AURORA BOREALIS, Polar Light, Northern Lights, or Streamers, is a beautifully luminous meteor, appearing in the form of beams or rays, and sometimes in that of arches or crowns. The rays are seldom stationary, generally flitting with greater or less velocity throughout the heavens; the arches are sometimes single; sometimes several concentric ones are seen. These lights, or meteoric coruscations, are most brilliant in the arctic regions, appearing mostly in the winter season and in frosty weather. In the Shetland Islands, the merry dancers, as they are there and elsewhere called, are the constant attendants of clear evenings, and serve to diminish materially the gloom of the long winter nights. They commonly appear at twilight near the horizon, and sometimes continue in that state for several hours without any sensible motion; after which they send forth streams of stronger light, which rise from the horizon in a pyramidal undulating form, shooting with great velocity up to the zenith, assuming columnar and other shapes, and varying in colour from a reddish yellow to the darkest russet. At other times they cover the whole hemisphere with their flickering and fantastical coruscations. On these occasions their motions are amazingly quick, and they astonish the spectator with rapid changes of form. They break out in places where none were seen before, skimming briskly along the heavens; then they are suddenly extinguished, leaving behind a uniform dusky track, which, again, is brilliantly illuminated in the same manner, and as suddenly left a dull blank. Some nights they assume the appearance of vast columns; exhibiting on one side tints of the deepest yellow, and on the other melting away till they become undistinguishable from the surrounding sky. They have generally a strong tremulous motion from end to end, which continues till the whole vanishes. According to the state of the atmosphere, their colours vary. They sometimes assume the hue of blood, on which occasion their appearance is considered portentous. Then the rustic sages become prophetic, and terrify the gazing spectators with the dread of war, pestilence, and famine. This superstition is not peculiar to the northern islands; nor are these appearances of recent date. The
ancients called them chasmata, and trabes, and bolides, according to their forms or colours.
In early times, it is said, these meteoric lights were extremely rare, and on that account were the more taken notice of. From the days of Plutarch to those of our sage historian Sir Richard Baker, they were supposed to portend great events, and lively imaginations shaped them into aerial conflicts:
Fierce fiery warriors fight upon the clouds,
In ranks, and squadrons, and right form of war.
Dr Halley tells us, that when he observed a great aurora borealis in 1716, he had begun to despair of ever seeing one at all; none having appeared, at least of any considerable extent, from the time he was born until then. But notwithstanding this long interval, it seems that, in some periods, the aurora borealis had been seen much more frequently; and perhaps this, as well as many other natural phenomena, may be subject to periodical changes and variations.
The only thing resembling a distinct history of this phenomenon is that which has been given by the learned Dr Halley, in the Philosophical Transactions, No. 347. The first account he gives is of the appearance of what is called by him burning spears, which were seen at London on the 30th January 1560. This account is taken from a book entitled A Description of Meteors, by W. F. D.D., reprinted at London in 1654. The next appearance, according to the testimony of Stow, was on the 7th October 1564. In 1574 also, according to Camden, and Stow above-mentioned, an aurora borealis was observed two nights successively, viz. on the 14th and 15th of November, having much the same appearances as that described by Dr Halley in 1716, and which we now so frequently observe. Again, an aurora was twice seen in Brabant, in the year 1575, viz. on the 13th of February and 28th of September. Its appearances at both these times were described by Cornelius Gemm, professor of medicine in the university of Louvain, who compares them to spears, fortified cities, and armies fighting in the air. After this, Michael Mæstlin, tutor to the great Kepler, assures us, that at Baknang in the county of Wurtemberg in Germany, these phenomena, which he styles chasmata, were seen by himself no less than seven times in 1580. In 1581 they again appeared, in an extraordinary manner in April and September, and in a less degree at some other periods of the same year. In September 1621, a similar phenomenon was observed all over France, and described by Gassendus, who gave it the name of aurora borealis; yet neither this, nor any similar appearance posterior to 1574, is described by English writers till the year 1707, which, as Dr Halley observes, shows the prodigious neglect of curious matters that then prevailed. From 1621 to 1707, indeed, there is no mention made of an aurora borealis having been seen by any body; and, considering the number of astronomers who during that period were in a manner continually poring over the heavens, we may very reasonably conclude that nothing of the kind really made its appearance until after an interval of eighty-six years. A small one was seen in November 1707; and during that and the following year, the same appearances were repeated five times. The next on record is that mentioned by Dr Halley in March 1716, which from its brilliancy attracted universal attention, and was considered by the vulgar as marking the introduction of a foreign race of princes. Since that time these meteors have been much more frequent. Many of those observed in this island have been described. The following account of a splendid aurora borealis, as seen from the Gosport Observatory on the 7th January 1831, will give an idea of the brilliancy of this meteor, and of its colours, lustres, arches, and beams.
"In the afternoon of this day there was a peculiar brightness in the atmosphere near the horizon, for several degrees on each side of the true north point, which indicated the approach of an aurora; indeed we have reason to suspect that it was a faint appearance of one; while the sun shone in all his splendour, without the interposition of cloud or vapour. Shortly after sunset an aurora borealis gradually rose above the northern horizon, and at a quarter past five o'clock it had assumed the form of an arch of resplendent light, ten degrees in height, and seventy degrees in width. From this time till half-past five it continued to increase in the intensity of its light, expanding to the western point of the horizon, and fifty-five degrees to the eastward of north, which made the chord of the aurora a hundred and fifty-five degrees. Now, a bright flame-coloured rainbow-like arch, between three and four degrees broad, and pretty well defined at its upper edge, emanated from the curved edge of the aurora to an altitude of thirty-five degrees; and while it remained apparently stationary, a beautiful rainbow-like arch, still more brilliant, formed about ten degrees south of the zenith, by streamers suddenly springing up from the N. E. by E. and W. by S. points of the horizon, and meeting in the zenith, so that these two bows presented themselves at the same time. At thirty-five minutes past five, the latter bow, in some parts four and in others six degrees in width, divided a little to the eastward of its vertex; and the long streamers which formed it passed off gently to the southward in very bright patches, two in the south-east and one in the south-west quarters, like luminous clouds, and continued in sight nearly a quarter of an hour. One of these bright patches nearly covered Orion several minutes. At forty minutes past five another rainbow-like arch, equally wide and bright, was formed by long streamers from about the same points of the horizon, whose point of convergence was the same, and its course through the feet of Gemini, near the Pleiades, through Aries, the square of Pegasus, the head of Equuleus, and the bow of Antinous. It passed off gradually towards the south; and at a quarter before six the planet Mars, then near the meridian, and about forty-five degrees in altitude, rested, as it were, conspicuously on it. At six it had gone far towards the southern horizon, and could scarcely be perceived, leaving the sky unusually clear and bright. By this time the bow over the aurora had much increased in altitude, and was nearly effaced.
"At a few minutes past six, after a great many coloured columns of light had risen from the north-east and north-west quarters, and passed the zenith, the aurora sunk considerably towards the horizon; but its upper edge remained bright and very well defined. Some of the streamers or columns were long, others short, and the widest generally remained long enough to pass through a gradation of prismatic colours. At half-past six the aurora again increased in altitude, and vivid coruscations radiated from every part of its arch, and, on intermixing with each other, formed wide columns, which were so grand with crimson tints as to astonish every spectator. Between seven and eight the aurora had spread at least two thirds over the heavens, and as far as the shoulders of Orion on the eastern side of the meridian, when large perpendicular columns, and short pointed luminous coruscations, rising from the aurora, glittering like spears and conical points in nearly parallel rows, now mixing and then dividing, all passed through red, orange, lake, crimson, green, and purple tints; so that the appearance altogether over so great an extent of the heavens was exceedingly grand, particularly when contrasted with the cerulean sky, and its spangled constellations, in the southern portion of the hemisphere.
"At ten minutes before eight the aurora was in its greatest splendour. At five minutes before eight another luminous rainbow-like arch-stretched across the heavens from the eastern point of the horizon, and displayed several prismatic colours while passing southward. Soon after eight a large dark space, in and near the horizon, presented itself several degrees on each side of the magnetic north, and the aurora, still far over the heavens, gradually diminished. At nine it again ascended, and wide columns rose from every part of its arch, and passed through the same colours as before-mentioned. Between nine and ten the magnetic needle, which in the early part of the evening stood at twenty-four degrees west of the true north, was disturbed, and receded upwards of half a degree northward, either by the influence of the aurora, or by a change of wind from north-east to southwest, and of course a change in its electrical state. At a quarter before eleven there was a grand display of about twelve or fourteen glowing columns from the aurora, several of which passed beyond the zenith, when a perfect red rainbow-like arch, ten degrees above the aurora, was visible. At eleven another bow three degrees and a half wide rose from the aurora, and passed through Aries, Cassiopeia, Ursa Minor, and the square of Ursa Major, until, reaching the zenith, it gradually disappeared. At half-past eleven the aurora again began to sink slowly, and did not rise afterwards. At five minutes before twelve, a large brilliant meteor, the only one observed through the night, passed under Ursa Major. At one o'clock A.M. the highest part of the aurora, about the magnetic north, had sunk to within six or seven degrees of the horizon; yet bright coruscations occasionally emanated from it till two, when the observations were discontinued, as no more interesting meteoric appearances were likely to occur."
The auroras observed, one at Cumberland House and another at Fort Enterprise, in North America, are described by Lieutenant Hood, in Captain Franklin's Narrative. The account of the former is as follows:—
"For the sake of perspicuity," says Mr Hood, "I shall describe the several parts of the aurora observed at Cumberland House, which I term beams, flashes, and arches. The beams are little conical pencils of light, ranged in parallel lines, with their pointed extremities towards the earth, generally in the direction of the dipping needle. The flashes seem to be scattered beams, approaching nearer to the earth, because they are similarly shaped, and infinitely larger. I have called them flashes, because their appearance is sudden, and seldom continues long. When the aurora first becomes visible, it is formed like a rainbow, the light of which is faint, and the motion of the beams undistinguishable. It is then in the horizon. As it approaches the zenith, it resolves itself at intervals into beams, which, by a quick undulating motion, project themselves into wreaths, afterwards fading away, and again brightening, without any visible expansion or concentration of matter. Numerous flashes are seen in different parts of the sky. That this mass, from its short distance above the earth, would appear like an arch to a person situated at the horizon, may be demonstrated by the rules of perspective, supposing its parts to be nearly equidistant from the earth. An undeniable proof of it, however, is afforded by the observations of the 6th and 7th of April, when the aurora, which filled the sky at Cumberland House, from the northern horizon to the zenith, with wreaths and flashes, assumed the shape of arches at some distance to the southward."
"But the aurora does not always make its first appearance as an arch. It sometimes rises from a confused mass of light in the east or west, and crosses the sky towards
the opposite point, exhibiting wreaths of beams, or coronæ boreales, in its way. An arch, also, which is pale and uniform at the horizon, passes the zenith without displaying any irregularity or additional brilliancy; and we have seen three arches together very near the northern horizon, one of which exhibited beams, and even colours; but the other two were faint and uniform. On the 7th of April an arch was visible to the southward, exactly similar to that in the north, and it disappeared in fifteen minutes. It had probably passed the zenith before sunset. The motion of the whole body of aurora is from the northward to the southward, at angles not more than twenty degrees from the magnetic meridian. The centres of the arches were as often in the magnetic as in the true meridian.
"The colours do not seem to depend on the presence of any luminary, but to be generated by the motion of the beams, and then only when that motion is rapid, and the light brilliant. The lower extremities quiver with a fiery red colour, and the upper with orange. We once saw violet in the former. The number of auroræ visible in September was two, in October three, in November three, in December five, in January five, in February seven, in March sixteen, in April fifteen, and in May eleven. Calm and clear weather was the most favourable for observation; but it is discernible in cloudy weather, and through mists. We could not perceive that it affected the weather. The magnetic needle, in the open air, was disturbed by the aurora, whenever it approached the zenith. Its motion was not vibratory, as observed by Mr Dalton; and this was, perhaps, owing to the weight of the card attached to it. It moved slowly to the east or west of the magnetic meridian, and seldom recovered its original direction in less than eight or nine hours. The greatest extent of its aberration was 45". A delicate electrometer, suspended at the height of fifty feet from the ground, was never perceptibly affected by the aurora; nor could we distinguish its rustling noise, of which, however, such strong testimony has been given to us, that no doubt can remain of the fact."
The account of the aurora observed at Fort Enterprise is also interesting and instructive with reference to the theory of the phenomenon.
"The shapes of the aurora observed at Fort Enterprise, at its entry into the horizon, and progress through the sky, may be reduced under two general descriptions. In the first I shall class those which are formed like rainbows or arches in the earliest stage of their appearance. They rise with their centres sometimes in the magnetic meridian, and sometimes several degrees to the eastward or westward of it. The number visible at the same time seldom exceeds five, and is seldom limited to one. The altitude of the lowest, when first seen, is never less than four degrees. As they advance towards the zenith, their centres (or the parts most elevated) preserve a course nearly in the magnetic meridian, or parallel to it. But the eastern and western extremities vary their respective distances, and the arches become irregularly broad streams in the zenith, each dividing the sky into two unequal parts, but never crossing one another till they separate into parts. Those arches which were bright at the horizon increase their brilliancy in the zenith, and discover the beams of which they are composed when the interior motion is rapid. This interior motion is a sudden glow, not proceeding from any visible concentration of matter, but bursting out in several parts of the arch, as if an ignition of combustible matter had taken place, and spreading itself rapidly towards each extremity. In this motion the beams are formed, such as are described in the preceding observations upon the subject. They have two motions; one at right angles to their length or sideways; and the other a tremulous
and short vibration, in which they do not exactly preserve their parallelism to each other. By the first they project themselves into wreaths, serpentine forms, or irregular broken curves. The wreaths, when in the zenith, present the appearance of coronæ boreales. The second motion is always accompanied with colours; for it must be observed, that beams are often formed without any exhibition of colours; and I have not, in that case, perceived the vibratory motion. The beams, in different auroræ, and sometimes in the same, are of different magnitudes, arising, probably, from their various distances. These evolutions, often repeated, destroy the shape and coherence of the several arches, though they doubtless retain the arched appearance to the eye of a spectator at the southern horizon; for it would be absurd to suppose that these changes occur only in the zenith of one particular place. The observations at different places in 1820 afford satisfactory proof to the contrary; and the number of arches often increased or diminished in their advance to the zenith, by a dismemberment of which, from their distance, we could not distinguish the particulars. However, their several parts passed gradually to the southward, where they assumed the form of an arch. They are also sometimes distributed into flashes, and other detached portions, which pass to the south-eastward. The revelation of an arch from north to south occupies a space of time varying from twenty minutes to two hours. At Cumberland House, the arches were in many instances almost stationary for several hours, a proof that if their motion was not slower, their distance from the earth was greater, than at Fort Enterprise. The arches, which are faint at the horizon, very frequently pass the zenith without any increase of brilliancy or apparent internal motion.
The second general class of auroræ are those which propagate themselves from different points of the compass, between north and west towards the opposite points; sometimes also originating in the south-east quarter, and extending themselves towards the north-west. They may be subdivided, like the former, into the distant arches which pass to the southward without much visible change in their appearance, and those which discover beams, and separate at intervals into wreaths, flashes, and irregular segments, exhibiting all the phenomena described above. In explaining the mode by which the two general classes of auroræ are conducted into the horizon, I shall call the motion of the arches, which is in a plane seldom deviating more than two points from the magnetic meridian, the direct motion; and that by which the auroræ propagate themselves nearly at right angles to the magnetic meridian, the lateral motion. Let us suppose a mass of auroræ to be modelled at its birth in a longitudinal form, crossing the meridians at various angles, the whole to be impelled with a direct motion towards the magnetic south, but the parts having different velocities, and each extremity continually removing itself by a lateral motion from the centre, so as to increase the length of the mass. If the centre enter the northern horizon, it will appear like an arch, the real extremities being invisible; and its direct motion will carry it to the southward in that form. But if one extremity first enter the horizon, it will extend itself by its lateral motion to the opposite point, passing, at the same time, by its direct motion to the southward. Of the unequal velocities of the parts, we had many instances, in the direct motion of the arches, by which the centres were often carried from the zenith sixty or seventy degrees to the southward, while the extremities did not materially alter their positions. Nor can this be accounted for by any application of the rules of perspective, because arches exactly similar sometimes changed the bearings of their extremities in proportion to the advance of their
centres; and at Cumberland House, on two occasions, the extremities of arches arrived at the east and west points of the compass, while their centres remained only ten degrees above the northern horizon.
It was for a long time matter of doubt whether this meteor made its appearance only in the northern hemisphere, or whether it was also to be observed near the south pole. But the occurrence of the aurora in the antarctic regions was clearly ascertained by Mr Forster, who, in his voyage round the world with Captain Cook, observed it in the high southern latitudes, though with phenomena somewhat different from those which are seen here. On the 17th February 1773, when the navigators were in latitude 58° south, "a beautiful phenomenon was observed," says Mr Forster, "during the preceding night, which appeared again this and several following nights. It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spreading on the whole southern part of the sky. These columns were sometimes bent sideways at their upper extremities; and though in most respects similar to the northern lights (aurora borealis) of our hemisphere, yet differed from them in being always of a whitish colour, whereas ours assume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point." The accounts of subsequent navigators, particularly Weddell, though given more in detail, differ in no material respect from that just quoted, and therefore need not be cited at length. They prove that this splendid meteor is not confined to the northern, but occurs also in the southern regions of the globe, though with considerable diversity in the accompanying phenomena.
Colour.—The colours of the polar lights are of various tints. The rays or beams are steel gray, yellowish gray, pea-green, celandine green, gold yellow, violet blue, purple, sometimes rose red, crimson red, blood red, greenish red, orange red, and lake red. Some of the beams appear as if tinged with black, and resemble dense columns of smoke. The arches are sometimes nearly black, passing into violet blue, gray, gold yellow, or white bounded by an edge of yellow. The colours are also sometimes vivid and prismatic. Maupertuis describes a very remarkable red-coloured polar light which he saw at Oswe Zornea on the 18th December 1786. An extensive region of the heavens towards the south appeared tinged of so lively a red, that the whole constellation Orion seemed as if dyed in blood. The light was for some time fixed, but soon became movable; and after having successively assumed all the tints of violet and blue, it formed a dome, of which the summit approached the zenith in the south-west. Its splendour was so great as to be in no degree affected by the bright light of the moon. Maupertuis adds, that he observed only two of these northern lights in Lapland, which are of very rare occurrence in that country, although the aurora there assumes a great variety of tints; hence they are considered by the natives as of portentous omen, and as the forerunner of some great calamity. These red-coloured polar lights have of late years been observed in the Shetland Islands; in many parts of Scotland, as in the neighbourhood of Edinburgh; and in England, from its northern to its southern extremity.
Lustre of the Polar Lights.—The lustre varies in kind as well as intensity. Sometimes it is pearly, sometimes imperfectly vitreous, sometimes also metallic. Its degree of intensity varies from a very faint radiance to a light nearly equalling that of the moon.
Progressive Motions.—The reverend James Farquharson states that "the aurora borealis follows a determinate order