Abstract

Using a photoelectric telephotometer and a calibration source, drift curves of the luminance of the sun along a diameter were obtained for different zenith distances of the sun on clear days. After extrapolation to zero air mass, the luminance <i>B</i> (0,0) at the center of the sun’s disk observed outside the earth’s atmosphere was obtained as 243 000±5000 candles/cm<sup>2</sup>. The data obtained for the radial distribution of luminance being comparatively less accurate, the standard limb darkening data of Abbot and others were used to give the average luminance B¯= 193 000±4000 candles/cm<sup>2</sup> and the extraterrestrial total solar illumination Ē<sub>0</sub>= 12 200±300 lumens/ft<sup>2</sup> for the mean solar distance. Previous determinations of Ē<sub>0</sub> found in the literature are reviewed and compared with these results.

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