Calculating the Phase Angle between the Sun / ISS and an observer on the earth

Question

To help me with calculating the visual magnitude of the International Space Station I need to be able to calculate the phase angle.

Can anyone help me calculate t

Answer 1

Since angle_b is practically zero (max 0.00016 degrees at perihelion and ISS directly overhead), you could just do angle_a = math.pi - angle_c.

This has about the same accuracy, since you already have made some minor errors with distance a.