The zenith (, ) is the imaginary point on the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

directly "above" a particular location. "Above" means in the

vertical direction In astronomy, geography, and related sciences and contexts, a ''Direction (geometry, geography), direction'' or ''plane (geometry), plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. ...

(

plumb line A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical direction as a reference line, or plumb-line. It is a precursor to the spirit level and used to est ...

) opposite to the gravity direction at that location (

nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...

). The zenith is the "highest" point on the celestial sphere. The direction opposite of the zenith is the

Origin

The word ''zenith'' derives from an inaccurate reading of the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

expression (), meaning "direction of the head" or "path above the head", by

Medieval Latin Medieval Latin was the form of Literary Latin used in Roman Catholic Church, Roman Catholic Western Europe during the Middle Ages. It was also the administrative language in the former Western Roman Empire, Roman Provinces of Mauretania, Numidi ...

scribes in the

Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...

(during the 14th century), possibly through

Old Spanish Old Spanish (, , ; ), also known as Old Castilian or Medieval Spanish, refers to the varieties of Ibero-Romance spoken predominantly in Castile and environs during the Middle Ages. The earliest, longest, and most famous literary composition in O ...

. It was reduced to ''samt'' ("direction") and miswritten as ''senit''/''cenit'', the ''m'' being misread as ''ni''. Through the

Old French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th Tropical-area-mactan-philippines

The term ''zenith'' sometimes means the highest point, way, or level reached by a celestial body">culmination">highest point, way, or level reached by a celestial body on its daily apparent path around a given point of observation. 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. 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 The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and ''azimuth''. Therefore, the horizontal coordin ...

(or

elevation angle The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and ''azimuth''. Therefore, the horizontal coordin ...

). The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the

Tropic of Cancer The Tropic of Cancer, also known as the Northern Tropic, is the Earth's northernmost circle of latitude where the Sun can be seen directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun ...

and the

Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reach ...

. The point where this occurs is known as the

subsolar point The subsolar point on a planet or a moon is the point at which its Sun is perceived to be directly overhead (at the zenith); that is, where the Sun's rays strike the planet exactly perpendicular to its surface. The subsolar point occurs at the ...

. In

Islamic astronomy Medieval Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th–13th centuries), and mostly written in the Arabic language. These developments mostly took place in th ...

, the passing of the Sun over the zenith of

Mecca Mecca, officially Makkah al-Mukarramah, is the capital of Mecca Province in the Hejaz region of western Saudi Arabia; it is the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red Sea, in a narrow valley above ...

becomes the basis of the

qibla observation by shadows Twice every year, the Sun culminates at the zenith of the ''Kaaba'' in Mecca, the holiest site in Islam, at local solar noon, allowing the qibla (the direction towards the ''Kaaba'') to be ascertained in other parts of the world by observing the ...

twice a year on 27/28 May and 15/16 July. At a given location during the course of a day, the Sun reaches not only its zenith but also its

, at the antipode of that location 12 hours from

solar noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clo ...

. In

astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, the altitude in the horizontal coordinate system and the zenith angle are

complementary angles In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...

, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the

that passes through the zenith, nadir, and the

celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...

s. A

zenith telescope A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both. A classic zenith telescope, also known ...

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 mirrors meant these telescopes could only point straight up. On the

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

, ''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") are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night pass 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 Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface ...

this allows

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

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 Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

can also be determined from the

right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...

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" of a sidereal compass rose of a sidereal compass.

Origin

Zenith star

See also

References

Further reading