The amount of light energy from the Sun falling on any area S is proportional to the size of the solid angle within which this body is seen from the Sun. As the area moves away from the sun, the size of this solid angle decreases proportionally to the square of the distance between the area and the Sun. Hence the energy falling on the area, and, consequently, also the light pressure, should decrease proportionally to the square of the distance as the area recedes from the Sun.
