The problem of scaling the impinging solar radiation spectrum, to the excitance of blackbodies on earth at moderate temperatures is discussed. Typically diagrams with these two kinds of spectra are used to demonstrate wavelength separation as the background for solar, optical selectivity. All spectra can satisfactorily be approximated with Planck curves, but the solar spectrum must be reduced in intensity because of the long distance Earth - Sun. Some difficulties with using astronomical data for this scaling are noted. As an alternative, the T5-dependence of the Planck curve maxima, when drawn on a wavelength axis, is used to demonstrate that the curves can be scaled based on their total energy and solar absorber stagnation. The ratios of the peaks of the impinging solar radiation to the excitance spectra for black surfaces on earth at various ordinary temperatures are reported. It is pointed out that the choice of independent variable effects the positions of the Planck function peaks, as well the shape of the spectra. The peak values increase proportionally to T3 when frequency is used instead of wavelength and the width of the curves grows linearily with temperature. Effects of using a logarithmic wavelength scale are also noted.