The area of a sphere whose radius is equal to the earth's distance from the sun in aphelion being to the sun's area as 218.1² to 1, while the reflecter of the solar pyrometer intercepts a sunbeam of 3,130 square inches section, it follows that the reflector will receive the radiant heat developed by 3,130 / 218.1² = 0.0658 square inch of the solar surface.