In the Schwarzschild geometry of a static spacetime, elliptical test-particle orbits precess at a rate that (famously) agrees with observations of the inner solar system. Yet the model system considered is isolated, spherically symmetric with only the radial coordinate non-Euclidean. I can't figure out what the axes of the ellipse precess "relative to" in this highly symmetric situation. Neither the "fixed stars" nor the CMB provide any explicit reference frame for the model. Does the analyst somehow provide an implicit reference? Indeed I fail to see what physically causes the GR precession in such a symmetric model situation. How does the feature of orbital precession get built into the model?