174 AKNUAL REPORT SMITHSONIAN INSTITUTION, 1942 



(.issue coil veil tiiis solar energy into chemical energy and how the sun's 

 rays falling on living animals create vitamins so essential to healthy 

 development. Paleontologists and geologists have shown how the 

 sun's energy has been captured in the past and preserved for us through 

 the ages, even though we are now forced to waste much of that pre- 

 cious energy in the prosecution of the war. There are, in addition, 

 certain effects of the sun's radiation not directly perceptible to our 

 senses but nevertheless of considerable importance to our modern lives. 

 I refer to the great physiochemical action of the sun's radiation on 

 our outer atmosphere. Through this intervening medium the sun 

 affects the earth's magnetism, produces polar lights, and makes pos- 

 sible transmission of radio messages over great distances. Through 

 study of these effects we are able in turn to obtain a better understand- 

 ing of solar phenomena. In particular, we have learned that the sun 

 not only sends out wave radiation but particle radiation as well, and 

 that the intensity of ultraviolet radiation from the sun far exceeds 

 what should be expected from observations of its visible radiation 

 alone. Our subject is this aspect of the sun and its manifold conse- 

 quences on the earth. 



SOLAR PHENOMENA'' 



The sun is an ever-changing, seething sphere 864,300 miles in dia- 

 meter. When viewed through a telescope, its surface appears gran- 

 ular; this is because of small variations in temperature over rela- 

 tively small areas (a few hundred square miles). Most obvious of 

 solar surface disturbances are the very large dark areas — in reality 

 luminous, but dark by contrast — known as sunspots, which frequently 

 appear. These vary greatly in size and on rare occasions may be al- 

 most 20 earth-diameters across. A spot about 30,000 miles or 2 earth- 

 diameters across can be seen with the eye through a piece of smoked 

 glass. Its day-to-day motion shows that the sun rotates in the same 

 direction as the earth. Sunspots tend also to occur in pairs, though 

 having smaller companions, with the larger or more stable spot in the 

 direction of the sun's rotation from left to right as seen from the earth. 

 They are solar tornadoes in which the whirling gases show features 

 resembling the field of an electromagnet. The great flames called 

 prominences, seen more readily near the edge of the sun, may remain 

 relatively steady for several days, occasionally in form of arches. 

 Eruptive prominences, attaining heights of hundreds of thousands of 

 miles, may appear in rapid succession, in arched form, above an area 

 subsequently occupied by sunspots. The following motion of the gas 



- The author is indebted to members of the staff of the Mount Wilson Observatory, espe- 

 cially to Director W. S. Adams and to Drs. Seth B. Nicholson and R. S. Richardson, from 

 whose articles much of the matter in this section on solar phenomena is taken, and for 

 permission to reproduce some of the Observatory's beautiful solar photographs. 



