I am wondering how space geographers would measure curvature of space around a large isolated star. i am thinking of the set up where there are two nearby spheres surrounding the star whose circumferences are already known. The remaining step is to measure the length of a radial geodesic segment connecting the two spheres. This it seems would give measurements in geodesic polar coordinates and would allow the computation of curvature using the usual formulas. How then does one find a geodesic ray and the measure its length?