The motional properties of the Earth's bow shock are analyzed using Geotail observations from beyond 70 RE behind the Earth. It is verified, with the help of an MHD model, that the shock motion mechanism can be explained as a consequence of the interaction between an upstream Alfv¿nic disturbance and the shock. From 68 samples of bow shock crossings observed in the flank region, the motion was found to be described by a large-scale flutter, accompanied by two- or three-dimensional motions, rather than as a simple one-dimensional oscillation. We observed that the local shock normal direction is distributed ~¿20¿ around the direction of its static shape. The surface velocity of the bow shock, Vshock, increases with increasing deviation of the shock normal direction, n, from its equilibrium direction. We have further shown that there is a linear relationship between Vshock and n. Upon a directional change of the solar wind magnetic field, the bow shock moves so that the incident solar wind flux in the shock rest frame is kept approximately constant. Results from MHD model calculations correlate well with such observations, where the bow shock minimizes the thermodynamic variations in the magnetosheath in response to upstream perturbations. Some extraordinary observations that the shock normal vector has an antisunward component can also be accounted for in this context. ¿ 2000 American Geophysical Union