STORMY WEATHER ON THE SUN — W. 0. ROBERTS 169 



SO that even though the bright face of the sun is obscured, some of 

 its light is scattered toward us from the clouds of electrons near the 

 sun. The electrons move so fast that the well-known absorption 

 lines of the spectrum of sunlight are smoothed away by the Doppler 

 effect. Also the light scattered by the electrons is polarized, in exact 

 and gratifying accordance with optical theory. 



The "dust corona," on the other hand, has recently been deduced 

 independently by van de Hulst of Holland and Allen of Australia. 

 These two astrophysicists agree in attributing it to particles between 

 the sun and the earth. Of course, they cannot be too near the sun, 

 because there the particles would all be vaporized by the heat. The 

 dust corona, then, is not really a part of the sun at all. It is a sort 

 of dust halo only apparently radiating from the sun and is perfectly 

 symmetrical, unlike the electron corona, which exhibits pronounced 

 streamers in some directions and which changes in shape with changes 

 in solar activity as the electron streams change their density. 



Measurement of the absolute brightness, the spectrum, the inten- 

 sity changes, the brightness in different colors, the relations of coro- 

 nal structure to surface features — these and many other important 

 scientific problems provide the incentives for eclipse research. 



But eclipses are rare events, and their locations often difficult to 

 reach. Weather vagaries increase the hazards of eclipse observing. 

 On the average, less than one total eclipse a year takes place on the 

 earth, and even then it lasts for just a few minutes at any given loca- 

 tion. It is expensive and risky, however valuable, to travel to eclipses 

 for scientific studies. 



Thus, it is no wonder that many famous astronomers have tradi- 

 tionally directed their efforts to observing the atmosphere of the sun 

 without total eclipses. The first real success came in Guntoor, India, 

 on August 19, 1868, when P. C. Jules Janssen, of France, the day 

 after a famous solar eclipse that he had gone to India to observe, 

 first placed the slit of a powerful spectroscope at the very edge of the 

 sun and looked successfully for the appearance of the lines of the 

 spectrum of solar prominences at the sun's edge. Today many 

 observatories operate spectroheliographs that are the direct descend- 

 ants of Janssen's experiment. Most notable are those at the ^IcMath- 

 Hulbert Observatory in Michigan, at Mount Wilson in California, 

 at Greenwich in England, at Meudon in France, and at Kodaikanal 

 in India. 



Nor is it any wonder that the success of Janssen, soon duplicated 

 independently by Lockyer of England, stimulated others to try to 

 observe the coronal spectrum by similar means. Today we know why 

 the corona did not yield to the same method but withheld its feeble 

 light from noneclipse research until the brilliant efforts of Bernard 

 Lyot of France were crowned with success in 1930. 



