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19-424: The direction opposite of the nadir is the zenith . 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 (e.g., meteorology ), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The term can also be used to represent

38-422: A sidereal compass rose of a sidereal compass. [REDACTED] Media related to Zenith (topography) at Wikimedia Commons 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 coordinate system

57-436: A person's spirits , the quality of an activity or profession, or the nadir of American race relations . Zenith The zenith ( UK : / ˈ z ɛ n ɪ θ / , US : / ˈ z iː n ɪ θ / ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction ( plumb line ) opposite to the gravity direction at that location ( nadir ). The zenith

76-573: 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 ,

95-409: Is sometimes called the az/el system , the alt/az system , or the alt-azimuth system , among others. In an altazimuth mount of a telescope , the instrument's two axes follow altitude and azimuth. This celestial coordinate system divides the sky into two hemispheres : The upper hemisphere, where objects are above the horizon and are visible, and the lower hemisphere, where objects are below

114-515: Is the "highest" point on the celestial sphere. The word zenith derives from an inaccurate reading of the Arabic expression سمت الرأس ( samt al-raʾs ), 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 ,

133-407: Is used to indicate the progression to the lowest point of a clinical symptom (e.g. fever patterns) or a laboratory count. In oncology , the term nadir is used to represent the lowest level of a blood cell count while a patient is undergoing chemotherapy . A diagnosis of neutropenic nadir after chemotherapy typically lasts 7–10 days. The word is also used figuratively to mean a low point, such as with

152-473: 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 (or elevation angle ). The Sun reaches the observer's zenith when it

171-467: 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,

190-499: The Earth's surface, in contrast to a geocentric celestial system . The horizontal coordinate system is fixed to a location on Earth, not the stars. Therefore, the altitude and azimuth of an object in the sky changes with time, as the object appears to drift across the sky with Earth's rotation . In addition, since the horizontal system is defined by the observer's local horizon, the same object viewed from different locations on Earth at

209-411: 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

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228-409: 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 can also be determined from the right ascension 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

247-401: The horizon and cannot be seen, since the Earth obstructs views of them. The great circle separating the hemispheres is called the celestial horizon , which is defined as the great circle on the celestial sphere whose plane is normal to the local gravity vector. In practice, the horizon can be defined as the plane tangent to a quiet, liquid surface, such as a pool of mercury . The pole of

266-502: The location's antipode and is 90° below the horizon. Nadir also refers to the downward-facing viewing geometry of an orbiting satellite , such as is employed during remote sensing of the atmosphere , as well as when an astronaut faces the Earth while performing a spacewalk . A nadir image is a satellite image or aerial photo of the Earth taken vertically . A satellite ground track represents its orbit projected to nadir on to Earth's surface. Generally in medicine, nadir

285-399: The lowest point that a celestial object reaches along its apparent daily path around a given point of observation (i.e. the object's lower culmination ). This can be used to describe the position of the Sun , but it is only technically accurate for one latitude at a time and only possible at the low latitudes . The Sun is said to be at the nadir at a location when it is at the zenith at

304-406: The same time will have different values of altitude and azimuth. The cardinal points on the horizon have specific values of azimuth that are helpful references. Horizontal coordinates are very useful for determining the rise and set times of an object in the sky. When an object's altitude is 0°, it is on the horizon. If at that moment its altitude is increasing, it is rising, but if its altitude

323-442: The station, relative to the earth. 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 this allows latitude to be determined, since

342-420: The upper hemisphere is called the zenith . The pole of the lower hemisphere is called the nadir . The following are two independent horizontal angular coordinates : A horizontal coordinate system should not be confused with a topocentric coordinate system . Horizontal coordinates define the observer's orientation, but not location of the origin, while topocentric coordinates define the origin location, on

361-590: 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

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