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Zenith
 
 
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In broad terms, the zenith is the direction pointing directly above a particular location (perpendicular
Perpendicular

In geometry, two line or plane , are considered perpendicular to each other if they form congruence adjacent angles angles . The term may be used as a noun or adjective....
, orthogonal). Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms. Specifically, in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, geophysics
Geophysics

Geophysics, a major discipline of the Earth sciences, is the study of the Earth by the quantitative observation of its physical properties, especially by Seismology, Electromagnetism, Radioactive decay, galvanic and potential field methods....
 and related sciences (e.g., meteorology
Meteorology

Meteorology is the interdisciplinary scientific study of the Earth's atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century....
), the zenith at a given point is the local vertical direction
Vertical direction

In astronomy, geography, geometry and related sciences and contexts, a Direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the Gravitation Field , i.e., with the direction of the gravitational force at that point....
 pointing away from the direction of the force of gravity
Gravitation

Gravitation is a natural phenomenon that gives weight to objects. In everyday life, attraction due to gravity is the result of the presence of relatively large bodies, such as the Earth and the Moon....
 at that location.

Zenith is also used for the highest point reached by a celestial body during its apparent orbit around a given point of observation.






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In broad terms, the zenith is the direction pointing directly above a particular location (perpendicular
Perpendicular

In geometry, two line or plane , are considered perpendicular to each other if they form congruence adjacent angles angles . The term may be used as a noun or adjective....
, orthogonal). Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms. Specifically, in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, geophysics
Geophysics

Geophysics, a major discipline of the Earth sciences, is the study of the Earth by the quantitative observation of its physical properties, especially by Seismology, Electromagnetism, Radioactive decay, galvanic and potential field methods....
 and related sciences (e.g., meteorology
Meteorology

Meteorology is the interdisciplinary scientific study of the Earth's atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century....
), the zenith at a given point is the local vertical direction
Vertical direction

In astronomy, geography, geometry and related sciences and contexts, a Direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the Gravitation Field , i.e., with the direction of the gravitational force at that point....
 pointing away from the direction of the force of gravity
Gravitation

Gravitation is a natural phenomenon that gives weight to objects. In everyday life, attraction due to gravity is the result of the presence of relatively large bodies, such as the Earth and the Moon....
 at that location.

Zenith is also used for the highest point reached by a celestial body during its apparent orbit around a given point of observation. Often used in this sense about the Sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
, it only corresponds to the first concept of zenith for one latitude
Latitude

Latitude, usually denoted symbolically by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the horizontal lines shown running east-to-west on maps ....
 at a time, and never at all for latitudes outside the tropics
Tropics

The Tropics, seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23?26' N latitude, and the Tropic of Capricorn in the southern hemisphere at 23?26' S latitude....
.

Origin

The word zenith derives from the inaccurate reading of the Arabic
Arabic language

Arabic is a Central Semitic language, thus related to and classified alongside other Semitic languages languages such as Hebrew language and Aramaic language....
 word samt ('path'), pronounced sent, by scribes in the Middle Ages
Middle Ages

File:Karl 1 mit papst gelasius gregor1 sacramentar v karl d kahlen.jpgThe Middle Ages of European history are a period in history which lasted for roughly a millennium, commonly dated from the fall of the Roman Empire in the 5th century to the beginning of the Early Modern Period in the 16th century, marked by the division of Western Christi...
 (during the 14th century), in the expression samt arrâs ('path above the head').

The arabic word for Zenith is Zawâl, meaning "decline", that is, when the sun ceases to rise and starts to decline.

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
    Horizontal coordinate system

    The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the Fundamental plane . This conveniently divides the sky into the upper sphere that you can see, and the lower hemisphere that you cannot ....
     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
    Meridian (astronomy)

    This article is about the astronomical concept. For other uses of the word, see meridian .In the sky, a meridian is an imaginary great circle on the celestial sphere....
     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
    Vertical direction

    In astronomy, geography, geometry and related sciences and contexts, a Direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the Gravitation Field , i.e., with the direction of the gravitational force at that point....
    , as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).
Tropical Area Mactan Philippines

See also

  • Geodesy
    Geodesy

    Geodesy , also called geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space....
  • History of geodesy
    History of geodesy

    Humanity has always been interested in the Earth. During very early times this interest was limited, naturally, to the immediate vicinity of home and residency, and the fact that we live on a near spherical globe may or may not have been apparent....
  • Midheaven
    Midheaven

    According to modern astrologers, the Midheaven or Medium Coeli is the zenith of a path in the sky traced by the point of intersection between the ecliptic and the meridian of a certain terrestrial location....
  • Nadir
    Nadir

    The nadir is the direction pointing directly below a particular location . Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms....
  • Subsolar point
  • Vertical deflection
    Vertical deflection

    The vertical deflections are important parameters of the local gravity field. They are widely used in geodesy, for surveying networks and for geophysical purposes....
  • Zenith distance