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AURORA POLARIS ( Aurora Borealis and Australis, Polar Light, Northern Lights), a natural phenomenon which occurs in many forms, some of great beauty.

1. Forms. - Various schemes of classification have been proposed, but none has met with universal acceptance; the following are at least the principal types. (1) Arcs. These most commonly resemble segments of circles, but are not infrequently elliptical or irregular in outline. The ends of arcs frequently extend to the horizon, but often one or both ends stop short of this. Several arcs may be visible at the same time. Usually the under or concave edge of the arc is the more clearly defined, and adjacent to it the sky often seems darker than elsewhere. It is rather a disputed point whether this dark segment - through which starlight has been seen to passrepresen is a real atmospheric condition or is merely a contrast effect. (2) Bands. These may be nearly straight and regular in outline, as if broken portions of arcs; frequently they are ribbon-like serpentine forms showing numerous sinuosities. (3) Rays. Frequently an arc or band is visibly composed of innumerable short rays separated by distinctly less luminous intervals. These rays are more or less perpendicular to the arc or band; sometimes they are very approximately parallel to one another, on other occasions they converge towards a paint. Longer rays often show an independent existence. Not infrequently rays extend from the upper edge of an arc towards the zenith. Combinations of rays sometimes resemble a luminous fan, or a series of fans, or part of a hollow luminous cylinder. Rays often alter suddenly in length, seeming to stretch down towards the horizon or mount towards the zenith. This accounts for the description of aurora as " Merry Dancers." (4) Curtains or Draperies. This form is rare except in Arctic regions, where it is sometimes fairly frequent. It is one of the most imposing forms. As a rule the higher portion is visibly made up of rays, the light tending to become more continuous towards the lower edge; the combination suggests a connected whole, like a curtain whose alternate portions are in light and shade. The curtain often shows several conspicuous folds, and the lower edge often resembles frilled drapery. At several stations in Greenland auroral curtains have been observed when passing right overhead to narrow to a thin luminous streak, exactly as a vertical sheet of light would seem to do to one passing underneath it. ( 5) Corona. A fully developed corona is perhaps the finest form of aurora. As the name implies, there is a sort of crown of light surrounding a comparatively or wholly dark centre. Farther from the centre the ray structure is usually prominent. The rays may lie very close together, or may be widely separated from one another. (6) Patches. During some displays, auroral light appears in irregular areas or patches, which sometimes bear a very close resemblance to illuminated detached clouds. ( 7) Diffused Aurora. Sometimes a large part of the sky shows a diffuse illumination, which, though brighter in some parts than others, possesses no definite outlines. How far the different forms indicate real difference in the nature of the phenomenon, and how far they are determined by the position of the observer, it is difficult to say. Not infrequently several different forms are visible at the same time.

2. Isochasms. - Aurora is seldom observed in low latitudes. In the southern hemisphere there is comparatively little inhabited land in high latitudes and observational data are few; thus little is known as to how the frequency varies with latitude and longitude. Even in the northern hemisphere there are large areas in the Arctic about which little is known. H. Fritz (2) has, however, drawn a series of curves which are believed to give a good general idea of the relative frequency of aurora throughout the northern hemisphere. Fritz' curves, shown in the illustration, are termed isochasms, from the Greek word employed by Aristotle to denote aurora. Points on the same curve are supposed to have the same average number of auroras in the year, and this average number is shown adjacent to the curve. Starting from the equator and travelling northwards we find in the extreme south of Spain an average of only one aurora in ten years. In the north of France the average rises to five a year; in the north of Ireland to thirty a year; a little to the north of the Shetlands to one hundred a year. Between the Shetlands and Iceland we cross the curve of maximum frequency, and farther north the frequency diminishes. The curve of maximum frequency forms a slightly irregular oval, whose centre, the auroral pole, is according to Fritz at about 81° N. lat., 70° W. long. Isochasms reach a good deal farther south in America than in Europe. In other words, auroras are much more numerous in the southern parts of Canada and in the United States than in the same latitudes of Europe.

3. Annual Variation

Table I. shows the annual variation observed in the frequency of aurora. It has been compiled from several authorities, especially Joseph Lovering (4) and Sophus Tromholt (5). The monthly figures denote the percentages of the total number seen in the year. The stations are arranged in order of latitude. Individual places are first considered, then a few large areas.

The Godthaab data in Table I. are essentially those given by Prof. A. Paulsen (6) as observed by Kleinschmidt in the winters of 1865 to 1882, supplemented by Lovering's data for summer. Starting at the extreme north, we have a simple period with a well-marked maximum at midwinter, and no auroras during several months at midsummer. This applies to Hammerfest, Jakobshavn, Godthaab and the most northern division of Scandinavia. The next division of Scandinavia shows a transition stage. To the south of this in Europe the single maximum at mid-winter is replaced by two maxima, somewhere about the equinoxes.

4. In considering what is the real significance of the great difference apparent in Table I. between higher and middle latitudes, a primary consideration is that aurora is seldom seen until the sun is some degrees below the horizon. There is no reason to suppose that the physical causes whose effects we see as aurora are in existence only when aurora is visible. Until means are devised for detecting aurora during bright sunshine, our knowledge as to the hour at which these causes are most frequently or most powerfully in operation must remain incomplete. But it can hardly be doubted that the differences apparent in Table I. are largely due to the influence of sunlight. In high latitudes for several months in summer it is never dark, and consequently a total absence of visible aurora is practically inevitable. Some idea of this influence can be derived from figures obtained by the Swedish International Expedition of 1882-1883 at Cape Thorsden, Spitsbergen, lat. 78° 28' N. (7). The original gives the relative frequency of aurora for each degree of depression of the sun below the horizon, assuming the effect of twilight to be nil (i.e. the relative frequency to be loo) when the depression is 18 5° or more. The following are a selection of the figures: - Angle of depression.. 4 5° 5° 15.5°

Relative frequency.. 0393 44 9 74 5 95.9

These figures are not wholly free from uncertainties, arising from true diurnal and annual variations in the frequency, but they give a good general idea of the influence of twilight.

If sunlight and twilight were the sole cause of the apparent annual variation, the frequency would have a simple period, with a maximum at midwinter and a minimum at midsummer. This is what is actually shown by the most northern stations and districts in Table I. When we come, however, below 65° lat. in Europe the frequency near the equinoxes rises above that at midwinter, and we have a distinct double period, with a principal minimum at midsummer and a secondary minimum at midwinter. In southern Europe - where, however, auroras are too few to give smooth results in a limited number of years - in southern Canada, and in the United States, the difference between the winter and summer months is much reduced. Whether there is any real difference between high and mean latitudes in the annual frequency of the causes rendered visible by aurora, it is difficult to say. The Scandinavian data, from the wealth of observations, are probably the most representative, and even in the most northern district of Scandinavia the smallness of the excess of the frequencies in December and January over those in March and October suggests that some influence tending to create maxima at the equinoxes has largely counterbalanced the influence of sunlight and twilight in reducing the frequency at these seasons.

5. Fourier Analysis

With a view to more minute examination, the annual frequency can be expressed in Fourier series, whose terms represent waves, whose periods are 12, 6, 4, 3, &c. months. This has been done by Lovering (4) for thirty-five stations. The nature of the results will best be explained by reference to the formula given by Lovering as a mean from all the stations considered, viz.: - 33+3 3 sin (30t+100°52') +2 53 sin (60t+309°5') +o 16 sin (90t+213°31') +o

56 sin (120t+162°45') +0 27 sin (150t+32°38'). The total number of auroras in the year is taken as 100, and t denotes the time, in months, that has elapsed since the middle of January.

FIG. I. - TWO Types Of Auroral Arcs.

FIG. 2. - TWO Types Of Auroral Rays.

PLATE 11.

(From the Internationale Polarforschung, 1882-1883, by permission of the Kaiserlichen A kademie der Wissenschaften, Vienna.) FIG. 3.-Auroral Bands. FIG. 4 .-Auroral Curtain Below An Arc. FIG, 5.-Auroral Corona. Place. Latitude. Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. Hammerfest.. . Jakobshavn.. . Godthaab. .. . St Petersburg.. . ° 701 69 64 60 20.9 14.6 15.5 6.5 17.6 13.0 12 '4 9.1 8.8 9.2 9'7 16.8 0 0.5 4.9 13'8 0 0 0 3.5 0 0 0 1.2 0 0 0 1.4 0 0 1.2 5.9 4.4 9.2 8.7 1 3.8 9.9 15.1 13.3 1 3' 1 17.6 18.4 17.0 7.6 20'9 20.0 17.4 7'3 Christiania . 60 8.6 11.4 14.0 II.2 o.6 0 0.2 6.5 14.6 12.2 10 3 10.3 Upsala 60 8.4 12.9 1 4'9 7.4 0.7 0.2 . 0.4 7.1 12.4 14.3 10.7 10'7 Stockholm.. . 59 7'9 Io o 14.7 16.4 3.8 0.0 0.0 5.6 12.9 11.4 Io o 7.3 Edinburgh. .. . 56 9.5 12.6 14 o 9.5 3.4 0.0 1.7 6 o 12.6 13.5 II. 8 5.2 Berlin. ... . 522 7.0 10 8 16.4 15.5 11.4 o 6 2.9 2.9 6.5 13.2 8.5 4.1 London. .. . 511 8.6 10.5 to 2 10 7 4.0 I I . 1.9 5.6 14.5 16.9 9.6 6.4 Quebec 47 3'6 14.8 8.3 14.2 4'1 5.9 7.7 5.9 11.2 12.4 7'7 4.1 Toronto. . 432 5.4 9.5 8.7 11.8 9.o 6.2 8 o 6.4 8.5 11.1 8.7 6.7 Cambridge, Mass. 422 5.1 8.2 II. 8 10-2 6.4 5.1 10.3 8-5 13.3 9.2 6.8 5.1 New Haven, Conn. . 412 7.7 7.3 8.9 8.2 7.6 5.7 8.9 8'1 11.9 7.6 Io 6 7.5 Scandinavia.. . N. of 682° 16.4 13.8 14.8 1.6 0.0 0.0 0.0 0.4 7.8 15.1 1 4.4 15'7 „ 682° to 65° 15.3 14.6 13.7 2.9 0.0 0.0 0.0 I I 9.7 14.6 14.0 14.1 65° to 612° 13.2 12.3 14.5 5.4 0.2 0.0 0.0 2.8 13.1 14.2 12.8 11.5 612° to 58° 9.5 II 2 13.5 10.9 I 3 0.1 0.4 5.7 13.6 13.8 10.4 9.6 S. of 58° 8.2 11.9 12.6 13.3 1.5 0.1 o 6 4.9 14.9 13.5 Io 3 8.2 Y „ ork ork State. . 45° to 4 0 2° 6 '3 7.4 9.t It. o 7.4 6.6 8.8 10.4 II. 7 9.7 6.2 5'4 [[Table I]].-Annual Frequency (Relative). Putting t=o, 1, &c., in succession, we get the percentages of the total number of auroras which occur in January, February, and so on. The first periodic term has a period of twelve, the second of six months, and similarly for the others. The first periodic term is largest when t X 30° -}- loo° 52' = 450°. This makes t=11.6 months after the middle of January, otherwise the 3rd of January, approximately. The 6-month term has the earliest of its two equal maxima about the 26th of March. These two are much the most important of the periodic terms. The angles loo° 52', 309° 5', &c.,are known as the phase angles of the respective periodic terms, while 3.0 3, 2 '53, &c., are the corresponding amplitudes. Table II. gives a selection of Lovering's results. The stations are arranged according to latitude. TABLE II. Speaking generally, the annual term diminishes in importance as we travel south. North of 55° in Europe its phase angle seems fairly constant, not differing very much from the value 110° in Lovering's general formula. The 6-month term is small, in the two most northern stations, but south of 60° N. lat. it is on the whole the most important term. Excluding Jakobshavn, the phase angles in the 6-month term vary wonderfully little, and approach the value 309° in Lovering's general formula. North of lat. 50° the 4-month term is, as a rule, comparatively unimportant, but in the American stations its relative importance is increased. The phase angle, however, varies so much as to suggest that the term mainly represents local causes or observational uncertainties. Lovering's general formula suggests that the 4-month term is really less important than the 3-month term, but he gives no data for the latter at individual stations. 6. Sunlight is not the only disturbing cause in estimates of auroral frequency. An idea of the disturbing influence of cloud may be derived from some interesting results from the Cape Thorsden (7) observations. These show how the frequency of visible auroras diminished as cloud increased from o (sky quite clear) to 10 (sky wholly overcast). Grouping the results, we have: Amount of cloud. .. o I to 3 4 to 6 7 to 9 10 Relative frequency.. loo 82 57 46 8 Out of a total of 1714 hours during which the sky was wholly overcast the Swedish expedition saw auroras on 17, occurring on 14 separate days, whereas 226 hours of aurora would have occurred out of an equal number of hours with the sky quite clear. The figures being based on only one season's observations are somewhat irregular. Smoothing them, Carlheim-Gyllenskold gives f = too' -7.3c as the most probable linear relation between c, the amount of cloud, and f, the frequency, assuming the latter to be loo when there is no cloud. 7. Diurnal Variation.-The apparent daily period at most stations is largely determined by the influence of daylight on the visibility. It is only during winter and in high latitudes that we can hope to ascertain anything directly as to the real diurnal variation of the causes whose influence is visible at night as aurora. Table III. gives particulars of the number of occasions when aurora was seen at each hour of the twenty-four during three expeditions in high latitudes when a special outlook was kept. The data under A refer to Cape Thorsden (78° 28' N. lat., 15° 42' E. long.); those under B to Jan Mayen (8) (71° o' N. lat., 8° 28' W. long.), both for the winter of 1882-1883. The data under C are given by H. Arctowski (9) for the "Belgica" Expedition in 1898. They may be regarded as applying approximately to the mean position of the " Belgica," or 701-° S.lat., 862° W. long. The method of counting frequencies was fairly alike, at least in the case of A and B, but in comparing the different stations the data should be regarded as relative rather than absolute. The Jan Mayen data refer really to Göttingen mean time, but this was only twenty-three minutes late on local time. In calculating the percentages of forenoon and afternoon occurrences half the entries under noon and midnight were assigned to each half of the day. Even at Cape Thorsden, the sun at midwinter is only I I° below the horizon at noon, and its effect on the visibility is thus not wholly negligible. The influence of daylight is presumably the principal cause of the difference between the phenomena during November, December and January at Cape Thorsden and Jan Mayen, for in the equinoctial months the results from these two stations are closely similar. Whilst daylight is the principal cause of the diurnal inequality, it is not the only cause, otherwise there would be as many auroras in the morning (forenoon) as in the evening (afternoon). The number seen in the evening is, however, according to Table III., considerably in excess at all seasons. Taking the whole winter, the percentage seen in the evening was the same for the " Belgica " as for Jan Mayen, i.e. for practically the same latitudes South and North. At Cape Thorsden from November to January there seems a distinct double period, with minima near noon and midnight. The other months at Cape Thorsden show a single maximum and minimum, the former before midnight. Station. Annual Term. 6-Month Term. 4-Month Term. Amp. Phase. Amp. Phase. Amp. Phase. ° ° ° Jakobshavn. .. . 10.40 123 1.13 206 1.41 333 Godthaab. .. .. . 8.21 III 1.54 316 0.64 335 St Petersburg. ... . 2.81 96 5'99 309 0.57 208 Christiania. .. .. . 4.83 116 4'99 317 0.76 189 Upsala. .. ... . 5.41 119 4.57 322 0.86 296 Stockholm. .. .. . 3.68 91 5.80 303 I 31 180 Makerstown (Scotland) 5'79 102 4'47 310 2.00 342 Great Britain. .. . 3.87 126 4.24 287 0.40 73 Toronto. ... . 0.18 12 2.13 260 0.52 305 Cambridge, Mass. . I 02 262 2.84 339 1'28 253 New Haven, Conn. ' 0.99 183 1.02 313 0.57 197 New York State.. .. . 1 34 264 2.29 325 0.54 157 The same phenomenon appears at Jan Mayen especially in November, December and January, and it is the normal state of matters in temperate latitudes, where the frequency is usually greatest between 8 and io P.M. An excess of evening over morning occurrences is also the rule, and it is not infrequently more pronounced than in Table III. Thus at Tasiusak (6 5° 37' N. lat., 37° 33' W. long.) the Danish Arctic Expedition (10) of 1904 found seventy-five out of every hundred occurrences to take place before midnight. TABLE III. - Diurnal Variation. 8. The preceding remarks relate to auroras as a whole; the different forms differ considerably in their diurnal variation. Arcs, bands and, generally speaking, the more regular and persistent forms, show their greatest frequencies earlier in the night than rays or patches. Table IV. shows the percentages of e. (evening) and m. (morning) occurrences of the principal forms as recorded by the Arctic observers at Cape Thorsden, Jan Mayen and Tasiusak. TABLE IV. Arcs. Bands. Rays. Patches. e. m. e. m. e. m. e. m. Cape Thorsden . 76 24 66 34 5 2 4 8 5 1 49 Jan Mayen . 78 22 68 32 60 40 60 40 Tasiusak. . 85 15 85 15 6 5 35 62 38 At Cape Thorsden diffused auroral light had percentages e. 65, m. 35, practically identical with those for bands. At Tasiusak, 8 P.M. was the hour of most frequent occurrence for arcs and bands, whereas patches had their maximum frequency at I I P.M. and rays at midnight. 9. Lunar and other Periods. - The action of moonlight necessarily gives rise to a true lunar period in the visibility of aurora. The extent to which it renders aurora invisible depends, however, so much on the natural brightness of the aurora - which depends on the time and the place - and on the sharpness of the outlook kept, that it is difficult to gauge it. Ekholm and Arrhenius(11) claim to have established the existence of a true tropical lunar period of 27.32 days, and also of a 26-day period, or, as they make it, a 25.9 2 9 -day period. A 26-day period has also been derived by J. Liznar (12), after an elaborate allowance for the disturbing effects of moonlight from the observations in 1882-1883 at Bossekop, Fort Rae and Jan Mayen. Neither of these periods is universally conceded. The connexion between aurora and earth magnetic disturbances renders it practically certain that if a 26-day or similar period exists in the one phenomenon it exists also in the other, and of the two terrestrial magnetism is probably the element least affected by external complications, such as the action of moonlight. 10. Sun-spot Connexion. - The frequency of auroral displays is much greater in some years than others. At most places the variation in the frequency has shown a general similarity to that of sun-spots. Table V. gives contemporaneous data for the frequency of sun-spots and of auroras seen in Scandinavia. The sun-spot data prior to 1902 are from A. Wolfer's table in the Met. Zeitschrift for 1902, p. 1 95; the more recent data are from his quarterly lists. All are observed frequencies, derived after Wolf's method; maxima and minima are in heavy type. The auroral data are from Table E of Tromholt's catalogue (5), with certain modifications. In Tromholt's yearly data the year commences with July. This being inconvenient for comparison with sun-spots, use was made of his monthly values to obtain corresponding data for years commencing with January. The Tromholt-Schroeter data for Scandinavia as a whole commenced with 1761; the figures for earlier years were obtained by multiplying the data for Sweden by 1.356, the factor being derived by comparing the figures for Sweden alone and for the whole of Scandinavia from July 1761 to June 1783. In a general way Table V. warrants the conclusion that years of many sun-spots are years of many auroras, and years of few sun-spots years of few auroras; but it does not disclose any very definite relationship between the two frequencies. The maxima and minima in the two phenomena in a good many cases are not found in the same years. On the other hand, there is absolute coincidence in a number of cases, some of them very striking, as for instance the remarkably low minima of 1810 and 1823. Hour. H Dec. Nov. and Jan. Feb., March, Sept. and Oct. Sept. to March (N. Lat.). March to Sept. (S. Lat.). A B A B A B A B C I 14 7 14 8 27 23 55 38 24 2 10 6 15 6 20 25 45 37 23 3 9 4 15 5 15 21 39 30 10 4 10 5 21 7 14 18 45 30 4 5 13 5 20 3 to 10 43 18 2 6 I I 3 15 4 2 3 28 10 I 7 9 2 13 3 I 2 23 7 0 8 5 I 6 I 0 0 II 2 O 9 7 2 9 0 0 0 16 2 0 10 10 0 5 0 0 0 15 0 0 I I 9 0 6 o o o 15 0 0 Noon io o 4 0 0 0 14 0 0 I 10 0 6 o 0 0 16 0 0 2 14 0 10 0 0 0 24 0 0 3 18 I 20 3 0 0 38 4 0 4 16 7 19 7 I I 36 15 0 5 12 II 22 10 5 2 39 23 3 6 14 10 21 16 8 5 43 31 3 7 16 13 23 16 20 9 59 38 14 8 15 12 22 18 24 24 61 54 25 9 14 15 18 17 27 28 59 60 31 10 12 15 19 15 31 25 62 55 29 II IO 12 18 17 33 26 61 55 26 Midnight 9 9 13 II 28 22 50 42 26 Totals . 277 140 354 167 266 244 897 551 221 Percentages Forenoon 42 28 42 25 39 46 41 35 35 Afternoon 58 72 58 75 61 54 59 65 65 Years of Y 1764-1872 . 1764-1817 . 1818-1872 . Spots. Auroras. Spots. Auroras. Spots. Auroras. Many sun-spots. Few „ 93.4 13.4 99.9 61.5 86.7 13.6 70.7 51.6 100.1 13.1 129.1 71.3 I I. During the period 1764 to 1872 there have been ten years of maximum, and ten of minimum, in sun-spot frequency. Taking the three years of greatest frequency at each maximum, and the three years of least frequency at each minimum, we get thirty years of many and thirty of few sun-spots. Also we can split the period into an earlier half, 1764 to 1817, and a later half, 1818 to 1872, containing respectively the earlier five and the later five of the above groups of sun-spot maximum and minimum years. The annual means derived from the whole group, and the two sub-groups, of years of many and few sun-spots are as follows: In each case the excess of auroras in the group of years of many sun-spots is decided, but the results from the two sub-periods do not harmonize closely. The mean sun-spot frequency for the group of years of few sun-spots is almost exactly the same for the two subperiods, but the auroral frequency for the later group is nearly 40% in excess of that for the earlier, and even exceeds the auroral frequency in the years of many sun-spots in the earlier sub-period. This inconsistency, though startling at first sight, is probably more apparent than real. It is almost certainly due in large measure to a progressive change in one or both of the units of frequency. In the case of sun-spots, A. Schuster (13 ) has compared J. R. Wolf and A. Wolfer's frequencies with data obtained by other observers for areas of sun-spots, and his figures show unquestionably that the unit in one or other set of data must have varied appreciably from time to time. Wolf and Wolfer have, however, aimed persistently at securing a definite standard, and there are several reasons for believing that the change of unit has been in the auroral rather than the sun-spot frequency. R. Rubenson (14), from whom Tromholt derives his data for Sweden, seems to accept this view, assigning the apparent increase in auroral frequency since 1860 to the institution by the state of meteorological stations in 1859, and to the increased interest taken in the subject since 1865 by the university of Upsala. The figures themselves in Table,V. certainly point to this conclusion, unless we are prepared to believe that auroras have increased enormously in number. If, for instance, we compare the first and the last three I I-year cycles for which Table V. gives complete data, we obtain as yearly means 1 7491 7 81.. Sun-spots 56.4 Auroras 77.5 1844-1876.. „ 55.8 „ I12.2 The mean sun-spot frequencies in the two periods differ by only I %, but the auroral frequency in the later period is 45% in excess of that in the earlier. 33- Year Period. Sun-spots. Auroras. 1 7 61 - 1 793 65'6 76., 1 794- 1826 20.3 39.5 1827-1859 56.1 84'4 3-Year Period. 3 Total Sun-spot Frequency. Total Nights of Aurora. 1865-1868 48 274 1869-1872 339 138 1876-1879 23 273 The above figures would be almost conclusive if it were not for the conspicuous differences that exist between the mean sun-spot frequencies for different II-year periods. Schuster, who has considered the matter very fully, has found evidence of the existence of other periods-notably 8.4 and 4.8 years-in addition to the recognized period of 11.125 years, and he regards the difference between the maxima in successive II-year periods as due at least partly to an overlapping of maxima from the several periodic terms. This cannot, however, account for all the fluctuations observed in sun-spot frequencies, unless other considerably longer periods exist. There has been at least one 33-year period during which the mean value of sun-spot frequency has been exceptionally low, and, as we shall see, there was a corresponding remarkable scarcity of auroras. The period in question may be regarded as extending from 1794 to 1826 inclusive. Comparing it with the two adjacent periods of thirty-three years, we obtain the following for the mean annual frequencies: 12. The association of high auroral and sun-spot frequencies shown in Table V. is not peculiar to Scandinavia. It is shown, for instance, in Loomis's auroral data, which are based on observations at a variety of European and American stations (Ency. Brit. 9th ed. art. Meteorology, Table XXVIII.). It does not seem, however, to apply universally. Thus at Godthaab we have, according to Adam Paulsen (15), comparing 3-year periods of few and many sun-spots: The years start in the autumn, and 1865-1868 includes the three winters of 1865 to '66, '66 to '67, and '67 to '68. Paulsen also gives data from two other stations in Greenland, viz. Ivigtut (1869 to 1879) and Jakobshavn (1873 to 1879), which show the same phenomenon as at Godthaab in a prominent fashion. Greenland lies to the north of Fritz's curve of maximum auroral frequency, and the suggestion has been made that the zone of maximum frequency expands to the south as sun-spots increase, and contracts again as they diminish, the number of auroras at a given station increasing or diminishing as the zone of maximum frequency approaches to or recedes from it. This theory, however, does not seem to fit all the facts and stands in want of confirmation. 13. Auroral Meridian.-It is a common belief that the summit of an auroral arc is to be looked for in the observer's magnetic meridian. On any theory it would be rather extraordinary if this were invariably true. In temperate latitudes auroral arcs are seldom near the zenith, and there is reason to believe them at very great heights. In high latitudes the average height is probably less, but the direction in which the magnetic needle TABLE V. Copyright Statement These files are public domain. Bibliography Information Chisholm, Hugh, General Editor. Entry for 'Aurora Polaris'. 1911 Encyclopedia Britanica. https://www.studylight.org/​encyclopedias/​eng/​bri/​a/aurora-polaris.html. 1910. terms of use • privacy policy • rights and permissions • contact sl • about sl • link to sl To report dead links, typos, or html errors or suggestions about making these resources more useful use the convenient contact form StudyLight.org © 2001-2022 Powered by Lightspeed Technology Ads FreeProfile Year. Frequency. Year. Frequency. Year. Frequency. Year. Frequency. Sun-spot. Auroral. Sun-spot. Auroral. Sun-spot. Auroral. Sun-spot. Auroral. 1749 80'9 103 1789 i18 1 89 1829 67'0 93 1869 73'9 160 1750 83.4 134 1790 89.9 90 1830 71.0 132 1870 139.1 195 1 75 1 47.7 53 1791 66.6 54 1831 47.8 89 1871 111.2 185 1 75 2 47'8 III 1792 60.o 64 1832 27.5 54 1872 101.7 200 1 753 30.7 96 1793 46'9 29 1833 8.5 79 1873 66'3 189 1754 12.2 65 1794 41'0 37 1834 13.2 81 1874 44'7 158 1 755 9.6 34 1795 21.3 34 1835 56.9 58 1875 17.1 133 1756 10 2 60 1796 16.0 37 1836 121.5 98 1876 11.3 137 1 757 32'4 83 1797 6.4 61 1837 138.3 137 1877 12.3 126 1 75 8 47.6 80 1798 4.1 35 1838 103.2 159 1878 3.4 .. 1 759 54'0 113 1799 6.8 28 1839 85.8 165 1879 6 o 1760 62.9 86 1800 14.5 30 1840 63.2 82 1880 32.3 1761 85.9 124 1801 34.0 34 1841 36.8 75 1881 54'3 1762 61.2 114 1802 45.0 65 1842 242 91 1882 59'7 1763 45'1 89 1803 43.1 73 1843 10.7 66 1883 63.7 1764 36.4 107 1804 47.5 ioi 1844 15.0 81 1884 63.5 1765 20.9 76 1805 42.2 85 1845 40-I 26 1885 52.2 1766 11.4 51 1806 28.1 62 1846 61.5 50 1886 25'4