1\0V. 12, 1885] 



NA TURE 



35 



RADIANT LIGHT AiVD HEAT' 



IV. 



Radiation and Absorption — Celestial Applications. 



THE continuous emission of light and heat from the 

 sun and stars through long periods, consisting of 

 millions of years, cannot fail to strike us with amazement, 

 more especially if we regard the great intensity of this 

 radiation. It has been conjectured that the amount of 

 solar heat received by the earth in one year would liquefy 

 a layer of ice 100 feet thick, covering the whole surface of 

 the earth. Now if we, bear in mind that the solar heat 

 reaching the earth at any time is only irgTjgg'ijijgoij of that 

 which leaves the sun, we may obtain some conception of 

 the enormous radiation from our luminary. It has been 

 calculated by Sir William Thomson that if the sun were 

 a hot solid body, such as carbon, its surface would cool in 

 a few minutes of time. It therefore becomes an object 

 of great scientific interest and importance to discover 

 what is the nature of the peculiar machinery which 

 enables the sun to continue, without interruption, dis- 

 charging, as it does, into space such enormous quantities 

 of radiant energy. The reply to this question can best be 

 given by a detailed study of the surface of the sun. 

 Whether viewed telescopically or spectroscopically, this 



surface is by no means that of a globe of uniformly 

 luminous heated matter. Let us begin by e.xamining this 

 surface telescopically. 



Shortly after the invention of the telescope Galileo and 

 Scheiner showed that the disc of the sun is far from being 

 uniformly luminous, since it frequently presents the ap- 

 pearance of having large spots on its surface. This is a 

 fact which had been previously known to the Chinese. 

 Further research showed that these black spots exhibit at 

 least two degrees of darkness, consisting of a central 

 intensely dark umbra, surrounded by a penumbra, or 

 semi-dark border. We know now that even the umbra 

 is not absolutely black, but consists of matter at a tem- 

 perature comparatively low as regards the sun, but com- 

 paratively high as regards the earth. It was likewise 

 found in the course of telescopic research that there are 

 patches which are brighter, not darker than the average 

 solar surface m- photosphere, 3.ViA these bright patches have 

 been termed /tjcv/te. 



Thus we have on the solar surface things with three 

 degrees of brightness, consisting of the normal solar 

 surface or photosphere, of the spots which are darker 

 than it, and of the faculas, which are the brightest of all. 



* Continued from vo'.. xxxii. p. 551. 



The facula; are more especially to be found in the neigh- 

 bourhood of spots. 



These are the phenomena which may generally be 

 viewed on the sun's surface on any occasion by means of 

 an ordinary telescope. Nevertheless, there are occasions 

 on which we shall find no spots. Schwabe, a German 

 observer, after forty years' patient study of the sun's 

 surface, was successful in detecting a periodicity of these 

 phenomena. There are certain years of maximum and 

 other years of minimum sun-spot frequency, and the 

 average distance from one maximum to the next, or from 

 one minimum to the next, is about eleven years. 



I have said nothing hitherto about the rotation of the 

 sun, which was discovered by means of the apparent 

 motion of the sun spots over the solar disc. This rotation 

 takes place in about twenty-six days, and its plane is not 

 far removed from the ecliptic, or that in which the earth 

 moves around the sun, the two motions being likewise in 

 the same direction. It has been discovered by Carrington 

 that, as a rule, spots are confined to the regions around 

 the solar equator, never by any chance appearing at the 

 poles. 



The nature of these spots has been a subject of much 



i\Si«:vA'«^^ 



discussion. Professor Wilson, of Glasgow, was the first to 

 bring forward evidence indicating that they are below the 

 general level of the solar surface — pits, in fact, the bottoms 

 of which are intensely black, while the sloping sides are 

 less so. This evidence consisted in the fact that when 

 near the sun's border that portion of the penumbra of a 

 spot which is next the visual centre is hidden from our 

 view, a behaviour which is illustrated in Fig. II. Again, 

 it has been pointed out by the Kevv obser\-ers that the 

 bottom of a spot is blacker because it is colder than the 

 general surface, and they have likewise brought forward 

 evidence to show that this diminution of temperature has 



