A study of the apsidal angle and a proof of monotonicity in the logarithmic potential case

Abstract

This paper concerns the behaviour of the apsidal angle for orbits of central force system with homogeneous potential of degree $-2 \le \alpha \le 1$ and logarithmic potential. We derive a formula for the apsidal angle as a fixed end-points integral and we study the derivative of the apsidal angle with respect to the angular momentum $\ell$. The monotonicity of the apsidal angle as function of $\ell$ is discussed and it is proved in the logarithmic potential case.