An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A fluid with uniform properties throughout, but meteorologists sometimes designate as homogeneous a fluid with constant density.

→ homogeneous, → fluid.

A → linear differential equation if the right-hand member is zero, Q(x) = 0, on interval I.

→ homogeneous; → linear; → differential; → equation.

→ Turbulence in which spatial derivatives of all mean turbulent quantities are negligible.

→ homogeneous, → turbulence.

A model Universe which is homogeneous and → isotropic on large scales. It is modeled by a → Robertson-Walker cosmology. A homogeneous Universe is filled with a constant density and negligible pressure. Any small spatial region is characteristic for the whole Universe.

→ homogeneous; → Universe.

A → mapping between two mathematical → objects that preserves the object structure. A general → morphism.

→ homo-; → morphism.

A molecule that is composed of only one type of → chemical element, e.g. the → molecular hydrogen and → ozone.

→ homo-; → nuclear; → molecule.

The altitude at which → molecular diffusion replaces → eddy diffusion as the dominant vertical transport mechanism. Light gases separate out from heavier ones above this altitude. The flux of hydrogen through the homopause is limited by diffusion.

→ homo-; + pause "break, cessation, stop," from M.Fr. pause, from L. pausa "a halt, stop, cessation," from Gk. pausis "stopping, ceasing," from pauein "to stop, to cause to cease."

Hamgen, → homogeneous, + marz "frontier, border, boundary," → frontier. frontier.

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 nebula of gas and dust (about 17" x 12" in size), which surrounds the massive star Eta Carinae and lies about 7500 light-years away. The surrounding material was ejected by the massive star in 1843 during its violent eruption, and is now expanding at about 500 km/sec.

Homunculus, "a diminutive human being; little man" (since the nebula resembled a small human to early observers), from L. homin-, homun-, homo "eartly being," humus "the earth" (cf. Pers. zamin "earth, ground," Mid.Pers. zamig "earth;" Av. zam- "the earth;" Skt. ksam; Gk. khthôn, khamai "on the ground;" PIE root *dh(e)ghom "earth") + → -ula, -ule; → nebula.

Miq, → nebula; âdamak "little man."

1) A curved or bent piece of metal or other hard material for catching, holding, or hanging something.
2) Something curved or bent like a hook.

M.E. hoke, O.E. hoc "hook, angle;" cf. M.Du. hoek, Du. haak, Ger. Haken "hook."

Qollab "a hook, a hooked device," probably ultimately from Proto-Ir. gart- "to turn;" cf. Pers. gard-, gardidan, gaštan "to turn, to wind;" cognate with dialectal qellidan "to roll."

The law stating that if a body is deformed the → strain produced is directly proportional to the applied → stress. If the elastic limit is not exceeded, the material returns to its original shape and size on the removal of the stress. Hooke's law forms the basis of the theory of → elasticity.
More specifically, within certain limits, the force required to stretch an elastic object such as a metal spring is directly proportional to the extension of the spring. It is commonly written: F = -kx, where F is the force, x is the length of extension/compression and k is a constant of proportionality known as the spring constant.

Named after Robert Hooke (1635-1703), British scientist who described the relationship in 1676; → law.

1) To move by a quick springy leap or in a series of leaps. (Of a person) Move by jumping on one foot. (Of a bird or other animal) move by jumping with two or all feet at once.
2) An act of hopping; short leap.

M.E. hoppen; O.E. hoppian; cognate with Ger. hopfen, O.N. hoppa.

Kopidan, from kop; cf. (Bašâgardi) kup, (Lârestâni) komp, (Bardesiri) gopak, (Sistâni) job, (Kermâni) pok, pokidan "jump, leap."

1) An imaginary circle that delimits the sky and the Earth.
2) The fundamental great circle of the → horizon system, defined by the intersection of the → celestial sphere and a level plane passing through the observer. → celestial horizon.
3) In → Robertson-Walker models, the boundary separating objects already observed from those not yet observed, or the boundary separating objects observable from unobservable (J. Plebanski, A. Krasinski, 2006, An Introduction to General Relativity and Cosmology, Cambridge Univ. Press).
4) → cosmic horizon.
5) → event horizon.
See also:
→ apparent horizon, → artificial horizon, → astronomical horizon, → dip of the horizon, → distance to the horizon, → geometric horizon, → horizon coordinate system, → horizon problem, → horizon system, → particle horizon, → sea horizon, → sensible horizon, → sound horizon, → true horizon, → visible horizon.

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.

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.

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; → problem.

Same as → horizon coordinate system.

→ horizon; → system.

1) Of or pertaining to the → horizon.
2) At right angles to the → vertical; parallel to level ground.
See also:
→ blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → horizontal branch, → horizontal branch star, → horizontal eclipse, → horizontal parallax, → horizontal refraction, → red horizontal branch star, → supra-horizontal branch star, → zero age horizontal branch star.

From → horizon + → -al.

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.

A star lying on the → horizontal branch.

→ horizontal; → branch; → star.

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.