# What is the Azimuth plane and how did it originate?

Background: As I understand it, "up" and "down" navigation aboard the ISS are referred to as Nadir and Zenith with down (towards Earth) being Nadir (which also happens to refer to the lowest note in a musical passage btw) and away from Earth being called Zenith.

Correct me if I'm wrong, but another related term is Azimuth which refers to the plane at right angles to both Nadir and Zenith (essentially if Zenith and Nadir are up and down, Azimuth would be left and right), but what I don't understand is how is it orientated longitudinally?

If I were standing on the North pole, Zenith would be towards space above me, Nadir towards the South pole, but where would does the Azimuth originate? Where would its 0 degree be? And does it change when I rotate?

Also, does it have any relation to geographic references (Longitude/Latitude - Magnetic/True North) or is it purely relative to the observer?