Editing Zenith

{{short description|Imaginary point directly above a particular location, on the imaginary celestial sphere}}
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{{other uses}}
{{redirect|Zenith angle|the novel|The Zenith Angle}}
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[[File:Zenith-Nadir-Horizon.svg|thumb|upright=1.25|right|Diagram showing the relationship between the zenith, the [[nadir]], and different types of [[horizon]]]]
The '''zenith''' ({{IPAc-en|UK|ˈ|z|ɛ|n|ɪ|θ}}, {{IPAc-en|US|ˈ|z|iː|-}})<ref>{{cite LPD|3}}</ref><ref>{{cite EPD|18}}</ref> is the imaginary point on the [[celestial sphere]] directly "above" a particular location. "Above" means in the [[vertical direction]] ([[Vertical and horizontal|plumb line]]) opposite to the [[gravity direction]] at that location ([[nadir]]). The zenith is the "highest" point on the celestial sphere. The direction opposite of the zenith is the [[nadir]].

==Origin==
The word ''zenith'' derives from an inaccurate reading of the [[Arabic language|Arabic]] expression {{lang|ar|سمت الرأس}} ({{Transliteration|ar|DIN|samt al-raʾs}}), meaning "direction of the head" or "path above the head", by [[Medieval Latin]] scribes in the [[Middle Ages]] (during the 14th century), possibly through [[Old Spanish]].<ref>{{cite book |last=Corominas |first=J. |title=Breve diccionario etimológico de la lengua castellana |language=es |date=1987 |location=Madrid |isbn=978-8-42492-364-8 |edition=3rd |pages=144}}</ref> It was reduced to ''samt'' ("direction") and miswritten as ''senit''/''cenit'', the ''m'' being misread as ''ni''. Through the [[Old French]] ''cenith'', ''zenith'' first appeared in the 17th century.<ref>{{cite Dictionary.com|zenith |access-date=2012-03-21}}</ref>

==Relevance and use==
[[File:Azimuth-Altitude schematic.svg|thumb|Angles and planes of a celestial sphere]]
[[File:Tropical-area-mactan-philippines.jpg|thumb|right|The shadows of trees are the shortest on Earth when the Sun is directly overhead (at the zenith). This happens only at [[solar noon]] on certain days in the [[tropics]], where the trees' [[latitude]] and the Sun's [[declination]] are equal.]]
{{see also|Culmination}}

The term ''zenith'' sometimes means the [[culmination|highest point]], way, or level reached by a [[celestial body]] on its daily apparent path around a given point of observation.<ref>{{cite dictionary|title=Zenith|url=http://www.merriam-webster.com/dictionary/zenith|dictionary=[[Merriam-Webster]]|access-date=March 21, 2012}}</ref> This sense of the word is often used to describe the [[position of the Sun]] ("The sun reached its zenith..."), but to an astronomer, the [[Sun]] does not have its own zenith and is at the zenith only if it is directly overhead.

{{anchor|Angle}}In a scientific context, the zenith is the direction of reference for measuring the '''zenith angle''' (or '''zenith angular distance'''), the angle between a direction of interest (e.g. a star) and the local zenith - that is, the complement of the [[altitude angle]] (or [[elevation angle]]).

The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the [[Tropic of Cancer]] and the [[Tropic of Capricorn]]. The point where this occurs is known as the [[subsolar point]]. In [[Islamic astronomy]], the passing of the Sun over the zenith of [[Mecca]] becomes the basis of the [[qibla observation by shadows]] twice a year on 27/28 May and 15/16 July.<ref>{{cite web|last=van Gent|first=Robert Harry|title=Determining the Sacred Direction of Islam|url=https://www.staff.science.uu.nl/~gent0113/islam/qibla.htm|year=2017|website=Webpages on the History of Astronomy}}</ref><ref>{{cite news|last=Khalid |first=Tuqa| url=https://www.cnn.com/2016/05/26/middleeast/sun-kabaa-alignment/index.html |title=Sun will align directly over Kaaba, Islam's holiest shrine, on Friday | work =CNN | year=2016}}</ref>

At a given location during the course of a day, the Sun reaches not only its zenith but also its [[nadir]], at the [[antipodal point|antipode]] of that location 12 hours from [[solar noon]].

In [[astronomy]], the altitude in the horizontal coordinate system and the zenith angle are [[complementary angles]], with the horizon perpendicular to the zenith. The astronomical [[meridian (astronomy)|meridian]] is also determined by the zenith, and is defined as a circle on the [[celestial sphere]] that passes through the zenith, nadir, and the [[celestial pole]]s.

A [[zenith telescope]] is a type of telescope designed to point straight up at or near the zenith, and used for precision measurement of star positions, to simplify telescope construction, or both. The [[NASA Orbital Debris Observatory]] and the [[Large Zenith Telescope]] are both zenith telescopes, since the use of [[liquid-mirror telescope|liquid mirrors]] meant these telescopes could only point straight up.

On the [[International Space Station]], ''zenith'' and ''nadir'' are used instead of ''up'' and ''down'', referring to directions within and around the station, relative to the earth.

===Zenith star===

Zenith stars (also "star on top", "overhead star", "latitude star")<ref name="Lewis 1972">{{cite web | last=Lewis | first=David | title=We, the navigators : the ancient art of landfinding in the Pacific | publisher=Australian National University Press | date=1972 | url=https://openresearch-repository.anu.edu.au/handle/1885/114874 | access-date=2023-06-01}}</ref> are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night pass  [[Culmination|culminate]] (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location. In [[celestial navigation]] this allows [[latitude]] to be determined, since the declination of the star equals the latitude of the observer.  If the current time at Greenwich is known at the time of the observation, the observers [[longitude]] can also be determined from the [[right ascension]] of the star. Hence "Zenith stars" lie on or near the circle of declination equal to the latitude of the observer ("zenith circle"). Zenith stars are not to be confused with "steering stars"<ref name="Lewis 1972"/> of a [[Compass rose#Sidereal|sidereal compass rose]] of a [[Sidereal time|sidereal]] compass.

==See also==
{{commons category-inline|Zenith (topography)}}
*[[Analemma]]
*[[Azimuth]]
*[[Geodesy]]
*[[History of geodesy]]
*[[Horizon zenith angle]]
*[[Horizontal coordinate system]]
*[[Keyhole problem]]
*[[Vertical deflection]]

==References==
{{Reflist}}

==Further reading==
*{{cite book|last=Glickman|first=Todd S.|title=Glossary of meteorology|date=2000|publisher=[[American Meteorological Society]]|isbn=978-1-878220-34-9}}
*{{cite book|last=McIntosh|first=D. H.|title=Meteorological Glossary|date=1972|publisher=Chemical |isbn=978-0-8206-0228-8|edition=5th}}
*{{cite book|last=Picoche|first=Jacqueline|title=Dictionnaire étymologique du français|date=2002|publisher=[[Le Robert]]|location=Paris|isbn=978-2-85036-458-7}}

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[[Category:Astronomical coordinate systems]]
[[Category:Horizontal coordinate system]]
[[Category:Technical factors of astrology]]
[[Category:Astrological house systems]]
[[Category:Topography]]
[[Category:Orientation (geometry)]]