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Ger. sonne. Fr. soleil, Lat. sol, Gr. IjXtos, from which comes helio- in various English compounds), the name of the central body of the solar system, the luminous orb from which the earth receives light and heat; (see Sunshine); hence by analogy other heavenly bodies which form the centre of systems are called suns.
To understand the phenomena of the sun, we should reproduce them upon the earth; but this is clearly impossible since they take place at temperatures which volatilize all known substances. Hence our only guides are such general laws of mechanics and physics as we can hardly believe any circumstances will falsify. But it must be remembered that these require extrapolation from experience sometimes sufficiently remote, and it is possible they may lead to statements that are obscure, if not contradictory. The body of the sun must consist of uncombined gases; at the surface the temperature is some 2000° C. above the boiling point of carbon, and a little way within the body it may probably exceed the critical point at which increase of pressure can produce the liquid state in any substance. But as the mean density exceeds that of water, and probably falls but little from the centre to the surface, these gases are gases only in the sense that if the pressure of neighbouring and outward parts gravitating towards the centre were relaxed, they would expand explosively, as we see happening in the eruptive prominences. They have lost completely the gaseous characteristic of producing a line spectrum, and radiate like incandescent solids. The surface region which yields a continuous spectrum is called the photosphere; it possesses optically a sharp boundary, which is generally a perfect sphere, but shows occasionally at the rim slight depressions or more rarely elevations. Enclosing the photosphere is a truly gaseous envelope which is called the chromosphere, and which shows a spectrum of bright lines when we can isolate its emission from that of the photosphere. This envelope is also sharply defined, but its normal appearance is compared to the serrations which blades of grass show on the skyline of a hill, and it is disturbed by the outbursts, called prominences, of which details are given below. Outside this again is an envelope of matter of enormous extent and extreme tenuity, whether gaseous or partly minute liquid or solid drops, which is called the corona. It has no sharp boundary, its brightness diminishes rapidly as we recede from the limb, and such structure as it shows consists of long streaks or filaments extending outwards from the limb in broad curved sweeps. Finally there is the envelope of still vaster extent and of unknown constitution which gives the zodiacal light; its greatest extent is along the ecliptic, but it can also be certainly traced for 35° in a perpendicular direction. The lower gaseous cloaks absorb a large part of the light admitted by the photosphere, and especially at the limb and for the more refrangible rays the loss of intensity is very marked.
In the instants when a sharp image of the photosphere is seen or photographed, it shows a granulated appearance like white flakes strewed fairly evenly upon a dark ground. The figs. 1, 2, 3, 4 (plate) show enlargements from photo- General graphs by Hansky at Pulkowa (June 25, 1905); Appearance they are separated by intervals from 25 to 80 of photo- seconds, and he has succeeded in showing identity sphere. in many of the granules, or more properly, clouds represented. Thus they exhibit at once general appearance and its changes. The diameters range from 400 m. or less up to 1200 m., and the speeds relative to the spot range up to 2 or 3 m. per second. M. Hansky believes these motions may be the consequences of matter rising from below and thrusting the surface groups aside. Usually the changes are such that it is impossible even to recognize the formations in successive photographs. Besides granulations the sun's disk shows, as a rule, one or more spots or groups of spots. Each spot shows with more or less completeness a ring-shaped penumbra enclosing a darker umbra; the umbra, which looks black beside' the photosphere, is actually about as brilliant as limelight. In the neighbourhood surrounding the penumbra the granules appear to be packed more closely, forming brilliant patches called faculae. In the shape of a spot there is neither rule nor permanence, though those that are nearly circular seem to resist change better than the others. They arise from combinations of smaller spots, or from nothing, in a short period, say a day. They are never wholly quiescent. Bridges, more brilliant than the rest of the photosphere, form across them, and they may divide into two parts which separate from one another with great velocity. The largest spots are easily seen by the naked eye, if the brilliancy of the disk is veiled; the umbra may be many - ten or more - diameters of the earth in breadth. The length of their life is difficult to assign, because there is some tendency for a new group to arise where an old one has disappeared; but one is recorded which appeared in the same place for eighteen months; the average is perhaps two months. They are carried across the disk by the sun's rotation, partaking in the equatorial acceleration; they also show marked displacements of their own, whether with, or relative to, the neighbouring photosphere does not appear; at the beginning of their life they usually outrun the average daily rotation appropriate to their latitude. Spots are rarely found on the equator, or more than 35° N. or S. of it, and at 45° are practically unknown. Their occurrence within these zones follows statistically a uniform law (see Aurora). Other information about the spots is given below, in connexion with their spectra. It may be said that nothing definite has been established as to what they are. The statement known as A. Wilson's theory (1774), that they are hollows in the photosphere, long supposed to be proved by perspective effects as the spot approached the limb, is discredited by F. Howlett's careful drawings, which, however, do not establish the contrary. To draw a trustworthy conclusion it is necessary that the spot should be quiescent, show a well-developed and fairly symmetrical penumbra, and be observed near the limb and also near the centre, and these conditions are satisfied in so few cases as to withdraw all statistical force from the conclusion. Figs. 5, 6, 7, 8 (plate) are reproductions of the Greenwich photographs of the sun from the 30th of January to the 8th of February 1905. The first, taken alone, might seem to bear out Wilson's theory, but the others show that the penumbra is really very unsymmetrical and much broader on the side towards the limb, apart from anything which perspective may have to say. The photosphere does not rotate in one piece, lower latitudes outrunning higher. This was discovered by R. C. Carrington from observations of the spots, extending from 1853 of to 1861, from which he determined also the position of the sun's axis. But conclusions from the spots are full of anomalies. E. W. Maunder and Mrs Maunder found that different spots in the same zone differ more than do the means for different zones, while a long-lived spot settles down to give more consistent results than are furnished by spots of one apparition. In the span of two complete sunspot periods no evidence was found of periodic or other change with lapse of time. The problem still awaits complete discussion. The irregularities incidental to use of the spots are escaped by comparing the relative Doppler displacements of the same spectral line as given by the receding and advancing limbs of the sun. The observation is a delicate one, and was first successfully handled by N. C. Duner in 1890. But his determinations, repeated recently (Ada upsal. IV. vol. i., 1907) as well as those of J. Halm at Edinburgh (Ast. Nach. vol. 173, 1907), are superseded by a photographic treatment of the problem by W. S. Adams (Astrophys. Journ., xxvi., 1907).
The diagram (fig. 9) shows Adams's value for the angular velocity t for different latitudes ¢, the dots representing the actual observations. Fig. Io shows the consequent distortion of a set of meridians after one revolution (at lat. 30°). An important feature added to the discussion by Adams is the different behaviour of spectral lines 880 860' 840' 820' 800 780' 760 740' 720' 700 0 consider first a frictionless fluid. The equations of surfaces of equal angular motion would be of the form r =R (i --6 cos 2 0), where e is proportional to the square of the angular motion, supposed small, and R increases as e diminishes. Consider the traces these surfaces cut on any sphere r =a: we have de/de = 2e sin a cos e/{cos t - aR2 dR/de}, which is positive and has a maximum in the middle latitudes; so that, proceeding from the pole to the equator along any meridian, the angular velocity would continually in crease, at a rate which was greatest in the middle latitudes.
This is exactly what the ob servations show. Now if this state be supposed established in a frictionless fluid, the con sideration of internal friction would simply extend the char acteristics found at any spot to the neighbourhood, and there fore if the boundary were a sphere and so for a frictionless fluid an exception, it would cease to be an exception when we allow for viscosity. But this theory gives no clue to the results relating to hydrogen, which belongs to a high level, and which Adams has shown to move with an angular velocity decidedly greater than the equatorial angular velocity below it, and not to show any sign of falling off towards the poles.
It is useful to form a conception of the mechanical state within the sun's body. Its temperature must be dominated directly or indirectly by the surface radiation, and since the matter is gaseous and so open to redistribution, the same is true of density and pressure. .It is true that within the body radiations must be stifled within a short distance of their source; none the less, they will determine a temperature gradient, falling from the centre to the borders, though for the most part falling very slowly, and we may ask what relative temperatures in different parts would maintain themselves if once established. Stefan's law of radiation according to the fourth power of the temperature is too difficult to pursue, but if we are content with cognate results we can follow them out mathematically in a hypothetical law of the first power. We then find that the density would increase as we go outwards, at first slowly, but finally with extreme rapidity, the last tenth of the radius comprising half the mass. The radiation from such a body would be practically nil, no matter how hot the centre was. Of course such a state would be statically unstable. It would never get established because currents would arise to exchange the positions of the hotter, less dense, inner parts and the cooler, more dense, outer ones. By this interchange the inner parts would be opened out and the total radiation raised. Since the only cause for these convection currents is the statical instability produced by radiation, and the rapid stifling of radiations within the body produces there a temperature gradient falling very slowly, they would be for the most part extremely slight. Only near the surface would they become violent, and only there would there be a rapid fall of temperature and density. Through the main body these would remain nearly constant. Indeed it seems that, in the final distribution of density throughout the part which is not subject to violent convection currents, it must increase slightly from the centre outwards, since the currents would cease altogether as soon as 'a uniform state was restored. In the outer strata a different state must prevail. Rapidly falling temperature must (and visibly does) produce furious motions which wholly outrun mere restoration of statical balance. Portions change places so rapidly and so continually, that we may take it, where any average is reached, the energy is so distributed that there is neither gain nor loss when such a change occurs. This is the law of convective equilibrium. But in the sun's atmosphere gravitation alone is a misleading guide. Convective equilibrium, which depends upon it, gives far too steep a temperature gradient, for it yields a temperature of 6000° only 200 m. within the free surface, whereas the chromosphere is of an average thickness of 5000 m., and attains that temperature only at its base. Probably the factor which thus diminishes the effective 10° 20' 30' 40' 50° 60' 70' 80' 90° FIG. 9.
which are believed to originate at different levels. The data given above refer to the mean reversing layer. Lines of lanthanum and carbon which are believed to belong to a low level showed systematically smaller angular velocity than the average. This promises to be a fertile field for future inquiry. Pending more conclusive evidence from the spectroscope, the interpretation of the peculiar surface rotation of the sun appears to be that the central parts of the body are rotating faster than those outside them; for if such were the case the observed phenomenon would arise. For FIG.
I o.
Enlarged Photographs Of The Solar Surface. Taken by M. A. Hansky at the Observatory of Pulkowa (1905, June 25), at intervals from 25s. to 80s. XxVI.86.
1905, Jan. 30d. 12h. 8m. 27s.
1905, Jan. 31d. i ih. 17m. 27s.
1905, Feb. 2d. 'oh. 50m. 28s.
1905, Feb. 8d. 13h. 3m. 5s.
Photographs Of The Sun, Taken At The Royal Observatory, Greenwich. Observer: E. W. Maunder. Instrument, Thompson Photoheliograph. Focal length, 9 ft. Aperture, 9 in.
condensing power of gravitation at the sun's borders is the pressure of radiation.
The radiations from the sun must be considered in two parts, corresponding respectively to the continuous spectrum and the line-spectrum. The latter is considered below; " The Black .,, it is i nd i cat i ve of the chemical elements from which the lines can proceed, and its state at the time of emission; the former is indicative only of the rate of loss of energy from the sun by radiation, and is inwoven with a remarkable group of physical theory and experiment, known as the theory of the black body, or as black radiation. The " black body " is an ideal body with surface so constituted as to reflect no part of any radiations that fall upon it; in the case of such a body Kirchhoff and Balfour Stewart showed that unless energy were to be lost the rate of emission and absorption must be in fixed ratio for each specific wave-length.
The name has no reference to the appearance of the body to the eye; when emitting energy, its radiations will he of all wave-lengths, and if intense enough will appeal to the eye as luminous between about wave-lengths 7600 and 4000 tenth-metres; this intensity is a question of temperature, and as it is exquisitely inappropriate to speak of the bulk of the solar radiations as black, the writer will speak instead of amorphous radiations from an ideal radiator. The ideal radiator is realized within any closed cavity, the walls of which are maintained at a definite temperature. The space within is filled with radiations corresponding to this temperature, and these attain a certain equilibrium which permits the energy of radiation to be spoken of as a whole, as a scalar quantity, without express reference to the propagation or interference of the waves of which it is composed. It is then found both by experiment and by thermodynamic theory that in these amorphous radiations there is for each temperature a definite distribution of the energy over the spectrum according to a law which may be expressed by 0 5 0(OX)dX, between the wave-lengths X, A+dX; and as to the form of the function 4), Planck has shown (Sitzungsber. Berlin Akad. 544) that an intelligible theory can be given which leads to the form j(OX) = c i /{exp(c 2 /A9) - I }, a form which agrees in a satisfactory way with all the experi ments. Fig. 11 shows the resulting distribution of energy. The enclosed area for each temperature represents the total emission of energy for that temperature, the abscissae are the wavelengths, and the ordinates the corresponding intensities of emission for that wave-length. It will be seen that the maximum ordinates lie upon the curve A9 = constant dotted in the figure, and so, as the temperature of the ideal body rises, the wave-length of most intense radiation shifts from the infra-red X towards the luminous part of the spectrum. When we speak of the sun's radiation as a whole, it is assumed that it is of the character of the radiations from an ideal radiator at an appropriate temperature.
The first adequate determination of the character as well as amount of solar radiation was made by S. P. Langley in 18 93 at Mount Whitney in California (14,000 ft.), with the bolometer, an exceedingly sensitive instrument which he invented, and which enabled him to feel his way thermally over the whole spectrum, noting all the Constant. chief Fraunhofer lines and bands, which were shown by sharp serrations, or more prolonged depressions of the curve which gave the emissions, and discovering the lines and bands of the invisible ultra-red portion. The bolograph thus obtained must be cleared of the absorption of the earth's atmosphere, and that of the transmitting apparatus - a spectroscope and siderostat. The first in itself requires an elaborate study. The first essential is an elevated observatory; the next is a long series of bolographs taken at different times of the year and of the day, to examine the effect of interposing different thicknesses of air and its variation in transparency (chiefly due to water vapour). It is found that atmospheric absorption is generally greater in summer than in winter, a difference of 20% being found between March and August; morning hours show a rapid and often irregular increase of transparency, culminating shortly after noon, after which the diminution is slow and comparatively regular.
The resulting allowances and conclusion are illustrated in fig. 12, taken from an article by Langley in the Astrophysical Journal (1903), xvii. 2. The integrated emission of energy is given by the area of the outer smoothed curve (4), and the conclusion from this one holograph is that the " solar constant " is 2.54 calories. The meaning of this statement is that, arguing away the earth's atmosphere, which wastes about one-half what is received, a square (From Astrophysical Journal, xvii. 2, by permission of the University of Chicago Press.) FIG. 12.
centimetre, exposed perpendicularly to the sun's rays, would receive sufficient energy per minute to raise 2.54 grams of water I ° C. Langley's general determination of the constant was greater than this-3o to 3.5 calories; more recently C. G. Abbot at Mount Wilson, with instruments and methods in which Langley's experience is embodied, has reduced it greatly, having proved that one of Langley's corrections was erroneously applied. The results vary between 1.89 and 2.22, and the variation appears to be solar, not terrestrial. Taking the value at 2 . 1 the earth is therefore receiving energy at the rate of 1.47 kilowatts per square metre, or 1.70 horse-power per square yard. The corresponding intensity at the sun's surface is 4.62 X Io 4 as great, or 6.79 X Io 4 kilowatts per square metre = 7.88 X Io 4 horse-power per square yard - enough to melt a thickness of 13.3 metres (=39.6 ft.) of ice, or to vaporize 1.81 metres (=5.92 ft.) of water per minute.
If we assume that the bolograph of solar energy is simply a graph of amorphous radiation from an ideal radiator, so that the con- Temperature stants cl, c 2 , of Planck's formula determined terrestrially apply to it, the hyperbola of maximum intensity is XO = 2, 921 X 10 7; and as the sun's maximum intensity occurs for about X =4900, we find the absolute temperature to be 5960° abs. If we calculate from the total energy emitted, and not from the position of maximum intensity, the same result is obtained within a few degrees. But to call this the temperature of the sun's surface is a convention, which sets aside some material factors. We may ask first whether the. matter of which the surface is composed is such as to give an ideal radiator; it is impossible to answer this, but even if we admit a departure as great as the greatest known terrestrial exception, the estimated temperature is diminished only some Io %. A second question relates to the boundaries. The theory refers to radiation homogeneous at all points within a single closed boundary maintained at uniform temperature; in the actual case we have a double boundary, one the sun's surface, and the other infinitely remote, or say, non-existent, and at zero temperature; and it is assumed that the density of radiation in the free space varies inversely as the squares of the distance from the sun. Though there is no experiment behind this assumption it can hardly lead to error.
A third question is more difficult. The temperature gradient at the confines of the photosphere must certainly ascend sharply at first. When we say the sun's temperature is 6000°, of what level are we speaking ? The fact is that radiation is not a superficial phenomenon but a molar one, and Stefan's law, exact though it be, is not an ultimate theory but only a convenient halting-place, and the radiations of two bodies can only be compared by it when their surfaces are similar in a specific way. One characteristic of such surfaces is fixity, which has no trace of parallel in the sun. The confines of the sun are visibly in a state of turmoil, for which a sufficient cause can be assigned in the relative readiness with which the outer portions part with heat to space, and so condensing produce a state of static instability, so that the outer surface of the sun in place of being fixed is continually circulating, portions at high temperatures rising rapidly from the depths to positions where they will part rapidly with their heat, and then, whether perceived or not, descending again. It is clear that at least a considerable part of the solar radiations comes from a more or less diffuse atmosphere. With the help of theory and observation the part played by this atmosphere is tolerably precise. Its absorptive effects upon the radiations of the inner photosphere can be readily traced progressively from the centre to the rim of the sun's disk, and it has been measured as a whole by Langley, W. E. Wilson and others, and for each separate wave-length by F. W. Very (Astrophys. Journ., vol. xvi.). The entries in the table on following page express the reduction of intensity for different wave-lengths X, when the slit is set at distances yXradius from the centre of the disk.
Building upon these results A. Schuster has shown (Astrophys. Journ., vol. xvi.) that, if for the sake of argument the solar atmosphere be taken as homogeneous in temperature and quality, forming a sheet which itself radiates as well as absorbs, the radiation which an unshielded ideal radiator at 6000° would give is represented well, both in sum and in the distribution of intensity with respect to wave-length, by another ideal radiator - now the actual body of ///4, *...
FIG. II.
the sun - at about 6700 0, shielded by an atmosphere at an average temperature of 5500°, and that such an atmosphere itself provides about 0.3 of the total radiations that reach us.
In connexion with this subject it may be mentioned that the highest measured temperature produced terrestrially, that of the arc, is about 3500° to 4000° abs.
The energy which the sun pours out into space is, so far as we know, and except for the minute fraction intercepted by the disks of the planets (ii?oooo") absolutely lost for the pur. poses of further mechanical effect. The amount is such that, supposing the average specific heat of the sun's body as high as that of water, there would result a general fall of temperature of 2.0° to 2.5° C. in the lapse of each year. Hence, if no other agency is invoked, at an epoch say xXmoo years ago, the sun's heat would have been greater than now by the factor 1-+x/3n, where n X 6000° is taken for the sun's present mean temperature. It seems possible that n is not a large number, and if we take x equal, say, to 200, we come to the most recent estimate - the astronomical - of the date of the earth's glacial epoch, when the sun's radiation was certainly not much more than it is now, while this factor would differ materially from unity. Hence loss does not go on without regeneration, and we are apparently at a stage when there is an approximate balance between them. It is in fact an impossibility that loss should go on without regeneration, for if any part of the sun's body loses heat, it will be unable to support the pressure of neighbouring parts upon it; it will therefore be compressed, in a general sense towards the sun's centre, the velocities of its molecules will rise, and its temperature will again tend upwards. In consequence of the radiation of heat the whole body will be more condensed than before, but whether it is hotter or colder than before will depend on whether the contraction set up is more or less than enough to restore an exact balance. If we are dealing with comparatively recent periods there is no evidence of progressive change, but if we go to remote epochs and suppose the sun to have once been diffused in a nebulous state, it is clear that its shrinkage, in spite of radiation, has left it hotter, so that the shrinkage has outrun what would suffice to maintain its radiation. It is equally clear that there is a point beyond which contraction cannot go, and thereafter, if not before, the body will begin to grow colder. There is thus a turning-point in the life of every star. The movement towards contraction and consequent rise of temperature which radiation sets up, like other motions, overruns the equilibriumpoint, only however by a minute amount; the accumulated excesses from all past time now stored in the sun would maintain its radiations at their present rate for nX3000 years, that is, for a few thousand years only.
There is a superior limit to the quantity of energy which can be derived from contraction. If we suppose the sun's mass once existed in a state of extreme diffusion, the energy yielded by collecting it into its present compass would not suffice to maintain its present rate of radiation for more than 17,000,000 years in the past; nor if its mean density were ultimately to rise to eight times its present amount, for more than the same period in the future. This supposes the present density nearly uniform; if it is not uniform, any amount added to the former period is subtracted from the latter. A contraction of 0.2" or 90 m. in the sun's radius would maintain the present emission for 3500 years. Such a rate of change would be quite insensible, and we can affirm that for recent times there is no reason to look for any other factor than contraction; but if we consider the remote past it is a different matter. We know nothing quantitatively of the radiations from a nebulous body; and it is quite possible that the loss of radiant energy in this early stage was very small; but it is at least as certain as any other physical inference that 17,000,000 years ago the earth itself was of its present dimensions, a comparatively old body with sea and living creatures upon it, and it is impossible to believe that the sun's radiations were wholly different; but, if they were not, they have been maintained from some other source than contraction.
The fall of meteoric matter into the sun must be a certain source of energy; if considerable, this external supply would retard the sun's contraction and so increase its estimated age, but to bring about a reconciliation with geological theory, very nearly the whole amount must be thus supplied. It is easy to calculate that this would be produced by an annual fall of matter equal to one nineteen millionth of the sun's mass, which would make an envelope eight metres thick, at the sun's mean density; this would be collected during the year from a spherical space extending beyond the orbit of Jupiter. The earth would intercept an amount of it proportional to the solid angle it subtends at the sun; that is to say, it would receive a deposit of meteoric matter about one-tenth of a millimetre, of density say 2, over its whole surface in the course of the year. So far there is nothing impossible in the theory. But there are two fatal objections. The sun is a small target for a meteorite coming from infinity to hit, and if this considerable quantity reaches its mark, a much greater amount will circulate round the sun in parabolas, and there is no evidence of it where it would certainly make itself felt, in perturbations of the planets. A second objection is that it fails in its purpose, because 20,000,000 years ago it would give a sun quite as much changed as the contraction theory gave. If we examine chemical sources for maintenance of the sun's heat, combustion and other forms of combination are out of the question, because no combinations of different elements are known to exist at a temperature of 6000°. A source which seems plausible, perhaps only because it is less easy to test, is rearrangement of the structure of the elements' atoms. An atom is no longer figured as indivisible, it is made up of more or less complex, and more or less permanent, systems in internal circulation. Now under the law of attraction according to the inverse square of the distance, or any other inverse power beyond the first, the energy of even a single pair of material points is unlimited, if their possible closeness of approach to one another is unlimited. If the sources of energy within the atom can be drawn upon, and the phenomena of radio-activity leave no doubt about this, there is here an incalculable source of heat which takes the cogency out of any other calculation respecting the sources maintaining the sun's radiation. An equivalent statement of the same conclusion may be put thus: supposing a gaseous nebula is destined to condense into a sun, the elementary matter of which it is composed will develop in the process into our known terrestrial and solar elements, parting with energy as it does so.
The continuous spectrum leads to no inference, except that of the temperature of the central globe; but the multitude of dark lines by which it is crossed reveal the elements composing pe ct rum o the truly gaseous cloaks which enclose it. A table of the sun.
these lines is a physical document as exact as it is intricate. The visual portion extends from about w.1.3700 to 7200 tenth-metres; the ultra-violet begins about 2970, beyond which point our atmosphere is almost perfectly opaque to it; the infrared can be traced for more tlian ten times the visual length, but the gaps which indicate absorption-lines have not been mapped beyond 9870. The ultra-violet and the visual portion are recorded photographically; Rowland's classical work shows some 5700 lines in the former, and 14,200 in the latter, on a graduated scale of intensities from moo to o, or 0000, for the faintest lines; between a quarter and a third of these lines have been identified, fully 2000 belonging to iron, and several hundred to water vapour and other atmospheric absorption. The infra-red requires special appliances; it has been examined visually by the help of phosphorescent plates (Becquerel), and with special photographic plates (Abney); but the most efficient way is to use the bolometer or radiomicrometer; by this means some 500 or 600 lines have been mapped.
The first problem of the spectrum is to identify the effects of atmospheric absorption, especially oxygen, carbonic acid and water vapour; this is done generally by comparing the spectra of the sun at great and small zenith-distances, or by reducing the atmospheric effect by observing from a great elevation, as did P. J. C.
J anssen from the summit of Mont Blanc, but the only unquestionable test is to find those lines which are not touched by Doppler effect when the receding and advancing limbs of the sun are compared (Cornu); by this method H. F. Newall has verified the presence of cyanogen in the photosphere, and it had previously served to disprove the solar origin of certain oxygen lines. In fact, doubt long surrounded the presence of oxygen in the sun, and was not set at rest until K. D. T. Runge and F. Paschen in 1896 identified an unmistakable oxygen triplet in the infra-red, which is shown terrestrially only in the vacuum tube, where the spectrum is very different from that of atmospheric absorptions. The absence of lines of the spectrum of any element from the solar spectrum is no proof that the element is absent from the sun; apart from the possibility that the high temperature and other circumstances may show it transformed into some unknown mode, which is perhaps the explanation of the absence of nitrogen, chlorine and other non-metals; if the element is of high atomic weight we should expect it to be found only in the lowest strata of the sun's atmosphere, where its temperature was nearly equal to that of the central globe, and so any absorption line which it showed would be weak. This is undoubtedly the case with lead and silver, and probably with mercury also. In Rowland's table lines from the arc-spectra of the following are identified. The order is approximately that of the numbers of identified lines. Excepting strontium, those which are low upon the list are represented also by lines of small intensity. The chromosphere adds the three last of the list. The strongest lines are those due to calcium, iron, hydrogen, sodium, nickel, in the order named.
The spectrum taken near the limb of the sun shows increased general absorption, but also definite peculiarities of great interest in connexion with the spectra of the spots, which it will be convenient to describe first.
When the slit of the spectroscope is set across a spot, it shows, as might be expected, a general reduction of brightness as we pass from the photosphere to the penumbra; and a still greater one as we pass to the umbra. This is not a uniform shade over the whole length of the spectrum, but shows in bands or flutings of greater or less darkness, which in places and at intervals have been resolved by Young, Duner and other unquestionable observers into hosts of dark lines. Besides this the spectrum shows very many differences from the mean spectrum of the disk, the interpretation of which is at present far from clear. Generally speaking, the same absorption lines are present, but with altered intensities, which differ from one spot to another. Some lines of certain elements are always seen fainter or thinner than on the photosphere, or even wholly obliterated; others sometimes show the same features, but not always; other lines of the same elements, perhaps originating at a level above the spot, are not affected; there are also bright streaks where even the general absorption of the spot is absent, and sometimes such a bright line will correspond to a dark line on the photosphere; most generally the lines are intensified, generally in breadth, sometimes in darkness, sometimes in both together, sometimes in one at the expense of the other; certain lines not seen in the photosphere show only across the umbra, others cross umbra and penumbra, others reach a short distance over the photosphere. A few of the lines show a double reversal, the dark absorption line being greatly increased in breadth and showing a bright emission line in its centre. The umbra of a spot is generally not tormented by rapid line-of-sight motions; where any motion has been found G. E. Hale and W. S. Adams make its direction downwards; but round the rim and on bridges the characteristic distortions due to eruptive prominences are often observed. There appears to be some connexion between prominences and spots; quiescent prominences are sometimes found above the spots, and W. M. Mitchell records an eruptive prominence followed next day in the same place by the appearance of a small spot. It does not appear that the affected lines follow in any way the sun-spot cycle. The radiation from a spot changes little as it approaches the sun's limb; in fact Hale and Adams find that the absorption from the limb itself differs from that of the centre of the disk in a manner exactly resembling that from a spot, the same lines being strengthened or weakened in the same way, though in much less degree, with, however, one material exception: if a line is winged in the photosphere the wings are generally increased in the spot, but on the limb they are weakened or obliterated. If the spot spectrum is compared with that of the chromosphere it appears that the lines of most frequent occurrence in the latter are those least affected in the spot, and the high level chromospheric lines not at all; the natural interpretation is that the spot is below the chromosphere. As to whether the spots are regions of higher or lower temperature than the photosphere, the best qualified judges are reserved or discordant, but recent evidence seems to point very definitely to a lower temperature. Hale and Adams have shown that the spectrum contains, besides a strong linespectrum of titanium, a faint banded spectrum which is that of titanium oxide, and a second banded part remarked by Newall has been identified by A. L. Fowler as manganese hydride. The band spectrum, which corresponds to the compound or at least to the molecule of titanium, certainly belongs to a lower temperature than the line spectrum of the same metal. Hence above the spots there are vapours of temperature low enough to give the banded spectra of this refractory metal, while only line spectra of sodium, iron and others fusible at more moderate temperatures are found (see also Spect Roheliogra Ph).
The chromosphere, which surrounds the photosphere, is a cloak of gases of an average depth of 5000 m., in a state of luminescence less intense than that of the photosphere. Hence when the photosphere is viewed through it an absorption spectrum is shown, but when it can be viewed separately a bright line spectrum appears. Most of the metallic vapours that produce this lie too close to the photosphere for the separation to be made except during eclipses, when a flash spectrum of bright lines shines out for, say, five seconds after the continuous spectrum has disappeared, and again before it reappears (see Eclipse). F. W. Dyson has measured some eight hundred lines in the lower chromosphere and identified them with emission spectra of the following elements: hydrogen, helium, carbon with the cyanogen band, sodium, magnesium, aluminium, silicon, calcium, scandium, titanium, vanadium, chromium, manganese, iron, zinc, strontium, yttrium, zirconium, barium, lanthanum, cerium, neodymium, ytterbium, lead, europium, besides a few doubtful identifications; it is a curious fact that the agreement is with the spark spectra of these elements, where the photosphere shows exclusively or more definitely the arc lines, which are generally attributed to a lower temperature. In the higher chromosphere the following were recognized: helium and parhelium, hydrogen, strontium, calcium, iron, chromium, magnesium, scandium and titanium.
In the higher chromosphere on occasions metallic gases are carried up to such a level that without an eclipse a bright line spectrum of many elements may be seen, but it is always possible to see those of hydrogen and helium, and by opening the slit of the spectroscope so as to weaken still further the continuous spectrum from the photosphere (now a mere reflection) the actual forms of the gaseous structures called prominences round the sun's rim may be seen. In the visual spectrum there are four hydrogen lines and one helium line in which the actual shapes may be examined. The features seen differ according to the line used, as the circumstances prevailing at different levels of the chromosphere call out one line or another with greater intensity. The helium formations do not reach the sun's limb, and it is another puzzling detail that the spectrum of the disk shows no absorption line of anything like an intensity to correspond with the emission line of helium in the chromosphere. The prominences are of two kinds, quiescent and eruptive. Some of the former are to be seen at the limb on most occasions; they may hang for days about the same place; they reach altitudes of which the average is perhaps 20,000 m., and show the spectral lines of hydrogen and helium. Sometimes they float above the surface, sometimes they are connected with it by stems or branches, and they show delicate striated detail like cirrus cloud. The eruptive prominences, called also metallic, because it is they which show at their bases a complete bright line spectrum of the metallic elements, rush upwards at speeds which it is difficult to associate with transfers of matter; the velocity often exceeds loo m. a second; W. M. Mitchell watched one rise at 250 m. a second to the height of 70,000 m., and in five minutes after it had faded away and the region was quiet. This is remarkable only in point of velocity. Much greater heights occur. Young records one which reached an elevation of 350,000 m., or more than three-quarters of the sun's radius. Since identification of spectral lines is a matter of extreme refinement, any cause which may displace lines from their normal places, or otherwise change their features, must be examined scrupulously. We have seen above numerous applications of the Doppler effect. Two other causes of displacement call for mention in their bearing on the solar spectrum - pressure and anomalous dispersion. The pressure which produces a continuous spectrum in gases at a temperature of 6000 0 must be very great. Recent experiments on arc spectra at pressures up to 100 atmospheres by W. J. Humphreys and by W. .
ffi Dueld show several suggestive peculiarities, though their bearing on solar phenomena is not yet determined. The lines are broadened (as was already known), the intensity of emission is much increased, but some are weakened and some strengthened, nor is the amount of broadening the same for all lines, nor is it always symmetrical, being sometimes greater on the red side; but besides the effect of unsymmetrical broadening, every line is displaced towards the red; different lines again behave differently, and they may be arranged somewhat roughly in a few groups according to their behaviour; reversals are also effected, and the reversed line does not always correspond with the most intense part of the emission line. For example, in the iron spectrum three groups about wave-length 4500 are found by Duffield to be displaced respectively 0-17, 0.34, 0.66 tenth-metres, at 100 atmospheres. This shift towards the red J. Larmor suggests is due to relaxation of the spring of the surrounding ether by reason of the crowding of the molecules; a shift of 0.17 tenth-metres would, if interpreted by Doppler's principle, have been read as a receding velocity of I I km. per second. It is clear that these results may give a simple key to some puzzling anomalies, and on the other hand, they may throw a measure of uncertainty over absolute determinations of line-of-sight velocities.
The possible applications of anomalous dispersion are varied and interesting, and have recently had much attention given to them. W. H. Julius holds that this sole fact robs of objective reality almost all the features of the sun, An al ou. including prominences, spots, faculae and flocculi, and p even the eleven-year period. Though few follow him so far, an explanation of the principle will make it clear that there are numerous possible opportunities for anomalous dispersion to qualify inferences from the spectrum. Theoretically anomalous dispersion is inseparable from absorption. When a system vibrating in a free period of its own encounters, say through the medium of an enveloping aether, a second system having a different free period, and sets it in vibration, the amplitude of the second vibration is inconsiderable, except when the periods approach equality. In such a case the two systems must be regarded as a single more complex one, the absorbed vibration becomes large, though remaining always finite, and the transmitted vibration undergoes a remarkable change in its period. This is illustrated in fig. 13, where the effect of a single absorbing system upon vibrations of all wave-lengths is shown. The line n shows the factor by which the index of refraction of the transmitted vibration is multiplied, and the curve p the intensity of the absorbed vibration for that wave-length. The relative increase of index takes place on the side where the wavelength is greater than that of the absorbing system. The effect of such a change may be to bend FIG. 13. back the coloured ribbon of the spectrum upon itself, but just where this is done all its light will be robbed to maintain the absorbing system in vibration. Theory is here much less intricate than fact, but it seems to cover the most important features and to be well confirmed. Omitting extreme examples, like fuchsin, where the spectrum is actually cut in two, it is of more general importance to detect the phenomenon in the ordinary absorption lines of the metallic elements. This has been done most completely by L. Puccianti, who measured it by the interferometer in the case of more than a hundred lines of different metals; he found its degree to differ much in different lines of the same spectrum.
Differences of refractive index produce their greatest dispersive effects when incidence on the refracting surface is nearly tangential. W. H. Julius has used this fact in an admirable experiment to make the effects visible in the case of the D lines of sodium. A burner was constructed which gave a sheet of flame 75 o mm. long and 1 mm. thick and to which sodium could be supplied in measured quantity. Light from an arc lamp was so directed that only that part reached the spectroscope which fell upon the flame of the burner at grazing incidence, and was thereby refracted. As the supply of sodium was increased, the lines, besides becoming broader, did so unsymmetrically, and a shaded wing or band appeared on one side or the other according as the beam impinged on one side or the other of the flame. These bands Julius calls dispersion bands, and then, assuming that a species of tubular structure prevails within a large part of the sun (such as the filaments of the corona suggest for that region), he applies the weakening of the light to explain, for instance, the broad dark H and K calcium lines, and the sun-spots, besides many remoter applications. But it should be noted that the bands of his experiment are not due to anomalous dispersion in a strict sense. They are formed now on one side, now on the other, of the absorption line; but the rapid increase of refractive index which accompanies true anomalous dispersion, and might be expected to produce similar bands by scattering the light, appears both from theory and experiment to belong to the side of greater wave-length exclusively. Julius's phenomenon seems inseparable from grazing incidence, and hence any explanation it supplies depends upon his hypothetical tubular structure for layers of equal density. There are other difficulties. In calcium, for instance, the g line shows in the laboratory much stronger anomalous dispersion than H and K; but in the solar spectrum H and K are broad out of all comparison to g. Hale has pointed out other respects in which the explanation fails to fit facts. In connexion with the question whether the phenomena of the sun are actually very different from what they superficially appear, A. Schmidt's theory of the photosphere deserves mention; it explains how the appearance of a sharp boundary might be due to a species of mirage.
Consider the rays which meet the eye (at unit distance) at an angle d from the centre of the sun's disk; in their Theory previous passage through the partially translucent por. tions of this body we have the equation sin d = rµ sin i (fig. 14). Now generally ,u will decrease as r increases, but the initial value of µ is not likely to be more than, say, twice its final value of unity, while r increases manifold in the same range, hence in general rµ will increase with r, and therefore for a given value of d, i will continually increase as we go inwards up to 90°, which it will attain for a certain value of r, and this will be the deepest FIG. 14.
level of the sun's body from which rays will reach the eye at the given angle d. But if there is a region, say from r to r" throughout which rµ decreases as r increases, any ray which cuts the outer envelope r at an acute angle will cut the inner one r" also, and can be traced still further inwards before the angle i amounts to 90°.
Apart then from absorption there will be a discontinuous change in brightness in the apparent disk at that value of the angular radius d which corresponds to tangential emission from the upper lever r' of this mirage-forming region. Of course we are unable to say whether such a region is an actuality in the sun, on the earth it is an exception and transient, but the greater the dimensions of the body the more probable is its occurrence. The theory can be put to a certain test by considering its implications with respect to colour. The greater µ is, the greater would be the value of d, the apparent angular radius, corresponding to horizontal emission from a given level r, and that whether we accept Schmidt's theory or not. Hence if the sun's diameter were measured through differently coloured screens, the violet disk must appear greater than the red. Now measures made by Auwers with the Cape heliometer showed no difference, amounting to oI ", and so far negative the idea that the rays reach us after issuing from a level where is sensibly different from unity. Presumably, then, the inner emissions are absorbed and those which reach us start from very near the surface.
The sun's distance is the indispensable link which connects terrestrial measures with all celestial ones, those of the moon alone excepted; hence the exceptional pains taken to deter mine it. The transits of Venus of 1874 and 1882 were observed by expeditions trained for the purpose beforehand with every possible foresight, and sent out by the British,. French and German governments to occupy suitable stations distributed over the world, but they served only to demonstrate that no high degree of accuracy can ever be expected from this. method. It is the atmosphere of Venus that spoils the observation. Whatever be the subsequent method of reduction, the instant is required when the planet's disk is in internal contact with that of the sun; but after contact has plainly passed it still remains connected with the sun's rim by a " black drop," with the result that trained observers using similar instruments set up a few feet from one another sometimes differed by half a minute of time in their record. It is little wonder, then, that the several reductions of the collected results were internally discordant so as to leave outstanding a considerable " probable error," but showed themselves able to yield very different conclusions when the same set was discussed by different persons. Thus from the British observations of 1874 Sir G. B. Airy deduced a parallax of 8.76" and E. J. Stone 8.88"; from the French observations of the same date Stone deduced 8.88" and V. Puiseux 8.91". The first really adequate determinations of solar parallax were those of Sir David Gill, measured by inference from the apparent diurnal shift of Mars among the stars as the earth turned diurnally upon its axis; the observations were made at the island of Ascension in 1878. The disk of Mars and his colour are certain disadvantages, and Gill. afterwards superseded his own work by treating in the same way the three minor planets Victoria, Iris and Sappho - the last was observed by W. L. Elkin. These planets are more remote than Mars, but that loss is more than outweighed by the fact that they are indistinguishable in appearance from stars. The measures were made with the Cape heliometer and have never been superseded, for the latest results with the minor planet Eros exactly confirm Gill's result-8.80" - while they decidedly diminish the associated probable error. The planet Eros was discovered in 1899, and proved to have an orbit between the earth and Mars, while every one of the other five or six hundred. known asteroids lies between Mars and Jupiter. Its mean distance from the sun is 1.46 times i that of the earth; but, besides, the eccentricity of its orbit is large (0.22), so that at the most favourable opportunity it can come within one-seventh of the distance of the sun. This favourable case is not realized at every opposition, but in 1900 the distance was as little as one-third of that of the sun, and it was observed from October 1900 to January 1901 photographically upon a concerted but not absolutely uniform plan by many observatories, of which the chief were the French national observatories, Greenwich,. Cambridge, Washington and Mount Hamilton. The planet showed a stellar disk varying in magnitude from 9 to 12. On some plates the stars were allowed to trail and the planet was followed, in others the reverse procedure was taken; in either case the planet's position is measured by referring it to " comparison stars " of approximately its own magnitude situated within 25' to 30' of the centre of the plate, while these stars are themselves fixed by measurement from brighter " reference stars," the positions of which are found by meridian observations if absolute places are desired. The best results seem to be obtained by comparing an evening's observations. with those of the following morning at the same observatory; the reference can then be made to the same stars and errors in their position are therefore virtually eliminated; even if the observations of a morning with those of the following evening are used the probable error is doubled. The observations at Greenwich thus reduced gave errors =0.0036" and oE00080" respectively. The general result is 8.800" t4:). 0044". To collate the whole of the material accumulated at different parts of the world is a much more difficult task; it requires first of all a most carefully constructed star-catalogue, upon which the further discussion may be built. The discussion was completed in 1909 by A. R. Hinks, and includes the material from some hundreds of plates taken at twelve observatories; in general it may be said the discussion proves that the material is distinctly heterogeneous, and that in places where it would hardly be expected. The result is nearly the same as found at Greenwich alone, 8.806" =0.0026", or a mean distance of 92,830,000 m. =1.493 X10 13 cm. with an error which is as probably below as above 30,000 miles.
The sun's distance enters into other relations, three of which permit of its determination, viz. the equation of light, the constant of aberration, and the parallactic inequality of the moon; the value of the velocity of propagation of light enters in the reduction of the two first, but as this is better known than the sun's parallax, no disadvantage results. The equation of light is the time taken by light to traverse the sun's mean distance from the earth; it can be found by the acceleration or retardation of the eclipses of Jupiter's satellites according as Jupiter is approaching opposition or conjunction with the sun; a recent analysis shows that its value is 498.6", which leads to the same value of the parallax as above, but the internal discrepancies of the material put its authority upon a much lower level. The constant of aberration introduces the sun's distance by a comparison between the velocity of the earth in its orbit and the velocity of light. Its determination is difficult, because it is involved with questions of the changing orientation of the earth's axis of rotation. S. C. Chandler considers the value 20.52" is well established; this would give a parallax of 8.78". The chief term in the lunar longitude which introduces the ratio of the distances of the sun and moon from the earth explicitly is known as the parallactic inequality; by analysis of the observations P. H. Cowell finds that its coefficient is 124.75", which according to E. W. Brown's lunar theory would imply a parallax 8.778".
The best discussion of the sun's apparent diameter has been made by G. F. J. A. Auwers, in connexion with his reduction of The The the German observations of the transit of Venus of 1874 and 1882. It was found that personality played an I mportant part; the average effect might be t", but frequently it reached 3", 4", 5" or even 10", with the same instrument and method, ndr was it fixed for the same observer. Some 15,000 observations, from 1851 to 1883, taken by one hundred observers at Greenwich, Washington, Oxford and Neuchatel, cleared as far as possible of personal equation, showed no sign of change that could with probability be called progressive or periodic, particularly there was no sign of adhesion to the sun-spot period. Better determinations of the actual value came from the heliometer, and gave an angular diameter of 31' 59.26" =0.10", and the value of the polar diameter exceeded the equatorial by 0.038" =0.023". The conclusion is that the photosphere is very sharply defined and shows no definite departure from a truly spherical shape. Using the parallax 8.80", the resulting diameter of the sun is 864,000 m. =1.390 X 10 11 cm.
If we regard the sun as one of the stars, the first four questions we should seek to answer are its distance from its neighbours, proper motion, magnitude and spectral type. In some The Star. respects the systematic prosecution of these inquiries a has only begun, and properly considered they involve vast researches into the whole stellar system. It would take us too far to treat them at any length, but it may be convenient to summarize some of the results. Thesun's nearest neighbour is a Centauri, which is separated from it by 270,000 times the earth's distance, a space which it would take light four years to traverse. It is fairly certain that not more than six stars lie within twice this distance. No certain guide has been found to tell which stars are nearest to us; both brightness and large proper motion, though of course increased by proximity, are apparently without systematic average relation to parallax.
The sun's proper motion among the stars has been sought in the past as the assumption that the universe of stars showed as a whole no definite displacement of its parts, and, on this assumption, different methods of reduction which attributed apparent relative displacement of parts to real relative displacement of the sun agreed fairly well in concluding that the " apex of the sun's way " was directed to a point in right ascension 275°, declination 37° (F. W. Dyson and W. G. Thackeray), that is to say, not far from the star Vega in the constellation Lyra, and was moving thither at a rate of twelve miles per second. But recent researches by J. C. Kapteyn and A. S. Eddington, confirmed by Dyson, show that there is better ground for believing that the universe is composed mainl y of two streams of stars, the members of each stream actuated by proper motions of the same sense and magnitude on the average, than that the relative motions of the stars with one another are fortuitous (see Star). This removes completely the ground upon which the direction of the sun's way has hitherto been calculated, and leaves the question wholly without answer.
A star is said to rise one unit in magnitude when the logarithm of its brightness diminishes by 0.4. Taking as a star of magnitude I a Tauri or a Aquilae, where would the sun stand in this scale ? Several estimates have been made which agree well together; whether direct use is made of known parallaxes, or comparison is made with binaries of well-determined orbits of the same spectral type as the sun, in which therefore it may be assumed there is the same relation between mass and brilliancy (Gore), the result is found that the sun's magnitude is - 26.5, or the sun is Io n times as brilliant as a first magnitude star; it would follow that the sun viewed from a Centauri would appear as of magnitude 0.7, and from a star of average distance which has a parallax certainly less than o1 ", it would be at least fainter than the fifth magnitude, or, say, upon the border-line for naked-eye visibility. We cannot here do more than refer to the spectral type of the sun. It is virtually identical with a group known as the " yellow stars," of which the most prominent examples are Capella, Pollux and Arcturus; this is not the most numerous group, however; more than one half of all the stars whose spectra are known belong to a simpler type in which the metallic lines are faint or absent, excepting hydrogen and sometimes helium, which declare themselves with increased prominence. These are the white stars, and the most prominent examples are Sirius, Vega and Procyon. It is commonly though not universally held that the difference between the white and yellow stars arises from their stages of development merely, and that the former represent the earlier stage. This again is disputed, and there is indeed as yet slight material for a decisive statement.
Summary of Numerical Data. Parallax: 8.806" t 0.003".
Mean distance from earth: 92,830,000 m. =1.493 X Io 13 cm.
(Time taken by light to traverse this distance: 498.6"). Diameter: Angular, at mean distance, 1919.3". Linear, 109 Xearth's equatorial diameter =864,000 M. = 1.390 X io ll cm.
Mass: 332,000 X mass of the earth.
Mean density: 256 X mean density of earth =1.415.
Equator; inclination to ecliptic: 7° 15'.
Longitude of ascending node (1908.0), 74° 28.6'. Rotation period; latitude o° :24.46d 300:26.43d 60°:29.634 80°:30.56d Solar constant, or units of energy received per minute per square centimetre at earth's mean distance: 2 I calories.
Effective temperature, as an ideal radiator or " black body ": 6000° abs.
Bibliography. - Nearly all the chief data respecting the sun have lately been and still are under active revision, so that publications have tended to fall rapidly out of date. The most important series is the Astrophysical Journal, which is indispensable, and in itself almost sufficient; among other matter it contains all the publications of Mount Wilson Solar Observatory (Professor G. E. Hale), H. A. Rowland's Tables of Wave-Lengths, many theoretical papers, and some reproductions of important papers issued elsewhere. But there are also papers which cannot be disregarded in Monthly Notices and Memoirs of the Royal Astronomical Society, and in Astronomische Nachrichten.
S. P. Langley's Researches on Solar Heat are published by the War Department (Signal Service, xv.) (Washington, 1884), and Gill's parallax researches in Cape Annals, vols. vi., vii. Auwer's discussion of the sun's diameter is in the discussion of the transit of Venus observations for 1874 and 1882. The best single volume upon the whole subject is C. A. Young's The Sun, 2nd ed. (Inter. Sci. Series), and an excellent summary of solar spectroscopy, as far as rapid progress permits, is in Frost's translation of Scheiner, Astronomical Spectroscopy (1894). Scheiner's volume, Strahlung u. Temperatur d. Sonne (1899), contains a great quantity of interesting matter carefully collected and discussed. For authoritative declarations upon the latest moot points the Transactions of the International Union for Solar Research (Manchester) may be consulted, vol. i. having been issued in 1906, and vol. ii. in 1908. (R. A. SA.)
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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Sun'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/s/sun.html. 1910.
