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

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.

The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator.

→ horizontal; → parallax.

The angular distance of an object below the horizon when it appears to lie on the horizon.

→ horizontal; → refraction.

In computer science, a scaling in which the processing power is increased/decreased by adding/removing nodes with similar resources. See also → vertical scaling.

→ horizontal; → scaling.

1a) The bony pointed outgrowth, usually in pairs, on the heads of some animals.
1b) Astro.: Either of the ends of the → crescent Moon.
2) Something resembling a horn.
3) A wind instrument, originally an animal horn used as a wind instrument.
See also: → feedhorn

M.E. horn(e), from O.E. horn "horn of an animal," also "wind instrument" (originally made from animal horns), from P.Gmc. *khurnaz (cf. Ger. Horn, Du. horen), from PIE *ker- "head, horn, top, summit" (cf. Pers. soru "horn," sar "head," Gk. kara "head," karena "head, top," keras "horn;" L. cornu "horn," cerebrum "brain;" Skt. śiras- "head, chief").

1, 2) Mid.Pers šâk; cf. Skt. sakha- "a branch, a limb;" Arm. cax; Lith. šaka; O.S. soxa; PIE *kakhâ "branch."
3) Karnâ "a trumpet-like wind instrument," variant sornâ "a wind instrument," probably related to soru, sorun "horn," sar "head;" Mid.Pers. sar "head," sru "horn;" Av. sarah- "head," srū- "horn, nail;" cognate with E. horn, as above, from PIE *ker- "head, horn."

The Clock. A faint constellation in the southern hemisphere, at about 3h right ascension, 55° south declination. Its brightest star, α Horologii, is of magnitude 3.9. Abbreviation: Hor; Genitive: Horologii.

Horologium "clock," from L., from Gk. horologion, from horolog(os) "timeteller," from horo-, combining form of hora "hour" (→ year) + -log-, stem of legein "to speak, tell" (+ -os adj. suffix) + -ion diminutive suffix.
Originally named Horologium Oscillitorium by Abbé Nicolas Louis de Lacaille (1713-1762) to honour the inventor of the pendulum clock, Christian Huygens (1629-1695).

Sâ'at "clock," from Ar.

A schematic drawing showing the positions of the Sun, Moon, and planets at the time of a person's birth for baseless astrological purposes.

From M.Fr. horoscope, from L. horoscopus, from Gk. horoskopos "nativity, horoscope," also "one who casts a horoscope," from hora "hour" + skopos "watching."

Zâyecé "horoscope, thema," from Mid.Pers. zâycag "horoscope," from zâyidan, zâdan, "to give birth, bring forth;" Av. zan- "to bear, give birth to a child, be born," infinitive zazāite, zāta- "born;" cf. Skt. jan- "to produce, create; to be born," janati "begets, bears;" Gk. gignomai "to happen, become, be born;" L. gignere "to beget;" PIE base *gen- "to give birth, beget."

1) A large, solid-hoofed, herbivorous quadruped, Equus caballus, domesticated since prehistoric times.
2) In a → planispheric astrolabe, the small prominence that, inserted into a slit in the pin, prevents the parts of the instrument from coming loose when in use. The part owes its name to the fact that astrolabe-makers would often shape it into a horse's head (online museo galileo, VirtualMuseum).
See also: → horse latitude, → Horsehead Nebula, → horsepower, → horseshoe mounting, → horseshoe orbit.

Horse, O.E. hors, from P.Gmc. *khursa- (cf. M.Du. ors, Du. ros, O.H.G. hros, Ger. Roß "horse"), of unknown origin; → latitude.

Asb "horse," from Mid.Pers. asp; O.Pers. asa- "horse;" Av. aspa- "horse," aspā- "mare," aspaiia- "pertaining to the horse;" cf. Skt. áśva- "horse, steed;" Gk. hippos; L. equus; O.Ir. ech; Goth. aihwa-; O.E. eoh "horse;" PIE base *ekwo- "horse."

The belts of latitude over the oceans, located around 30° north and south of the equator, characterized by predominantly calm or light winds and hot and dry weather.

→ horse; → latitude.
The origin of the term horse latitudes is not clear, despite numerous speculations. A likely explanation appears in Spanish in a natural history text (Historia General y Natural de las Indias by Lopez de Gomara) published in 1535. Therefore, the term derives from El Golfo de las Yeguas, which translates to "The Mares' Sea." The sailors called it this because in the 1500's there was active shipping of horses, particularly brood mares, from Spain to the Canary Islands, and many of the horses died during the transit of this area.

A huge → dark cloud of → interstellar dust that is shaped like a horse's head. It is luminous at its edges because it is in front of the bright → emission nebula IC 434. Its height and width are about 5 and 2.5 → light-years respectively. It is located at a distance of about 1500 light-years in the constellation → Orion. Also known as Barnard 33.

→ horse; → head; → nebula.

A unit that is used to measure the → power of engines and motors.
1) Metric horsepower is equal to the power required to carry a load of 75 kg over a distance of one meter in one second. It is equivalent to 746 → watts.
2) British (US) horsepower is the rate of work when 33,000 foot-pounds of work are done per minute. The horsepower was defined by James Watt (1736-1819), the inventor of the steam engine, to compare the output of steam engines with the power of draft horses. He determined that a horse is typically capable of a power of 550 foot-pounds per second.

→ horse; → power.

Asb-e boxâr "vapor horse," translation of Fr. cheval-vapeur, from asb, → horse, + boxâr, → vapor.

An equatorial mounting in which the upper end of the polar axis frame is made into a horseshoe shape to accommodate the telescope tube.

Horseshoe, from → horse + shoe, from O.E. scoh "shoe," from P.Gmc. *skokhaz (cf. Dan., Swed. sko, O.S. skoh, Du. schoen, O.H.G. scuoh, Ger. Schuh); → mounting.

Barnešând, → mounting; na'l "horseshoe, shoe," loanword from Ar.

A periodic orbit which passes around the → Lagrangian points L4, L3, and L5, but neither of the two primaries. This orbit is shaped like a horseshoe when viewed in a reference frame rotating with the primaries. Such orbits occur in the solar system, for example in the case of the satellites → Janus and → Epimetheus, which share the same orbit around → Saturn. The smaller Epimetheus encompasses both the L4 and L5 points associated with the larger Janus and performs a horseshoe orbit relative to Saturn and Janus. The satellites experience a close approach every 4 years during which their orbits are exchanged. → tadpole orbit.

→ horseshoe mounting; → orbit.