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The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" point on the celestial sphere.
The word "zenith" derives from an inaccurate reading of the Arabic expression سمت الرأس (samt al-ras), meaning "direction of the head" or "path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), possibly through Old Spanish. It was reduced to "samt" ("direction") and miswritten as "senit"/"cenit", the "m" being misread as "ni". Through the Old French "cenith", "zenith" first appeared in the 17th century.
The term zenith sometimes means the 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 Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the Tropic of Cancer and the Tropic of Capricorn. In Islamic astronomy, the passing of the Sun over the zenith of Mecca becomes the basis of the qibla observation by shadows 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 nadir, at the antipode of that location 12 hours from solar noon.
In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles.
A zenith 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. 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, zenith and nadir are used instead of up and down, referring to directions within and around the station, relative to the earth.
Geodesy is the Earth science of accurately measuring and understanding Earth's geometric shape, orientation in space and gravitational field. The field also incorporates studies of how these properties change over time and equivalent measurements for other planets. Geodynamical phenomena include crustal motion, tides and polar motion, which can be studied by designing global and national control networks, applying space and terrestrial techniques and relying on datums and coordinate systems.
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.
A solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countries, the seasons of the year are determined by reference to the solstices and the equinoxes.
An azimuth is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth.
The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane. It is often used to specify a location in the sky using angles called elevation and azimuth. However, in general a location can include range or can be specified using any coordinate system, such as Cartesian coordinate system.
In observational astronomy, culmination is the instant of time of the transit of a celestial object across the observer's local meridian. These events were also known as meridian transits, used in timekeeping and navigation, and measured precisely using a transit telescope.
Spherical Earth or Earth's curvature refers to the approximation of figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. In the 3rd century BC, Hellenistic astronomy established the roughly spherical shape of the Earth as a physical fact and calculated the Earth's circumference. This knowledge was gradually adopted throughout the Old World during Late Antiquity and the Middle Ages. A practical demonstration of Earth's sphericity was achieved by Ferdinand Magellan and Juan Sebastián Elcano's circumnavigation (1519–1522).
In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon. Meridians, celestial and terrestrial, are determined by the axial-pencil of planes passing through the axis of Earth's rotation. For a location not at a geographical pole there is a unique plane in this axial-pencil through that location. The intersection of this plane with Earth's surface is the geographical meridian, and the intersection of the plane with the celestial sphere is the celestial meridian for that location and time.
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of light through air decreasing with increased density. Atmospheric refraction near the ground produces mirages. Such refraction can also raise or lower, or stretch or shorten, the images of distant objects without involving mirages. Turbulent air can make distant objects appear to twinkle or shimmer. The term also applies to the refraction of sound. Atmospheric refraction is considered in measuring the position of both celestial and terrestrial objects.
Spherical astronomy, or positional astronomy, is a branch of observational astronomy used to locate astronomical objects on the celestial sphere, as seen at a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of astrometry.
Meridian altitude is a method of celestial navigation to calculate an observer's latitude. It notes the altitude angle of an astronomical object above the horizon at culmination.
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 there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences, the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith.
The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a culmination, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the north celestial pole, the zenith, the south point of the horizon, the south celestial pole, and the nadir. Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis.
In astronomy, a vertical circle is a great circle on the celestial sphere that is perpendicular to the horizon. Therefore, it contains the vertical direction, passing through the zenith and the nadir. There is a vertical circle for any given azimuth, where azimuth is the angle measured east from the north on the celestial horizon. The vertical circle which is on the east–west direction is called the prime vertical. The vertical circle which is on the north–south direction is called the local celestial meridian (LCM), or principal vertical. Vertical circles are part of the horizontal coordinate system.
Sun path, sometimes also called day arc, refers to the daily and seasonal arc-like path that the Sun appears to follow across the sky as the Earth rotates and orbits the Sun. The Sun's path affects the length of daytime experienced and amount of daylight received along a certain latitude during a given season.
The Earth's equator is an imaginary planetary line that is about 40,075 km (24,901 mi) long in circumference. The equator divides the planet into the Northern Hemisphere and Southern Hemisphere and is located at 0 degrees latitude, the halfway line between the North Pole and South Pole.
In spherical astronomy, the parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hour circle of the object. It is usually denoted q. In the triangle zenith—object—celestial pole, the parallactic angle will be the position angle of the zenith at the celestial object. Despite its name, this angle is unrelated with parallax. The parallactic angle is zero or 180° when the object crosses the meridian.
The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere Rayleigh scattering of light from air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky to be blue. The polarization is characterized at each wavelength by its degree of polarization, and orientation.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
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 to be ascertained in other parts of the world by observing the shadows cast by vertical objects. This phenomenon occurs at 12:18 Saudi Arabia Standard Time on 27 or 28 May, and at 12:27 SAST on 15 or 16 July. At these times, the sun appears in the direction of Mecca, and shadows cast by vertical objects determine the qibla. At two other moments in the year, the sun passes through the nadir of the Kaaba, casting shadows that point in the opposite direction, and thus also determine the qibla. These occur on 12, 13, or 14 January at 00:30 SAST, and 28 or 29 November at 00:09 SAST.