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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?

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Ok so to answer my own question (turns out I just needed to do a little more reading up):

The reference plane for the Azimuth is based on True North with its 0 degree starting at the Meridian line and increasing clockwise 360 degrees until once again reaching the meridian. It is also possible to use True South to orientate, but then the Azimuth increases in degrees counter-clockwise through 360 degrees.

In the image below you can see how the Azimuth plane (Green) relates to the Equatorial plane (Also called the Celestial Plane)(Blue), in this orientation the Meridian line is the blue line heading almost vertically upwards out of the picture

enter image description here

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    $\begingroup$ It's best if you try to find a supporting link or two for your answer. Since you did some reading-up, just mention what you read (a short quote perhaps) and where you read it. Good Stack Exchange answers will generally support statements of fact with supporting links to help future readers verify the answer and do further reading. Welcome to Space! $\endgroup$ – uhoh Oct 1 '19 at 15:46

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