400 



NA TURE 



{Feb. 25, 1886 



our power to know very much more about the sun, which is the 

 star that lies nearest to us, than about the other stars, so far as 

 detailed structure, at all events, is concerned, for the reason that 

 the sun, although actually so far away, is relatively very near to 

 us. 



The Sun's Dislance ^ 



The distance of the sun we may take to be about 93,000,000 

 of miles, and, although that seems a long way in terrestri.il 

 reckonings, twice that distance is the smallest base line which 

 astronomers can use in dealing with the distance of the star 

 which is next nearest to us, to say nothing of the millions of 

 others more remote. The sun, from being relatively so near tu 

 us, appears as a body of a different order. The stars proper, how- 

 ever powerful the telescope with which we regard them, appear 

 to us as the finest points imaginable, whereas the sun gives us 

 the appearance of a circular disk, this disk being the projection of 

 a sphere. That part of the sun with which we are most familiar 

 is in fact a sphere of something like 860,000 miles in diameter ; 

 hence, taking the diameter of this world of ours roughly at 

 8000 miles, the diameter of the sun is more than 100 times a, 

 great. 



The moon, the nearest celestial body to us, journeys round us 

 at the relatively small distance of about 240,000 miles. The 



Fig. I. — The orbit of the Moon and the circumference of the Sun 

 compared. 



accompanying drawing will enable us to coa^pare the orbit or 

 l>ath of the moon round the earth and the space which it 

 incloses, with the actual circumference of the sun (Fig. i). 



In the centre we have the earth very much larger than it 

 ought to be, and the inner disk represents the space included in 

 the orbit of the moon. The outer one represents the disk of the 

 sun ; so that it is clearly seen that if the sun were a soap bubble 

 it would be quite easy for the earth's attendant satellite, the 

 moDn, to carry on its revolution inside it ; in fact the orbit of 

 the moon would not He very much past half-way from the centre 

 of the earth to the circumference of the sun. 



The Sun's Envelopes 



The ne.^t point which has been made clear by the work of 

 the last qnarter of a century, let us say, is that this central 

 nucleus which we see ordinarily and call the sun, is only, after 

 all, a very small part of it. Outside it there is a shell very ex- 

 quisite in colour. This might be described as a sort of sea 



^ The most recently determined value nf the sun's distance depends upon 

 Prof. Newcomb's determination of the velocity of light. This velocity Prof. 

 Newcomb values at 299,860 kilometres per second, with a probaijie error of 

 30 kilometres either way. Combining this with Nyr^n's value of the constant 

 of aberration 2o"'4g2, the S3lar parallax = 8"*794, which gives a distance cf 

 14961 millions of kilometres. This equals 92,965,020 British statute miles. 



surrounding the central nucleus, if that expression did not give 

 the idea that the nucleus itself was a solid, which it is not. 



This first envelope, the chromosphere, as it is called, repre- 

 sents a sort of atmosphere or sea surrounding what is named the 

 photosphere, to a height varying, say, from 5000 to io,ooo 

 miles. I have said that it is exquisite in colour. It is of an 

 intensely brilliant pearly white at the base, and of a magnifi- 

 cent scarlet, as a rule, higher up. But this is not the outside of 

 the sun by any means. Further from the centre, again, there is 

 another region, which we may call the inner corona, overlying 

 the photosphere and chromosphere ; the height of this atmo- 

 sphere — I mean the distance from the top of it on the average 

 down to the photosphere — we may again roughly take at some- 

 thing like 100,000 miles. Not very many years ago, in the 

 text-books we were told that the earth's atmosphere only ex- 

 tended to a height of 50 miles, so that it can be easily recognised 

 that we are dealing with an atmosphere on a very large scale 

 indeed when we come to touch the sun. 



These portions of the sun's atmosphere are only very rarely 

 seen under the best conditions. We can feel, so to speak, the 

 lower reaches every day in our observatories, but we can only 

 see them when the sun is eclipsed. When, in an eclipse, we can 

 get a good sight of the inner corona, what we see is very beauti- 

 ful indeed, because we not merely get a pearly shell of light, 

 which, roughly speaking, may be taken to be 100,000 miles 

 high, but we see stretching into it from and through the chromo- 

 sphere beautiful and curious objects called "red flames," or 

 " prominences," or " protuberances." 



Have we yet finished with the solar envelopes? No ; there is 

 still another. There is still an upper atmosphere, and to this I 

 must ask you to give a height of anything you like between half 

 a million of miles and a million and a half; and I speak thus 

 indefinitely for the reason that the exact limit is at the present 

 moment occupying those who are concerned in these matters. 

 The limit of course you will understand is a limit which can 

 only be determined during eclipses. Now the sun is only 

 eclipsed for something less than a week in a whole century over 

 the whole earth, and I suppose that if an observer of eclipses 

 were to give his whole time to them he could not spend more 

 on the average than six minutes every two years, so that the 

 time is not excessive in which the astronomer either has to 

 make observations or to make up his mind as to what he sees. 

 Vou must not, therefore, be surprised when I give you this large 

 choice. If we call it roughly a million of miles, we at all events 

 shall not be very far off the truth, even supposing the height to 

 be constant ; but it would appear indeed that the height varies every 

 time we have a chance of observing it. On that point we shall 

 have a great deal to say further on. 



When we cjme to this outer atmosphere, we pass from one 

 with a more or less concentric boundary, to one with a most 

 irregular outline, full of strange forms varying in an almost inex- 

 plicable manner from eclipse to eclipse. 



In the eclipse of 1878 one of the observers who took special 

 precautions to shield his eye from any brighter light at the 

 moment that the eclipse took place, imagined, or saw, the 

 fainter portions of the solar corona, or some solar surrounding, 

 extending to several diameters. The outer corona is not only 

 very strange in its appearance, but wonderful in colour, and 

 full of detail for a considerable distance from the dark moon. 



To sum up some of the principal points — by no means all of 

 them — we may say, first, that its outline is very irregular ; that 

 there seems to be a flattening, or very often two opposed flatten- 

 ings, at opposite ends of a solar diameter. This tends to make 

 the thing look very often more or less square. In all parts of it, 

 irregularly (by which I mean that you cannot predict quite 

 where they will he), you get radial rifts in which the light is 

 much less intense than elsewhere. 



I have, then, indicated that the sun that we see is not the 

 whole of the sun. Hence, when we study the stars we shall 

 probably find that we have not only to take into account the 

 phenomena presented by the sun as we ordinarily see it, but 

 others associated with those parts of the sun which are only 

 revealed to us from time to time, and the possibility that such 

 phenomena as we see on the sun may be enormously intensified 

 in other bodies. 



The Suns lioteilion 



The next question that we have to put concerning the sun is 

 this : — Since it is a sphere as the earth is, and since the earth 

 rotates on its axis, does the sun rotate on its axis? How are 

 we to answer that question ? 



