced, vivid auroras occurred over the greater part of both the northern and southern (magnetic) hemispheres. They were seen in latitudes where usually auroras are as infrequent as rain in Peru,--at Rome, in the West Indies, even within eighteen degrees of the equator.

The disturbance of the earth's electrical condition was well shown in other ways. Mr. C. V. Walker, the telegraphist, found that strong electrical currents affected the various telegraphic lines throughout England. These currents changed in direction every two or three minutes. In many places it was impossible to send telegraphic messages. In America some of the signalmen received severe electric shocks. "At a station in Norway," says Sir J. Herschel, "the telegraphic apparatus was set fire to; and at Boston, in North America, a flame of fire followed the pen of Bain's electric telegraph (which writes down the message upon chemically prepared paper)."

Many of my readers will doubtless remember the auroras of May 13, 1869, and October 24, 1870, both of which occurred when the sun's surface was marked by many spots, and both of which were accompanied by remarkable disturbance of the earth's magnetism.

It may, then, fairly be assumed that the occurrence of auroras depends in some way, directly or indirectly, on the condition of the sun. But what the real nature of that connection may be is not to be easily determined. It is clear that the eleven-year-period of sun-spots is not the only, or even the chief period affecting auroras, for we have seen that sometimes for a full century, or even more, very few auroras are seen. It is not by any means certain that the connection between the sun's condition and the occurrence of auroras is of the nature of cause and effect; quite probably sun-spots and auroras depend on some common cause as yet undetected,--and possibly never to be detected by man.

Regarding the auroral streamers as terrestrial lights only, but in some sense like the light reflected by planets in having their real source in the sun, we can no longer speak, as Humboldt was wont to do, of our planet possessing a power of emitting light of its own. Yet his manner of dealing with auroral light still possesses interest for us, especially in relation to the question whether these polar lights are emitted by other planets and may possibly be discerned from our earth. "It results from the phenomena of the aurora," said Humboldt, "that the earth is endowed with the property of emitting a light distinct from that of the sun. The intensity of this light is rather greater than that of the moon in its first quarter. It is at times, as on January 7, 1831, strong enough to admit of one's reading printed characters without difficulty. This light of the earth, the emission of which towards the poles is almost continuous" (this, however, is not strictly the case), "reminds us of the light of Venus, the part of which not lighted by the sun often glimmers with a dim phosphorescent light. Other planets may also possess a light evolved out of their own substance."

I would venture, however, to express strong doubts as to the possibility of discerning, either on Venus or on any other planet, the auroral gleams which may very probably illuminate at times their nocturnal skies. It must be remembered that the aurora, when at its brightest and covering a large part of the sky, only gives about as much light as the moon in her first quarter,--that is, as one half of a disc so small that 180,000 such discs would not equal the entire sky. The luminosity of the aurora is then in reality very small; probably far less than that of the earth's surface when illuminated by the full moon. A distant hill on which the rays of the full moon are falling seems strongly illuminated, and yet its light is really so faint that we could scarcely discern it at all save for the favouring effect of contrast. We know this, because we often see portions of the moon's surface which are illuminated by earthshine (when we see what is called the old moon in the new moon's arms), and these portions are quite faint by comparison with the rest of the moon; yet earthshine exceeds moonshine at least twelve times, and probably more nearly twenty times in splendour.

The glimmering phosphorescent light, supposed to have been seen on parts of Venus not lighted by the moon, is a phenomenon about which experienced telescopists are somewhat doubtful, though Webb speaks of the appearance as remarkably well attested, quoting, amongst others, the following cases. In 1715, Derham, in his "Astro-Theology," says that "the sphericity or rotundity is manifest in our moon, yea, and in Venus, too, in whose greatest falcations" (_i.e._, when they appear as crescents) "the dark parts of their globes may be perceived, exhibiting themselves under the appearance of a dull and rusty colour." In 1806, the phenomenon displayed itself beautifully to Harding three times and to Schröter once within five weeks. "Guthrie and others noticed it a few years ago, with small reflectors, in Scotland; Purchas, at Ross, in England; De Vico and Palomba, many times in Italy." Winnecke records a similar observation, though very faint, 1871, September 25, a little before noon. Van Hahn also says he saw it repeatedly, by day as well as by night, and with several instruments; he was, however, an inferior observer. The dark side is sometimes described as grey, sometimes as reddish. The phenomenon has, on the other hand, been looked for specially, on several occasions, by practised observers, using very fine instruments, who have failed to recognise any trace of it.

One of the most remarkable observations ever made on Venus must here be mentioned. Mädler states that on one occasion, when he was observing the planet, he saw a number of brushes of light diverging from the circular side (_i.e._, the outside of the planet's crescent), lasting as long as the planet could be seen that evening, and remaining unchanged when he changed the position of the telescopic eye-piece, or used a different one. "He attempts no explanation," says Webb, "but thinks it could not have been an optical illusion. This is certainly _possible_, but it is an instructive instance of the oversights which may be incidental even to great philosophers, that it never seems to have occurred to him to try another telescope!" It cannot be doubted that the evidence would have been greatly strengthened had he changed telescope as well as eye-piece; though it is not readily to be explained how a known telescope, frequently used as well before as after this strange appearance was seen, could for one evening only have played so strange a trick as Mädler's must have done, if what he saw was merely an instrumental illusion.

However, whether we have telescopic evidence or not respecting auroral lights surrounding the polar regions of other planets, we can have very little doubt that some among the planets, if not all of them, resemble our earth in this as in so many other respects. The aurora is a cosmical phenomenon, not one peculiar to our own earth. It is not, indeed, altogether certain that our sun himself may not be girt round by mighty auroral streamers, and that the light of these may not constitute a noteworthy portion of the corona of glory seen around him during the time of total eclipse.

This view, indeed, although it has not been definitely entertained as I have here expressed it, has been suggested by reasoning which led others to suppose that the coloured prominences around the sun may be auroras. Perceiving the nature of the connection between terrestrial magnetism and auroras, Balfour Stewart reasoned that we may extend our inquiries and ask, "If the sun's action is able to create a terrestrial aurora, why may he not also create an aurora in his own atmosphere?" It occurred independently to General Sabine, Prof. Challis, and himself, that the red flames visible during a total solar eclipse "may be solar auroræ." We now know that the solar flames are not auroræ, nor, properly speaking, flames at all, but great masses of glowing vapour. It is not, however, by any means so clear that the solar corona is not auroral in its nature. The following reasoning, applied by Balfour Stewart to the sun's prominences, applies with much greater force to the corona. After mentioning the height (from 70,000 to 80,000) which some prominences attain, he proceeds, "Considering the gravity of the sun, we are naturally unwilling to suppose that there can be any considerable amount of atmosphere at such a distance from his surface; and we are therefore induced to seek for an explanation of these red flames amongst those phenomena which require the smallest possible amount of atmosphere for their manifestation. Now the experiments of Mr. Gassiot and the observed height of the terrestrial aurora alike convince us that this meteor will answer our requirements best. And besides this, the curved appearance of these red flames, and their high actinic power, in virtue of which one of them, not visible to the eye, was photographed by Mr. De la Rue, are bonds of union between these and terrestrial auroræ."

All this and much more may be said of the solar corona. Its streamers extend not 70,000 or 80,000 miles, but 700,000 or 800,000 miles, from the surface of the sun, where the pressure must be far smaller than near the summits of even the loftiest prominences. They are curved and striated, like those of the aurora, whereas the shapes of the prominences bear only a distant resemblance to auroral streamers. They possess a high actinic (_i.e._, photographic) power, as is shewn by the readiness with which, during the total eclipse of December, 1871, they were photographed, no less than six well-defined negatives being taken both by Col. Tennant, at Ootacamund, and by Mr. Davis, at Baikal, during the brief continuance (only a few minutes) of total obscuration. In every respect the solar corona accords far better than do the solar coloured prominences with the appearance we should expect to recognise in solar auroras.

In particular, it has always seemed to me that the curved, especially the doubly curved, streamers of the corona can only be well explained by regarding the corona as in the main an auroral phenomenon. If mighty currents prevailed in the higher regions of a rare atmosphere, extending hundreds of thousands of miles from the sun's surface, appearances such as these curved streamers would undoubtedly be explained. But no one who considers the effect of the sun's tremendous attractive power on such an atmosphere can fail to perceive that, according to the known laws connecting gaseous pressure and density, the density of that atmosphere would be enormously great, even at a very great distance from the sun's surface, if the curved streamers really were caused by atmospheric currents. We know, on the contrary, from the behaviour of comets which have passed very near to the sun, that the atmosphere above his visible surface must be very rare indeed.

It must not be understood, however, that I regard the corona as simply a great solar aurora. It is certain that the whole region filled by the corona is occupied by immense numbers of scattered meteors, and extremely probable that large quantities of cometic matter exist within the same region. Vaporous masses may also be there, circling independently around the sun. But that this region is illuminated constantly by auroral light, varying greatly in intensity and position, seems very strongly indicated by all that we know about the corona, as seen during different total eclipses of the sun.

If we so viewed the solar corona, and found our earth, therefore, in this respect resembling the great central orb of the solar system, we could not but regard as extremely probable the theory that other planets also resemble the central body in this respect. We might then picture to ourselves every orb in the solar system carrying onward its faintly luminous crowns of boreal and austral light, not shining with constant lustre, or in the same constant position, but at one time leaping in coloured steamers to a great distance from the body they adorned, and anon sinking down and growing fainter and fainter, or occasionally disappearing altogether. Then, when some great disturbance affected the central sun, and caused _his_ auroral banners to shine out more brilliantly and to attain a greater extension, suddenly the auroral streamers of all the planets would leap out into new light and life, playing around the northern and southern magnetic poles of those orbs, even as electric brushes play around the positive and negative electrodes of a Geissler's tube. "Suddenly" at least so far as each planet is concerned, but not suddenly throughout the whole system. For the magnetic influences, like the light and heat of the sun, require time for their transmission. Yet, so rapidly do they travel that, in a few hours, the auroral illumination would extend from the central sun to the outermost limits of his system.

It remains that I should make a few remarks on the evidence which that wonderful instrument of research, the spectroscope, has afforded respecting the light of the aurora.

Angström was the first to observe the spectrum of the aurora borealis. He found that the greater part of the auroral light, as observed in 1867, was of one colour, yellow, but three faint bands of green and greenish blue colour were also seen. The aurora of April 15, 1869, was seen under very favourable conditions in America. Prof. Winlock, observing it at New York, found its spectrum to consist of five bright lines, of which the brightest was the yellow line just mentioned. One of the others seems to agree very nearly, if not exactly, in position with a green line, which is the most conspicuous feature of the spectrum of the solar corona. During the aurora of October 6, 1869, Flögel noticed the strong yellow line and a faint green band. Schmidt, on April 5, 1870, made a similar observation. He saw the strong yellow line, and from it there extended towards the violet end of the spectrum a faint greenish band, which, however, at times showed three defined lines, fainter, than the yellow line.

It was not till the magnificent aurora of October 24, 25, 1870, that any red lines were seen in the spectrum of an aurora. On that occasion the background of auroral light was ruddy, and on the ruddy background there were seen three deep red streamers very well defined. The ruddy streamers, on the night of October 25, converged towards the auroral crown, which was on that occasion singularly well seen. Förster of Berlin failed to see any red line or band despite the marked ruddiness of the auroral light. But Capron at Guildford saw a faint line in the red part of the spectrum; and Elger at Bedford observed a red band in the light of the red streamers, the band disappearing, however, when the spectroscope was directed on the white rays of the aurora.

As yet the auroral spectrum has not been interpreted. It is not a spectrum which can be (at present) artificially produced. We understand the spectrum of the sun and stars, because spectra of the same order can be produced in our laboratories. The spectra of the planets, so far as they differ from the spectrum of reflected sunlight in showing signs of the absorptive action of the planetary atmosphere, have been similarly interpreted. So also the spectra of the coloured solar prominences are understood, while those of nebulæ and comets, though not as yet thoroughly explained, have been partly interpreted, because of their partial agreement with the known spectra of earthly elements. But as yet neither the spectrum of the aurora nor that of the solar corona has been explained. The reason probably is, that the conditions under which the light of the aurora as of the corona is formed are not such as have been or perhaps can be attained or even approached in laboratory experiments.

VII.

_THE LUNAR HALO._

There are some phenomena of nature which suggest false ideas. For instance, when we look at the broad expanse of ocean on a moonlit night, and see a path of glory on its surface, directed towards the moon's place, we seem to be assured by the sense of sight that that broad track is illuminated while the waters all around are dark. A little consideration, however, assures us that the impression is a false one, that in this case seeing is not believing. The moon's rays really illumine the whole surface which lies before us, and we fail to receive light from other parts than the track below the moon, _not_ because they receive no light, but because the light which they receive is not reflected towards us. An observer, stationed a mile or two towards the right or towards the left of our station, sees a different track of light, while the part which seems bright to us seems dark to him.

The rainbow is another phenomenon of this deceptive kind. We seem to see an arch of many colours suspended in the air,--and when we learn that it is due to the presence of drops of water in the air, we are apt to infer that where we see the red arch there are drops lit up with red light, where the yellow, green, or violet arch, that the drops are aglow with yellow, green, or violet light. But in reality this is not so; the same drops which seem green to us will seem red to another observer, violet to another, and to yet other observers will show none of the prismatic colours, but only the dull grey colour of the cloud on which the rainbow is seen. We have here a pretty emblem of the varied aspects which events of the same real nature present to different persons, or according to the different circumstances under which the same person may see them. One shall see events in rosy tints, or with the freshness of spring hues, or with the melancholy symbolled by the

deeper indigo (as when The heavy-skirted evening droops with frost)--

while to others the same events shall show only the ordinary tints of common-place life.

The lunar halo is one of the phenomena thus deceptive to the view. We see all around the moon a circle or arc of light, nearly white, though sometimes faint tints of colour can be perceived in it, while the space within the circle seems manifestly darker than the space outside. The appearance of the halo as seen under favourable conditions is shown in fig. 11, on the next page. In this country the dark space round the moon is not generally so well seen as in countries where the air is clearer. But this is in reality the characteristic feature of the halo, as its name shows. For the name is derived from a Greek word signifying threshing-floor (the old threshing-floors being round), and thus naturally describes a round space relatively clear, surrounded on all sides by a ring of aggregated matter.

We seem in looking at the lunar halo, then, to see the moon at the centre of a dark space, surrounded by a ring of bright particles, outside which again are particles not quite so brightly illuminated as those forming the ring, but more brightly than those within the ring.