An Etymological Dictionary of Astronomy and Astrophysics

1. Of or relating to a college, academy, school, or other educational institution, especially one for higher education.
   

2. Pertaining to areas of study that are not primarily vocational or applied, as the humanities or pure mathematics.
   

3. Theoretical or hypothetical; not practical, realistic, or directly useful (Dictionary.com).

See also: → academy; → -ic.

A member of an association or institution for the advancement of arts, sciences, or letters.

Etymology (EN): From Fr. académicien, from académic-, → academic,

• -ian a suffix forming adjectives and nouns.

Etymology (PE): Farhangestânvand, from farhangestân, → academy,

• -vand a suffix forming adjectives and agent nouns, → actual.

1. A group of authorities and leaders in a field of scholarship, art, etc., who are often permitted to dictate standards, prescribe methods, and criticize new ideas.
   

2. An association or institution for the advancement of art, literature, or science (Dictionary.com).

Etymology (EN): From Fr. Académie, from L. Academia, from Gk. Akademeia “grove of Akademos,” a legendary Athenian of the Trojan War tales, whose estate, six stadia from Athens, was the enclosure where Plato taught his school.

Etymology (PE): Farhangestân, literally “site of culture and education,” from farhang, → culture, + -estân suffix of place and time, → summer.

(v.tr.) To increase the velocity of a body; to cause to undergo acceleration.
(v.intr.) To increase in speed.

See also: Verbal form of → acceleration.

The motion of an object subject to → acceleration. Opposite to → uniform motion.

Etymology (EN): Accelerated, from → accelerate; → motion.

Etymology (PE): Jonbeš, → motion; šetâbdâr “accelerated,” from šetâb→ accelerate + 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;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

Moving or proceeding with → acceleration; e.g.
→ accelerating system; → accelerating Universe.

See also: Verbal adj. from → accelerate.

→ accelerating Universe.

See also: → accelerating; → expansion; → Universe.

A material system that is subject to a constant force in each and every one of its instantaneous points of trajectory.

See also: → accelerating; → system.

The deduction based on the observation that the most distant → Type Ia supernovae
are fainter than that expected from their → redshifts in a matter-only dominated expanding Universe. The faintness is attributed to larger distances resulting from an accelerating Universe driven by presence of a new component with strongly negative pressure. This
component that makes the Universe accelerate is named → dark energy. The deceleration or acceleration of an expanding Universe, given by the general relativistic equation, is:
R^../R = -(4/3)πGρ(1 + 3w),

where R is the linear → cosmic scale factor of the expanding Universe, G the → gravitational constant, ρ the mean density of the Universe, and w the → equation of state parameter representing dark energy. The expansion accelerates whenever w is more negative than -1/3. The Nobel Prize in Physics 2011 was awarded to the initiators of this concept,
Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess, for their discovery of the accelerating expansion of the Universe through observations of distant supernovae. See also the original paper: Perlmutter et al. 1999, ApJ 517, 565.

See also: → accelerating; → universe.

The rate at which the velocity of an object changes with time.

Etymology (EN): Acceleration, from accelerate, from L. accelerare “quicken,” from → ad- “to” + celerare “hasten,” from celer “swift” (cf. Skt. car, carati “to move, go, drive,” Gk. keles “fast horse, horse race,” Av. kar- “to walk, move, go around,” Mod.Pers. cal, calidan “to move, to go, to walk” (jald? “quick, active, brisk”), Gilaki/Hamadâni jal “quick, fast,“Lori žil “motion, impulse”); PIE *kel- “to drive, set in swift motion.”

Etymology (PE): Šetâb “quickness, haste, speed,” Mid.Pers. ôštâp “hurry, haste,” ôštâftan “to hurry, hasten,” from *abi.stap-, from
abi- " to; in addition to; against”

• *stap- “to oppress,” Arm. (loanword) štap “haste, trouble.”

The acceleration that an object experiences because of gravity when it falls freely close to the surface of a massive body, such as a planet. Same as → gravitational acceleration.

See also: → acceleration; → gravity.

A measure of the departure from a constant rate of the acceleration of the Universe, expressed by:
q(t) = R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.

See also: → acceleration; → parameter.

A machine which accelerates charged atomic (ions of various elements) or subatomic particles (usually electrons and protons) to high energies. → cyclotron; → synchrotron.

Etymology (EN): Accelerator, from accelerate, → acceleration,

• -or agent suffix, from M.E. -or, -our, from O.F. -eor, -eur, from L. -or.

Etymology (PE): Šetâbgar, from šetâb→ acceleration + -gar agent suffix, → -or.

In a → semiconductor, an → impurity which creates a hole in the electronic structure of the crystal and causes p conduction. → donor; → impurity.

Etymology (EN): M.E. acceptour, from O.Fr. acceptour, from L. acceptor, from acceptare “take or receive willingly,” from accipere “to receive,” from → ad- “to” + capere “to take, hold, seize,” PIE base *kap- “to grasp” (cf. Skt. kapati “two handfuls,” Gk. kaptein “to swallow,” Goth. haban “have, hold,” O.E. habban “to have, hold,” probably Mod.Pers. qâp-, qâpidan, kapidan “to seize, rob,” Av. haf-, hap- “to keep, observe”).

Etymology (PE): Pazirandé agent noun from paziroftan “to receive, accept, admit,” Mid.Pers. padiriftan, padir- “to receive, accept,” from Proto-Iranian *pati- + *grab- “to grab, seize, take,” Av./O.Pers. grab- “to take, seize;” cp. Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab (E. grab); PIE *ghrebh- “to seize.”

1. To have the right to connect to a computer or a server and retrieve information.
   
2. The act or instance of accessing a computer.

Etymology (EN): M.E. accesse, from O.Fr. acces “onslaught,” from L. accessus “a coming to, an approach,” p.p. of accedere “to approach, enter upon” from → ad- “to” + cedere “go, move.”

Etymology (PE): Dastrasi “access,” from dast “hand” (Mid.Pers. dast; O.Pers. dasta-; Av. zasta-; cf. Skt. hásta-; Gk. kheir; L. praesto “at hand;” Arm. jern “hand;” Lith. pa-žastis “arm-pit;” PIE *ghes-to-) + rasi, verbal noun of 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;” cf. Skt. ar-, rcchati “reaches;”
Gk. erkhomai “to go, to reach;” L. oriri “to rise”);
dâštan “to have, hold, maintain, possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind;” cf.
Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

An unexpected and undesirable event; an unforeseen and unplanned event or circumstance.

Etymology (EN): M.E., from M.Fr., from L. accidentum, pr.p. of accidere “to happen, fall out” from ad- “to” + cadere “to fall.”

Etymology (PE): Došâmad, from doš- prefix meaning “bad, ill, abnormal” (Mid.Pers. duš-, duž-, O.Pers./Av. duš-, duž- “bad, ugly, evil,” cognate with
Skt. dus-, Gk. → dys- “bad, hard, unlucky”)

• âmad, from âmadan “to come, happen,” as in the current term pišâmad “event, happening.”

1. To welcome or salute with shouts or sounds of joy and approval; applaud.
   

2. To announce or proclaim with enthusiastic approval (Dictionary.com).

See also: From ac-, variant of → ad-, + → claim.

To bring to its goal or conclusion; carry out; perform; finish (Dictionary.com).

Etymology (EN): M.E. accomplice, from O.Fr. acompliss-, p.p. stem of acomplir “to fulfill, complete,” from L. → ad- “to” + complere “to fill up,” → complete.

Etymology (PE): Anjâm dâdan, literally “to make an end of, to finish; to carry out,” from anjâm “end, conclusion” (from Mid.Pers. hanjâm “end, completion, outcome;” ultimately
from Proto-Ir. *ham-gam-, from *ham-, → com-,

• *gam- “to come,” cf. Av. gam- “to come,” related to Pers. âmadan, → walk), + dâdan “to give, commit,” → yield.

1. An act or instance of carrying into effect; fulfillment.
   

2. Something done admirably or creditably (Dictionary.com).

See also: → accomplish; → -ment.

On a multiuser → computer system or → network, a method of identifying a particular → user, usually requiring a → password to enter.

Etymology (EN): M.E. a(c)ount(e), ac(c)ompte, from O.Fr. aco(u)nte, acont, “account, reckoning, terminal payment,” from a “to,” → ad- + cont “counting, reckoning of money to be paid,” from L.L. computus “a calculation,” from L. computare “calculate, → compute.”

Etymology (PE): Ažmar, from Kurd. žmârtin, žmêr-, variant of šomâr, šomârdan “to → count.”

To grow or increase gradually, by the process of → accretion.

See also: Back-formation from → accretion.

The gas involved in various accretion processes, such as that fed into an → accretion disk, pulled by a compact object, or used in the mass growth of a galaxy.

See also: → accretion; → gas.

That → accretes, such as
→ accreting star and → accreting neutron star.

See also: Adjective from → accrete + → -ing.

A → neutron star in a → binary system that accretes matter from the → campion star, either from the → stellar wind or from an → accretion disk that forms if the companion overflows its → Roche lobe. The → gravitational energy from the infalling matter provides at least part of the energy for the observed radiation and the accretion torques dominate the spin evolution. Despite these common properties, accreting → neutron stars display a wide variety of behaviors, depending on the neutron star → magnetic field strength,
mass of the companion and properties of → accretion
(A. K. Harding, 2013, Front. Phys. 8, 679).

See also: → accreting; → neutron; → star.

The star which → accretes matter, particularly in its protostellar phase or in a close binary system.

See also: → accreting; → star.

1. The process by which an object increases its mass under the influence of its
   → gravitational attraction.
   Accretion plays a key role in a wide range of astrophysical phenomena. In particular stars result from the accretion of material by a → protostar from a surrounding → molecular cloud. The accumulation of mass on the protostar involves the formation of an → accretion disk.
   Theoretical and observational investigations of protostars and newborn stars indicate the important role of → magnetic fields in this process. They favor the magnetospheric accretion model for mass transfer from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. Various → emission lines, such as the hydrogen → Balmer series, He I 5876 Å, Brγ 2.17 μm, and so forth are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet excess continuum emission is produced in the → accretion shocks. The accretion is accompanied by mass ejection through collimated → bipolar jets.
   

2. Accumulation of dust and gas onto larger bodies by → coalescence under the influence of their mutual → gravitational attraction or as a result of chance collisions.
   

See also:
→ accretion column, → accretion disk, → accretion flow, → accretion rate, → accretion shock, → accretion time.

Etymology (EN): L. accretionem (nom. accretio, gen. accretionis) “a growing larger,” from stem of accrescere, from ad- “to” + crescere “to grow”.

Etymology (PE): Farbâl from prefix → far- which conveys “increase, abundance” + bâl, from bâlidan “to grow, to wax great,” Mid.Pers. vâlitan, Av. varəd-, varədait- “to increase, augment, strengthen, cause to prosper,” Skt. vrdh-, vardhati.

The channel through which matter is accreted onto a body such as a → protostar, → white dwarf, → neutron star, or → black hole.
The accreting body possesses magnetic fields strong enough to disrupt the → accretion flow and carry the material through column-shaped channels directly on to a small fraction of the stellar surface near the magnetic poles.

See also: → accretion; → column.

A rotating disk of gas and dust formed around a center of strong gravity that pulls material off a surrounding or near-by gaseous object. Accretion disks are associated with several astrophysical objects such as → binary stars, → protostars, → white dwarfs, → neutron stars, and → black holes. Accretion disk forms because the infalling gas does not directly crash the accreting object due to its too high → angular momentum. The individual particles go into a circular orbit around the accretor because the circular orbit has the lowest energy for a given angular momentum. A spread in angular momentum values will give a population of particles moving on different orbits, so that a rotating disk of matter forms around the object.
The matter in the disk becomes very hot due to internal friction and
→ viscosity as well as the tug of the accreting object. Since this hot gas is being accelerated it radiates energy and loses angular momentum and falls onto the accretor. Theoretical and observational pieces of evidence point to the importance of → magnetic fields in the accretion process. According to current models, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. See also → flared disk, → self-shadowed disk, → protoplanetary disk, → alpha disk model.

See also: → accretion; → disk.

1. Flow of matter during an accretion process.
   

2. In a → binary system, flow of matter
   from the losing-mass → companion toward the compact one. The flow can be from a → stellar wind or through the → inner Lagrangian point.
   

3. → cold accretion flow, → hot accretion flow.

See also: → accretion; → flow.

The amount of mass → accreted during unit time. The accretion rate for the → collapse of a singular → isothermal sphere is expressed by:
dM/dt = 0.975 c_s³/G, where c_s is the isothermal → sound speed (Shu 1977, ApJ 214, 488). This relation can be written as:
dM/dt = 4.36 x 10^-6 (T / 20 K)^3/2 in units of solar masses per year, where T is the temperature. Observed temperatures of 10-20 K in regions of → low-mass star formation imply accretion rates of about 10^-6 to 10^-5 solar masses per year. Accretion rates for → massive stars amount to values of 10^-4 to 10^-3 solar masses per year.

See also: → accretion; → rate.

A → shock wave occurring at the surface of a compact object or dense region that is accreting matter with a → supersonic velocity from its environment. In the case of → young stellar objects the process is believed to take place by funneled streams in the form of → accretion columns that originate in the surrounding → accretion disk and flow along the → field lines of the → protostar → magnetosphere. The gas falls supersonically onto the surface of the central body and its impact
produces strong shocks of a few million → kelvin, a phenomenon that is observable in → X-rays.

See also: → accretion; → shock.

The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate.

See also: → accretion; → time.

An → astronomical object that accretes surrounding material. See also → accretion.

See also: Agent noun, from → accrete + → -or.

To gather or collect, often in gradual degrees; heap up (Dictionary.com).

See also: From ac-, → ad-, + → cumulate; → amass.

A device for storing electricity. An electric current is passed between two plates in a liquid. This causes charges (due to electrolysis) in the plates and the liquid. Same as → secondary cell.

Etymology (EN): From L. accumulator, from accumulare “to heap up,” from → ad- “in addition” + cumulare “heap up,” from cumulus “heap.”

Etymology (PE): Anbâré, from anbâr-, anbâštan, → amass; âkumgar, from âkumidan, → accumulate.

1. The state or quality of being → accurate.
   

2. The degree of nearness of a measured value to the standard or known value of the quantity, not to be confounded with → precision.

For example, a refrigerator holds a constant temperature of 5.0 °C. A thermometer is used seven times to read the temperature, with the following results: 6.4, 5.1, 6.3, 4.5, 5.3, 6.1, and 4.1. This distribution does not well match the actual temperature, therefore it lacks accuracy, and shows no tendency toward a particular value; it lacks precision, as well.

If the measured temperatures are 4.8, 5.3, 5.1, 5.0, 4.6, 5.2, and 5.0, the mean value is accurate, because it comes close to the actual temperature, but the distribution shows no clear tendency toward a particular value (lack of precision).

Now suppose that the measured temperatures are 6.2, 6.3, 6.1, 6.0, 6.1, 6.3, and 6.2. In this case every measurement is well off from the actual temperature (low accuracy), but the distribution does show a tendency toward a particular value (high precision).

Finally, if the measured temperatures are 5.0, 5.0, 4.8, 5.1, 5.0, 4.9, and 5.0, the distribution is very near the actual temperature each time (high accuracy), and does show a tendency toward a particular value (high precision).

Accuracy is often given to n → significant digits or n → decimal places.
For example e = 2.71828 … = 2.718 is rounded to two four significant figures or three decimal places. → accurate to n significant figures, → accurate to n decimal places.

Etymology (EN): From L. accuratus “prepared with care, exact,” p.p. of accurare “take care of,” from ad- “to” + curare “take care of.”

Etymology (PE): Rašmandi, from rašmand, from raš + adjective forming suffix -mand. Raš, from Av. root raz- “to right, correct, arrange;”
compare with Skt. raj “to reign, rule, direct,” Gk. oregein “to strech out,” L. rego “to direct, lead;” PIE *reg- “to move in a straight line.” Similarly, Av. râšta-, rašta- “straight,” Skt. rju “straight, right, upright,” Gk. orektos “elongated,” L. rectus “straight,” Ger. recht, E. right. In Mod.Pers. there are several derivatives:
râst, râšt (as in afrâšt(an)) “right; true,” rasté, rešté, raj, raž, râh, ris, râdé, radé, Lori rezg “row,” etc.

1. Conforming exactly to truth or to a standard; free from error.
   

2. Designating → accuracy.

See also: Accurate, from L. accuratus, → accuracy.

An expression specifying the number of meaningful digits to the right of the → decimal point. For example, e = 2.71828 … = 2.718 is said to be accurate to three decimal places and 2.72 to two decimal places.

See also: → accurate; → decimal; → place.

An expression specifying the number of meaningful digits used to express the value of a measured quantity. Same as accurate to n significant figures. For example, e = 2.71828 … = 2.718 is rounded to four significant digits, and 2.72 to three significant digits. → accurate to n decimal places.

See also: → accurate; → significant; → digit.

Able to be accused; open to an accusation (of); blameworthy, reprehensible (OxfordDictionaries.com).

See also: → accuse; → -able.

A charge or claim that someone has done something illegal or wrong; the action or process of accusing someone (OxfordDictionaries.com).

See also: → accuse; → -tion.

1. → accusative case.
   

   2. → accusatory.

Etymology (EN): M.E., from M.Fr., from L. accusativus, from ac-, → ad-, + -cusativus, combining form of causativus, → causative, a loan-translation of Gk. aitiatike, in the sense of pointing to the origin or cause, accusing.

Etymology (PE): 1) Kondâri, i.e. → objective case.
2) → accusatory.

The → grammatical case of a noun in some languages (such as Greek, Latin, G erman, Russian, Old Persian, Avestan, or Sanskrit), which shows that the noun is the → direct object of a → verb or a → preposition.

See also: → accusative; → case.

Containing an accusation; accusing (Dictionary.com).

See also: → accuse; → -tion.

To charge with the fault, offense, or crime (Dictionary.com).

Etymology (EN): M.E. ac(c)usen, from O.Fr. acuser “to accuse, indict, blame,” earlier “announce, report, disclose,” from L. accusare “to call to account,” from → ad- “to, toward, at, with regard to” + causari “give as a cause or motive,” from causa “reason,” → cause.

Etymology (PE): Mârzidan, from (Lâr, Gerâsh) mârz, (Farâmarzân) morz “blame, reproach, accusation,” maybe ultimately from Proto-Ir. *marc- “to destroy, damage;” cf. Av. mərənc- “to destroy,” (+ *para-) “to damage, injure,” (+ *ui-) “to ruin, spoil;” Mid.Pers. mwlncyn- / murnjên- “to destroy;” Khotanese mulch- “to cause to miscarry” (Cheung 2007).

A person or persons charged in a court of law with a crime, offense, etc.

See also: → accuse; → -tion.

A person who accuses, especially in a court of law.

See also: → accuse; → -tion.

The brightest star in the constellation → Eridanus. A → subgiant of → spectral type B5; apparent visual magnitude 0.5, about 140 → light-years distant (other names: HR 472, HD 10144). Recent interferometric observations show it to have a flattened shape imposed by fast rotation.

Etymology (EN): Achernar, from Ar. Axir an-Nahr “end of the river,” from axir “end” + nahr “river”.

Etymology (PE): Âxer-e nahr, from Axir an-Nahr.
Rudpâyân “river’s end,” from rud “river,” → Eridanus, + pâyân “end”.

A class of → stony meteorites that lack → chondrules. They are made of rock that has crystallized from a molten state. Achondrites are relatively rare, accounting for about 8% of all meteorite falls.

See also: Achondrite, from Gk. prefix a- (an- before stems beginning with a vowel or h) “not, without, lacking” + Gk. chondrite, from chondr-, from chondros “grain,”

• affix -ite.

Of or relating to an optical system which is capable of transmitting light without decomposing it into constituent colors.

See also: → a-; → chromatic.

Lens (or combination of lenses) that brings different wavelengths within a ray of light to a single focus, thus overcoming chromatic aberration.

See also: → achromatic; → lens.

A substance that releases hydrogen ions to form a solution with a pH of less than 7, reacts with a base to form a salt, and turns blue litmus red.

See also: From Fr. acide, from L. acidus “sour,” adj. of state from acere “to be sour,” acer “sharp, pungent, bitter;” from PIE base *ak- “sharp, pointed.”

Being or containing an acid; of a solution having an excess of hydrogen atoms (having a → pH of less than 7).

See also: → acid; → -ic.

1. To admit to be real or true; recognize the existence, truth, or fact of.
   

   2. To recognize the authority, validity, or claims of.
      

   3. To show or express appreciation or gratitude for (Dictionary.com).

Etymology (EN): A blend of M.E aknow “admit or show one’s knowledge” (from O.E. oncnawan “understand”) and M.E. knowlechen “to admit.”

Etymology (PE): Âdânidan, from âdân, from prefix â- + dân present stem of dânestan, → know; cf. Sogd. âzân, azân “to acknowledge, to confess,” from prefixed zân, variant of dân.

1. Recognition of the existence or truth of something.
   

   2. An expression of appreciation (Dictionary.com).

See also: → acknowledge; → -ment.

Pertaining to the sense of hearing, or to → sound waves.

See also: → acoustics.

One of several peaks appearing in the → CMB angular power spectrum of the → cosmic microwave background radiation which are ripples left by acoustic oscillations of the plasma-radiation fluid in the early Universe (→ baryon acoustic oscillations). When the Universe was small and very hot, the free electron density was so high that photons could not propagate freely without being scattered by electrons. Ionized matter, electrons and radiation formed a single fluid, with the inertia provided by the baryons and the radiation pressure given by the photons.

See also: → acoustic; → peak.

Same as → sound pressure.

See also: → acoustic; → pressure.

A type of → longitudinal wave that consists of mechanical → vibrations of small
→ amplitude propagated in an → elastic medium. Acoustic waves exhibit phenomena like → diffraction, → reflection, and → interference, but not → polarization. Also called → sonic and → sound waves. See also
→ acoustic wave equation. The branch of physics concerned with the properties of sonic waves is called → acoustics.

See also: → acoustic; → wave.

A → differential equation that describes the time evolution of the → scalar potential of the field φ. It is expressed by: ∇²φ = (1/c²)∂²φ/∂t², where c is → velocity of → longitudinal waves and ∇² is the → Laplacian operator.

See also: → acoustic; → wave; → equation.

1. The study of sound, especially of its generation, propagation, and reception.
   
2. Those qualities of an enclosure that together determine its character with respect to distinct hearing.

Etymology (EN): From Fr. acoustique, from Gk. akoustikos “pertaining to hearing,” from akoustos “heard, audible,” from akouein “to hear,” from copulative prefix a- + koein “to mark, perceive, hear,” from PIE root *(s)keu- “to notice, observe.”

Etymology (PE): Sedâyik from sedâ “sound” + Pers. suffix -ik, → -ics. Sedâ is most probably Persian,
since it exists also in Indo-Aryan languages: Skt. (late Vedic): sabda “articulate sound, noise,”
Pali and Prakriti: sadda “sound, noise,” Sindhi: sadu, sado “shout, call,” Gujrâti sad “call, voice, echo,” Marathi: sad “shouting to,” Konkani sad “sound,” Sinhali: sada “sound.” Therefore, sadâ in Arabic “reverbrating noise, echo” may be borrowed from Persian, or a coincidence. Note that for the author of the classical Persian dictionary Borhân-e Qâte’ (India, 1652 A.D.), the Arabic term is a loanword from Persian.

See also: → phone; →phonetics.

Relating to or occurring at sunset. → heliacal

Etymology (EN): Acronical, from Gk. akronukos, from akros
“tip, end,” cf. Av. aγra- “top, first,” Skt. agra- “first, foremost, climax”

• nuks, nuktos “night,” → night.

Etymology (PE): Šâmgâhi, adj. of šâmgâh “evening,” from šâm “evening, evening meal” + gâh “time.” The first component, šâm, from Mid.Pers. šâm “evening meal, supper,” from Av. xšāfnya- “evening meal,” from Av. xšap-, xšapā-, xšapan-, xšafn- “night” (O.Pers. xšap- “night,” Mid.Pers. šap, Mod.Pers. šab “night”); cf. Skt. ksap- “nigh, darkness;” Hittite ispant- “night.” The second component gâh “time,” 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 rising of a star in the sky at or just after sunset. → heliacal rising.

See also: → acronical; → rising.

The setting of a star at nightfall. → heliacal setting.

See also: → acronical; → setting.

A southern hemisphere bright star (α Crucis) of magnitude 0.77 lying at a distance of 321 → light-years. It is apparently made up of three components. The primary component, Acrux A, is a blue subgiant of apparent magnitude 1.34 and spectral type B0.5 IV. The B component lies at about 4.1 arcsec away from the A component, which represents a distance of at least 400 AU, that is, more than 10 times the distance between the sun and planet Pluto. The C component lies about 90 arcsec away from A.

Etymology (EN): Acrux, from A, from Alpha, designating the brightest star of the constellation + crux, the constellation name. The name Acrux is probably a coinage of the American astronomer, Elijah H. Burritt, who published several editions of an astronomical atlas between 1833 and 1856.

1. The process of doing or performing something;
   something done or performed.
   

2. (v.intr.) To carry out an action; to produce an effect.

Etymology (EN): Act, from O.Fr. acte, from L. actus “a doing” and actum “a thing done,” both from agere “to do, set in motion, drive,
urge, chase, stir up,” from PIE root *ag- “to drive, draw out or
forth, move” (cf. Gk. agein “to lead, guide, drive, carry off,”
agon “assembly, contest in the games,” agogos “leader;” Av. az- “to drive (away),” azaiti “drives,”
Mod.Pers. govâz “stick for driving cattle,” from Av. gauuāza-, from gao- “cow, ox, cattle” (→ Bootes)

• āza-, from az-, as above; Skt. aj- “to drive, sling,” ájati “drives,” ajirá- “agile, quick.” The E. agile “characterized by quickness, lightness, and ease of movement;
  mentally quick or alert” is from this root.

Etymology (PE): In major European languages there are two fundamental and very close verbs which convey “work, action, activity”. These are: 1) to do (in French faire, Spanish hacer, German machen) and
2) to act (French agir, Spanish actuar, German
handeln). In Persian there is only one word for these two concepts:
kardan; and this is obviously a big handicap.
An ad hoc equivalent for action has therefore been
koneš, from kardan “to do.” The problem is that
this solution, despite being widely used, confounds “to do” with
“to act,” and is incapable of forming all the related derivatives.
Therefore, we propose žir, which derives from Av. žirā- “active, agile, clever;” Mid.Pers. žir, zir “active, busy” (loaned in Arm. žir “active, busy, clear”), Mod.Pers. zirak “clever, alert, intelligent;” Kurd. žir “agile,” žiri “agility.”

Any member of the group of → chemical elements with → atomic numbers from 89 (→ actinium) to 103 (→ lawrencium), analogous to the → lanthanides.

See also: From the chemical element → actinium.

A silver-white radioactive → chemical element; symbol Ac. The first member of the → actinide series of the → periodic table. → Atomic number 89; → atomic weight 227.0278; → melting point about 1,050°C; → boiling point 3,200°C ± 300°C; → specific gravity 10.07; → valence +3. It is found with uranium minerals in pitchblende. Its longest lived → isotope is ²²⁷Ac with a → half-life of 21.77 years.

See also: From actin-, variant of actino-, from Gk. aktinos “ray, beam” + → -ium. The discovery of actinium is shared between two chemists who independently found the element. The earlier discovery was made by the French chemist André Debierne (1874-1949) in 1899 in pitchblende residues left after Pierre and Marie Curie had extracted → radium. The element was rediscovered in 1902 by the German chemist Friedrich Otto Giesel (1852-1927), who called it emanium.

Any instrument for measuring the intensity of radiation, especially that of the Sun, in its thermal, chemical, and luminous aspects.

Etymology (EN): Actinometer, from actino- combining form with the meaning “ray, beam,” from Gk. aktis, aktin
“ray,” + → -meter.

Etymology (PE): Partow, → ray; + -sanj, → -meter.

1. The process or state of acting or of being active.
   

2. According to → Newton’s third law of motion, an external force that is applied to a body and that is counteracted by an equal force in the opposite direction ( → reaction).
   

3. A quantity whose → dimension (ML²T^-1) coincides with that of → angular momentum, the → impulse of a force, or → energy x → time. The action plays an important part in → analytical mechanics,
   → quantum mechanics, and in a number of other fields of physics. Initially introduced in analytical mechanics, the concept of action has become a basic ingredient of modern physics, due
   to the role it has played in the generalization of → variational principle.
   

4. A scalar quantity computed as a function of the path followed by a system during its evolution between an initial instant t_i and a final instant t_f. It is defined by the → integral of the → Lagrangian between the two instants:
   S = ∫L dt
   

In the framework of the → field theory, the action is expressed by the integral of the → Lagrangian density over the corresponding space-time volume:
S = ∫L_d d⁴x.

In classical physics, the path actually followed by the system is the one for which S is stationary (→ least action problem).

5. → quantum of action.
   

6. Math.: The action is a → functional, a mathematical relationship which takes an entire path and produces a single number.

Etymology (EN): Action, from O.Fr. action, from L. actionem, from agere “to do,” → act.

Etymology (PE): Žireš, verbal noun from žir stem of žiridan “to act;” → act. Koneš, noun from kardan “to do, to make,” Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,”
karma “act, deed;” PIE base k^wer- “to do, to make.”

The instantaneous action of a body on another body independently of the distance separating them. The description of → gravity by → Newton’s law and → electrostatics by → Coulomb’s law are examples of action at a distance. According to Newton, → gravitation acts directly and instantaneously between two objects. For example, if the Sun should suddenly break apart, the Earth’s orbit would be affected instantaneously. However, action at a distance violates the → principle of relativistic causality. According to → general relativity, gravitational effects
travel at the → speed of light. For modern physics there is no instantaneous action at a distance.

See also: → action; → distance.

The time integral associated with the evolution of a physical system in the phase space.

See also: → action; → variable.

1. To induce activity in a system that is static, as in neutron activation of radioactivity.
   
2. To start the operation of an electrical device.

Etymology (EN): Activate, verb from → active.

Etymology (PE): Žirândan, transitive verb from žir,
→ act.

1. The process of inducing or creating a state of → activity.
   

2. The process of producing a → radioactive isotope by bombarding a → stable → nuclide with → nuclear particles (such as → protons, → neutrons, → alpha particles, heavy ions, etc.).

See also: Verbal noun of → activate; → -tion.

Chemistry: The minimum amount of energy that is required to activate → atoms or → molecules to a condition in which they can undergo a → chemical reaction. Most reactions involving neutral molecules cannot take place at all until they have acquired the energy needed to stretch, bend, or otherwise distort one or more → bonds. In most cases, the activation energy is supplied by → thermal energy.

See also: → activation; → energy.

1. Being in a state of action; not quiescent.
   

2. Functioning or capable of functioning.
   

3. Marked by vigorous activity.

Etymology (EN): M.Fr. actif, from L. activus, from actus, p.p. of agere, → act.

Etymology (PE): Žirâ, adj. from stem žir, → act + suffix -â.

A central region of an → active galaxy, which is
a → light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (→ gamma rays, → X-rays, → ultraviolet) and shows → variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 10⁴ km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure. See also → broad-line region, → narrow-line region, → quasar.

See also: → active; → galactic;
→ nucleus.

A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: → Seyfert galaxies, → quasars, and → blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object,
because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one → light-year in diameter cannot vary significantly in brightness over a period of less than one year.

See also: → active; → galaxy.

A technique for improving the → resolving power of a telescope by controlling the shape of the main mirror at a relatively slow rate. The → image quality is optimized automatically through
constant adjustments by in-built corrective → actuators operating at fairly low temporal frequency ~0.05 Hz or less. → adaptive optics.

See also: → active; → optics.

A solar → prominence with very rapid motion (up to 2,000 km s^-1), moving and changing in appearance over a few minutes of time, in contrast to a → quiescent prominence.

See also: → active; → prominence.

An area of the Sun exhibiting → solar activity with the presence of → sunspots, → flares,
→ faculae, → prominences, and other phenomena associated with intense magnetic fields.

See also: → active; → region.

The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.

See also: → active; → sun.

1. The doctrine or practice of vigorous action or involvement as a means of achieving political or other goals, sometimes by demonstrations, protests, etc. (Dictionary.com). See also → activist.
   

2. Philosophy: a) A theory that the essence of reality is pure activity, especially spiritual activity, or process.
   b) A theory that the relationship between the mind and the objects of perception depends upon the action of the mind (Dictionary.com).

See also: → active; → action; → -ism.

An especially active, vigorous advocate of a cause, especially a political cause (Dictionary.com).

See also: Agent noun from → activism.

For a radioactive substance, the average number of atoms disintegrating per unit time.

Etymology (EN): Activity, from → active + -ity.

Etymology (PE): Žirandegi, noun from žirandé, → active.

1. One who takes part; a participant.
   

   2. A theatrical performer.

See also: → act + → -or.

1. Existing in act or fact; real.
   

2. Philosophy: The character of what is realized in act (such as a statue), as opposed to what can potentially be realized (from a piece of rock).

Etymology (EN): M.E. actuel, from O.Fr. actuel “now existing, up to date,” from L. actualis “active, pertaining to action,” adj. form of actus, → act.

Etymology (PE): Žirvand, from žir, → act, + -vand a suffix forming adjectives denoting “possessed of, similarity, relation,” variants -âvand, -van (e.g. xodâvand “possessor, master,” pulâdvand “hard as steel, name of a hero,” dowlatvand “rich,” setarvan “mule-like, barren,” polvan “bridge-like, a raised path”), from Mid.Pers. -âwand (hunarâwand “skilled,” warzâwand “powerful”); Av. -vant (aurvant- “swift,” surunvant- “audible”); cf. Skt. -vant (amavant- “having attacking power”).

1. The quality or state of being actual or something that is actual; reality.
   

2. Philosophy: According to Aristotle, what has → form and can causally interact with other things, as opposed to → potentiality.

See also: → actual; → -ity.

The act or process of actualizing.

See also: Verbal noun of → actualize; → -tion.

To make actual or real; turn into action or fact.

See also: → actual; → -ize.

1. To put into mechanical action or motion.
   

2. To incite or move to action; motivate.

Etymology (EN): From L. actuatus, p.p. of actuare, from actus, → act.

Etymology (PE): Baržiridan, from bar- “on, upon, up,” → on-, + žiridan “to → act.”

The act or process of putting into action; activation.

See also: Verbal noun of → actuate.

A mechanism to activate process control equipment by use of pneumatic, hydraulic, or electronic signals.

See also: → actuate; → -or.

Sharpness; acuteness; keenness of perception.

Etymology (EN): M.E., from M.Fr. acuité, from M.L. acuitatem (nom. acuitas) “sharpness,” from L. acus “needle,” acuere “to sharpen,” from PIE root *ak- “be sharp.”

Etymology (PE): Tiznâ “sharpness,” from tiz, “→ sharp,”

• -nâ, a suffix that transforms adjective into noun (compare with tangnâ, derâznâ, farâxnâ, etc.).

The ability of the → eye to see separately two points close to each other. It is a measure of the → resolving power of the eye’s → optical system and depends on the density of cells in the → retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc.

See also: → acuity; → vision.