Diagram showing the relationship between the Zenith, the Nadir, and different types of Horizon. The nadir (from Arabic ندير nadeer نظير nathir, "opposite" is the astronomical term for the point directly The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates Note that the Zenith is opposite the Nadir.

In broad terms, the zenith is the direction pointing directly above a particular location (perpendicular, orthogonal). In Geometry, two lines or planes (or a line and a plane are considered perpendicular (or orthogonal) to each other if they form congruent In Mathematics, two Vectors are orthogonal if they are Perpendicular, i Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic g. , meteorology), the zenith at a given point is the local vertical direction pointing away from direction of the force of gravity at that location. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary In Astronomy, Geography, Geometry and related sciences and contexts a direction passing by a given point is said to be vertical if Gravitation is a natural Phenomenon by which objects with Mass attract one another

For reference, the vertical direction at the given location and pointing in the same sense as the gravitational force is called the nadir. The nadir (from Arabic ندير nadeer نظير nathir, "opposite" is the astronomical term for the point directly

Zenith is also used for the highest point reached by a celestial body during its apparent orbit around a given point of observation. [1] Often used in this sense about the Sun, it only corresponds to the first concept of zenith for one latitude at a time, and never at all for latitudes outside the tropics. The Sun (Sol is the Star at the center of the Solar System. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the The Tropics are centered on the Equator and limited in Latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' (23

Origin

The word zenith derives from the inaccurate reading of the Arabic word samt ('path'), pronounced sent, by scribes in the Middle Ages (during the 14th century), in the expression samt arrâs ('path above the head').

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 in astronomy. The horizontal coordinate system is a Celestial coordinate system that uses the observer's local Horizon as the fundamental plane.

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 plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. This article is about the astronomical concept For other uses of the word see Meridian. 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, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides). In Astronomy, Geography, Geometry and related sciences and contexts a direction passing by a given point is said to be vertical if Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood

Only in the tropics is it possible for the Sun to be at the zenith. The Sun (Sol is the Star at the center of the Solar System.

See also

• Zenith distance
• Geodesy
• History of geodesy
• Vertical deflection
• Nadir
• Midheaven
• Subsolar point

References

• Huschke, Ralph E. In Astronomy, a celestial coordinate system is a Coordinate system for mapping positions in the sky Geodesy (dʒiːˈɒdɪsi also called geodetics, a branch of Earth sciences, is the scientific discipline that deals See also the main article on Geodesy. Humanity has always been interested in the Earth. The vertical deflections (deflections of the Plumb line, astro-geodetic deflections are important parameters of the local Gravity field. The nadir (from Arabic ندير nadeer نظير nathir, "opposite" is the astronomical term for the point directly 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 The subsolar point on a Planet is where its Sun is perceived to be directly overhead (in Zenith) (1959) Glossary of Meteorology, American Meteorological Society, Boston, Second printing-1970.
• McIntosh, D. H. (1972) Meteorological Glossary, Her Majesty's Stationery Office, Met. O. 842, A. P. 897, 319 p.
• Picoche, J. (1992) Dictionnaire Etymologique du Français, Le Robert, Paris, ISBN 2-85036-458-4.