Solar corona

“ That pearly light all round the sun, and the long streamers that give it the appearance of an enormous star with six long points, form what is termed the solar corona, and this can only be seen from our earth during the very few minutes when an eclipse of the sun is at its totality. It is to see the corona and other surroundings of the sun, in order to study them, that astronomers go such very long distances—often thousands of miles—when there is a total eclipse expected, and not merely to see the eclipse itself. ”

“ It is so, for the reason that when the sun is eclipsed, as I have just told you, the solar corona, which cannot be seen at any other time owing to the overpowering brilliance of the solar orb, becomes plainly visible, and by studying the form and other particulars of the corona astronomers are able to draw most important conclusions concerning the constitution of the sun, the mechanism of its radiation, and its effects upon the earth. ”

“ The corona is undoubtedly affected both in shape and constitution by the periodic ebb and flow of solar activity, its low-tide form being winged, its high-tide form stellate; while the rays emitted by the gases contained in it fade, and the continuous spectrum brightens, at times of minimum sun-spots. The appendage, as a whole, must be of inconceivable tenuity, since comets cut their way through it without experiencing sensible retardation. ”

“ Surrounding the main body of the sun are a series of layers, shells, or atmospheres. Outside of all and very irregular in structure, indeed probably not a solar atmosphere at all, is the solar corona, parts of which behave much as if it were an atmosphere, but it appears to be bound up in some way with the sun's radiation. It has streamers that vary with the sun-spot period, but its constitution and function are very imperfectly known, because it has never been seen or photographed except at rare intervals on occasion of total eclipses of the sun. ”

“ Then there is the inner equatorial corona, apparently connected intimately with truly solar phenomena, quite as the polar rays are. The third element in the composite is the outer ecliptic corona, for the most part made up of long streamers. This is most fully developed at the time of the fewest spots on the sun. It is traceable much farther against the black sky with the naked eye than by photography. ”

“ By means, then, of the five great eclipses of 1860-71 it was ascertained: first, that the prominences, and at least the lower part of the corona, are genuine solar appurtenances; secondly, that the prominences are composed of hydrogen and other gases in a state of incandescence, and rise, as irregular outliers, from a continuous envelope of the same materials, some thousands of miles in thickness; thirdly, that the corona is of a highly complex constitution, being made up in part of glowing vapours, in part of matter capable of reflecting sunlight. ”

“ Until recently, the solar atmosphere could be seen only when the sun itself was hidden by the moon, a few minutes in a century. Now, however, the spectroscope has brought the chromosphere and the prominences within the range of daily observation, so that they can be studied with nearly the same facility as the spots and faculae, and a fresh field of great interest and importance is thus opened to science. But the corona as yet defies the new method, and can be seen only during the fleeting moments of a solar eclipse. ”

“ But the believers in an atmospheric corona were not even yet wholly satisfied. Nay, before the recent total eclipse one among them even went so far as to say that the observations and photographs of 1870 and 1871, while demonstrating the solar nature of the glory immediately surrounding the sun, proved the long rays extending much farther from the sun to be non-solar phenomena. ”

“ When an eclipse occurs at a spot-maximum, the distribution of the coronal features is found to have entirely changed. Instead of being sharply divided into polar brushes and equatorial wings, the streamers are distributed fairly evenly around the whole solar margin, in a manner suggesting the rays from a star, or a compass-card ornament. The existence of this periodic change has been repeatedly confirmed, and there can be no doubt that the corona reflects in its structure the system of variation which prevails upon the sun. ”

“ One thing is in the highest degree probable, namely, that electricity is largely concerned.
Going back to the question of Sun-spots regarded in their possible or probable association with the Corona, the present position of matters appears to be this: that there is a real connection between the general form of the Corona and disturbances on the Sun, taking Sun-spots as an indication of solar activity. When Sun-spots are at or near their maximum, the Corona has generally been somewhat symmetrical, with synclinal groups of rays making angles of 45° with its general axis. ”

“ Then follows his letter to the president, which runs thus: "I have seen the corona as I could not in 1868, when I gave myself wholly to the prominences. Nothing could be more beautiful or more brilliant; and there were definite forms which exclude all possibility of an origin in our own atmosphere." He proceeds to describe the coronal spectrum, confirming the American observations—with one notable exception: he recognized the solar dark lines in the spectrum of the corona, a proof that no inconsiderable portion of its light is reflected sunlight. ”

“ They leave no doubt that, at those eclipses, the spectrum of the inner corona contained no perceptible dark lines. Perrine's Sumatra photographs seem to establish that the spectrum of the great outer portion is substantially a copy of the solar spectrum. The simplest interpretation of these observations is that the outer corona is largely composed of minute particles which reflect and diffract the sunlight falling upon them, whereas the portions near the hot solar surface are mostly incandescent, shining by their own light. ”

“ He has calculated that if the meteoric shower were sufficiently heavy to make up for the sun’s whole expenditure of heat, the matter of the corona must be so dense as seriously to perturb the orbits of certain comets which pass very close to his surface,—a result which is found not to be the case. ”

“ The negatives taken to depict the outer corona show from three to four solar diameters extension for the longest streamers. The equatorial 'wings,' as they recede from the sun, are finally lost in an illuminated sky, without any indication of having actually come to an end. ”

“ Now, it happens that we know quite well that the pressure exerted by the real solar atmosphere, even close by the bright surface which forms the visible globe of the sun, is nothing like so great as it would be if the corona formed part of that atmosphere. ”

“ If the existence of great solar disturbances causes the corona to be greatly enlarged, as compared with the corona seen when the sun shows no spots, we should expect to find the corona moderately enlarged only when the sun shows a considerable but not the maximum number of spots. ”

“ Retardation of sun-spots by continuous development to the rear, and recession in front, as the sun rotates on its axis. The short arrows represent lines of planetary energy; the long arrows show the direction of the sun’s rotation.
The dark inner disk represents the solar core, the white circle the photosphere, the mottled area the chromosphere and faculæ, and the dark outer ring the corona. Loops and tufted sprays are shown, caused by inflowing faculæ in front, caught by the uprush of active portions of the sun-spot towards rear. ”

“ The picture represents an eclipse of the sun; the dark body of the moon has screened the sun's shining disc and taken the glare out of our eyes; we see a silvery halo surrounding the great orb on every side. It is the sun's atmosphere, or "crown" (corona) , stretching for millions of miles into space in the form of a soft silvery-looking light; probably much of its light is sunlight reflected from particles of dust, although the spectroscope shows an element in the corona that has not so far been detected anywhere else in the universe and which in consequence has been named Coronium. ”

“ Extending much farther from the sun, how far is not as yet known, is the radiated corona. It is much fainter than the inner corona, and its light grows fainter and fainter with distance from the sun, until lost to view on the dark but not black background of the sky. Through this faint and softly graduated corona extend radiations of somewhat greater brightness. It is between these radiations that those dark gaps or rifts appear, which have figured so much in the narratives of recent eclipse observations. The dark gaps are, indeed, more striking features than the radiations which form them ”

“ The important result of a comparison of the radiations of the inner corona, the full moon, and the daylight sky somewhat remote from the sun is that while the three are roughly of equal visual brightness, the corona is effectively a cool and far from intense source, while the moon and sky are effectively warm and many fold richer in radiation. Hence it would appear plausible that the corona merely sends out visible rays and that its light is not associated with the great preponderance of long wave-length rays proper to the radiation from bodies at a high temperature. ”

“ Of the corona I cannot tell you much, except that it is far more subtle than anything we have spoken of yet; that it is always strongest when the sun is most spotted; that it is partly made up of self-luminous gases whose bright lines we can see, especially an unknown green ray; while it also shines partly by reflected light from the sun, for we can trace in it faint dark lines; lastly it fades away into the mysterious zodiacal light, and so the sun ends in mystery at its outer fringe as it began at its centre. ”

“ Wesley of London has succeeded eminently in work of this character, and his drawings of the sun's corona, visible at total eclipses from 1871 onward, in possession of the Royal Astronomical Society, are the finest in existence. They give a vastly better idea of the corona, as the eye sees it, than any single photograph possibly can. ”

“ The outer and far more extensive portion, which in texture and rarity seems to resemble the tails of comets, is known as the coronal atmosphere, since to it is chiefly due the corona, or glory, which surrounds the darkened sun during an eclipse. ”

“ Yet no matter what its form, it is a magnificent background during the sun's eclipse for the narrow rim of deep crimson projects beyond the inky edges of the moon.
The corona seems to be caused by electrical discharges in a medium of fine dust and rarefied gases, chiefly coronium and protofluorine. Various electrical phenomena take place in the strong magnetic fields of the sun as a whole and in the sun-spots, and the changing fields of magnetic force cause the changing appearance of the corona. ”

“ Both its unspringing shape, and the violet rays of calcium strongly emitted by it, contradicted the supposition that "white prominences" represent a downrush of refrigerated materials.
The corona of 1886, as photographed by Dr. Schuster and Mr. Maunder, showed neither the petals and plumes of 1871, nor the streamers of 1878. It might be called of a transition type. [561] Wide polar rifts were filled in with tufted radiations, and bounded on either side by irregularly disposed, compound luminous masses. ”

“ Cavendish Laboratory, Cambridge, 19th March, 1879 Sir, I have received with much pleasure the table of the satellites of Jupiter which you have been so kind as to send me, and I am encouraged by your interest in the Jovial system to ask you if you have made any special study of the apparent retardation of the eclipses as affected by the geocentric position of Jupiter.
I am told that observations of this kind have been somewhat put out of fashion by other methods of determining quantities related to the velocity of light, but they afford the only method, so far as I know, of getting any estimate of the direction and magnitude of the velocity of the sun with respect to the luminiferous medium. ”

“ The sky spread over with one continuous cloud, whitened by the light of the moon, which, though her dim shape was seen, did not throw forth so strong a light as to chequer the earth with shadows. At once the clouds seemed to cleave asunder, and left her in the centre of a black-blue vault. She sailed along, followed by multitudes of stars, small, and bright, and sharp. ”

“ immense France has her freaks of pettiness. That is all.
To this there is nothing to say. Peoples, like planets, possess the right to an eclipse. And all is well, provided that the light returns and that the eclipse does not degenerate into night. Dawn and resurrection are synonymous. The reappearance of the light is identical with the persistence of the I.
Let us state these facts calmly. ”

“ Whenever a total eclipse of the sun was visible in an accessible region parties were sent out to observe it. ”

“ Winter Stars
I went out at night alone;
The young blood flowing beyond the sea
Seemed to have drenched my spirit's wings—
I bore my sorrow heavily.
But when I lifted up my head
From shadows shaken on the snow,
I saw Orion in the east
Burn steadily as long ago.
From windows in my father's house,
Dreaming my dreams on winter nights,
I watched Orion as a girl
Above another city's lights.
Years go, dreams go, and youth goes too,
The world's heart breaks beneath its wars,
All things are changed, save in the east
The faithful beauty of the stars. ”