Does the retrograde movement affect the true angular distance between 2 planets? I need to calculate the relative angular path between two planets in the geocentric view or in other words the number of degrees of a circle one planet travels away from another planet around the Earth. In the heliocentric system I simply subtract the longitude of one planet from the longitude of the other and add n*360 if they made full cycles in between. Would the same work for the geocentric coordinates? Does the loop a retrograding planet makes in the sky actually count as its path? Should I consider the retrograde movement as an optical illusion or include it in the formula and how?
In our moving Earth-based point of view,
retrograde apparent motion
means a planet's geocentric ecliptic longitude is temporarily decreasing
instead of increasing as usual.
The word "apparent" avoids confusion with a
retrograde orbit.
All major planets' orbits are prograde,
with heliocentric longitude increasing at all times1.
Most retrograde-orbiting Solar system objects are comets.

As seen from Earth, the apparent angular separation
between two planets near the ecliptic
is approximately2 the difference
between their geocentric longitudes at that point in time,
regardless of their rates of change in longitude.
Even if you plot this value versus time,
your calculation need not break apparent motion down
into positive and negative cases.


Except where longitude wraps around from 360$^\circ$ to 0$^\circ$.
When longitude difference is small (near conjunction), latitude difference is also significant.