An Etymological Dictionary of Astronomy and Astrophysics

A → quadruple system of stars consisting of two close pairs, AB Dor A / AB Dor C and AB Dor Ba / AB Dor Bb, separated by about 9 arcseconds. The brightest star of the system, AB Dor A, is a → pre-main sequence star of → spectral type K1, with strong emission at all wavelengths, from radio to X-rays. The companion of this star, i.e. component C, lying 0.185 arcseconds apart, is a very low-mass star of 0.090
→ solar masses with a spectral type M 5.5 (Boccaletti et al. 2008, A&A 482, 939 and references therein).

See also: A, a, B, b, and C designating letters of alphabet; → Dorado.

“From the beginning,” as opposed to in medias res “starting in the middle of the story.” In physics and chemistry “from first principles;” → ab initio calculation.

Etymology (EN): L. “from the beginning,” from ab “from,” → ab- + initio, ablative of initium “a beginning, an entrance,” from inire “to go into, begin,” from in- “into, in” + ire “to go” (cf. Gk. ienai “to go,” Skt. eti “goes,” O.Pers. aitiy “goes,” Av. aeiti “goes,” Mod.Pers. âyad “comes,” PIE *ei- “to go, to walk”).

Etymology (PE): Hac bon, from Mid.Pers. hac “from” (O.Pers. hacā “from,” Av. hacā “from, out of,” cf. Skt. sácā “with,” PIE base *sek^w- “to follow,” Av. hac-, hax- “to follow,” hacaiti “follows,” Skt. sácate “accompanies, follows,” Gk. hepesthai “to follow,” L. sequi “to follow,” Lith. seku “to follow”)

• bon “beginning, base, root” (Av. būna- “base, depth,” cf. Skt. bundha-, budhná- “base, bottom,” Pali bunda- “root of tree”).

In physics and chemistry, a calculation that relies on basic and established laws without additional assumptions or special models. Experimental input in ab initio calculations is limited to the determination of values of fundamental physical constants.

See also: → ab initio; → calculation.

A → photometric system defined by reference to → monochromatic magnitudes in such a way that, when monochromatic → flux f_ν is measured in ergs sec^-1 cm^-2 Hz^-1, the magnitude will be:

AB = -2.5 logf_ν - 48.60.

The constant is set so that AB is equal to the V magnitude for a source with a flat → spectral energy distribution. The → zero point is defined by the flux of the star → Vega at 5546 Å. In this system, an object with constant flux per unit frequency interval has zero color.

See also: → magnitude; → system.

From, away, off.

Etymology (EN): M.E., from O.Fr., from L. ab “from, away from, out of, with” Gk.

See also: → apo-.

1. To leave completely and finally; forsake utterly; desert.
   

2. To give up; discontinue; withdraw from (Dictionary.com).

Etymology (EN): M.E. abando(u)nen, from M.Fr. abandoner, from O.Fr. abandoner from adverbial phrase à bandon “at will, at discretion,” from à “at, to,” → ad-, + bandon “power, jurisdiction,” from L. bannum “proclamation.”

Etymology (PE): Râcidan, related to Pers. parhêz, parhiz “to keep away from, abstain, avoid,” gurêz, goriz “to flee, run away;” Av. raēc- “to leave, let;” → heritage.

The reciprocal of the → dispersive power of a substance. Also known as constringence.

See also: → Abbe sine condition; → number.

In → geometric optics, a condition for eliminating → spherical aberration and → coma in an → optical system. It is expressed by the relationship: sin u’/U’ = sin u/U, where u and U are the angles, relative to the → optical axis,
of any two rays as they leave the object, and u’ and U’ are the angles of the same rays where they reach the image plane. A system which satisfies the sine condition is called → aplanatic.

See also: Named after Ernst Karl Abbe (1840-1905), a German physicist; → sine; → condition.

The → locus of the intersection of input and output → conjugate rays in an
optical system satisfying the → Abbe sine condition.

See also: → Abbe sine condition; → sphere.

1. If a → power series → converges for some nonzero value x₀, then it converges absolutely for any value of x, for which |x| < |x₀|.
   

2. If a power series → diverges for some nonzero value x₀, then it diverges for any value of x, for which |x| > |x₀|.

See also: Named after the Norwegian mathematician Niels Henrik Abel (1802-1829); → theorem.

A catalog of 4073 rich → galaxy clusters grouped by constellation and by catalog number. Published first in 1958, it contained the clusters visible from the northern hemisphere. A supplement for the southern hemisphere survey was published in 1988.

See also: George O. Abell (1927-1983), American astronomer; → catalog.

Diverge or deviate from the straight path; produce → aberration.

Etymology (EN): Aberrate, from aberrare “go astray,” from ab- “away” + errare “to wander.”

Etymology (PE): Birâhidan, from birâh “a devious path; a wanderer, who deviates, errs,” from bi- “without” + râh “way”.

1. An imperfection in the imaging properties of a → lens or → mirror.
   The main aberrations are → chromatic aberration, → spherical aberration, → coma, → astigmatism, → distortion, and → field curvature.
   

2. → aberration of starlight.
   

See also: → aberration angle→ aberration constan→ aberration of starlight→ aberration orbit→ annual aberration→ chromatic aberration→ constant of aberration→ diurnal aberration→ optical aberration→ planetary aberration→ relativistic aberration→ secular aberration→ spherical aberration→ stellar aberration.

Etymology (EN): Aberration, from L. aberrationem, from aberrare “go astray,” → aberrate.

Etymology (PE): Birâheš, from birâidan, → aberrate.

The angle tilt required by the → stellar aberration phenomenon in order that a moving telescope points directly to a star.

See also: → aberration; → angle.

Same as → constant of aberration.

See also: → aberration; → constant.

→ aberration of starlight

See also: → aberration; → light.

An apparent displacement in the observed position of a star. It is a result of the finite speed of light combined with the relative motion of the Earth through space. Suppose that you walk through a vertically falling rain with an umbrella over your head. The faster you walk, the further you must lower the umbrella in front of yourself to prevent the rain from striking your face. For starlight to enter a telescope, a similar phenomenon must occur, because the Earth is in motion. The telescope must be tilted in the direction of motion by an angle: tan θ =(v/c), where v the Earth velocity and c the speed of light. The aberration of starlight was discovered by the English astronomer James Bradley (1693-1762) in 1729 by observing → Gamma Draconis. The tilt angle is θ = 20’’.50, from which the Earth’s orbital speed, 29.80 km s^-1, can be deduced, using the above equation. See also → annual aberration; → diurnal aberration; → secular aberration. → Special relativity modifies the classical formula for aberration, predicting results which differ substantially from those of classical physics for objects moving at a substantial fraction of the speed of light; → relativistic aberration.

See also: → aberration; → star;
→ light.

The apparent path described by a star on the → celestial sphere due → annual aberration. A star at the → ecliptic pole is seen to move around a circle of angular radius about 20".50, once a year. A star on the → ecliptic oscillates to and fro along a line of angular half-length 20".50. At an intermediate → celestial latitude, β, the aberration orbit is an ellipse, with semi-major axis 20".50 and semi-minor axis (20".50) sin β.

See also: → aberration; → orbit.

Of or pertaining to → aberration.

See also: → aberration; → -al.

A → Besselian day number denoted by C or D.

See also: → aberration; → -al; → day; → number.

The → locus of points on the → celestial sphere occupied by a star during the annual → revolution of the → Earth about the → Sun due to → annual aberration. annual aberration.

See also: → aberrational; → ellipse.

The → erosion of a surface through a process such as → vaporization or → friction.

Etymology (EN): L. ablatio, ablation, from ablatus, from ab- “away” + latus “carried.”

Etymology (PE): Farsâb from far-, prefix denoting “abundance, excess” + sâb present stem of sâbidan “to rub, wear out,” variants sâyidan,
pasâvidan “to touch,” Khotanese sauy- “to rub,” Sogdian ps’w- “to touch,” Proto-Iranian *sau- “to rub.”

Not typical, usual, or regular; not normal; deviant. → anomalous, → anomaly.

Etymology (EN): M.L. anormalis, blend of L.L. abnormis “deviating from a rule,” from ab- “off, away from” + L. norma “rule.”

Etymology (PE): Bihanjâr, from Pers. bi- “without” +
Pers. hanjâr “rule, habit, law, staright road.”

1. To terminate a procedure before it has finished naturally.
   

2. In computer science, to terminate an application intentionally or unexpectedly.
   For example, to stop a search or a print job; or unexpected
   termination of a program because of bugs or difficulties.

Etymology (EN): From L. abortus, p.p. of aboriri “to miscarry” (a child) from → ab- “from, away, off,” + oriri “to rise, be born;” cf. Pers. rasidan
“to attain; to arrive” (Mid.Pers. rasidan “to arrive, to mature;” O.Pers./Av. rasa- present stem of ar- “to move, go or come toward;” Skt. ar-, rcchati “reaches;”
Gk. erkhomai “to go, to reach”).

Etymology (PE): Fagânidan, from fagâné “abortion”, related to fegandan, fekandan, afkandan “to throw, to cast away;” Mid.Pers. abgandan “to throw;” O.Pers. avakan- “to throw, place on,” from Proto-Iranian *kan- “to throw, place, put.”

The stopping of a process; a result of such termination.

See also: Verbal noun of → abort.

In plane Cartesian coordinates, the distance of any point from the vertical axis (y-axis). The distance from the horizontal axis (x-axis) is called → ordinate.

Etymology (EN): From L abscissa (linea) “(a line) cut off,” from p.p. of abscindere “to cut off,” from → ab- “off, away” + scindere “to cut.”
The word abscissa was first used by Stefano degli Angeli (1623-1697), a professor of mathematics in Rome.

Etymology (PE): Pirâ, present stem of pirâstan “to prune, clip, trim; to adorn, embellish (especially by cutting, clipping, or taking away),” related to ârâstan “to arrange, adorn,” from Mid.Pers. payrâstan, patrâstan “to arrange, adorn,” ultimately from Proto-Iranian *pati-rad-. The first component *pati- “to, toward, near to, against;” cf. Mid.Pers. pât-, from O.Pers. paity “against, back, opposite to, toward, face to face, in front of;” Av. paiti;
Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti. The second component *rad- “to direct, to prepare;” cf. 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.”

1. State of being away or not being present.
   

2. Lack; deficiency.

See also: Noun from → absent.

1. Not in a certain place at a given time; away, missing (opposed to present).
   

2. Lacking; nonexistent (Dictionary.com).

Etymology (EN): M.E., from M.Fr. absent, from L. absentem (nominative absens), p.p. of abesse “to be away from,” from → ab- “away” + esse “to be,” → present.

Etymology (PE): Apâst, on the model of L. absentem, as above, from apâ- “away from,” → apo-, + ast “is,” → present.

A person who is absent, especially from work or school (Dictionary.com).

Etymology (EN): From → absent + -ee a suffix forming nouns that denote a person who is the object or beneficiary of the act specified by the verb, from Fr. -é, ending of p.p.s used as nouns.

Etymology (PE): Apâstgar, from apâst, → absent, + -gar, → -or.

Voluntary non attendance at work, without valid reason. Absenteeism means either habitual evasion of work, or willful absence as in a strike action. It does not include involuntary or occasional absence due to valid causes, or reasons beyond one’s control, such as accidents or sickness (BusinessDictionary.com).

See also: → absent; → -ism.

1. General: Not limited by restrictions or exceptions; perfect in quality or nature; unqualified in extent or degree; complete.
   

2. Physics: Relating to measurements or units of measurement derived from fundamental units of length, mass, and time.
   

3. Physics: Having an ideal value as opposed to a conditional or relative one.
   

See also: → absolute acceleration, → absolute dating, → absolute humidity, → absolute luminosity, → absolute magnitude, → absolute permeability, → absolute space, → absolute temperature, → absolute tensor, → absolute time, → absolute viscosity, → absolute zero.

Etymology (EN): From M.Fr. absolut, from L. absolutus “unrestricted,” p.p. of absolvere “to set free,” from ab- “away” + solvere “to loosen,” from PIE *leu-. → solve.

Etymology (PE): Avast from negation prefix → a- + vast, variant of bast, basté “tied, bound,” from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind, → band.

Synonyms: Synonymous: motalq(مطلق), from Ar.

For a body that moves with respect to a rotating → reference frame, the vector sum of the observed acceleration, the → Coriolis acceleration, and the
→ centrifugal acceleration. See also the
→ Coriolis theorem.

See also: → absolute; → acceleration.

Any method of measuring the age of an event or object in years. For example, in geology, this method can, unlike → relative dating, give us the age of a rock or fossil in x number of years. The most widely used and accepted method of absolute dating is → radioactive dating. See also: → radiocarbon dating, → radiometric dating.

See also: → absolute; → dating.

The difference between the measured value of a quantity x₀ and its (true) actual value x, given by Δx = x₀ - x. See also: → relative error.

See also: → absolute; → error.

In a system of moist air, the ratio of the mass of water vapor to the total volume of the system. → humidity.

See also: → absolute; → humidity.

A star’s → intrinsic brightness, i.e. the total amount of energy radiated by the star per second. → Luminosity is often expressed in units of watts or erg/sec. The Sun’s absolute luminosity is 3.86 × 10³³ erg/sec.

See also: → absolute; → luminosity.

1. The → magnitude a star would have if it were at a distance of 10 → parsecs in a void space, without → interstellar absorption. The absolute magnitude is usually deduced from the → visual magnitude, measured through a V filter (→ UBV system), when it is written as M_V. If it is defined for another wavelength, it gets another index (U, B, etc). If the radiation on all wavelengths is included, it becomes absolute → bolometric magnitude, M_bol. The Sun has the absolute magnitude + 4.8. Most of the stars have absolute magnitudes ranging between -9 (→ supergiants) and + 19 (→ red dwarfs) (M.S.: SDE).
   

2. The brightness a → comet or → asteroid would have if it were at a distance of 1 → astronomical unit both from the Sun and the Earth and were completely illuminated by the Sun (M.S.: SDE).

See also: → absolute; → magnitude.

A measurement in which the comparison is directly with quantities whose units are basic units of the system. For example, the measurement of speed by measurements of distance and time is an absolute measurement, but the measurement of speed by a speedometer is not an absolute measurement. Note that the word absolute measurement implies nothing about → precision or → accuracy (IEEE Standard Dictionary of Electrical and Electronics Terms).

See also: → absolute; → measurement.

→ magnetic permeability.

See also: → absolute; → permeability.

A fixed space in which physical phenomena occur and whose properties do not depend on what occupies it, nor on the observer. It is a distinguished frame of reference that could show bodies to be truly moving or truly at rest. Absolute space is one of the basic assumptions of → Newtonian mechanics, but it was abandoned in Einstein’s → special relativity. See also → absolute time; → space-time.

See also: → absolute; → space.

Also called → thermodynamic temperature, the value of a → temperature in the → Kelvin scale. It is is equal to the temperature on the → Celsius scale -273.15 °C.

See also: → absolute; → temperature.

A → tensor of → weight  → zero.

See also: → absolute; → tensor.

A universal time supposed to be the same for all observers at any place in the Universe. Absolute time is one of the foundations of → Newtonian mechanics, but it fails to account for physical phenomena in → reference frames
with relative motion. Its abandoning was one of the starting points of → special relativity. See also → absolute space; → space-time.

See also: → absolute; → time.

For any → real number a, the non-negative value of a without regard to its sign; denoted by |a|. Same as → modulus. The absolute value of a is always either → positive or → zero, but never negative. The absolute value of a number may be thought of as its → distance from zero. The following rules hold:
|ab| = |a||b|
|a + b|  ≤  |a| + |b|
|a - b|  ≥  |a| - |b|
For a> 0, |x|  ≤  a if and only if -a  ≤  x  ≤  a.

See also: → absolute; → value.

Same as → viscosity and → dynamic viscosity.

See also: → absolute; → viscosity.

The → zero point of the → Kelvin scale of → temperature; the theoretical point at which all molecular activity ceases, -273.16 °C.

See also: → absolute; → zero.

1. General: To suck up or take up.
   

2. Physics: To retain → energy (radiation or sound, for example) or → matter wholly, without → reflection or → transmission.

Etymology (EN): M.Fr. absorber, from L. absorbere “to swallow up,” from ab- “from” + sorbere “suck in;” PIE root *srebh- “to suck, absorb;” cf. Arm. arbi “I drank;” Gk. rhophein “to sup up, gulp down;” Lith. srebiu “to drink greedily;” Proto-Iranian *hrab- “to sip, suck (in),” cf. Pashto raw-, Shaghni râv-, Sariqoli rov-, Ishkashimi ruv- “to suck” (Cheung 2007), Pers. (prefixed) zâlu “leech.”

Etymology (PE): Daršamidan, from daršam, from dar- “in, into”

• šam, variant of šâm, as in âšâm, âšâmidan “to drink, to sip;” Av. šam- “to drink, sip, swallow;” Skt. cam, camati “to sip, drink, lick up, absorb.”

Capable of being absorbed.

Etymology (EN): Absorbable, adj. from → absorb + → -able.

Etymology (PE): Daršamidani adj. from aršamidan “to absorb”

• -i, → -able.

The ability of a solution or a layer of a substance to absorb → radiation, i.e. the ratio of the intensity of the light incident on the solution to the intensity transmitted by it, in logarithmic scale.

See also: → absorb; → -ance.

1. Any material or → substance that → absorbs.
   

2. Relating to or capable of → absorption.

Etymology (EN): Absorbent, from → absorb + -ent.

Etymology (PE): Daršamandé from aršam present stem of daršamidan, → absorb, + -andé suffix forming adj. and/or agent name.

Any medium that absorbs something.

See also: → absorb; → -er.

Optics: A strip or annulus of material, such as glass or film, whose density increases progressively from one end to the other. The function of the wedge is to weaken the light beam in an optical system.

See also: Verbal adj. from → absorb; → wedge.

1. General: The process or fact of absorbing.
   

2. Physics: The action of energy or matter penetrating or being assimilated into a body of matter with no reflection or emission. → adsorption,
   → desorption, → sorption.
   

See also: → absorption band, → absorption coefficient, → absorption curve, → absorption feature, → absorption line, → absorption nebula, → absorption spectrum, → atmospheric absorption, → discrete absorption component, → foreground absorption, → intergalactic absorption, → internal absorption, → interstellar absorption, → mass absorption coefficient, → photoabsorption, → selective absorption, → self-absorption.

See also: Verbal noun of → absorb; → -tion, from L. absorptionem.

1. A series of very closely spaced absorption lines in stellar spectra resulting from the absorption of light by molecules. Bands caused by titanium oxide (→ TiO bands) and carbon compounds occur in the spectra of low temperature M and C stars.
   

2. A range of wavelengths, usually in electromagnetic radiation, that are absorbed by a given substance. Absorption bands are characteristic of molecules and correspond to changes of electron orbits in the molecules. See also → anomalous dispersion.

See also: → absorption; → band.

The fraction of normally incident light that is absorbed per unit path length or by a unit mass of absorbing medium.

See also: → absorption; → coefficient.

A graphic representation of the amount of radiant energy absorbed by a material as a function of the wavelength.

See also: → absorption; → curve.

A dip in a spectrum indicating that light has been absorbed on the path from the emitting source to the observer. It may be a line, corresponding to a well-defined atomic or molecular transition, or a more complex feature whose physical origin is not necessarily understood.

See also: → absorption; → feature.

A dark line in the spectrum of a source produced by a lower temperature gas cloud lying between the source and the observer.

See also: → absorption; → line.

A dark cloud of dust and gas that absorbs light from and impedes the view of background stars; dark nebula.

See also: → absorption; → nebula.

A spectrum of absorption lines or bands, produced when light from a hot source, itself producing a continuous spectrum, passes through a cooler gas.

See also: → absorption; → spectrum.

The ratio of energy absorbed by a body to energy incident upon the same body.

Etymology (EN): Absorptivity, from absorptive adj. from → absorb + → -ity, suffix expressing state or condition.

Etymology (PE): Daršamâyi, from daršamâ quality adj. from daršam, present stem of daršamidan + -yi noun suffix.

For a spatial object, the ratio between its absorption of solar radiation and its infrared emission.

See also: → absorptivity; → emissivity; → ratio.

To keep oneself from doing or partaking of something; refrain. → avoid.

Etymology (EN): M.E. absteynen, absteinen, abstenen, from O.Fr. abstiner, abstenir, astenir “hold (oneself) back, refrain voluntarily,” from L. abstinere / abstenere “withhold, keep back,” from → ab-“off, away from” + tenere “to hold,” from PIE root *ten- “to stretch,” → tension.

Etymology (PE): Parhiz, from Mid.Pers. pahrêzidan, pahrêxtan “to keep away from, abstain,” ultimately from Proto-Ir. *pati-raic-, from prefix *pati-

• *raic- “to leave, let abandon;” cf. Av. raēc- “to leave, let;” Pers. (+ *ui-) gorêz-, goriz-, gorixtan “to run away, flee;” Skt. rec- “to leave, release;” Gk. leipo “I let;” L. linquô “I leave;” PIE base *leik^w- “to leave, let.”

The act or practice of abstaining from indulging a desire or appetite.

See also: Verbal noun of → abstain.

1a) (adj.) A designation of what derives from → abstraction. Contrasted to → concrete.

1b) Dissociated from any concrete reality or specific instance.

2a) (n.) An abstract thing or state.

2b) A summary of a scientific article, document, speech, etc.

Etymology (EN): M.E., from L. abstractus “drawn away,” p.p. of abstrahere “to draw away, withdraw,” from abs- “away,” from → ab-

• trahere “to draw,” → attract.

Etymology (PE): 1) Âhanjidé, p.p. of âhanjidan, variants âhixtan, âxtan “to draw, pull, extract,” → object.

2. Cekidé, literally “dropped, oozed out,” from cekidan “to drop, distill, fall in small portions, as water,” from cek, cekké “a drop, as water,” probably an onomatopoeia.

An entity that does not exist in space or time and is not perceptible. Some examples are universals, sets, geometrical figures, and numbers.

See also: → abstract; → object.

1. The mental process in which an element or quality is separated from a total object. Also the result of this process.
   

2. Chemistry: → abstraction reaction.

See also: Verbal noun from → abstract; → -tion.

Chemistry: A bimolecular chemical reaction that involves removal of an atom or ion from a molecule. For example, hydrogen abstraction from methane:
CH₄ + Cl → CH₃ + HCl.

See also: → abstraction; → reaction.

Ridiculously senseless, illogical, or untrue; contrary to all reason. → reductio ad absurdum, → absurdity. See also → nonsense.

Etymology (EN): M.Fr. absurde, from L. absurdus “out of tune; foolish,” from surdus “deaf, stupid, inaudible.”

Etymology (PE): Yâvé, literally “unrelated speech,” ultimately from *ui-vaka-, from *ui- “apart, away,” → transit, + *vaka- “word, speech,” → word.

The quality or state of being absurd; something that is absurd.

See also: → absurd + → -ity.

1. The relative amount of a given → chemical element with respect to other elements.
   

2. The amount of an → isotope relative to other isotopes of the same element in a given sample. → overabundance; → underabundance.

Etymology (EN): M.E., from O.Fr. abundance, from L. abundantia “fullness,” from abundare “to overflow,” from L. → ab- “away” + undare “to surge,” from unda “water, wave.”

Etymology (PE): Farâvâni “abundance,” from farâvân “abundant,” from feré “much, more; increase; priority;”
Mid.Pers. frêh “more, much;” O.Pers. fra- “before, forth;”
Av. frā, fərrā “forth, forward;” PIE base *pro-; cf. L. pro “on behalf of, in place of, before, for;” Gk. pro- “before, in front of.”