An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 60 Search : sphere
hemisphere
  نیم‌سپهر، نیمکره   
nimsepehr (#), nimkoré (#)

Fr.: hémisphère   

Half of a sphere bounded by a great circle, especially one of the halves into which the earth or the celestial sphere is divided.

From L. hemisphærium, from Gk. hemisphairion, from hemi- "half," (from PIE base *semi-; cf. Skt. sami, L. semi-, O.H.G. sami- "half," and O.E. sam-) + sphaira, → sphere.

From nim-, → half + sepehr, koré, → sphere.

Hill sphere
  سپهر ِ هیل   
sepehr-e Hill

Fr.: sphère de Hill   

The spherical region around a → secondary in which the secondary's gravity is more important for the motion of a particle about the secondary than the tidal influence of the → primary. The radius is described by the formula: r = a (m/3M)1/3, where, in the case of the Earth, a is the semi-major axis of the orbit around the Sun, m is the mass of Earth, and M is the mass of the Sun. The Hill sphere for the Earth has a radius of 0.01 → astronomical units (AU). Therefore the Moon, lying at a distance of 0.0025 AU, is well within the Hill sphere of the Earth.

Named for George William Hill (1838-1914), an American astronomer who described this sphere of influence; → sphere.

homocentric spheres
  سپهرهای ِ هم‌مرکز   
sepehrhâ-ye ham-markaz

Fr.: sphères homocentriques   

Concentric → spheres of Eudoxus.

homo-; → center; → -ic; → sphere.

hydrosphere
  آب‌سپهر   
âbsepehr (#)

Fr.: hydrosphère   

A term denoting the water portion of the Earth's surface.

hydro-; → sphere.

infosphere
  ازداسپهر   
azdâsepehr

Fr.: infosphère   

The global network of all the world's communications, databases, and sources of information.

information; → sphere.

ionosphere
  یون‌سپهر   
yonsepehr (#)

Fr.: ionosphère   

The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation.

ion + → sphere.

Jupiter's atmosphere
  جو ِ هرمز، هواسپهر ِ ~   
javv-e Hormoz, havâsepehr-e ~

Fr.: atmosphère de Jupiter   

The gaseous envelope surrounding Jupiter. It is about 90% → hydrogen and 10% → helium (by numbers of atoms, 75/25% by mass) with traces of → methane, → water, and → ammonia. This is very close to the composition of the primordial → solar nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. The outermost layer is composed primarily of ordinary → molecular hydrogen and helium. Visually, Jupiter is dominated by two atmospheric features; a series of ever-changing atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the → Great Red Spot.

Jupiter; → atmosphere.

lithosphere
  سنگ‌سپهر، لیتوسپهر   
sangsepehr (#), litosepehr

Fr.: lithosphère   

The solid portion of the → Earth, as compared to the → atmosphere and the → hydrosphere. The lithosphere consists of semi-rigid plates that move relative to each other on the underlying → asthenosphere. The process is known as → plate tectonics and helps explain → continental drift.

litho-; → sphere.

lower atmosphere
  هواسپهر ِ زیرین، جو ِ ~   
havâsepehr-e zirin, javv-e ~

Fr.: atmosphère inférieure   

Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the → troposphere and lower → stratosphere); hence used in contrast to the common meaning for the → upper atmosphere. In other contexts, the term implies the lower troposphere (Meteorology Glossary, American Meteorological Society).

lower; → atmosphere.

lunar exosphere
  برون‌سپهر ِ ماه   
borun-sepehr-e mâh

Fr.: exosphère lunaire   

An extremely thin gathering of gas surrounding the → Moon. It is made up of → atoms and → ions generated at the Moon's surface by interaction with → solar radiation, → plasma in the Earth's → magnetosphere, or → micrometeorites.

lunar; → exosphere.

magnetosphere
  مغنات‌سپهر   
meqnâtsepehr

Fr.: magnétosphère   

The region around a celestial body in which the magnetic field of the body dominates the external magnetic field. Each planet with a magnetic field (Earth, Jupiter, Saturn, Uranus, and Neptune) has a magnetopause. The Earth's magnetosphere is a dynamic system that responds to solar variations. It prevents most of the charged particles carried in the → solar wind, from hitting the Earth. Since the solar wind is → supersonic, a → bow shock is formed on the sunward side of the magnetosphere. The solar wind ahead is deflected at a boundary called → magnetopause. The region between the bow shock and the magnetopause is called the → magnetosheath. As the solar wind sweeps past the Earth, the terrestrial magnetic field lines are stretched out toward the night side to form a → magnetotail.

From → magnet + → sphere.

mesosphere
  مسوسپهر   
mesosepehr

Fr.: mésosphère   

The layer of the atmosphere located between the → stratosphere and the → ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.

meso-; → sphere.

middle atmosphere
  هواسپهر ِ میانی، جو ِ ~   
havâsepehr-e miyâni, javv-e ~

Fr.: atmosphère moyenne   

The region lying between the → troposphere and the → thermosphere comprising the → stratosphere and the → mesosphere (Meteorology Glossary, American Meteorological Society).

middle; → atmosphere.

MOLsphere
  مول-سپهر   
MOLsepehr

Fr.: MOLsphère   

A hypothetical, large molecular sphere, as a component of the → stellar atmosphere, suggested to encompass → red giant and → supergiant stars. This suggestion has offered satisfactory explanations to the spectrum of → Mu Cephei (Tsuji 2003), but it is not clear whether MOLspheres are common features of the atmospheres of all such stars.

MOL, from → molecular; → sphere.

northern hemisphere
  نیم‌سپهر ِ هودری   
nimsepehr-e hudari

Fr.: hémisphère nord   

The half of the → Earth or another → astronomical object between the → north pole and the → equator.

northern; → hemisphere.

oblique sphere
  سپهر ِ یکور   
sepehr-e yekvar

Fr.: sphère oblique   

The celestial sphere when the circles parallel to the equator are cut obliquely by the horizon plane, which divides them into two unequal parts. In other words, the sphere when its axis is oblique to the horizon of the place.

oblique; → sphere.

osculating sphere
  سپهرِ آبوسنده، کره‌یِ آبوسنده   
sepehr-e âbusandé, kore-ye ~

Fr.: sphère osculatrice   

For a curve C at a point p, the limiting sphere obtained by taking the sphere that passes through p and three other points on C and then letting these three points approach p independently along C.

osculating; → sphere.

photon sphere
  سپهر ِ فوتون   
sepehr-e foton

Fr.: sphère de photons   

A surface where if a photon is emitted from one of its points the photon follows a closed orbit and returns periodically to its departure point. Such a surface exists only near sufficiently → compact objects where the → curvature of → space-time is very important. In other words, a body can take a stable orbit around a → black hole provided that it moves with the → speed of light. However, only photons can have such a velocity; hence the term "photon sphere." For a non-rotating → Schwarzschild black hole, the photon sphere has a radius of R = 3GM/c2 = 3 RS/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and RS is the → Schwarzschild radius. For a rotating, → Kerr black hole, the situation is much more complex due to the → Lense-Thirring effect. In that case circular paths exist for radii whose values depend on the rotation direction. More specifically, in the equatorial plane there are two possible circular light paths: a smaller one in the direction of the rotation, and a larger one in the opposite direction.

photon; → sphere.

photosphere
  شیدسپهر، رخشان‌سپهر   
šidsepehr, raxšânsepehr

Fr.: photosphère   

1) The visible surface of the Sun (temperature 5700 K), just below the → chromosphere and just above the → convective zone. The solar photosphere is a thin layer of roughly 300 km wide. Its temperature decreases uniformly with height, from about 6,600 K (pressure 0.868 → millibars) at its bottom, to about 4,400 K (pressure 125 mb), where it merges with the chromosphere. The photosphere has a "rice-grain" appearance, called → granulation, caused by rising (hot) and falling (cool) material in the → convective cells just below the photosphere. Other main features of the photosphere are → sunspots, → faculae, and → supergranulation.
2) The region of a star which gives rise to the continuum radiation emitted by the star.

photo- + → sphere.

plane-parallel atmosphere
  هواسپهر ِ پراسو-تخت، جو ِ ~   
havâsepehr-e parâsu-taxthâ, javv-e ~

Fr.: atmosphère plan-parallèle   

An approximation used in many stellar atmosphere models that depict the atmosphere as being only one-dimensional and bounded at the top and bottom by horizontal plane surfaces normal to the direction of gravity.

plane; → parallel; → atmosphere.

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