Nadir, << NAY duhr, >> is the point in space directly below where one stands. To an observer on earth, the sky appears to be a half-dome whose edge forms a great circle resting on the flat surface of the earth. Imagine a plumb line suspended from the center of this dome, directly above your head, and passing through the center of the earth and into space as far as the central point of the invisible half-dome beneath the earth. The two points marking the ends of the imaginary plumb line are, respectively, the zenith and the nadir. They are the poles of the horizon, and each is 90° from the horizon. See also Zenith .
