The aurora effects
Polar auroras, which are subdivided into aurora borealis and aurora australis, according to the hemisphere in which they occur, are assuredly the finest of the optical phenomena of nature, but are, even at the present day, one of the least understood. While by their sudden appearance, their superb colouring, their rapid movement, their infinitely varied form, the northern lights have from all time excited the attention of the multitude, their mysterious nature and the relations which seem to connect them with terrestrial magnetism, and even with certain cosmic phenomena, such as the spots in the sun, make them the subject of the study of meteorologists and physicists.
The aurora borealis was already known to the Greeks and Eomans, although it is a rare phenomenon in districts so far south as the shores of the Mediterranean. Aristotle touches on them briefly in his 'Meteorology.'1 He says that they sometimes present the appearance of the smoke of the straw which is burnt in the country, and his observation is exact. Other forms of the aurora borealis are designated by him under the name of brands (SaXol) and goats (alyss): the author enters into no explanation, but this last name may plausibly be referred to certain luminous rays, of which we shall speak later, which show rapid alternative movements in the direction of their length, and thus appear to leap. Further on, among the phenomena which may be observed on calm nights, Aristotle enumerates gulfs and abysses (^da.fiaTa, fiodvvoi) and sanguine colours. The first two expressions doubtless refer to what is now known as the obscure segment, the dark part of the heaven which is seen in the form of an arc, below the aurora. This identification appears the more probable that Aristotle adds: 'The gulfs seem to have a certain depth because of the contrast which the light makes with the black and blue colour of the sky. Often even, when they contract, brands or torches issue from them (Ba\ol in Greek); the gulf then seems to converge.' These few lines appear to be absolutely incomprehensible in certain French translations; as rendered above they correspond with sufficient accuracy to certain aspects of the aurora borealis.
Cicero merely mentions twice the aurora, which he indicates by the term torches. Pliny, the naturalist, is much more explicit, and he cites the precise epochs at which the phenomenon was observed.
'Beams (trabes) are seen to shine in the heaven,which are called BokoI in Greek, as it happened atthe time when the Lacedaemonians, vanquished at sea,lost the dominion over Greece. In the sky appears agulf which is called chasmu (the Greek word translated directly into Latin). And, besides, there is seenin the heaven (and nothing is more terrible fortrembling mortals) blood-coloured flames whichafterwards fall upon the earth, as it happened in thethird year of the hundred and seventh Olympiad,when King Philip ruled over Greece. Under theconsulate of C. Csecilius and Cn. Papirius, and onmany other occasions, a light was seen in heavenwhich made the night almost as light as day. It issaid that at the time of the wars of the Cimbri, andalso often before and since, the clashing of arms andthe sound of trumpets were heard in the sky. But inthe third consulate of Marius the dwellers in Ameriaand Tuderta saw in the heavens two armies rushingone against the other from the east and from thewest; that of the west was defeated. The heaven itself caught fire: this is no extraordinary thing, andit has often been seen when the clouds are exposedto great heat.'
In this quotation from Pliny we find for the firsttime the trace of that popular superstition whichobtained almost down to our own day, and whichattributed the great auroras to armies combating inthe sky.
Seneca gives a few more details, curiously exact,showing that observers in classic times were oftenmore accurate than those of several centuries later.Speaking of the different sorts of heavenly fires,Seneca describes a certain number of appearanceswhich are clearly the aurora borealis; but instead ofusing Latin words to describe them he borrows theGreek terms of Aristotle, which we gave above. Hereis the passage from Seneca:
'Sometimes flames are seen in the sky, either stationary or full of movement. Several kinds are known: the abysses, when beneath a luminous crown the heavenly fire is wanting, forming as it were the circular entrance to a cavern; the tuns (pithita), when a great rounded flame in the form of a barrel is seen to move from place to place, or to burn immovable; the gulfs (chasmata), when the heaven seems to open and to vomit flames which before were hidden in its depths. These fires present the most varied colours: some are a vivid red; others resemble a faint and dying flame; some are white; others scintillate; others finally are of an even yellow, and emit neither rays nor projections. Among these phenomena should be ranged those appearances as of the heavens on fire so often reported by historians; sometimes these fires are high enough to shine among the stars; at others, so low that they might be taken for the reflection of a distant burning homestead or city. This is what happened under Tiberius, when the cohorts hurried to the succour of the colony of Ostia, believing it to be on fire. During the greater part of the night the heaven appeared to be illuminated by a faint light resembling a thick smoke.'
The mistake mentioned by Seneca is so natural that it has often occurred since. At Copenhagen, in 1709, during a large and brilliant aurora, several battalions turned out under arms and beating drums. The Greek and Eoman authors often confounded certain faint auroras with comets, and it is probable that the catalogues of early appearances of comets contain many errors of this description. The mistake is manifest when it is stated in the description that the comet appeared in the north and only lasted a few hours, or even less; in this case it was simply a ray of the aurora borealis. Such, for instance, was the supposed comet of October 11,1527, inFrance, in Germany, and almost the whole of Europe: it was of immense length; its summit was like an arm, bent at the elbow; it was only visible towards the north, and lasted but an hour and a quarter. It was of an orange-red colour, 'and joined to it were dark rays in the form of tails, lances, bloody swords, figures of men, and heads cut off, bristling with hair and beards.' From all these details it is easy to recognise in the phenomenon an aurora borealis. Among early records of the aurora we will cite yet another, mentioned by Gregory of Tours. He says he saw the appearance himself, and his description is interesting, both from its simplicity and accuracy, and because the form now known as the ' boreal crown' is clearly indicated for the first time: 'Two nights in succession we saw signs in the heavens, that is to say rays of light which rose in the north, as often happens. A great light took possession of a part of the sky and seemed to overflow it. . . . There was in the middle of the heaven a cloud of light, towards which all the rays converged, in the form of a tent of which the sides, much wider at the foot, ascended, narrowing to the summit, where they united, often in the shape of a hood.' Gregory of Tours in several other places mentions the aurora borealis, and his descriptions are always equally simple: he considers the aurora as a curious manifestation, but attaches to it no idea of the supernatural. Some centuries later astrology had so troubled the minds of men that the aurora borealis had become a source of terror: bloody lances, heads separated from the trunk, armies in conflict, were clearly distinguished. At the sight of them people fainted (according to Cornelius Gemma), others went mad. Pilgrimages were organised to avert the wrath of Heaven, manifested by these terrible signs. Thus, according to the Journal of Henri III., in the month of September 1583 eight or nine hundred persons of all ages and both sexes, with their lords, came to Paris in procession, dressed like penitents or pilgrims, from the villages of Deux-Gemeaux and Ussy-en-Brie, near La Ferte-Gaucher, 'to say their prayers and make their offerings in the great church at Paris; and they said that they were moved to this penitential journey because of signs seen in heaven and fires in the air, even towards the quarter of the Ardennes, whence had come the first such penitents, to the number of ten or twelve thousand, to Our Lady of Eheims and to Liesse.' The chronicler adds that this pilgrimage was followed a few days afterwards by five others, and for the same cause.
These superstitious terrors lasted at least till the end of the seventeenth century. La Mothe le Vayer, who was tutor to the Duke of Orleans, brother of Louis XIV., and afterwards to Louis XIV. himself, and who entered the Academy in 1639, alludes to these popular beliefs in his 78th letter, entitled 'De la Credulite:
'I will take my second example (of credulity) from the writings of Baptiste le Grain, for whom I have a great esteem: he says in his sixth book that he observed in Paris in 1615, about eight o'clock in the evening of the 26th of October, men of fire in heaven, who fought with lances, and who by this terrifying spectacle foretold the fury of the wars which followed. Yet I was with him in the same town, and I protest, having studied attentively until eleven at night the phenomenon of which he speaks, that I saw nothing similar to his description, but only an appearance which is sufficiently common, in the form of pavilions in the sky flaming up and fading out again, as is usual with such meteors. A number of persons now living will testify to what I say.'
It was towards this epoch that, as a result of the observations of Gassendi, and later of those of Cassini, of Eoemer, &c., the aurora ceased to be regarded, at any rate by the educated classes, as a supernatural phenomenon, presaging horrible calamities. Yet even at the present day, in rural districts, simple folk might be prone to attribute a supernatural origin or significance to the aurora if its appearance should chance to coincide with striking events or great disasters. Perhaps some vestiges of this ancient superstition might have been found at the time of the remarkable aurora borealis which was observed throughout France during the nights of October 24 and 25, 1870.
The Norsemen believed that the aurora represented the Valkyries riding through the air on their sombre horses. This belief, indicated by Tacitus in his description of Germany, is given more explicitly in several passages of the Edda. But since this phenomenon is of common occurrence in the north, familiarity early deprived it of all its terrors. About the year 1250, less than a century after the composition of the Edda, a Norwegian, who probably lived near the town of Nansos (to the north of Trondhiem), wrote a work, at once philosophical and political, entitled 'The King's Mirror' (Konungs Skuggsja), which is remarkable on more than one count: in it the aurora is thus described:
'The nature and constitution of the northern lights are such that the darker the night the brighter they shine: they are never seen by day, but only at night, especially in profound obscurity, and rarely by moonlight. They appear like a great flame seen from a distance, as though that of a vast fire. From the smoke of it pointed shafts of unequal and very variable size seem to dart upwards into the air, so that now the one and now the other is the higher, and they seem to quiver like flames. When these rays are at their highest and brightest they give so much light that one can find one's way about quite easily out of doors, and even hunt if this is desired. In houses with windows it is light enough within for men to see each other's faces. This light is so variable that it sometimes becomes obscured, as though it were
covered by thick smoke or cloud; and soon it seems to be stifled by this smoke and near extinct. But as soon as the cloud begins to dissipate and grow less thick, the light increases and glows again, and sometimes great sparks seem to fly from it, as from a redhot iron taken from the forge. As the night advances and dawn approaches, this light begins to pale, and it fades altogether when day breaks. Certain people maintain that this light is a reflection of the fire which surrounds the seas of the north and of the south; others say that it is the reflection of the sun when it is below the horizon; for my part I think that it is produced by the ice which'radiates at night the light which it has absorbed by day.'
Note, by the way, this first attempt at an explanation of the polar auroras. Doubtless it is not very convincing and would not be considered satisfactory at the present day; but we must not smile at it, since it is nearly the same as that which many centuries later was suggested both by Descartes and Sir John Franklin.
We may add, in concluding this historical sketch, that Gassendi, in 1621, first used the term aurora borealis. In the seventeenth century and even in a part of the eighteenth, as this appearance was not known to exist in the southern hemisphere, the light was called borealis when it appeared in the northern quarter of the sky, and australis when it appeared to the south, but always to an observer in the northern hemisphere.
Although polar auroras were seen in Chili as early as 1640, the first certain observations taken in the southern hemisphere and known in Europe were those of Antonio de Ulloa. During the voyage round Cape Horn in 1745 he more than once had occasion to observe a polar aurora, and he maintains that they must be as frequent in the southern hemisphere as in our own. Since then the name of aurora borealis has been restricted to those which are seen in the northern hemisphere, round the north pole, and that of aurora australis to those which are seen round the south pole. The name polar aurora, therefore, is to be preferred as being more general.
In England and the United States the name proposed by Gassendi has been adopted in scientific phraseology to designate this phenomenon. The sailors of the north of England call them northern lights, or streamers. In Germany and candinavian countries the ancient name of northern light which we find in 'The King's Mirror' has been retained, Nordlicht in German, Nordljus in Swedish, Nordlys in Danish.