Note: Do not rely on this information. It is very old.
Nadir is the point in the heavens immediately beneath our feet; it is therefore the zenith (q.v.) of a place on the opposite side of the earth. Every place has its own zenith and nadir, these two points forming a straight line with the centre of the earth. Since this straight line is a vertical line for the place in question, and the horizon is a plane perpendicular to it, the zenith and nadir are respectively the upper and lower poles of the horizon.