# Zenith

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Zenith

In broad terms, the zenith is the direction pointing directly "above" a particular location (perpendicular, orthogonal). Since the concept of "being above" is itself somewhat vague, scientists define the zenith in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the zenith at a given point is the local vertical direction pointing away from direction of the force of gravity at that location.

For reference, the vertical direction at the given location and pointing in the same sense as the gravitational force is called the nadir. Zenith is the opposite of nadir.

Zenith is also used for the highest point reached by a celestial body during its apparent orbit around a given point of observation. [http://www.merriam-webster.com/dictionary/zenith] Often used in this sense about the Sun, it only corresponds to the first concept of zenith for one latitude at a time, and never at all for latitudes outside the tropics.

Origin

The word "zenith" derives from the inaccurate reading of the Arabic word "samt" ('path'), pronounced "sent", by scribes in the Middle Ages (during the 14th century), in the expression "samt arrâs" ('path above the head').

Relevance and use

The zenith is used in the following scientific contexts:
*It serves as the direction of reference for measuring the zenith angle, which is the angular distance between a direction of interest (e.g., a star) and the local zenith, relative to the point for which the zenith is defined.
*It defines one of the axes of the horizontal coordinate system in astronomy.

Discussion

*The zenith directions corresponding to three different locations (not on the same vertical direction) are divergent.
*Strictly speaking, the zenith is only approximatively contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).

ee also

*Geodesy
*History of geodesy
*Midheaven
*Subsolar point
*Vertical deflection
*Zenith distance

References

*Huschke, Ralph E. (1959) "Glossary of Meteorology", American Meteorological Society, Boston, Second printing-1970.
*McIntosh, D. H. (1972) "Meteorological Glossary", Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.
*Picoche, J. (1992) "Dictionnaire Etymologique du Français", Le Robert, Paris, ISBN 2-85036-458-4.

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• zénith — [ zenit ] n. m. • v. 1360; chenit 1324; d une mauvaise lecture (ni pour m) de l ar. samt, semt, proprt « chemin » dans l expr. samt ar râs « chemin au dessus de la tête »; cf. azimut 1 ♦ Point de la sphère céleste situé sur la verticale… …   Encyclopédie Universelle

• Zenith — Zénith Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom …   Wikipédia en Français

• Zenith — – Zeitschrift für den Orient Fachgebiet Orient, Naher Osten, Südasien, Islam, Wirtschaft, Kultur und Internationale Politik Sprache deutsch Verlag Deutscher Levante Verlag …   Deutsch Wikipedia

• Zenith — e nith (?; 277), n. [OE. senyth, OF. cenith, F. z[ e]nith, Sp. zenit, cenit, abbrev. fr. Ar. samt urras way of the head, vertical place; samt way, path + al the + ras head. Cf. {Azimuth}.] [1913 Webster] 1. That point in the visible celestial… …   The Collaborative International Dictionary of English

• Zenith FM — Zénith FM Pour les articles homonymes, voir Zénith. Zénith FM est une radio associative qui émet dans la partie sud de l Ille Vilaine sur le 87.7 MHz. Depuis le 14 mai 2007, une deuxième radio indépendante a été ouverte à Vitré sur le 91.9… …   Wikipédia en Français

• Zenith 93 — Zénith 93 Pour les articles homonymes, voir Zénith. Zénith 93 Album par Véronique Sanson Sortie 10 novembre 1993 Label WEA …   Wikipédia en Français

• Zenīth — (arab.), 1) (Scheitelpunkt), der Punkt am Himmel, welcher der Verlängerung einer, vom Mittelpunkt der Erde aus, durch den Standpunkt des Beobachters gezogenen Linie entspricht, also von einem nach aufwärts verlängerten Faden des Bleilothes… …   Pierer's Universal-Lexikon

• zenith — (n.) late 14c., from O.Fr. cenith (Fr. zénith), from M.L. cenit, senit, bungled scribal transliterations of Arabic samt road, path, abbreviation of samt ar ras, lit. the way over the head. Letter m misread as ni . The Medieval Latin word could as …   Etymology dictionary

• zenith — [zē′nith; ] Brit, & often Cdn [, zen′ith] n. [ME senyth < MFr cenith < ML cenit < senit, scribal error for Ar samt, road, path (as in samt al raʾs, zenith, lit., way of the head) < L semita, path, way] 1. the point directly overhead… …   English World dictionary

• Zenith — oder Scheitelpunkt, der höchste Punkt am Himmel, in gerader Linie über unserem Haupte gedacht, wo wir gerade stehen; diesem entgegengesetzt befindet sich der Nadir (s. d.) …   Damen Conversations Lexikon

• Zenith — Zenith, Scheitelpunkt, derjenige Punkt am Himmelsgewölbe, welcher senkrecht über dem Haupte des Beobachters steht u. diesem als der höchste Punkt am Himmel gilt. Der Bogen des größten Kreises zwischen dem Z. u. einem Gestirne heißt die Z. distanz …   Herders Conversations-Lexikon