CHAPTER 3 



Sunlight as a Source of Radiation 



J. A. Sanderson and E. O. Hulburt 



Naval Research Laboratory 

 Washington, D.C. 



The sun as o radiator. Sunlight on top of the atmosphere. Solar ultraviolet radiation 

 at the earth's surface. Atmospheric ozone. Calculated ultraviolet intensity at the earth's 

 surface. Observed ultraviolet intensity at the earth's surface. Solar infrared spectrum 

 at the earth's surface. References. 



THE SUN AS A RADIATOR 



As a star, the sun is quite ordinary ; it is placed by astronomers in spec- 

 tral class Go of yellow stars which, on the average, are about one one- 

 hundredth as bright as average blue stars and a hundred times brighter 

 than average red stars. It is located near the middle of the main sequence 

 into which stars fall when absolute magnitudes are plotted against spec- 

 tral class. As the source of a spectrum of radiations extending from 

 X-ray to radio wave lengths which fall on the earth, conveniently situated 

 to utilize them in sustenance of life, it is a fascinating object of never- 

 ending study. The story of the sun has been well written by Abetti 

 (1938), by Menzel (1949), and by Hoyle (1949). Hoyle devoted his 

 effort to specialized problems of solar physics, while Abetti and Menzel 

 gave more general discussions of knowledge of the sun. The radiations 

 of the sun probably derive their energy from nuclear reactions deep in 

 the gaseous interior, where the temperature is about 40 million degrees 

 and the density is 76 compared with a density of 1.4 for the entire sun. 

 These radiations finally emerge from the outer layers, mainly at tempera- 

 tures of 4000°-G000°K, and then travel unmodified for 8 brief minutes 

 until they reach the svn-face of the earth, at which point they are the 

 principal interest of this chapter. 



Since the sun is gaseous throughout, transitions in temperature, the 

 states of matter, and the character of the radiation emitted are gradual 

 along the radius. Yet several marked distinctions in these properties 

 exist, which permit the division of the outer reaches of the incandescent 

 mass of gas into several regions, each characterized l\v its radiative prop- 

 erties. The photosphere, or light sphere, is the innermost and most 



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