Orbital sketch of the binary system Vela X-1 as observed from Earth. The blue disk centered at (0,0) is the optical companion HD 77581. The black solid line is the NS elliptical orbit. The gray dashed rays and the respective numbers indicate the orbital phases. Phase zero is taken at the mid-eclipse time. The red straight solid line crossing the ellipse represents the major axis, pointing the periastron and the apastron at the intersections with the NS orbit.