  In a central force system the apsidal angle is the angle at the centre of
force between two consecutive apsis and measures the precession rate of the
line of apsis. The apsidal angle has applications in different fields and the
Newton's apsidal precession theorem has been extensively studied by
astronomers, physicist and mathematicians. The perihelion precession of
Mercury, the dynamics of galaxies, the vortex dynamics, the JWKB quantisation
condition are some examples where the apsidal angle is of interest.
  In case of eccentric orbits and forces far from inverse square, numerical
investigations provide evidence of the monotonicity of the apsidal angle with
respect to the orbit parameters, such as the orbit eccentricity. However, no
proof of this statement is available.
  In this paper central force systems with $f(r)\sim \mu r^{-(\alpha+1)}$ are
considered. We prove that for any $-2<\alpha<1$ the apsidal angle is a
monotonic function of the orbital eccentricity, or equivalently of the angular
momentum. As a corollary, the conjecture stating the absence of isolated cases
of zero precession is proved.
