Are explanations of the Poynting-Robertson effect correct?

Physics of the Poynting-Robertson (P-R) effect is discussed and compared with the statements published in the past thirty years. Relativistically covariant formulation reveals the essence of the P-R effect and points out to nonphysical explanations in scientific papers and monographs. Although the final equation of motion $m$ $d\vec{v} / dt$ $=$ ($S A'\bar{Q'}_{pr}$ $/$ $c$) ${(1 - \vec{v} \cdot \vec{e} / c) \vec{e} - \vec{v} / c}$ has been usually correctly presented and used, its derivation and explanation of its essence is frequently incorrect. The relativistically covariant form of the equation of motion yields the P-R effect as an action of the radiation pressure force on a moving spherical body. No "P-R drag", as a particular relativistically covariant equation of motion, exists. Omission of the nonphysical term "P-R drag" excludes any confusion in the published definitions. The difference between the effects of solar electromagnetic and corpuscular (solar wind) radiation is stressed. The force acting on the particle due to the solar wind (the simple case of radial solar wind velocity is considered) is $\vec{F}_{sw}$ $=$ $F_{sw}$ [ (1 $-$ $\vec{v} \cdot \vec{e} / v_{sw}$) $\vec{e}$ $-$ $x'$ $\vec{v} / v_{sw}$ ], where $F_{sw}$ is the force on the stationary particle, $v_{sw}$ is the heliocentric solar-wind speed, and, the value of $x'$ depends on material properties of the particle (1 $<$ $x'$ $<$ 3). Secular evolution of orbital elements is presented. Initial conditions are included.