  Based on a recent paper by Rothe and Sch\"afer on compact binary systems,
explicit expressions for canonical center and relative coordinates in terms of
standard canonical coordinates are derived for spinless objects up to second
post-Newtonian approximation of Einstein's theory of gravity. The inverse
relations, i.e. the dependence of the standard canonical coordinates on the
canonical center and relative coordinates, are also given up to the second
post-Newtonian approximation. The famous Pythagorean-theorem-type
Lorentz-invariant relation between the system's total energy or Hamiltonian
squared, the rest energy or mass squared - solely depending on relative
coordinates -, and the total linear momentum squared are explicitly shown
through second post-Newtonian approximation.
