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1 The appearance of the corona during the total eclipse of 30 August 1905
2 Spectro-heliogram of the whole ball of the sun taken at the Yerkes Observatory, 12 August 1903
1 Solar prominence 80,000 miles high, photographed with the spectroheliograph on Mount Wilson
2 The chromosphere and prominences photographed in projection against the sun's disk, showing the hydrogen vortices centering in sun-spots discovered at Mount Wilson
The absorption and re-radiation from successive layers is almost instantaneous, the velocity of heat transference approaching 180,000 miles a second.
In drawing our conclusions the intensity of gravitation on the sun must be borne in mind. It has nearly 28 times the force of gravity on the earth. A man of ordinary size would weigh two tons at the surface of the sun, and would, therefore, be instantly crushed to death by his own weight, were it possible for him otherwise to exist there. Consequently, the pressure to which the vapors of the sun are subject increases with enormous rapidity below the surface.
The average specific gravity of the materials composing the sun can be determined by astronomical theory with great exactness. It is known that the mean specific gravity is about 40 per cent greater than that of water, and one-quarter that of the earth. It is doubtless much smaller than this at the surface, and, therefore, increases toward the centre. A calculation of the resulting pressure shows that near the centre of the sun the pressure produced by the enormous mass and gravitation of the matter composing the solar orb amounts about 5,000,000 tons per square inch. This pressure is so far beyond any that we can produce at the earth's surface that we are unable to say what effect it would have upon matter.
Yet another unknown factor is the temperature of the interior. At no great distance toward the centre the temperature exceeds our powers of determination – it may even be 1,000,000°. As the highest temperature which it is possible to produce artificially probably does not amount to 12,000°, it is impossible to say what effect such a temperature would have upon matter. Thus we have two opposing causes, the one an inconceivable degree of heat, such that, were matter exposed to it on the surface of the earth, it would explode with a power to which nothing within our experience can be compared, and a pressure thousands of times any we can produce, tending to condense and solidify this intensely heated matter. One thing which we can say with confidence as to the effect of these causes is that no chemical combinations can take place in matter so circumstanced. The distinction between liquid and gaseous matter is lost under such conditions. Whether the central portions are compressed into a solid, or remain liquid, it is impossible to say.
Modern research shows that the sun, as a whole, is a complex body, the various parts of which are in very different conditions. Beginning at the centre and passing outward, we have first the vast, invisible interior which forms the globe itself, and which our sight can never penetrate. Surrounding this interior is the visible photosphere, or seeming surface, which we see with the naked eye or the telescope, the appearance of which has been fully described. So far as ordinary direct observation could show, this would be the whole of the sun. But the spectroscope, as well as eye observation during total eclipses, has shown most complex surroundings of the sun, which would otherwise have been invisible. The surroundings are formed of two envelopes, the chromosphere and the corona.
The earliest accurate observers of total eclipses with the telescope noticed that dur. ing the total phase red cloud-like masses were seen here and there projecting beyond the limb of the dark moon, Moreover, at the beginning or the end of the eclipse, it is found that these projections are connected with a red border extending round the sun. There is, therefore, an envelope which radiates red light and surrounds the sun, and which is invisible except during eclipses. Quite independent of this envelope is a bright effulgence which is seen during a total eclipse. These phenomena are fully described in the article ECLIPSE. What we have now to do is to set forth what they indicate.
· The red envelope which rests immediately on the photosphere is called chromosphere. It is comparatively thin — so thin as to be almost immediately covered when the sun is totally eclipsed. Its nature was first made known by the spectroscope, which showed it to be composed mostly of hydrogen, helium, and calcium vapor. Its principal and lower parts differ in constitution. At the photosphere it comprises nearly all the substances which exist in the latter. This was shown in a very beautiful way by observations of the reversing layer, first made by Young at the total eclipse of 1870. The explanation of the phenomena there described is that the photosphere is hot enough to shine by its own light, and, being a gas, to give bright spectral lines. But the photosphere is so much hotter than the chromosphere that the latter is, in comparison, a cool gas which absorbs the spectral lines from the light radiated by the photosphere. The question of the density of the chromosphere and reversing layer, as its base is called, has given rise to very varied estimates.
The fact that the spectroscope shows bright lines as the last ray of true sunlight disappears at the beginning of a total eclipse shows that the gas from which these lines emanate must be so rare as to be transparent through a distance of thousands of miles. We are, therefore, justified in concluding that the gases of the chromosphere are extremely rare, and the same is probably true of the principal regions of the photosphere.
Among the most extraordinary phenomena exhibited by the sun are the mountainous elevations of the chromosphere, which we see as the red protuberances already described. These are of two kinds, the eruptive and the cloud-like. The latter present to us the appearance of vast clouds floating in an atmosphere of the sun. It seems certain, however, that they cannot be what they seem, because there can be no atmosphere there to support them. They are probably held up by an impulsion of the solar rays, which will be described presently. The eruptive prominences seem to be due to outbursts of intensely hot gases, mostly hydrogen, from the sun. These are thrown up with a velocity of several hundred miles per second, like immense mountains of fire. They sometimes rise to a height of many thousand miles, their ascent being doubtless aided by the impulsion of the solar rays; then they fall back again to the sun. The chromosphere and prominences can now be photographed in projection against the sun's disc with the spectroheliograph... When such photographs are made with the light of the red hy
drogen line, they show great vortex phenomena, centering in sun spots and closely related to the vortices in the photosphere which constitute the spots themselves. See Plate III.
The violent forces seen in action in the chromosphere are in singular contrast to the soft white light of the corona. Much mystery still surrounds the constitution of the latter. It was supposed to be an atmosphere of the sun; but this view is rendered untenable by the fact that an atmosphere supported by its own weight would more than double in density for every mile that it was nearer its base. It probably consists of exceedingly minute molecules of gaseous matter, similar to those which make up the tail of a comet, and possibly having some resemblance to the latter. The newly-discov
an electrical thermometer. The unit usually employed is the calory, which is the amount of heat required to raise one gram of water 1° C. The solar constant is then the number of calories which would be received on each square centimeter of the earth's surface, exposed perpendicularly to the solar rays, if there were no loss in transmission through the atmosphere.
The following small table gives the mean values of the solar constant obtained from observations made at Washington, Mount Whitnėy and Mount Wilson. Observations were made at the second station, whose elevation is nearly three miles, in order to test the accuracy of the laws assumed for the absorption of solar rays by the atmosphere.
ered fact of the impulsion of the solar rays probably affords a clew for an explanation of these and similar phenomena. More than 40 years ago it was announced by Maxwell, as a result of his electro-magnetic theory of light, that light and heat emitted by the sun should exercise a very minute pressure on any object which they struck. Conclusion showed that this pressure was so slight that no apparatus then known was so delicate as to make it sensible. Within the last few years, however, E. F. Nichols and others have succeeded in showing experimentally that on very finely divided matter this action of light can be observed and measured. It follows that particles below a certain size will be repelled by the ,sun's light with greater force than they are attracted toward it, and will thus be driven from the sun when in its neighborhood, or supported temporarily at a certain height above the sun. Hale has proved, by spectroscopic methods similar to those employed in his discovery of magnetic fields in sun spots, that the entire sun is a magnet, with a field about 80 times as intense as the magnetic field of the earth and with its magnetic axis inclined about six degrees to the sun's axis of rotation.
The Sun's Rotation.-As the light and heat which we receive from the sun are the source of all life on the earth, the important work is at once suggested to measure exactly how much radiant energy we receive from the sun in a given time, and especially, if possible, to find whether this is growing greater or less, or if it varies from time to time. Until so late as 1905 the measurements were comparatively very crude, but the sensitiveness of the instruments employed has recently been so increased and observations with them have been so carefully and continuously carried on that this quantity has now been well determined.
For measuring the amount of heat received on a square unit of the earth's surface, the so-called pyrheliometer is employed, an instrument which presents a surface of known area to the solar rays, the rise in temperature due to the heating being communicated to a stream of water (in the best form), and measured by
Prior to 1905 the true value of the constant was in much doubt; numbers ranging from 1.76 to 4.10 were stated for it, and the average value 3.0, was frequently accepted. There can be no doubt that the value 1.95 is very near the truth, and this may be regarded as the best value now obtainable. It has been well established, however, especially by the recent work of Abbot, that this fundamental constant varies slightly and irregularly. In 1919 a solar station was established at Calama, Chile, at which it is planned to make constant measures of the sun's radiation for several years, in conjunction with northern observations, with a special view of ascertaining, if possible, the law of this variation, and its effects upon terrestrial climates.
The Sun's Magnetism.- In the year 1908, Prof. G. E. Hale of the Mount Wilson Observatory, from an examination of the spectral lines in sun spots, discovered that around each spot there is a more or less powerful magnetic field. A powerful field will double many of the lines of the spectrum; a less powerful one will merely widen them. A few lines were found triple in sun spots, and afterward these same lines were found to become triple in the laboratory when viewed along the magnetic lines of force. Thus the lines of sun spots near the sun's limb tend to become triple, while those from spots near the centre of the solar disc are doubled merely. In many cases a pair of sun spots quite near together are found to have opposite polarity, and while in general the polarity of spots in the southern hemisphere is different from that in the northern, many cases of exception occur. It has, however, been well established that the sun, like the earth, has a north and south magnetic pole; the inclination of the sun's magnetic axis has been determined, and the fact has been established that the magnetic pole is in slow rotation about the pole of the sun.
A curious relation is found by the study of magnetic storms on the earth. The latter consist in occasional perturbations of the magnetic needle, which are very irregular in their character and are felt over the whole globe. They