apparent horizon ofoq-e padidâr Fr.: horizon apparent The circle determined by the intersection of the heavens with a → cone whose → vertex is the → eye, and whose elements are tangent to lines of the Earth's surface. Same as → visible horizon. Assuming that there is no → atmospheric refraction, apparent horizon coincides with → geometric horizon. See also → sea horizon. |
artificial horizon ofoq-e sâxtegi Fr.: horizon artificiel A shallow flat vessel filled with → mercury or some other viscous → liquid used in special → sextant for measuring altitudes of celestial bodies at sea in the absence of a → visible horizon. → artificial; → horizon. |
astronomical horizon ofoq-e axtaršenâsik Fr.: horizon astronomique The intersection of a plane perpendicular to the radius of the Earth through the observer's eye with the celestial sphere. Same as → true horizon. Because the → celestial sphere has an infinite radius, two observers at different heights above sea level, but placed on the same vertical line, have the same astronomical horizon. Because of → dip of the horizon, the astronomical horizon always lies above the → sea horizon. But on land it is usually hidden by trees, hills, and buildings which determine the observer's → apparent horizon. → astronomical; → horizon. |
blue horizontal branch star setâre-ye âbi-ye šâxe-ye ofoqi Fr.: étoile bleue de la branche horizontale A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0, they have evolved past the → red giant stage and are burning helium in their core. → blue; → horizontal; → branch, → star. |
celestial horizon ofoq-e âsmâni (#) Fr.: horizon céleste A great circle on the → celestial sphere having a plane that passes through the center of the Earth at a right angle to the line formed by an observer's → zenith and → nadir. |
cosmic horizon ofoq-e keyhâni (#) Fr.: horizon cosmologique The → observable region of the → Universe,
limited in extent by the distance → light has traveled during
the time elapsed since the beginning of the Universe
(→ Big Bang). No signal from the objects lying beyond the cosmic horizon
can be received because light has not yet had enough time to travel the distance.
The cosmic horizon can be defined in two ways: |
dip of the horizon našib-e ofoq Fr.: inclinaison de l'horizon The angle created by the observer's line of sight to the → apparent horizon and t he → true horizon. Neglecting the → atmospheric refraction, dip of the horizon can be expressed by θ (radians) = (2h/R)^{1/2}, where h is the observer's height and R the Earth's radius. An an example, for a height of 1.5m above the sea, and R = 6.4 x 10^{6} m, the dip angle is about 0.00068 radians, or 0.039 degrees, about 2.3 minutes of arc, quite appreciable by the eye. See also → distance to the horizon. Same as → dip angle. |
distance to the horizon apest-e ofoq Fr.: distance à l'horizon The distance separating an observer and the → apparent horizon of the place. Neglecting the → atmospheric refraction, it is given by: d = (2Rh)^{1/2}, where R is the radius of the Earth and h is the observer's height. This can be approximated to: d (km) = 3.57(h)^{1/2} for a typical value of R = 6378 km. The atmospheric refraction, however, makes the thing more complex, depending on the temperature and density variations along the line of sight. Generally, refraction pushes the apparent horizon about 10% farther. |
event horizon ofoq-e ruydâd (#) Fr.: horizon d'événement 1) The surface surrounding a → black hole with the property
that any light ray emitted inside it cannot escape to the outer space because of the
strength of the → gravitational field. The radius of the
event horizon is called the → Schwarzschild radius.
See also → photon sphere. |
extreme horizontal branch star (EHB) setâre-ye šâxe-ye ofoqi-ye ostom Fr.: étoile de la branche horizontale extrême The hottest variety of stars on the → horizontal branch with temperatures ranging from 20,000 to 40,000 K. EHB stars are distinguished from normal horizontal branch stars by having extremely thin, inert hydrogen envelopes surrounding the helium-burning core. They are hot, dense stars with masses in a narrow range near 0.5 Msun. These stars have undergone such extreme mass loss during their first ascent up the giant branch that only a very thin hydrogen envelope survives. Stars identified as EHB stars are found in low metallicity globular clusters as an extension of the normal HB. → extreme; → horizontal; → branch; → star. |
field horizontal branch star setâre-ye šâxe-ye ofoqi-ye meydâni Fr.: étoile de la branche horizontal du champ A → horizontal branch star with high velocity. → field; → horizontal; → branch; → star. |
geometric horizon ofoq-e hendesi Fr.: horizon géométrique Where the apparent → sea horizon would be if there were no → atmospheric refraction. |
horizon ofoq (#) Fr.: horizon 1) An imaginary circle that delimits the sky and the Earth. From O.Fr. orizon, from orizonte, from L. horizontem (nom. horizon), from Gk. horizon kyklos "bounding circle," from horizein "bound, limit, divide, separate," from horos "boundary." Ofoq, from Ar. |
horizon coordinate system râžmân-e hamârâhâ-ye ofoqi Fr.: coordonnées horizontales The coordinate system based on the position of the observer. The horizontal plane is the fundamental plane and the coordinates are → altitude and → azimuth. → horizon; → coordinate; → system. |
horizon problem parâse-ye ofoq Fr.: problème de l'horizon A problem with the standard cosmological model of the Big Bang related to the observational fact that regions of the Universe that are separated by vast distances nevertheless have nearly identical properties such as temperature. This contradicts the fact that light moves with a finite speed and, as a result, certain events which occur in the Universe are completely independent of each other. Inflationary cosmology offers a possible solution. |
horizon system râžmân-e ofoqi Fr.: coordonnées horizontales Same as → horizon coordinate system. |
horizontal ofoqi (#) Fr.: horizontal 1) Of or pertaining to the → horizon. |
horizontal branch (HB) šâxe-ye ofoqi (#) Fr.: branche horizontale A set of roughly horizontal points in the → Hertzsprung-Russell diagram of a typical → globular cluster. It displays a stage of stellar evolution which immediately follows the → red giant branch (RGB) in stars with an initial mass < 1.2 Msun. When the star's ascent of the RGB is terminated by the → helium flash, it moves down to the HB. The star's → effective temperature on the HB is higher than it was on the RGB, but the luminosity is considerably less than at the helium flash. Usually HB stars have two energy sources: in addition to the → helium burning in their cores, they experience → hydrogen fusion in a surrounding shell. The thickness of the shell determines the color of the HB stars. A thin shell, involving low → opacity, makes the star look blue. The HB domain encompasses a very large effective temperature range with several members: → extreme HB, → blue HB, → RR Lyrae, → red HB, and → red clump stars. The locations depend on many parameters, including stellar mass, metallicity, age, helium abundance, and rotation. → horizontal; → branch. |
horizontal branch star setâre-ye šâxe-ye ofoqi Fr.: étoile de la branche horizontale A star lying on the → horizontal branch. → horizontal; → branch; → star. |
horizontal eclipse mâhgereft-e ofoqi Fr.: selenelion A type of → lunar eclipse that occurs when both the Sun and the eclipsed Moon can be observed at the same time. This is possible only when lunar eclipse occurs just before sunset or just after sunrise. At that case, both bodies will appear just above the horizon at nearly opposite points in the sky. Also called → selenelion and → selenehelion. → horizontal; → eclipse. |