... The simple pair of formulas derived in problem (8) . ... around the z axis, , giving the above result and producing (x',y'), while z remains the same, so that in the new system z' = z. Then we rotate (x',y',z') by an angle ? around the x' axis, using the same formulas but with (y',z', ?) in place of (x,y, ?) ... Knowing its semi-major axis a, eccentricity e and mean anomaly M, one can derive its true anomaly f (using appropriate formulas, not given in #12a) and from it the radial distance r . ...