Sun disk

“ The largest ever seen has not had an extent approaching that of our imagined black disk, even when the whole dimensions of the spot—nucleus, umbra, and penumbra—have been taken into account. Moreover, all round a spot there is always a region of increased brightness, making up to a great degree, if not altogether, for the darkness of the spot itself. So that unquestionably the summer heat in the Southern Hemisphere exceeds the summer heat in our hemisphere to a much more marked degree than the heat given out by the sun when he is without spots exceeds the heat of a spotted sun. ”

“ You can photograph the Sun with an ordinary camera and make his disk [Pg 188] as large as you want to, but you will get nothing more on your plate than a white spot. The Moon can likewise be photographed, but if you focus it sharp on the plate it will not be much larger than a mere point of light, and if you get a disk large enough to see there will only be a small white spot on your finished print.
To make good photographs of the Sun, Moon and planets, a big telescope driven by clockwork to offset the turning of the Earth is needed. ”

“ One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. The solar system occupies a disk-shaped, or [Pg 14] flat circular, expanse, about 5,580,000,000 miles across and relatively very thin, the sun being in the center. From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote. ”

“ Well, judging from the observations I have made, we were probably right so far; but if you recollect, I accounted for the mysterious daily rise and set of the sun, if I may use the words, by changes in the density of the atmosphere bending the solar rays, and making the disk appear to rise and sink periodically, though in reality it does nothing of the kind. A similar effect is well-known on the earth. It produces the 'after glow' on the peaks of the Alps when the sun is far below the horizon ”

“ Those who are still doubting that our sun emits powerful cosmic rays evidently overlook that the solar disk, in whatever position it may be in the heavens, cuts off the radiations from beyond, replacing them by its own. ”

“ Relatively so close is she to both bodies that to show she does not need the abetting background of the night, but without waiting for the sun's withdrawal may nearly always be seen in the daytime in clear air if one knows where in the sky to look. Situate about seven tenths of our own distance from our common giver of light and heat, she gets about double the amount of solar radiation that falls to our lot, so that her surface is proportionately brilliantly illuminated. Being also relatively near us, she displays a correspondingly large disk. ”

“ Of three movable indices, the farthest from the center of the dial is fitted with an index on either side and marked with four segments of a circle. Immediately below are five numbers, divided into the days of setting the measure of the mean-synodic age of the moon, and into signs, degrees of the signs, and of the distance of the moon from the sun. These, in each revolution, revolve once around the solar disk superimposed on the mean synodic-lunar disk, and also around the lunar disk. ”

“ All our best tests show the spectrum of this light to be continuous, and therefore reflected; which indicates that it is a ring of small masses of meteoric matter surrounding the sun, revolving with it and reflecting its light. One bit of stone as large as the end of one's thumb, in a cubic mile, would be enough to reflect what light we see looking through millions of miles of it. Perhaps an eye sufficiently keen and far away would see the sun surrounded by a luminous disk, as Saturn is with his rings. ”

“ All regions of the sun's disk radiate heat uniformly, and the sun's own atmosphere absorbs so much that we should receive 1.7 times more heat if it were removed. So far as is known, solar light and heat are radiated equally in all directions, so that only a very minute fraction of the total amount ever reaches the earth, that is, 1 2200 millionth part of the whole. ”

“ Sun spots in violent disturbance are rarely observed. As the sun turns round on his axis, the spots will often be carried across the disk from the center to the edge, when they become very much foreshortened. The sun's period of rotation is 28 days, so that if a spot lasts more than two weeks without breaking up, it may reappear on the eastern limb of the sun after having disappeared at the western edge. ”

“ These, says Sir John Herschel, cover the whole disk of the sun (except the spaces occupied by the spots) in countless millions, and lie crossing each other in every imaginable direction. Mr. Dawes denies that they are so general, but they are universally recognized in the vicinity of t he solar spots, taking a radial direction around the penumbra, and giving it a jagged or coarsely-thatched appearance. ”

“ Remarkable obscurations of the sun's disk, during which the stars have been seen at mid-day (as, for instance, in the obscuration of 1547, which continued for three days, and occurred about the time of the eventful battle of MÂhlberg) , can not be explained as arising from volcanic ashes or mists, and were regarded by Kepler as owing either to a 'materia cometica', or to a black cloud formed by the sooty exhalations of the solar body. ”

“ There were no clouds, the sun was going down in a limpid, gold-washed sky. Just as the lower edge of the red disk rested on the high fields against the horizon, a great black figure suddenly appeared on the face of the sun. We sprang to our feet, straining our eyes toward it. In a moment we realized what it was. On some upland farm, a plough had been left standing in the field. The sun was sinking just behind it. Magnified across the distance by the horizontal light, it stood out against the sun, was exactly contained within the circle of the disk; ”

“ It procures for us the benefit of a much longer day, at all seasons of the year, than we should enjoy, did not this property of the atmosphere produce this effect. It is owing to the same cause that the disks of the sun and moon appear elliptical or oval, when seen in the horizon, their horizontal diameters appearing longer than their vertical—which is caused by the greater refraction of the rays coming from the lower limb, which is immersed in the densest part of the atmosphere. ”

“ If it is not a question of seeing a spot spring into view, but of watching one already in existence, we shall, in general, see such a spot appear on the Eastern limb of the Sun just after having turned the corner, so to speak. The spots traverse the Sun’s disc in lines which are apparently oblique with reference to the diurnal movement and the plane of the ecliptic, and after about 13 days they will disappear at the Western limb if they have not done so before by reason of physical changes in their condition. ”

“ It is often said that the Sun exhibits a granulated structure. If we wish to realise in the most precise manner what is meant by the word “granulation” as applied to the structure of the Sun, we must abandon the method of projection and examine the Sun directly with a powerful eyepiece, taking advantage of a moment when the atmosphere is perfectly calm, and before the eyepiece has had time to get hot. It may then be seen that the Sun’s surface is covered with a multitude of little grains, nearly all of about the same size, but of different shape, though for the most part more or less oval. ”

“ The solar system of circulation, instead of being adapted, like that of the earth, to the distribution of heat received from without, was seen to be directed towards the transportation towards the surface of the heat contained within. Polar and equatorial currents, tending to a purely superficial equalisation of temperature, were replaced by vertical currents bringing up successive portions of the intensely heated interior mass, to contribute their share in turn to the radiation into space which might be called the proper function of a sun. ”

“ But to establish this fact, it is necessary to embrace a considerable portion of the solar disc, and it is this which it is impossible to realise when we look at the sun in a very powerful instrument, the field of which is, by the very fact of its power, very small. ”

“ Hence, wherever a spectator is placed with regard to a luminous body, every point of that part of its surface which is toward him will be visible, when no intervening object intercepts the passage of the light. Hence, likewise, it follows, that the sun illuminates, not only an immense plane extending along the paths of the planets, from the one side of the orbit of Uranus to the other, but the whole of that sphere, or solid space, of which the distance of Uranus is the radius. ”

“ We have seen how great is the difference in the estimates of geologists and those of physicists as to the time that has elapsed during the whole development of life. But the position we have now found for the sun, in the outer portion of the central star-cluster, may afford a clue to this problem. What we require is, some mode of keeping up the sun's heat during the enormous geological periods in which we have evidence of a wonderful uniformity in the earth's temperature, and therefore in the sun's heat-emission. ”

“ It may be some other form of solar radiation to which we are not accustomed on the earth, but which is specially effective here when the sun is uncovered because of the greater nearness of Venus. This atmosphere, notwithstanding its density, may well be diaphanous to the ultraviolet rays, owing to some peculiarity in its composition which I have not had time to study. At any rate, it is evident, from what we have seen, that the rays of the unclouded sun almost instantly affect the brain. ”

“ These planetary globes themselves, as they coalesced and contracted, left behind or threw off rings of their outer matter, which, in turn, became moons, and finally our solar system with its central sun was evolved as we now see it; development continued, the planets cooled and condensed, life appeared when the conditions became suitable, and the original progressive condensation of the central mass—the sun—still continuing, the evolution of light and heat continues, and will continue in a correlative degree. ”

“ When the materials of each solar system were thus set apart from the original mass of star dust or vapour, they began an independent development which led step by step, in the case of our own solar system at least, and presumably also in the case of the other suns, the fixed stars, to the formation of planets and their moons or satellites, all moving around the central sun. ”

“ And if there be any such atmosphere, the spectrum formed by the moon’s light, that is, by the sun’s light reflected from the moon, would be modified in such a manner as to exhibit absorption-lines different from those found in the spectrum of the direct solar rays, just as the absorption-lines vary according as the sun’s rays have to pass through a thinner or a denser stratum of the terrestrial atmosphere. ”

“ If, on the other hand, the region of the Earth’s atmosphere through which the Sun’s rays pass be highly saturated, it will be the blue rays which suffer absorption, whilst the red rays will be transmitted and will impart a ruddy hue to the Moon. Finally, if the Earth’s atmosphere is in a different condition in different places, saturated in some parts and not in others, a piebald sort of effect will be the result, and some portions of the Moon’s disc will be invisible, whilst others will be more or less illuminated. ”

“ In certain regions of the photosphere several dark patches are usually visible, which are called ‘sun-spots.’ At occasional times they are almost entirely absent from the solar disc. It has been observed that they occupy a zone extending from 10° to 35° north and south of the solar equator, but are not found in the equatorial and polar regions of the Sun. A sun-spot is usually described as consisting of an irregular dark central portion, called the umbra; surrounding it is an edging or fringe less dark, consisting of filaments radiating inwards called the penumbra. ”

“ So slight a change would not be perceptible; but it is not alone sun-spots which obscure the solar surface, its entire globe is enveloped with an obscuring veil. When studied with a powerful telescope the sun’s surface is seen to be thickly mottled with relatively obscure specks, so numerous that it has been estimated that they cut off from one-tenth to one-twentieth of the light that we should receive from it if the whole surface were as brilliant as its brightest parts. ”

“ During all this time the black lunar disc may be watched making its way steadily across the solar face. Notwithstanding the gradual obscuration of the sun, one does not notice much diminution of light until about three-quarters of his disc are covered. Then a wan, unearthly appearance begins to pervade all things, the temperature falls noticeably, and nature seems to halt in expectation of the coming of something unusual. The decreasing portion of sun becomes more and more narrow, until at length it is reduced to a crescent-shaped strip of exceeding fineness. ”