What name is given to the point on the celestial sphere, directly below the observer, opposite the zenith?
"is the point on the observer's meridian, where a line from the centre of the earth (treated in this case as a 'sphere') projected outwards through the observer and onto the Celestial Sphere points to his Zenith. A simple way of thinking about it is to imagine that when you are standing on Z, your feet point directly towards the centre of the earth, and your head points to your Zenith. (A line in the opposite direction, through your feet and the centre of the earth, would point to your 'Nadir') Thus 'Z', the observer's position on the earth, can"
"to that of the subsolar point. Parallactic angle 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 when the object crosses the meridian. Depending on the type of mount of the telescope, this angle may also affect the orientation of the celestial object's"
"Subsolar point The subsolar point on a planet 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. It can also mean the point closest to the sun on an astronomical object, even though the sun might not be visible. To an observer on a planet with an orientation and rotation similar to those of Earth, the subsolar point will appear to move westward, completing one circuit around the globe each day, approximately moving along the equator. However, it will"
"Azimuth An azimuth () (from the pl. form of the Arabic noun ""السَّمْت"" ""as-samt"", meaning ""the direction"") 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. When used as a celestial coordinate, the azimuth is the horizontal direction of a star or other astronomical object in the sky. The star is the point of interest, the reference plane is the local area (e.g. a"
"Parallactic angle 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 when the object crosses the meridian. Depending on the type of mount of the telescope, this angle may also affect the orientation of the celestial object's disk as seen in a telescope."
"Nadir The nadir (, ; from / ALA-LC: ""naẓīr"", meaning ""counterpart"") 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 (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 direction opposite the nadir is the zenith. Nadir is"
"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 still liquid surface, such as a pool of mercury. The pole of 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: The horizontal coordinate system is sometimes called other names, such as the az/el system, the alt/az system, or the alt-azimuth system, from the name of the mount used for telescopes, whose two axes follow"
"a scientific context, the zenith is the direction of reference for measuring the zenith angle, the angle between a direction of interest (e.g. a star) and the local zenith. 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. 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"
"Circle of equal altitude The circle of equal altitude also called circle of position, CoP, is the real line of position in celestial navigation. It is defined as the locus of the Earth on which an observer sees a star, at a given time, with the same observed altitude. The center of the CoP, is the substellar point or geographical position of the observed body, and its radius is the great circle distance equal to the zenith distance of the body. The equation links the following variables formula_1 Being ""B"" the latitude (+N/S), ""L"" the longitude (+E/-W). ""LHA"" = ""GHA"""
"Central meridian (planets) The central meridian of a celestial body that presents a disc to an observer (such as planet, moon, or star) is the meridian on the body's surface that goes through the centre of the body's disc as seen from the point of view of the observer. The term as generally used in observational astronomy refers to the central meridian of the celestial body as seen by a theoretical observer on Earth for whom the celestial body is at the zenith. An imaginary line is drawn from the centre of the Earth to the center of the other"
"the Earth as seen from the Moon changes as the Earth rotates. Central meridian (planets) The central meridian of a celestial body that presents a disc to an observer (such as planet, moon, or star) is the meridian on the body's surface that goes through the centre of the body's disc as seen from the point of view of the observer. The term as generally used in observational astronomy refers to the central meridian of the celestial body as seen by a theoretical observer on Earth for whom the celestial body is at the zenith. An imaginary line is drawn"
"of History of Science. ""A remarkable synthesis of Indian spherical astronomical knowledge occurs in a passage in Tantrasamgraha."" In astronomy, the spherical triangle formed by the zenith, the celestial north pole and the Sun is called the ""astronomical triangle"". Its sides and two of its angles are important astronomical quantities. The sides are 90° - φ where φ is the observer's terrestrial latitude, 90° - δ where δ is the Sun's declination and 90° - ""a "" where ""a"" is the Sun's altitude above the horizon. The important angles are the angle at the zenith which is the Sun's azimuth"
"is Caloris Planitia (""hot basin""), appropriately named because an observer near its centre would see the Sun loop around the zenith once per Mercurian day, and hence experience a very hot day indeed. After the Sun, the second-brightest object in the Mercurian sky is Venus, which is much brighter there than for terrestrial observers. The reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see only its night side. Indeed, even when Venus is brightest in the Earth's sky, we are actually seeing only a narrow crescent. For"
"Declination In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question. The root of the word ""declination"" (Latin, ""declinatio"") means ""a bending away"" or ""a bending down"". It comes from the same root as the words ""incline"" (""bend toward"") and ""recline"" (""bend backward""). In some 18th and 19th century astronomical texts, declination is given as ""North Pole Distance"""
"+28° 56'. Galactic anticenter The galactic anticenter is a direction in space directly opposite to the Galactic Center, as viewed from Earth. This direction corresponds to a point on the celestial sphere. From the perspective of an observer on Earth, the galactic anticenter is located in the constellation Auriga, and Beta Tauri is the bright star that appears nearest this point. In terms of the galactic coordinate system, the Galactic Center (in Sagittarius) corresponds to a longitude of 0°, while the anticenter is located exactly at 180°. In the equatorial coordinate system, the anticenter is found at roughly RA 05h"
"Galactic anticenter The galactic anticenter is a direction in space directly opposite to the Galactic Center, as viewed from Earth. This direction corresponds to a point on the celestial sphere. From the perspective of an observer on Earth, the galactic anticenter is located in the constellation Auriga, and Beta Tauri is the bright star that appears nearest this point. In terms of the galactic coordinate system, the Galactic Center (in Sagittarius) corresponds to a longitude of 0°, while the anticenter is located exactly at 180°. In the equatorial coordinate system, the anticenter is found at roughly RA 05h 46m, dec"
"starting point of the longitudinal coordinates. The origin is the zero distance point, the ""center of the celestial sphere"", although the definition of celestial sphere is ambiguous about the definition of its center point. The ""horizontal"", or altitude-azimuth, system is based on the position of the observer on Earth, which revolves around its own axis once per sidereal day (23 hours, 56 minutes and 4.091 seconds) in relation to the star background. The positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth."
"south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on Earth. (At the poles, the celestial equator coincides with the astronomical horizon.) At all latitudes, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the celestial equator itself. Astronomical objects near the celestial equator appear above"
"Celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit). An observer standing on Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north (or"
"distances, it was necessary to determine the observatory's colatitude, or distance of the celestial pole from the zenith, by observing the upper and lower culmination of a number of circumpolar stars. The difference between the circle reading after observing a star and the reading corresponding to the zenith was the zenith distance of the star, and this plus the colatitude was the north polar distance. To determine the zenith point of the circle, the telescope was directed vertically downwards at a basin of mercury, the surface of which formed an absolutely horizontal mirror. The observer saw the horizontal wire and"
"sphere. The declination symbol , (lower case ""delta"", abbreviated dec) measures the angular distance of an object perpendicular to the celestial equator, positive to the north, negative to the south. For example, the north celestial pole has a declination of +90°. The origin for declination is the celestial equator, which is the projection of the Earth's equator onto the celestial sphere. Declination is analogous to terrestrial latitude. The right ascension symbol , (lower case ""alpha"", abbreviated RA) measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the"
"camera rigging that utilizes more than 2 lenses to capture 360º footage, the nadir location follows the same logic as the astronomical definition in that it is located opposite of the zenith, or in the same direction as the gravitational pull in relation to the camera rigging. Nadir The nadir (, ; from / ALA-LC: ""naẓīr"", meaning ""counterpart"") 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"
"Lunar orbit In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon. The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion, or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion, or periselene, from names or epithets of the moon goddess. Low lunar orbit (LLO)—orbits below altitude—are"
"also move north and south between the tropics over the course of a year, so it is spiraling like a helix. The subsolar point contacts the Tropic of Cancer on the June solstice and the Tropic of Capricorn on the December solstice. The subsolar point crosses the Equator on the March and September equinoxes. When the point passes through Hawaii, the only U.S. state in which this happens, it is known as Lahaina Noon. Subsolar point The subsolar point on a planet is the point at which its sun is perceived to be directly overhead (at the zenith); that is,"
"is employed during remote sensing of the atmosphere, as well as when an astronaut faces the Earth while performing a spacewalk. The term nadir can also be used to represent the lowest point reached by a celestial body during its apparent orbit around a given point of observation. This can be used to describe the location 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 the location's antipode"
"points midway between the Celestial Poles is the Celestial Equator which has a declination of 0°. The visible declinations vary depending where you are on the globe. Only an observer on the North Pole of Earth would be unable to see any stars from the Southern Celestial Hemisphere at night (see diagram below). Once a declination has been found for the point on the horizon that a building faces it is then possible to say whether a specific body can be seen in that direction. While the stars are fixed to their declinations the Sun is not. The rising point"
"celestial body's GP and the observer's position. After some computations, referred to as sight reduction, this measurement is used to plot a line of position (LOP) on a navigational chart or plotting work sheet, the observer's position being somewhere on that line. (The LOP is actually a short segment of a very large circle on Earth that surrounds the GP of the observed celestial body. An observer located anywhere on the circumference of this circle on Earth, measuring the angle of the same celestial body above the horizon at that instant of time, would observe that body to be at"
"Gegenschein Gegenschein ( German for ""countershine"") is a faintly bright spot in the night sky, around the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon. Like the zodiacal light, the gegenschein is sunlight scattered by interplanetary dust. Most of this dust is orbiting the Sun in about the ecliptic plane, with a possible concentration of particles at the Earth–Sun Lagrangian point. It is distinguished from zodiacal light by its high angle of reflection of the incident sunlight on the dust particles. It forms a slightly more luminous, oval glow directly opposite the Sun within the"
"the same point on an infinite-radius celestial sphere will be looking along parallel lines, and observers looking toward the same great circle, along parallel planes. On an infinite-radius celestial sphere, all observers see the same things in the same direction. For some objects, this is over-simplified. Objects which are relatively near to the observer (for instance, the Moon) will seem to change position against the distant celestial sphere if the observer moves far enough, say, from one side of planet Earth to the other. This effect, known as parallax, can be represented as a small offset from a mean position."
"mirror meant these telescopes could only point straight up. 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 as a zenith sector employs a strong altazimuth mount, fitted with levelling screws. Extremely sensitive levels are attached to and the telescope has an eyepiece fitted with a micrometer. They are used for the measurement of small differences of zenith distance, and used in the determination of astronomic"
"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 March equinox to the (hour circle of the) point above the earth in question. When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. An old term, ""right ascension"" () refers to the ""ascension"", or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a"
"point on the surrounding sphere (of radius ) which is the projection parallel to the Earth's axis of a point on the ellipsoid at latitude . It was introduced by Legendre and Bessel who solved problems for geodesics on the ellipsoid by transforming them to an equivalent problem for spherical geodesics by using this smaller latitude. Bessel's notation, , is also used in the current literature. The parametric latitude is related to the geodetic latitude by: The alternative name arises from the parameterization of the equation of the ellipse describing a meridian section. In terms of Cartesian coordinates , the"
"Subparhelic circle The subparhelic circle is a rare halo, an optical phenomenon located below the horizon. It passes through both the subsun, below the sun, and the antisolar point, opposite to the sun. The subparhelic circle is the subhorizon counterpart to the parhelic circle located above the horizon. Located on the subparhelic circle are several relatively rare optical phenomena: The subsun, the subparhelia, the 120° subparhelia, Liljequist subparhelia, the diffuse arcs, and the Parry antisolar arcs. On the accompanying photo centred on the antisolar point, the subparhelic circle is viewable as a gently curved horizontal line intercepted by anthelic arcs."