Relationship between Periapsis and apoapsis longitude What is the relationship between Periapsis and Apoapsis longitude ? Are they 180° apart ? (ie) Periapsis + 180° = Apoapsis ? 

Related question: if apoapsis is not known, what orbital elements are required to compute it ?
The periapsis and apoapsis are 180&deg; apart.
According to Kepler's first law,
an orbit is an ellipse with the central body at one focus.
The periapsis (P), central body (F), and apoapsis (A) all lie on the major axis of that ellipse.