Dependence of the phase velocity of the turning points on the galactocentric radius for the first four shells according to Eq. (3). For parameters of the host galaxy potential, see Sect. 6.1. Black crosses show the true velocity of the first shell calculated for several radii according to the method described in Sect. 2.2. In fact, the turning point responsible for the current location of shell is not at the same radius as the shell edge at the same time, but the difference is small (Table 1).