The Edge of the Sun 1 



By Donald H. Menzel 



Director, Harvard College Observatory 

 Paine Professor of Astronomy, Harvard University 



[With 4 plates] 



If the title of this lecture, "The Edge of the Sun," perchance 

 suggests to you Rachel Carson's delightful book, "The Edge of the 

 Sea," the resemblance is not purely coincidental. I chose the title de- 

 liberately because of the many similarities that exist between the sun 

 and the sea. If any of you should quibble that I use the word "edge" 

 in a different sense than did Miss Carson, to indicate a boundary layer 

 rather than a boundary line, let me refer you to Webster, who approves 

 the double usage. 



When I agreed almost nine months ago to give this lecture on the 

 subject of "What's New on the Sun," I had no idea that the sun would 

 be so cooperative. As a result, much of what I would have told you 

 then has since been changed. Even in the several months that have 

 elapsed since the lecture and the preparation of the final manuscript, 

 solar activity has markedly increased. I have included some of the 

 latest illustrations and have employed the information derived from 

 them to help solve our basic problem : " What is the structure of the 

 sun's outer layers and how does solar activity originate?" 



Man's interpretation of the sun has been undergoing a gradual evo- 

 lution for thousands of years. We no longer look to the sun as a deity, 

 nor do we bring offerings to solar temples as a means of soliciting his 

 bounty. 



We recognize that the sun is a sphere of hot gas, whose vast output 

 of light and heat comes from atomic energy, released deep in its core. 

 The process of atomic fusion that gives rise to the sun's energy appears 

 to be similar to that employed in our great H bombs : fusion of protons, 

 the nuclei of hydrogen atoms. The energy so released in the sun's 

 interior is equal to that from a billion or so hydrogen bombs exploded 

 every second. Tonight we shall largely concern ourselves with the 

 profound effect that this energy has upon the sun's outer layers, just 



1 The twenty-third James Arthur Lecture, given under the auspices of the 

 Smithsonian Institution April 26, 1956. 



412575—57 15 215 



