Radiation falling on a surface exerts a pressure; its value is proportional to the net flux of energy divided by the speed
of light. If a particle is traveling in an orbit around the Sun, solar
radiation will appear to come from a position slightly shifted toward the
direction in which the particle is moving, just as to a moving pedestrian raindrops appear to fall not vertically but from a direction tilted toward the direction in which the person walks. This displacement is an aberration of light (see aberration, stellar) and causes a net retarding force to act on the particle (Poynting-Robertson effect) that will ultimately cause the particle to fall into the Sun.