High Quality Content by WIKIPEDIA articles! The Poynting?Robertson effect, also known as Poynting?Robertson drag, named after John Henry Poynting and Howard Percy Robertson, is a process by which solar radiation causes a dust grain in the solar system to slowly spiral inward. The drag is essentially a component of radiation pressure tangential to the grain's motion. The first description of this effect, given by Poynting in 1903, was still "luminiferous aether"-based. Later, in 1937, Robertson described the effect correctly in terms of general relativity.