Main Question or Discussion Point
Does anyone know how to manipulate the ephemeris data from JPL HORIZONS in order to find out the tangential velocity of a body relative to a topographic center, eg the Sun?
Well it might be easy but I don't know.Use the vector output capability rather than observer (right ascenscion/declination) output. This will give you the position vector and velocity vector in ICRF coordinates of some body relative to some origin. Computing the component of the velocity vector normal to the position vector is vector simple arithmetic.