A Popular History of Astronomy During the Nineteenth Century Part 7

The corona, both as to figure and extent, presented very different appearances at different stations. This was no doubt due to varieties in atmospheric conditions. At the Superga, for instance, all details of structure seem to have been effaced by the murky air, only a comparatively feeble ring of light being seen to encircle the moon.

Elsewhere, a brilliant radiated formation was conspicuous, spreading at four opposite points into four vast luminous expansions, compared to feather-plumes or _aigrettes_.[157] Arago at Perpignan noticed considerable irregularities in the divergent rays. Some appeared curved and twisted, a few lay _across_ the others, in a direction almost tangential to the moon's limb, the general effect being described as that of a "hank of thread in disorder."[158] At Lipeszk, where the sun stood much higher above the horizon than in Italy or France, the corona showed with surprising splendour. Its apparent extent was judged by Struve to be no less than twenty-five minutes (more than six times Airy's estimate), while the great plumes spread their radiance to three or four degrees from the dark lunar edge. So dazzling was the light that many well-instructed persons denied the totality of the eclipse. Nor was the error without precedent, although the appearances attending respectively a total and an annular eclipse are in reality wholly dissimilar. In the latter case, the surviving ring of sunlight becomes so much enlarged by irradiation, that the interposed dark lunar body is reduced to comparative insignificance, or even invisibility. Maclaurin tells us[159] that during an eclipse of this character which he observed at Edinburgh in 1737, "gentlemen by no means shortsighted declared themselves unable to discern the moon upon the sun without the aid of a smoked gla.s.s;" and Baily (who, however, _was_ shortsighted) could distinguish, in 1836, with the naked eye, no trace of "the globe of purple velvet" which the telescope revealed as projected upon the face of the sun.[160] Moreover, the diminution of light is described by him as "little more than might be caused by a temporary cloud pa.s.sing over the sun"; the birds continued in full song, and "one c.o.c.k in particular was crowing with all his might while the annulus was forming."

Very different were the effects of the eclipse of 1842, as to which some interesting particulars were collected by Arago.[161] Beasts of burthen, he tells us, paused in their labour, and could by no amount of punishment be induced to move until the sun reappeared. Birds and beasts abandoned their food; linnets were found dead in their cages; even ants suspended their toil. Diligence-horses, on the other hand, seemed as insensible to the phenomenon as locomotives. The convolvulus and some other plants closed their leaves, but those of the mimosa remained open.

The little light that remained was of a livid hue. One observer described the general coloration as resembling the lees of wine, but human faces showed pale olive or greenish. We may, then, rest a.s.sured that none of the remarkable obscurations recorded in history were due to eclipses of the annular kind.

The existence of the corona is no modern discovery. Indeed, it is too conspicuous an apparition to escape notice from the least attentive or least practised observer of a total eclipse. Nevertheless, explicit references to it are rare in early times. Plutarch, however, speaks of a "certain splendour" compa.s.sing round the hidden edge of the sun, as a regular feature of total eclipses;[162] and the corona is expressly mentioned in a description of an eclipse visible at Corfu in 968 A.D.[163] The first to take the phenomenon into scientific consideration was Kepler. He showed, from the orbital positions at the time of the sun and moon, that an eclipse observed by Clavius at Rome in 1567 could not have been annular,[164] as the dazzling coronal radiance visible during the obscuration had caused it to be believed. Although he himself never witnessed a total eclipse of the sun, he carefully collected and compared the remarks of those more fortunate, and concluded that the ring of "flame-like splendour" seen on such occasions was caused by the reflection of the solar rays from matter condensed in the neighbourhood either of the sun or moon.[165] To the solar explanation he gave his own decided preference; but, with one of those curious flashes of half-prophetic insight characteristic of his genius, declared that "it should be laid by ready for use, not brought into immediate requisition."[166] So literally was his advice acted upon, that the theory, which we now know to be (broadly speaking) the correct one, only emerged from the repository of antic.i.p.ated truths after 236 years of almost complete retirement, and even then timorously and with hesitation.

The first eclipse of which the attendant phenomena were observed with tolerable exactness was that which was central in the South of France, May 12, 1706. Ca.s.sini then put forward the view that the "crown of pale light" seen round the lunar disc was caused by the illumination of the zodiacal light;[167] but it failed to receive the attention which, as a step in the right direction, it undoubtedly merited. Nine years later we meet with Halley's comments on a similar event, the first which had occurred in London since March 20, 1140. By nine in the morning of May 3, 1715, the obscuration, he tells us, "was about ten digits,[168] when the face and colour of the sky began to change from perfect serene azure blue to a more dusky livid colour, having an eye of purple intermixt....

A few seconds before the sun was all hid, there discovered itself round the moon a luminous ring, about a digit or perhaps a tenth part of the moon's diameter in breadth. It was of a pale whiteness, or rather pearl colour, seeming to be a little tinged with the colours of the iris, and to be concentric with the moon, whence I concluded it the moon's atmosphere. But the great height thereof, far exceeding our earth's atmosphere, and the observation of some, who found the breadth of the ring to increase on the west side of the moon as emersion approached, together with the contrary sentiments of those whose judgment I shall always revere" (Newton is most probably referred to), "makes me less confident, especially in a matter whereto I confess I gave not all the attention requisite." He concludes by declining to decide whether the "enlightened atmosphere," which the appearance "in all respects resembled," "belonged to sun or moon."[169]

A French Academician, who happened to be in London at the time, was less guarded in expressing an opinion. The Chevalier de Louville declared emphatically for the lunar atmospheric theory of the corona,[170] and his authority carried great weight. It was, however, much discredited by an observation made by Maraldi in 1724, to the effect that the luminous ring, instead of travelling _with_ the moon, was traversed _by_ it.[171]

This was in reality decisive, though, as usual, belief lagged far behind demonstration. In 1715 a novel explanation had been offered by Delisle and Lahire,[172] supported by experiments regarded at the time as perfectly satisfactory. The aureola round the eclipsed sun, they argued, is simply a result of the _diffraction_, or apparent bending of the sunbeams that graze the surface of the lunar globe--an effect of the same kind as the coloured fringes of shadows. And this view prevailed amongst men of science until (and even after) Brewster showed, with clear and simple decisiveness, that such an effect could by no possibility be appreciable at our distance from the moon.[173] Don Jose Joaquim de Ferrer, however, who observed a total eclipse of the sun at Kinderhook, in the State of New York, on June 16, 1806, ignoring this refined optical _rationale_, considered two alternative explanations of the phenomenon as alone possible. The bright ring round the moon must be due to the illumination either of a lunar or of a solar atmosphere. If the former, he calculated that it should have a height fifty times that of the earth's gaseous envelope. "Such an atmosphere," he rightly concluded, "cannot belong to the moon, but must without any doubt belong to the sun."[174] But he stood alone in this unhesitating a.s.sertion.

The importance of the problem was first brought fully home to astronomers by the eclipse of 1842. The brilliant and complex appearance which on that occasion challenged the attention of so many observers, demanded and received, no longer the casual attention hitherto bestowed upon it, but the most earnest study of those interested in the progress of science. Nevertheless, it was only by degrees, and through a process of "exclusions" (to use a Baconian phrase) that the corona was put in its right place as a solar appendage. As every other available explanation proved inadmissible and dropped out of sight, the broad presentation of fact remained, which, though of sufficiently obvious interpretation, was long and persistently misconstrued. Nor was it until 1869 that absolutely decisive evidence on the subject was forthcoming, as we shall see further on.

Sir John Herschel, writing to his venerable aunt, relates that when the brilliant red flames burst into view behind the dark moon on the morning of the 8th of July, 1842, the populace of Milan, with the usual inconsequence of a crowd, raised the shout, "_Es leben die Astronomen!_"[175] In reality, none were less prepared for their apparition than the cla.s.s to whom the applause due to the magnificent spectacle was thus adjudged. And in some measure through their own fault, for many partial hints and some distinct statements from earlier observers had given unheeded notice that some such phenomenon might be expected to attend a solar eclipse.

What we now call the "chromosphere" is an envelope of glowing gases, by which the sun is completely covered, and from which the "prominences"

are emanations, eruptive or flame-like. Now, continual indications of the presence of this fire-ocean had been detected during eclipses in the eighteenth and nineteenth centuries. Captain Stannyan, describing in a letter to Flamsteed an occurrence of the kind witnessed by him at Berne on May 1 (o.s.), 1706, says that the sun's "getting out of the eclipse was preceded by a blood-red streak of light from its left limb."[176] A precisely similar appearance was noted by both Halley and De Louville in 1715; during annular eclipses by Lord Aberdour in 1737,[177] and by Short in 1748,[178] the tint of the ruby border being, however, subdued to "brown" or "dusky red" by the surviving sunlight; while observations identical in character were made at Amsterdam in 1820,[179] at Edinburgh by Henderson in 1836, and at New York in 1838.[180]

"Flames" or "prominences," if more conspicuous, are less constant in their presence than the glowing stratum from which they spring. The first to describe them was a Swedish professor named Va.s.senius, who observed a total eclipse at Gothenburg, May 2 (o.s.), 1733.[181] His astonishment equalled his admiration when he perceived, just outside the edge of the lunar disc, and suspended, as it seemed, in the coronal atmosphere, three or four reddish spots or clouds, one of which was so large as to be detected with the naked eye. As to their nature, he did not even offer a speculation, further than by tacitly referring them to the moon. The observation was repeated in 1778 by a Spanish Admiral, but with no better success in directing efficacious attention to the phenomenon. Don Antonio Ulloa was on board his s.h.i.+p the _Espagne_ in pa.s.sage from the Azores to Cape St. Vincent on the 24th of June in that year, when a total eclipse of the sun occurred, of which he has left a valuable description. His notices of the corona are full of interest; but what just now concerns us is the appearance of "a red luminous point" "near the edge of the moon," which gradually increased in size as the moon moved away from it, and was visible during about a minute and a quarter.[182] He was satisfied that it belonged to the sun because of its fiery colour and growth in magnitude, and supposed that it was occasioned by some crevice or inequality in the moon's limb, through which the solar light penetrated.

Allusions less precise, both prior and subsequent, which it is now easy to refer to similar objects (such as the "slender columns of smoke" seen by Ferrer)[183] might be detailed; but the evidence already adduced suffices to show that the prominences viewed with such amazement in 1842 were no unprecedented or even unusual phenomenon.

It was more important, however, to decide what was their nature than whether their appearance might have been antic.i.p.ated. They were generally, and not very incorrectly, set down as solar clouds. Arago believed them to s.h.i.+ne by reflected light,[184] but the Abbe Peytal rightly considered them to be self-luminous. Writing in a Montpellier paper of July 16, 1842, he declared that we had now become a.s.sured of the existence of a third or outer solar envelope, composed of a glowing substance of a bright rose tint, forming mountains of prodigious elevation, a.n.a.logous in character to the clouds piled above our horizons.[185] This first distinct recognition of a very important feature of our great luminary was probably founded on an observation made by Berard at Toulon during the then recent eclipse, "of a very fine red band, irregularly dentelated, or, as it were, creva.s.sed here and there,"[186] encircling a large arc of the moon's circ.u.mference. It can hardly, however, be said to have attracted general notice until July 28, 1851. On that day a total eclipse took place, which was observed with considerable success in various parts of Sweden and Norway by a number of English astronomers. Mr. Hind saw, on the south limb of the moon, "a long range of rose-coloured flames,"[187] described by Dawes as "a low ridge of red prominences, resembling in outline the tops of a very irregular range of hills."[188] Airy termed the portion of this "rugged lines of projections" visible to him the _sierra_, and was struck with its brilliant light and "nearly scarlet" colour.[189] Its true character of a continuous solar envelope was inferred from these data by Grant, Swan, and Littrow, and was by Father Secchi, after the great eclipse of 1860,[190] formally accepted as established.

Several prominences of remarkable forms, especially one variously compared to a Turkish scimitar, a sickle, and a boomerang, were seen in 1851. In connection with them two highly significant circ.u.mstances were pointed out. First, that of the approximate coincidence between their positions and those of sun-spots previously observed.[191] Next, that "the moon pa.s.sed over them, leaving them behind, and revealing successive portions as she advanced."[192] This latter perfectly well-attested fact was justly considered by the Astronomer Royal and others as affording absolute certainty of the solar dependence of these singular objects. Nevertheless sceptics were still found. M. Faye, of the French Academy, inclined to a lunar origin for them;[193] Feilitsch of Greifswald published in 1852 a treatise for the express purpose of proving all the luminous phenomena attendant on solar eclipses--corona, prominences and "sierra"--to be purely optical appearances.[194]

Happily, however, the unanswerable arguments of the photographic camera were soon to be made available against such hardy incredulity.

Thus, the virtual discovery of the solar appendages, both coronal and chromospheric, may be said to have been begun in 1842, and completed in 1851. The current Herschelian theory of the solar const.i.tution remained, however, for the time, intact. Difficulties, indeed, were thickening around it; but their discussion was perhaps felt to be premature, and they were permitted to acc.u.mulate without debate, until fortified by fresh testimony into unexpected and overwhelming preponderance.

FOOTNOTES:

[Footnote 131: Kosmos, Bd. iii., p. 409; Lalande, _Bibliographie Astronomique_, pp. 179, 202.]

[Footnote 132: R. Wolf, _Die Sonne und ihre Flecken_, p. 9.

Marius himself, however, seems to have held the Aristotelian terrestrial-exhalation theory of cometary origin. See his curious little tract, _Astronomische und Astrologische Beschreibung der Cometen_, Nurnberg, 1619.]

[Footnote 133: _Phil. Trans._, vol. xxvii., p. 274. _Umbrae_ (now called _penumbrae_) are s.p.a.ces of half-shadow which usually encircle spots.

_Faculae_ ("little torches," so named by Scheiner) are bright streaks or patches closely a.s.sociated with spots.]

[Footnote 134: _Mem. Ac. Sc._, 1776 (pub. 1779), p. 507. D. Ca.s.sini, however, first put forward about 1671 the hypothesis alluded to in the text. See Delambre, _Hist. de l'Astr. Mod._, t. ii., p. 694; and _Kosmos_, Bd. iii., p. 410.]

[Footnote 135: _Phil. Trans._, vol. lxiv., part i., pp. 7-11.]

[Footnote 136: _Rosa Ursina_, lib. iv., p. 507.]

[Footnote 137: R. Wolf, _Die Sonne und ihre Flecken_, p. 12.]

[Footnote 138: Sch.e.l.len, _Die Spectrala.n.a.lyse_, Bd. ii., p. 56 (3rd ed.).]

[Footnote 139: _Phil. Trans._, vol. lxiv., p. 20.]

[Footnote 140: _Ibid._, vol. lx.x.xv., 1795, p. 63.]

[Footnote 141: _Phil. Trans._, vol. xci., 1801, p. 303.]

[Footnote 142: The supposed opaque or protective stratum beneath the photosphere was named by him "planetary," from the a.n.a.logy of terrestrial clouds.]

[Footnote 143: _Ibid._, p. 305.]

[Footnote 144: _Novum Organum_, lib. ii. aph. 20.]

[Footnote 145: Brewster's _Life of Newton_, vol. ii., p. 103.]

[Footnote 146: _Beschaftigungen d. Berl. Ges. Naturforschender Freunde_, Bd. ii., p. 233.]

[Footnote 147: _Gentleman's Magazine_, 1787, vol. ii., p. 636.]

[Footnote 148: _Results_, etc., p. 432.]

[Footnote 149: _Ibid._, p. 434.]

[Footnote 150: See _ante_, p. 31.]

[Footnote 151: _Memoir of Francis Baily, Mem. R. A. S._, vol. xv., p.

524.]

[Footnote 152: _Mem. R. A. S._, vol. x., pp. 5-6.]

[Footnote 153: _Ibid._, pp. 14-17.]

[Footnote 154: _Mem. R. A. S._, vol. xv., pp. 4-6.]

[Footnote 155: _Ibid._, p. 16.]

[Footnote 156: _Annuaire_, 1846, p. 409.]

[Footnote 157: _Ibid._, p. 317.]

[Footnote 158: _Ibid._, p. 322.]

[Footnote 159: _Phil. Trans._, vol. xl., p. 192.]

[Footnote 160: _Mem. R. A. S._, vol. x., p. 17.]

[Footnote 161: _Ann. du Bureau des Long._, 1846, p. 309.]