The wordzenith derives from an inaccurate reading of theArabic expressionسمت الرأس (samt al-raʾs), meaning "direction of the head" or "path above the head", byMedieval Latin scribes in theMiddle Ages (during the 14th century), possibly throughOld Spanish.[3] It was reduced tosamt ("direction") and miswritten assenit/cenit, them being misread asni. Through theOld Frenchcenith,zenith first appeared in the 17th century.[4]
Angles and planes of a celestial sphereThe shadows of trees are the shortest on Earth when the Sun is directly overhead (at the zenith). This happens only atsolar noon on certain days in thetropics, where the trees'latitude and the Sun'sdeclination are equal.
The termzenith sometimes means thehighest point, way, or level reached by acelestial body on its daily apparent path around a given point of observation.[5] This sense of the word is often used to describe theposition of the Sun ("The sun reached its zenith..."), but to an astronomer, theSun does not have its own zenith and is at the zenith only if it is directly overhead.
In a scientific context, the zenith is the direction of reference for measuring thezenith angle (orzenith angular distance), the angle between a direction of interest (e.g. a star) and the local zenith - that is, the complement of thealtitude angle (orelevation angle).
At a given location during the course of a day, the Sun reaches not only its zenith but also itsnadir, at theantipode of that location 12 hours fromsolar noon.
Inastronomy, the altitude in the horizontal coordinate system and the zenith angle arecomplementary angles, with the horizon perpendicular to the zenith. The astronomicalmeridian is also determined by the zenith, and is defined as a circle on thecelestial sphere that passes through the zenith, nadir, and thecelestial poles.
Azenith 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. TheNASA Orbital Debris Observatory and theLarge Zenith Telescope are both zenith telescopes, since the use ofliquid mirrors meant these telescopes could only point straight up.
On theInternational Space Station,zenith andnadir are used instead ofup anddown, referring to directions within and around the station, relative to the earth.
Zenith stars (also "star on top", "overhead star", "latitude star")[8] are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night passculminate (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location. Incelestial navigation this allowslatitude 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 observerslongitude can also be determined from theright 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"[8] of asidereal compass rose of asidereal compass.
^Corominas, J. (1987).Breve diccionario etimológico de la lengua castellana (in Spanish) (3rd ed.). Madrid. p. 144.ISBN978-8-42492-364-8.{{cite book}}: CS1 maint: location missing publisher (link)
