  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.
