On 2018-04-11, wrote:
An acceleration of an orbiting body, in a central force, is defined as:

a(r,v) = -k/r^2(1 + v^2/c^2)

k = GM = const, v = dr/dt = r'

what is the precession of perihelion,
or the apsidal angle in this case?

Basically, solve it without the v/c correction (ellipse) and then
treat the extra term as a perturbation. The orbit will close slightly
slower given the sign of the correction. Use this to find the extra shift
of the closure angle (ie, where r dot is minimum). There is a trick to
simplify it but I have forgotten it :-)

(IIRC it is well described in Ohanian's text on General Relativity)

--
ciao, Bruce