Nadir (direction)

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The diagram shows the relationships between zenith, nadir and horizons: Nadir and zenith are opposite poles

As a nadir (from the Arabic نظير, DMG naẓīr , opposite, image ') is used in geometry and in celestial navigation to denote the base point opposite the zenith . The nadir lies on the extension of the plumb direction downwards and can also stand for the plumb direction as a direction indication. The nadir is an important point of the unit sphere for spherical astronomy and trigonometry .

In geodesy and photogrammetry , elevation angles are sometimes related to the nadir. Instead of zenith angle or zenith distance, one speaks of nadir distance .

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Wiktionary: Nadir  - explanations of meanings, word origins, synonyms, translations