Zenith
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219613112
2008-06-16T02:17:29Z
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[[Image:Zenith-Nadir-Horizon.svg|thumb|right|Diagram showing the relationship between the Zenith, the [[Nadir]], and different types of [[Horizon]]. Note that the Zenith is opposite the Nadir.]]
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 [[Gravitation|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 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_(astronomy)|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]]).
[[Image:Tropical-area-mactan-philippines.jpg|thumb|240px|right|Only between the tropics is it possible for the [[Sun]] to be at the zenith.]]
==See also==
* [[Zenith distance]]
* [[Geodesy]]
* [[History of geodesy]]
* [[Vertical deflection]]
* [[Nadir]]
* [[Midheaven]]
* [[Subsolar point]]
==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.
[[Category:Celestial coordinate system]]
[[Category:Arabic words and phrases]]
[[Category:Technical factors of astrology]]
[[Category:Astrological house systems]]
[[ar:سمت الرأس]]
[[ast:Cénit]]
[[bn:সুবিন্দু]]
[[bg:Зенит]]
[[ca:Zenit]]
[[cs:Zenit]]
[[da:Zenit]]
[[de:Zenit (Richtungsangabe)]]
[[et:Seniit]]
[[el:Ζενίθ]]
[[es:Cenit]]
[[eo:Zenito]]
[[eu:Zenit]]
[[fr:Zénith (astronomie)]]
[[gd:Druim an t-saoghail]]
[[gl:Cénit]]
[[hr:Zenit]]
[[id:Zenith]]
[[is:Hvirfilpunktur]]
[[it:Zenit]]
[[he:זנית]]
[[lt:Zenitas]]
[[hu:Zenit]]
[[nl:Zenit (astronomie)]]
[[ja:天頂]]
[[no:Senit]]
[[nn:Senit]]
[[pl:Zenit (astronomia)]]
[[pt:Zênite]]
[[ro:Zenit]]
[[ru:Небесная сфера#.D0.97.D0.B5.D0.BD.D0.B8.D1.82_.D0.B8_.D0.BD.D0.B0.D0.B4.D0.B8.D1.80]]
[[sk:Zenit]]
[[sl:Nadglavišče]]
[[sr:Зенит]]
[[fi:Zeniitti]]
[[sv:Zenit]]
[[th:จุดจอมฟ้า]]
[[vi:Thiên đỉnh]]
[[tr:Zenit]]
[[uk:Зеніт]]
[[zh:天頂]]