Chapter 12
Solar Radiation
12.1 The Nature of the Solar Radiation
The sun radiates in all regions of the spectrum, from radio waves to gamma rays. Our eyes are sensitive to less than one octave of this, from 400 to 750 THz (750 to 400 nm), a region known, for obvious reasons, as visible. Although narrow, it contains about 45% of all energy. At the distance of one astronomical unit, the power density of the solar radiation is about 
, a value called solar constant, which is not really constant—it varies a little throughout the year, being largest in January when the Earth is nearest the sun.
The expression power density is used to indicate the number ...
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