A definition of "closeness" of fit of a gray body curve to the sun's extraterrestrial spectrum is presented. F. S. Johnson's data are used for the solar spectrum. For the wavelength inter¬val 0.3 to 2.4 microns (pertinent to thermal design), the magni¬tude of C1 calculated for use in Planck's black body formula is 8.6504 X 10-17 watt-cm2 at a temperature of 5742 ± 0.5 °K.
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