Nadir refers to the lowest or worst point of something. When used in astronomy, nadir describes the point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer.
From Arabic. The source of nadir is naḍhīr, meaning "opposite"—the opposite, that is, of the zenith, the highest point of the celestial sphere which is positioned vertically above the observer. (The word zenith itself is a modification of another Arabic word).
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