An Etymological Dictionary of Astronomy and Astrophysics

A → conjunction word used to indicate alternatives.

Etymology (EN): M.E., from or, adverb “early, before,” from Old Norse ār akin to O.E. ær “early.”

Etymology (PE): Yâ, from Mid.Pers. ayâb, aviâp “or;” cf. P.Pers. ada, Av. adā, aδa “then.”

1. A globose berry with a yellowish to reddish-orange rind and a sweet edible pulp.
   

2. Any of a group of colors that are between red and yellow in hue (Merriam-Webster.com).

Etymology (EN): M.E., from O.Fr. orange, orenge, from M.L. pomum de orenge, from It. arancia, originally narancia, alteration of Ar. nâranj, from Pers. nârang, from Skt. narangah “orange tree.”

Etymology (PE): Nârenji, from nârenj “orange,” as above.

The path followed by a body moving in a gravitational field. For bodies moving under the influence of a centrally directed force, without significant perturbation, the shape of the orbit must be one of the conic section family of curves (circle, ellipse, parabola, or hyperbola).

Etymology (EN): L. orbita “wheel track, course, circuit.”

Etymology (PE): Madâr, from Ar.

A gradual change in the orbit of a spacecraft caused by aerodynamic drag of a planet’s outer atmosphere and other forces.

See also: → orbit; → decay.

1. Of or relating to an orbit.
   
2. A wave function that describes the state of an electron with a given energy (n, l, and m_l quantum numbers) in an atom (atomic orbital) or in a molecule (molecular orbital).

See also: Orbital, from → orbit + → -al.

1. Mechanics: The → angular momentum associated with the motion of a particle about an origin, equal to the cross product of the position vector (r) with the linear momentum (p = mv): L = r x p. Although r and p are constantly changing direction, L is a constant in the absence of any external force on the system. Also known as orbital momentum.
   

2. Quantum mechanics: The → angular momentum operator associated with the motion of a particle about an origin, equal to the cross product of the position vector with the linear momentum, as opposed to the → spin angular momentum. In quantum mechanics the orbital angular momentum is quantized. Its magnitude is confined to discrete values given by the expression: ħ &radic l(l + 1), where l is the orbital angular momentum quantum number, or azimuthal quantum number, and is limited to positive integral values (l = 0, 1, 2, …). Moreover, the orientation of the direction of rotation is quantized, as determined by the → magnetic quantum number. Since the electron carries an electric charge, the circulation of electron constitutes a current loop which generates a magnetic moment associated to the orbital angular momentum.

See also: → orbital; → angular; → momentum.

The → perpendicular to the → orbital plane.

See also: → orbital; → axis.

Objects in orbit around Earth created by humans, that no longer serve any useful purpose. They consist of everything from entire spent rocket stages and defunct satellites to explosion fragments, paint flakes, dust, and slag from solid rocket motors, and other small particles. Also called space junk and space waste.

See also: → orbital; → debris .

An electron contained within an atom which may be thought of as orbiting around the nucleus, in a manner analogous to the orbit of a planet around the Sun.

See also: → orbital; → electron .

Any of the six parameters needed to specify the → orbit of an object around a → primary body (such as a planet around the Sun or a satellite around the Earth) and give its position at any instant.

Two of them define the size and the form of the orbit: → semi-major axis (a) and → eccentricity (e).

Three angular values determine the orbit position in space: the → inclination (i) of the object’s → orbital plane to the reference plane (such as the → ecliptic), the → longitude of ascending node (Ω), and the → argument of periapsis (ω). And finally the sixth element is the → time of periapsis passage which allows calculating the body’s position along the orbit at any instant.

See also: → orbital; → element.

The → sum of the → potential energy and the → kinetic energy of an object in → orbit.

See also: → orbital; → energy.

An → orbital element that defines the angle between the orbital plane of a solar system body (planet, comet, asteroid) and the plane of the ecliptic. The orbital inclination of the Earth’s orbit is 0°; those of Mercury, Venus, and Mars are 7.01°, 3.39°, and 1.85° respectively.

See also: → orbital; → inclination .

The moving of a spacecraft between two different orbits resulting from a change in its velocity (acceleration). Generally, manoeuvres are caused by
→ thrust from the spacecraft’s motors.

See also: → orbital; → maneuver.

→ orbital maneuver.

See also: → orbital; → manoeuvre.

Theoretical prediction according to which a → giant planet, formed in the outer regions of a → protoplanetary disk, could migrate inward by losing → energy and → angular momentum as the result of → gravitational interactions with the remnants of the disk. This orbital migration could explain the presence of giant gaseous Jupiter-like planets (→ hot Jupiters) very close to their host stars.

See also: → orbital; → migration.

One of the two points of intersection of the orbit of a secondary body with the plane of reference through the primary.

See also: → orbital; → node.

→ orbital element.

See also: → orbital; → parameter.

The time interval between two successive passages of an object through the same point in its orbit around another object.

See also: → orbital; → period .

In → photometry of
→ binary stars or → two-body systems, the number of whole or fractional orbits completed, from the point the photometry begins. The point is conventionally chosen as the position at which the → primary star eclipses the → secondary star, and therefore the → light curve is at a minimum. The phase keeps counting indefinitely, thus the secondary star gets eclipsed at phase 0, 1, 2, 3, and so on. At these phases the primary lies between the secondary and the observer. An orbital phase of 0.5 corresponds to halfway through the binary orbit, 0.75 is three-quarters the way through, and so on.

See also: → orbital; → phase.

The photometric variability induced by the → orbital motion in a → two-body system.

See also: → orbital; → phase.

The plane defined by the motion of an object about a primary body.

See also: → orbital; → plane .

Same as → relativistic precession.

See also: → orbital; → precession.

The situation in which two orbiting objects exert a regular, periodic gravitational influence on each other and therefore their orbital frequencies are related by a ratio of two small → integers. Orbital resonance often results in an unstable interaction in which bodies exchange momentum and shift orbits until the resonance disappears. The resonance increases the eccentricity until a body approaches a planet too closely and the body is slung away.

See also: → orbital; → resonance.

The lessening in size of the orbit of a binary system composed of two compact objects (pulsars/black holes) due to loss of energy by the system, in particular through gravitational wave radiation. This loss will cause the two objects to approach closer to each other, the orbital period decreases and the binary companions will eventually merge.

Etymology (EN): → orbital; shrinkage, from shrink, from M.E. schrinken, O.E. scrincan, from P.Gmc. *skrenkanan (cf. M.Du. schrinken, Swed. skrynka “to shrink.”

Etymology (PE): Darhamkešidegi “shrinking, shriveling,” from state noun of < i>darhamkešidé, from darham- “together, in eachother, toward eachother” (For etymology of dar-, → in-; for etymology of ham-, → com-)

• kešidé “drawn, shrivelled, wrinkled,” from Mod./Mid.Pers. kešidan, kašidan “to draw, protract,
  trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
  Av. karš- “to draw; to plow,” karša- “furrow;” Proto-Iranian *kerš-/*xrah- “to draw, plow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
  Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow;” madâri, → orbital.

Same as → orbital velocity.

See also: → orbital; → speed.

The velocity of an object in a given orbit around a gravitating mass. For a perfect circular orbit, the velocity is described by the formula V =√[G(M + m)/r], where G is the gravitational constant, M the mass of the primary gravitating body, m the mass of the orbiting object, and r the radius of the orbit.

See also: → orbital; → velocity.

A → spacecraft or → satellite designed to orbit a planet or other → solar system body.

See also: → orbit; → -er.

In astronautics, the study of satellite orbits and precise
determination of orbital elements which gives the exact position of the satellite.

See also: → orbital + → -graphy.

1a) General: The way in which several items are arranged, as an indication of their relative importance or size or when each will be dealt with.

1b) Optics: → order of interference.

1c) Math.: The number of rows or columns of a
→ square matrix or → determinant.

1d) Math.: The highest → derivative appearing in a given → differential equation. For example,
d²y/dx² + 2y (dy/dx) - 3 = 0
is a differential equation of order two.

2. To arrange, regulate, or dispose things in their proper places. See also: → disorder, → first-order differential equation, → first-order partial derivative, → first-order spectrum, → interference order, → order of a tensor, → order of interference, → order of magnitude.

Etymology (EN): From O.Fr. ordre, from earlier ordene, from L. ordinem (nominative ordo) “row, rank, arrangement.”

Etymology (PE): Râyé, noun related to râyânidan “to regulate, set in order,” from Mid.Pers. râyânīdan “to arrange, organize,” from rây-
(Mod.Pers. ârây-, ârâyeš, ârâyidan “to arrange, adorn”),
ârây-, ârâstan “to arrange, adorn;” O.Pers. rād- “to prepare,” rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”); Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line;” see also → direct.

The → number of → vertices.

See also: → order; → graph.

The maximum number of the indices (→ index) of a tensor.

See also: → order; → tensor.

A whole number which characterizes a particular position of an interference fringe according to whether there is interference arising from one, two, three, etc. wavelength difference of path. Same as → interference order

See also: → order; → interference.

Value of a number or of a physical quantity given roughly, usually expressed as a power of 10. Thus, 2.5 x 10⁵ and 6.4 x 10⁵ are of the same order of magnitude, and 2 x 10⁷ is 2 orders of magnitude greater than either.

See also: → order; → magnitude.

Characterized by → order; arranged according to a rule. → ordered tree.

See also: → order + -ed.

In → graph theory, a → tree in which the → children of each → vertex are ordered.

See also: → ordered; → tree.

1. Of or relating to an order, as of animals or plants.
   

2. Of or relating to order, rank, or position in a series.
   

3. → ordinal number.

Etymology (EN): M.E., from O.Fr., from L.L. ordinalis “denoting order or place in a series,” from L. ordo (genitive ordinis) “row, series,” → order; → -al.

Etymology (PE): → order; → -al.

1. A number which defines the position or rank of something in a series, in contrast to → cardinal number. For example, first, second, tenth, etc.
   

2. In → set theory, the order type of a → well-ordered set. Also called ordinal.

See also: → ordinal; → number.

Usual; normal. → ordinary ray;
→ ordinary year.

Etymology (EN): M.E. ordinarie, from O.Fr. ordinarie, from L. ordinarius “regular, usual, orderly,” from ordo (genitive ordinis) “order”

• -arius-ary.

Etymology (PE): Šunik “ordinary,” from Mid.Pers. šônik, šônig “ordinary, customary,” from šôn “kind, manner, sort, way”

• -ik, → -ic.

A → differential equation in which the unknown function depends on only one → independent variable, as contrasted with a → partial differential equation.

See also: → ordinary; → differential; → equation.

The point M₀(x₀,y₀) of the curve F(x,y) = 0, where at least one of the partial derivatives ∂F/∂x and ∂F/∂y does not vanish. → singular point

See also: → ordinary; → point.

The ray that has an → isotropic speed in a → doubly refracting crystal. It obeys → Snell’s law upon refraction at the crystal surface.
See also → extraordinary ray

See also: → ordinary; → ray.

A → calendar year that contains 365 days and therefore is not a → leap year.

See also: → ordinary; → year.

Math.: In plane Cartesian coordinates, the distance of a point from the x-axis measured parallel to the y-axis. → abscissa.

Etymology (EN): Ordinate, from N.L. (linea) ordinate (applicata) “(line applied) in order;” from ordinatus “arranged.”

Etymology (PE): Ârâ, from ârâstan “to arrange, to set in order, adorn,”
Mid.Pers. ârây-, ârâstan “to arrange, adorn;” O.Pers. rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”), rād- “to prepare;”
Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line.”

A natural deposit containing a mineral of an element to be extracted.

Etymology (EN): Ore, merger of M.E. ore, O.E. ora “ore, unworked metal” and M.E. or(e) “ore, metal,” O.E. ar “brass, copper, bronze” (cf. O.N. eir “brass, copper;” Ger. ehern “brazen;” Erz “oar;” Goth. aiz “bronze;” O.H.G. ēr “ore”), from PIE
*aus- “gold;” cf. Mod/Mid..Pers. âhan “iron;” Av. aiianhaēna- “made of metal,” from aiiah- “metal;” Skt. áyas- “iron, metal;” L. aes “brass”

Etymology (PE): Kâné, from kân “mine,” from kandan “to dig” (Mid.Pers. kandan “to dig;” O.Pers. kan- “to dig,” akaniya- “it was dug;” Av. kan- “to dig,” uskən- “to dig out” (→ ex- for prefix us-); cf. Skt. khan- “to dig,” khanati “he digs”).

1. Biology: A structure consisting of tissues and performing some specific function in an organism, e.g. the heart, kidneys, liver, stomach.
   
2. A large musical keyboard instrument producing a wide range of musical effects using compressed air passed through pipes.
   
3. An organization or body acting on behalf of a larger institution.
   
4. A newspaper or magazine regarded as a means of communication, especially one communicating the views of a particular group such as a political party.

Etymology (EN): O.E. organe, from O.Fr. orgene “musical instrument,” from L. organa, pluriel of organum, from Gk. organon “implement, musical instrument, organ of the body,” literally “that with which one works,” from PIE *werg-ano-, from base *werg- “to do, to work” related to Gk. ergon “work” (cf. Av. varəz- “to work, do, perform, exercise;” Mod.Pers. varz-, varzidan “to labor, exercise, practise;” Arm. gorc “work;” Lith. verziu “tie, fasten, squeeze,” vargas “need, distress;” Goth. waurkjan; O.E. wyrcan “work,” wrecan “to drive, hunt, pursue”).

Etymology (PE): 1) Andâm, from Mid.Pers. handam “member, limb;” Av. handāma- “limb;” from Proto-Iranian *ham-dāman-, from prefix ham-→ com- + *dāman- “created;” Av. dā- “to give, grant; to put; to create; to determine” (O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” Mod.Pers. dâdan “to give;” cf.
Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer,” facere “to do, to make;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”); cf. Skt. sandháy- “joint, articulation.”

2), 3), 4) loanwords from Fr., as above.

1. Of, relating to, or derived from living organisms: organic matter.
   
2. Characterized by the systematic arrangement of parts; organized; systematic.
   
3. Chemistry: Of or designating carbon compounds.

Etymology (EN): From L. organicus, from Gk. organikos “of or pertaining to an organ,” from organon “instrument,” → organ, + → -ic.

Etymology (PE): 1) Orgânik, loan from Fr.; âli loan from Ar.; 2) sâzmândâr, from sâzmân, → organization, + dâr “having, possessor” (from dâštan “to have, to possess,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support”).

The study of compounds that contain carbon chemically bound to hydrogen.

See also: → organic; → chemistry.

A molecule that is normally found in or produced by living systems. Organic molecules typically consist of carbon atoms in rings or long chains, where other atoms (e.g. hydrogen, oxygen, and nitrogen) are attached, except for → carbon monoxide (CO) and → carbon dioxide (CO₂), etc.

See also: → organic; → molecule.

1. Biology: A living individual composed of mutually interdependent parts that maintain various vital processes.
   
2. Any organized body or system conceived of as analogous to a living being. → microorganism

Etymology (EN): From → organ + -ism a suffix appearing in loanwords from Gk. denoting several senses, among which state or condition, principles, doctrines.

Etymology (PE): Sâzvâré, from sâz present stem of sâxtan, sâzidan “to build, make, fashion; to adapt, adjust, be fit” (from
Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”) + -vâré, from -vâr a suffix meaning “resembling, like” (from Mid.Pers. -wâr; Av. -vara, -var; cf. Skt. -vara).

1. The act or process of organizing.
   
2. The state or manner of being organized.
   
3. A group of persons organized for some end or work.

Etymology (EN): M.E. organizacion, from M.L. organization-, from organizatus p.p. of organizare “organize”

• -ate.

Etymology (PE): Sâzmân, from sâz present stem of sâxtan, sâzidan “to build, make, fashion; to adapt, adjust, be fit” (from
Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”) + -mân verbal noun suffix used with present and past stems, as in zâymân, câymân; sâxtemân, goftemân.

Note 1: The suffix -mân occurs extensively in some dialects, in particular in Garkuye-yi where it replaces the infinitive suffix -idan: resâmon (= rasidan “to arrive”), kešâmon (= kešidan “to draw”), kartemon (= kardan “to do”), bartemon (= bordan “to carry, bear”), paxdemon (= poxtan “to cook”), esbârtemon (= sepordan “entrust, make over to”), bâfdemon (= bâftan “to weave”), ašnofdemon (= šenidan “to hear”), terakâmon (= tarakidan “to blast”), teknâmon (= tekândan “to shake”).

Note 2: -mân may be related to Av. suffix -man, which forms a number of derivative action-nous: rasman- “column, rank,” from raz- “to direct;”
vaēsman- “dwelling,” from vaes- “to be or keep ready as, to serve as;” nāman- “name;” asman- “stone, heaven.”

To form as or into a whole consisting of interdependent or coordinated parts.

Etymology (EN): From M.L. organizare “to contrive, arrange,” from L. organ(um) “instrument, organ,” → organ + -izare→ -ize.

Etymology (PE): Sâzmân dâdan, from sâzmân, → organization, + dâdan “to give” (O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer,” facere “to do, to make;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”); sâzmânidan, from sâzmân + -idan infinitive suffix.

A class of molecules that contain at least one → halogen atom bonded to → carbon. Organohalogens are abundant on the Earth where they are mainly produced through industrial and biological processes. They have been proposed as → biomarkers in the search for life on → exoplanets.

Simple halogen hydrides have been detected in → interstellar medium sources and in → comets. → Methyl chloride (CH₃Cl), the most abundant organohalogen in the Earth’s atmosphere, has both → natural and → synthetic production pathways (Fayolle et al., 2017, Nature Astronomy 1, 703).

See also: From organo-, → organ, + → halogen.

1. East. The countries of Asia, especially East Asia.
   

2a) To place in a definite relation to the points of the compass or other fixed or known directions.

2b) To turn toward the east or in any specified direction.

Etymology (EN): M.E., from O.Fr. orient “east,” from L. orient-, oriens rising sun, east," from pr.p. of oriri “to rise,” cognate with Pers. ras-, rasidan “to arrive;” O.Pers./Av. rasa- present stem of ar- to move, go or come forward;" cf. Skt. ar- “to reach, come toward, meet with,” rccháti “reaches;”
Gk. erkhomai “to go, to reach.”

Etymology (PE): 1) Xâvar, → east.
2) su dâdan literally “to give direction, orientation;” su yâftan “to acquire direction,” from su “direction, side,” from Mid.Pers. sôk “direction, side,” + dâdan “to give,” → datum; yâftan “t find, obtain,” → find.

1. The position in relation to a specific place or object.
   
2. The act or process of orienting; the state of being oriented.

See also: Verbal noun of → orient.

1. General: The beginning or starting point.
   
2. Math.: The point of intersection of two or more axes.

Etymology (EN): M.E., from L. origin-, origo “beginning, source, lineage,” from oriri “to rise,” cognate with Pers. ras-, rasidan “to arrive;” O.Pers./Av. rasa- present stem of ar- to move, go or come forward;" cf. Skt. ar- “to reach, come toward, meet with,” rccháti reaches;"
Gk. erkhomai “to go, to reach.”

Etymology (PE): Xâstgâh, from xâst past stem of xâstan, xizidan “to rise, get up” (Mid.Pers. xyz- “to stand up, rise;” Proto-Iranian *xiz- “to rise, ascend; increase”) + -gâh suffix of time and place (Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).

The Hunter. A prominent constellation, one of the largest in the sky, located on the celestial equator around 5h 30m right ascension, 0° declination. This constellation is rich in bright stars and nebulae. The brightest star is Rigel (β Orionis), visual magnitude 0.2. The second brightest star is → Betelgeuse (α Orionis), magnitude between 0.2 and 1.0. A key feature of Orion’s constellation is his Belt of three bright stars that form a nearly straight line across its central parts. It contains also the → Orion Nebula,
the only region of massive star formation visible to the unaided eye.

Etymology (EN): In Gk. mythology, Orion was a giant hunter and the enemy of Artemis the huntress, who according to some tales was responsible for his death. Other stories, though, tell how he pursued the Pleiades and with them was turned into a constellation to chase them forever across the sky.

Etymology (PE): Šekârgar, “→ hunter.”

A minor → spiral arm of the → Milky Way Galaxy close to which the → Sun is located. It is some 3,500 → light-years across and approximately 10,000 light-years in length. The solar system lies close to the inner rim of this spiral arm, about halfway along its length. Its name derives from the fact that the stars closest to the Sun which actually lie within the arm are in the constellation → Orion. Its other designations are → Local Arm, → Local Spur, → Orion Bridge, → Orion Spur, and → Orion-Cygnus Arm.

See also: → Orion; → arm.

A large OB stellar association centered on the → Orion Nebula. Lying some 1500 → light-years away, it is about 400 light-years across and contains the main stars of Orion, except → Betelgeuse.

See also: → Orion; → association.

A part of a → molecular cloud toward the → Orion Nebula viewed edge-on. It is the surface of interaction between the → H II region and its → associated molecular cloud. Same as the → Orion Bright Bar.

See also: → Orion; → bar.

Same as → Orion Arm.

See also: → Orion; → bridge.

A prominent emission ridge in the → Orion Nebula located approximately 2’ southeast of the → Trapezium cluster. Various observations have suggested that it is an escarpment in the main → ionization front of the Nebula seen almost edge-on. The Orion Bar is one of the nearest and best-studied → photodissociation regions.

See also: → Orion; → bright; → bar.

A controversial proposition according to which a coincidence would exist between the mutual positions of the three stars of → Orion’s Belt and those of the main Giza pyramids. More specifically, Khufu, Khafre, and Menkaure would be the monumental representation of → Alnitak, → Alnilam, and → Mintaka, respectively.

See also: → Orion; → correlation; → theory.

A giant cloud, or complex of clouds, of interstellar gas and dust associated with the Orion nebula (M42). It is about 1,500 light-years away and measures about 240 light-years across. Besides M42 and M43 it contains a number of famous objects, including Barnard’s Loop, the Horsehead Nebula, and the reflection nebulae around M78. Within this cloud, stars have formed recently, and are still in the process of formation.

See also: → Orion; → molecular cloud.

The best known → ionized nebula and one of the nearest regions to the Sun in which stars are presently being formed. It is visible to the naked eye in the constellation → Orion south of → Orion’s Belt as a fuzzy patch. It lies about 1,500 → light-years away and measures about 30 light-years across. The Orion Nebula is ionized and made visible by a small group of → O-type and → B-type stars known as the → Trapezium cluster. Other designations: M 42, NGC 1976. See also: → Orion molecular cloud; → Huygens Region ;
→ Orion association; → Orion Bar; → Orion Bright Bar.

See also: → Orion; → nebula.

An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:

Orin OB1a, which contains the stars to the northwest of → Orion’s Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;

Orion OB1b, containing the group of stars located in and around the Belt;

Orion OB1c, including the stars around → Orion’s Sword; and

Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

See also: → Orion; → OB association.

Same as → Orion Arm.

See also: → Orion; → → spur.

Same as → Orion Arm.

See also: → Orion; → Cygnus; → arm.

Three prominent stars in the central regions of the constellation → Orion that align to form the “belt” of the mythological Hunter. They are → Alnitak (ζ Ori), → Alnilam (ε Ori), and → Mintaka (δ Ori). The easternmost star Alnitak is separated from the middle one, Alnilam, by 1°.36, and the westernmost Mintaka has an angular distance of 1°.23 from Alnilam. Their distance is between 800 and 1,300 → light-years from Earth. They probably formed inside the same → molecular cloud less than 10 million years ago.

See also: → Orion; → belt.

An astronomical → asterism in the constellation → Orion forming an almost vertical line beneath → Orion’s Belt. From north to south, the most prominent objects in the Sword are the cluster NGC 1981, the star → 42 Orionis, the famous → Orion Nebula, and the Sword’s brightest star → Iota Orionis (→ Hatsya).

None of these objects is particularly bright in itself, but their proximity to one another and the nebulosity across much of this region makes the Sword stand out clearly in the night sky.

See also: → Orion; → sword.

A meteor shower that appears to emanate from the constellation → Orion.
It peaks between October 14 and 20 with about 20 meteors per hour.

See also: → Orion + → -ides.

1. A child who has lost both parents through death, or, less commonly, one parent.
   

2. A young animal that has been deserted by or has lost its mother (Dictionary.com). → orphaned protostar.

Etymology (EN): M.E., from L.L. orphanus “destitute, without parents,” from Gk. orphanos “bereaved;” akin to L. orbus “bereaved,” Skt. arbhah “weak, child,” Armenian orb “orphan,” O.Irish orbe “heir,” O.C.S. rabu “slave,” Ger. Erbe, O.E. ierfa “heir,” O.H.G. arabeit, Ger. Arbeit “work,” O.E. earfoδ “hardship, suffering, trouble.”

Etymology (PE): Yatim “fatherless,” from Ar.

A → protostellar object which has been dynamically ejected from a newborn → multiple star system, either into a tenuously bound orbit or into an escape, thus depriving it from gaining much additional mass. Recent observations have shown that → Class I protostellar sources have a population of distant companions at separations ~ 1000 to 5000 → astronomical unit (AU)s. Moreover, the companion fraction diminishes as the sources evolve. According to N-body simulations of unstable → triple systems embedded in dense cloud cores, many companions are ejected into unbound orbits and quickly escape, but others are ejected with insufficient momentum to climb out of the potential well of the cloud core and associated binary. These loosely bound companions reach distances of many thousands of AU before falling back and eventually being ejected into escapes as the cloud cores gradually disappear (B. Reipurth et al. 2010, arXiv:1010.3307).

See also: → orphan; → protostar.

A mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model.

See also: Named after Charles Boyle, 4th Earl of Orrery (1676-1731), for whom the device was first made.

Prefix denoting right, straight, correct.

Prefix indicating that an organic compound contains a benzene ring substituted in the 1.2 position.

Etymology (EN): Ortho-, from Gk. orthos “set upright, straight, true, correct, regular;” cf. L. arduus “high, steep,” O.Ir. ard “high;” also cognate with Av. ərəduua-, as below; from PIE *eredh- “high.”

Etymology (PE): Ardâ-, from Av. ərəduua- “upright, erect, risen;” cf. Skt. ūrdhvá- “upright, tending upward, high;” Gk. ortho, as above.

The → water molecule in which the → nuclear spin of the constituent → hydrogen atoms are → parallel (→ orthohydrogen). In astrophysics the ratio between ortho- and → para-water is used to determine temperatures in → interstellar medium.

See also: → ortho-; → water.

Of a triangle, the point where the three → altitudes of the → triangle converge.

See also: → ortho-; → center.

In elementary geometry, pertaining to or involving right angles or perpendiculars.
Of a system of real functions, defined so that the integral of the product of any two different functions is zero.
Of a linear transformation, defined so that the length of a vector under the transformation equals the length of the original vector.
Of a square matrix, defined so that its product with its transpose results in the identity matrix.

Etymology (EN): → ortho- + gonia “angle,” related to gony “knee;” L. genu “knee;” Mod.Pers. zânu “knee;” Av. žnav-, žnu- “knee;” Skt. janu-; PIE base *g(e)neu-, see below, + → -al

Etymology (PE): Ardâkonj, from ardâ-, → ortho-, + konj “angle, corner, confined place” (variants xong “corner, angle,” Tabari kânj, Kurd. kunj, Hamadâni kom), maybe from the PIE base *g(e)neu-, as above, and related to Mod.Pers. zânu “knee” (Av. žnu-), Skt. kona- “angle, corner,” Gk. gony, gonia, L. cuneus “a wedge,” Albanian (Gheg dialect) kân “angle, corner,” Albanian (Toks) kënd “angle, corner.”

A set of functions, any two of which, by analogy to orthogonal
vectors, vanish if their product is summed by integration over a
specified interval.

See also: → orthogonal; → function.

Perpendicular lines.

See also: → orthogonal; → line.

Math.: An → isogonal trajectory where the family of curves are cut at right angles.

See also: → orthogonal; → trajectory.

Two non-zero vectors which are perpendicular, i.e. their → scalar product is zero.

See also: → orthogonal; → vector.

1. The property of → orthogonal functions.
   

2. The following conditions satisfied by the → Fourier series:
   ∫ cos (mx) sin (nx) dx = 0 (summed from -π to +π) for all m, n
   ∫ cos (mx) cos (nx) dx = 0 (summed from -π to +π) for all m ≠ n, = 2π for m = n = 0, = π for (m = n) > 0
   ∫ sin (mx) sin (nx) dx = 0 (summed from -π to +π) for m ≠ n, = π for (m = n) > 0.

See also: → orthogonal; → -ity.

Molecular hydrogen in which the nuclei (protons) of the two hydrogen atoms contained in the molecule have spins in the same direction. → parahydrogen

See also: → ortho-; → hydrogen.

Two non-zero vectors that are → orthogonal and have magnitude 1.

See also: → orthogonal; → vector.

Of or relating to a crystalline structure characterized by three mutually perpendicular axes of different length. → orthorhombic crystal system.

See also: → ortho-; → rhombic.

A → crystal system that has three mutually perpendicular axes, each of which is of a different length than the others.

See also: → orthorhombic; → crystal; → system.

Of or relating to an optical system corrected for → distortion.

See also: → ortho-; → -scopic.

A telescopic eyepiece that produces a wide field of view (between 40° and 50°). The eyepiece consists of a single element lens that is normally plano-convex, and a cemented triplet that is usually symmetrical.

See also: → orthoscopic; → eyepiece.