An Etymological Dictionary of Astronomy and Astrophysics
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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.

Based on chemical composition, the atmosphere is divided into two broad layers: the → homosphere and the heterosphere. The heterosphere has heterogeneous chemical composition, with layered structure, of nitrogen, oxygen, helium, and hydrogen, respectively. The heterosphere begins from about 90 km from the Earth's surface and extends to space.

→ hetero-; → shere.

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.

Concentric → spheres of Eudoxus.

→ homo-; → center; → -ic; → sphere.

Based on chemical composition, the Earth atmosphere is divided into two broad layers: the homosphere and the → heterosphere. The homosphere extends from the surface of the Earth up to the height of about 90 km. It is characterized by an almost homogeneous composition of nitrogen (78%), oxygen (21%), argon (10%), carbon dioxide as well as traces of constituents like dust particles, → aerosols and cloud droplets.

→ homo-; → shere.

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

→ hydro-; → sphere.

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

→ information; → sphere.

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.

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.

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.

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.

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.

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.

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.

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

→ middle; → atmosphere.

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.

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

→ northern; → hemisphere.

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.

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.

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 R_S/2, where G is the → gravitational constant, M is the mass, c is the → speed of light, and R_S 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.