Every mass point is either in motion or not. Motion is said to be relative, but this will be only appearance. For mass points in motion, there are motions in which vector of motion (magnitude and direction) vary and motions in which there are no varying. In motions varying, inertial force that vary (qualitatively or quantitatively) are recognized. In motions without varying, inertial force are not recognized.
The above will be depending on existence of a uniform isotropic aether in space. Every motion relative to aether (absolute rest frame) will be measurable by optical way easily.ｙ
