Zenith

Thezenith(UK:/ˈzɛnɪθ/,US:/ˈziːnɪθ/)^[1]is the imaginary point on thecelestial spheredirectly "above" a particular location. "Above" means in thevertical direction(plumb line) opposite to thegravity directionat that location (nadir). The zenith is the "highest" point on the celestial sphere.

Origin and history

The wordzenithderives from an inaccurate reading of theArabicexpressionسمت الرأس(samt al-raʾs), meaning "direction of the head" or "path above the head", byMedieval Latinscribes in theMiddle Ages(during the 14th century), possibly throughOld Spanish.^[2]It was reduced tosamt( "direction" ) and miswritten assenit/cenit,thembeing misread asni.Through theOld Frenchcenith,zenithfirst appeared in the 17th century.^[3]

Relevance and use

The shadows of trees are the shortest on Earth when the Sun is directly overhead (at the zenith). This happens only atsolar noonon certain days in thetropics,where the trees'latitudeand the Sun'sdeclinationare equal.

The termzenithsometimes means thehighest point,way, or level reached by acelestial bodyon its daily apparent path around a given point of observation.^[4]This sense of the word is often used to describe theposition of the Sun( "The sun reached its zenith..." ), but to an astronomer, theSundoes 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).

The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between theTropic of Cancerand theTropic of Capricorn.InIslamic astronomy,the passing of the Sun over the zenith ofMeccabecomes the basis of theqibla observation by shadowstwice a year on 27/28 May and 15/16 July.^[5]^[6]

At a given location during the course of a day, the Sun reaches not only its zenith but also itsnadir,at theantipodeof 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 astronomicalmeridianis also determined by the zenith, and is defined as a circle on thecelestial spherethat passes through the zenith, nadir, and thecelestial poles.

Azenith telescopeis 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 Observatoryand theLarge Zenith Telescopeare both zenith telescopes, since the use ofliquid mirrorsmeant these telescopes could only point straight up.

On theInternational Space Station,zenithandnadirare used instead ofupanddown,referring to directions within and around the station, relative to the earth.

Zenith star

Zenith stars (also "star on top", "overhead star", "latitude star" )^[7]are stars whichculminate(pass) through the zenith. Used incelestial navigationit allowed the navigator to determine theirlatitudalposition, since at every latitude different stars pass one's zenith, or with other words lie on thegreat circleof the zenith ( "zenith circle" ). Zenith stars are not to be confused with "steering stars"^[7]of asidereal compass roseof asiderealcompass.

References

^Jones, Daniel(2011).Roach, Peter;Setter, Jane;Esling, John(eds.).Cambridge English Pronouncing Dictionary(18th ed.). Cambridge University Press.ISBN 978-0-521-15255-6.
^Corominas, J. (1987).Breve diccionario etimológico de la lengua castellana(in Spanish) (3rd ed.). Madrid. p. 144.ISBN 978-8-42492-364-8.{{cite book}}:CS1 maint: location missing publisher (link)
^"zenith".Dictionary.com Unabridged(Online). n.d.Retrieved2012-03-21.
^"Zenith".Merriam-Webster.RetrievedMarch 21,2012.
^van Gent, Robert Harry (2017)."Determining the Sacred Direction of Islam".Webpages on the History of Astronomy.
^Khalid, Tuqa (2016)."Sun will align directly over Kaaba, Islam's holiest shrine, on Friday".CNN.
^^a ^bLewis, David (1972)."We, the navigators: the ancient art of landfinding in the Pacific".Australian National University Press.Retrieved2023-06-01.

Zenith

Origin and history

Relevance and use

Zenith star

See also

References

Further reading