Diagram marking several orbital elements for an orbit inclined relative to the reference plane which is the sky plane. Ω and ω are not coplanar. We follow the binary star convention where positive Z (not drawn) is into the sky plane (below the sky plane drawn here), such that the ascending node is the point where the orbiting body crosses the reference plane (red circle) toward positive Z (Green 1985). In this particular sketch, the planet lies out of the sky plane (nearest Earth), which means that the descending node (blue circle) follows periastron passage. The periastron vector lies in the plane of the orbit and represents the direction of the true semi-major axis. This does not necessarily correspond to the apparent semi-major axis of an inclined orbit projected onto the sky plane. At the current epoch, Fomalhaut  b has passed through periastron, but it has not yet reached the descending node (i.e., it still resides behind the sky plane).