Solar Flux Density Reaching Earth
		q  Solar Constant (S)     The solar energy density at the mean distance of Earth from the sun (1.5  x 1011 m)         S = L / (4 p d2)            = (3.9 x 1026 W) / [4 x 3.14 x (1.5  x 1011 m)2]            = 1370 W/m2