An Etymological Dictionary of Astronomy and Astrophysics

A prefix meaning “again, anew” to indicate repetition (as rebuild, retell), “back, backward” to indicate “withdrawal” or “backward motion” (react, recall),
or sometimes merely intensive (refine, resolve).

Etymology (EN): M.E., from O.Fr., from and L. re- “again, back.”

Etymology (PE): Vâ-, variant bâz-, prefix denoting “reversal, opposition; separation; repetition; open; off; away,” from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] “away, from;”
Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind.”

A prefix meaning “again, anew” to indicate repetition (as rebuild, retell), “back, backward” to indicate “withdrawal” or “backward motion” (react, recall),
or sometimes merely intensive (refine, resolve).

Etymology (EN): M.E., from O.Fr., from and L. re- “again, back.”

Etymology (PE): Vâ-, variant bâz-, prefix denoting “reversal, opposition; separation; repetition; open; off; away,” from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] “away, from;”
Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind.”

1. To act in response to an agent, influence, or stimulus.
   
2. To act in a reverse direction or manner; to act in opposition, as against some force.
   
3. Chem.: To undergo a → chemical reaction.

See also: → re-; → act.

1. To act in response to an agent, influence, or stimulus.
   
2. To act in a reverse direction or manner; to act in opposition, as against some force.
   
3. Chem.: To undergo a → chemical reaction.

See also: → re-; → act.

The opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

Etymology (EN): From → react + -ance a suffix of nouns.

Etymology (PE): Vâžirâyi state noun of vâžirâ agent noun of vâžiridan→ react.

The opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

Etymology (EN): From → react + -ance a suffix of nouns.

Etymology (PE): Vâžirâyi state noun of vâžirâ agent noun of vâžiridan→ react.

1. General: A person or thing that reacts.
   

2. A substance on the left side of a → chemical equation.

Etymology (EN): From → react + -ant.

1. General: A person or thing that reacts.
   

2. A substance on the left side of a → chemical equation.

Etymology (EN): From → react + -ant.

1. General: Action in response to some influence, event, stimulus.
   

2. An equal and opposite force exerted by a body against a force acting upon it. → principle of action and reaction.
   

3. A → nuclear reaction; → chain reaction.
   

4. → chemical reaction.

See also: Verbal noun of → react.

1. General: Action in response to some influence, event, stimulus.
   

2. An equal and opposite force exerted by a body against a force acting upon it. → principle of action and reaction.
   

3. A → nuclear reaction; → chain reaction.
   

4. → chemical reaction.

See also: Verbal noun of → react.

The speed with which a → chemical reaction takes place. In other words, the change in → concentration of a → substance divided by the → time interval during which the change is observed.

See also: → reaction; → rate.

The speed with which a → chemical reaction takes place. In other words, the change in → concentration of a → substance divided by the → time interval during which the change is observed.

See also: → reaction; → rate.

Tending to react. Pertaining to or characterized by reaction.

Etymology (EN): From → react + -ive a suffix of adjectives expressing tendency, disposition, function, connection, etc.

Etymology (PE): Vâžirandé verbal adj. of vâžiridan→ react; vâžireši, adj. of vâžireš→ reaction.

Tending to react. Pertaining to or characterized by reaction.

Etymology (EN): From → react + -ive a suffix of adjectives expressing tendency, disposition, function, connection, etc.

Etymology (PE): Vâžirandé verbal adj. of vâžiridan→ react; vâžireši, adj. of vâžireš→ reaction.

1. General: The quality or condition of being reactive.
   

2. Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
   

3. Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

See also: From → reactive + → -ity.

1. General: The quality or condition of being reactive.
   

2. Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
   

3. Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

See also: From → reactive + → -ity.

1. A device for introducing reactance into an electrical circuit (e.g. a capacitor).
   

2. Same as → nuclear reactor.

See also: From → react + → -or.

1. A device for introducing reactance into an electrical circuit (e.g. a capacitor).
   

2. Same as → nuclear reactor.

See also: From → react + → -or.

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

See also: → reactor; → core.

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

See also: → reactor; → core.

In computing and data processing, the number of words, characters, fields, etc. sensed by an input sensing device per unit of time.

Etymology (EN): Reading, verbal noun of read, from M.E. reden, O.E. rædan, redan “to counsel, read;” cf. Du. raden, Ger. raten “to advise, counsel, guess;” akin to Skt. rādh- “to succeed, accomplish;” Gk. arithmos “number amount;” L. ratio; Pers. rây, râ “because of, for the sake of;” → reason; → rate.

Etymology (PE): Nerx, → rate; xâneš verbal noun of xândan “to read; to sing; to call;” Mid.Pers. xwân- “to resound; to call;” Av. x^van- “to sound,” Proto-Iranian *huan- “to call;” cf. Skt. svan-, sváranti “to sound, make a sound, sing;” L. sonus “sound,” sonare “to sound;” O.E. swinn “music, song,” PIE base *suen- “to sound” (Cheung 2007).

In computing and data processing, the number of words, characters, fields, etc. sensed by an input sensing device per unit of time.

Etymology (EN): Reading, verbal noun of read, from M.E. reden, O.E. rædan, redan “to counsel, read;” cf. Du. raden, Ger. raten “to advise, counsel, guess;” akin to Skt. rādh- “to succeed, accomplish;” Gk. arithmos “number amount;” L. ratio; Pers. rây, râ “because of, for the sake of;” → reason; → rate.

Etymology (PE): Nerx, → rate; xâneš verbal noun of xândan “to read; to sing; to call;” Mid.Pers. xwân- “to resound; to call;” Av. x^van- “to sound,” Proto-Iranian *huan- “to call;” cf. Skt. svan-, sváranti “to sound, make a sound, sing;” L. sonus “sound,” sonare “to sound;” O.E. swinn “music, song,” PIE base *suen- “to sound” (Cheung 2007).

The noise added in the process of reading a detector such as a CCD.

See also: → reading rate; → noise.

The noise added in the process of reading a detector such as a CCD.

See also: → reading rate; → noise.

A chemical substance that, because of the reactions it causes, is used to detect, measure, or prepare another substance.

Etymology (EN): From re(act) + → agent.

Etymology (PE): Vâkonešgar, from vâ-, → re-, + konešgar→ agent.

A chemical substance that, because of the reactions it causes, is used to detect, measure, or prepare another substance.

Etymology (EN): From re(act) + → agent.

Etymology (PE): Vâkonešgar, from vâ-, → re-, + konešgar→ agent.

1. General: Having actual physical existence; genuine, not artificial.
   

2. Math.: Involving, relating to, or having elements of the set of rational or irrational numbers only.
   

3. Philo.: Existent or pertaining to the existent as opposed to the nonexistent; actual as opposed to possible or potential.
   

See also:
→ real dynamical variable, → real equator, → real equinox, → real fluid, → real gas, → real image, → real number, → real object, → real-time operation, → real-time processing, → realism, → realist, → realistic, → reality, → true.

Etymology (EN): M.E., from O.Fr. reel, from L.L. realis “relating to things; actual,” from L. res “matter, thing,” of unknown origin.

Etymology (PE): Hasyâ, from O.Pers. hašya- “true, real;” Av. haiθa- “true, real;” cf. Khotanese hiththo “true, real;” Ossetic aecaeg “true, real, genuine;” Skt. satyá- “true, real, genuine;” PIE base *es- “to be” (Mod.Pers. hastan, astan “to be;” O.Pers./Av. ah-, Skt. as-).

1. General: Having actual physical existence; genuine, not artificial.
   

2. Math.: Involving, relating to, or having elements of the set of rational or irrational numbers only.
   

3. Philo.: Existent or pertaining to the existent as opposed to the nonexistent; actual as opposed to possible or potential.
   

See also:
→ real dynamical variable, → real equator, → real equinox, → real fluid, → real gas, → real image, → real number, → real object, → real-time operation, → real-time processing, → realism, → realist, → realistic, → reality, → true.

Etymology (EN): M.E., from O.Fr. reel, from L.L. realis “relating to things; actual,” from L. res “matter, thing,” of unknown origin.

Etymology (PE): Hasyâ, from O.Pers. hašya- “true, real;” Av. haiθa- “true, real;” cf. Khotanese hiththo “true, real;” Ossetic aecaeg “true, real, genuine;” Skt. satyá- “true, real, genuine;” PIE base *es- “to be” (Mod.Pers. hastan, astan “to be;” O.Pers./Av. ah-, Skt. as-).

A → dynamical variable which does not have an → imaginary number part.

See also: → real; → dynamical; → variable.

A → dynamical variable which does not have an → imaginary number part.

See also: → real; → dynamical; → variable.

Same as → true celestial equator.

See also: → real; → equator.

Same as → true celestial equator.

See also: → real; → equator.

Same as → true equinox.

See also: → real; → equinox.

Same as → true equinox.

See also: → real; → equinox.

The opposite of an ideal fluid; a fluid which possesses viscosity and therefore exhibits certain frictional phenomena. Viscosity arises due to cohesive forces between molecules and molecular momentum exchange between fluid layers. These effects appear as tangential or shear stresses between moving fluid layers.

See also: → real; → fluid.

The opposite of an ideal fluid; a fluid which possesses viscosity and therefore exhibits certain frictional phenomena. Viscosity arises due to cohesive forces between molecules and molecular momentum exchange between fluid layers. These effects appear as tangential or shear stresses between moving fluid layers.

See also: → real; → fluid.

Actual gas whose molecules do not necessarily possess the properties assigned to those of an → ideal gas.

See also: → real; → gas.

Actual gas whose molecules do not necessarily possess the properties assigned to those of an → ideal gas.

See also: → real; → gas.

In an optical system, the image of an object produced by the convergence of the light rays that make up the image. → virtual image; → real object.

See also: → real; → image.

In an optical system, the image of an object produced by the convergence of the light rays that make up the image. → virtual image; → real object.

See also: → real; → image.

A number that can be represented by a point on a line. The set of real numbers includes all rational and irrational numbers, but not the imaginary numbers.

See also: → real; → number.

A number that can be represented by a point on a line. The set of real numbers includes all rational and irrational numbers, but not the imaginary numbers.

See also: → real; → number.

In an optical system, a collection of points which actually serves as a source of light rays.

See also: → real; → object.

In an optical system, a collection of points which actually serves as a source of light rays.

See also: → real; → object.

The operation of a computer during the actual time that the related physical processes take place so that the results can be used to guide the physical processes.

See also: → real; → time; → operation.

The operation of a computer during the actual time that the related physical processes take place so that the results can be used to guide the physical processes.

See also: → real; → time; → operation.

Data processing that takes place instantaneously upon data entry or receipt of a command.

See also: → real; → time;
→ processing.

Data processing that takes place instantaneously upon data entry or receipt of a command.

See also: → real; → time;
→ processing.

1. An inclination or concern for the actual or real, as distinguished from the abstract or speculative.
   

2. Philo.: The doctrine that material objects exist independently of our perception of them.
   

3. Art, literature: The attempt to describe the objects or social conditions as they actually are, without idealization or presentation in abstract form.

See also: → real; → -ism.

1. An inclination or concern for the actual or real, as distinguished from the abstract or speculative.
   

2. Philo.: The doctrine that material objects exist independently of our perception of them.
   

3. Art, literature: The attempt to describe the objects or social conditions as they actually are, without idealization or presentation in abstract form.

See also: → real; → -ism.

1. A person who tends to view or represent things as they really are; an adherent of → realism.
   

2. Of or pertaining to realism or to a person who embodies its principles or practices.

See also: → real; → -ist.

1. A person who tends to view or represent things as they really are; an adherent of → realism.
   

2. Of or pertaining to realism or to a person who embodies its principles or practices.

See also: → real; → -ist.

1. Interested in, concerned with, or based on what is real or practical.
   

2. Philo.: Of or pertaining to realists or → realism.

See also: From → real + -istic, from L. -isticus, Gk. -istikos, from -ikos, → -ic.

1. Interested in, concerned with, or based on what is real or practical.
   

2. Philo.: Of or pertaining to realists or → realism.

See also: From → real + -istic, from L. -isticus, Gk. -istikos, from -ikos, → -ic.

1. The state or quality of being real. A real thing or fact.
   

2. Philo.: The totality of what is, as opposed to what merely seems to be.

See also: → real + → -ity.

1. The state or quality of being real. A real thing or fact.
   

2. Philo.: The totality of what is, as opposed to what merely seems to be.

See also: → real + → -ity.

In reality; actually; indeed.

See also: → real; → -ly.

In reality; actually; indeed.

See also: → real; → -ly.

1. A basis or cause, as for some belief, action, fact, event, etc.
   

2. A statement presented in justification or explanation of a belief or action.
   

3. The mental powers concerned with forming conclusions, judgments, or → inferences especially in orderly rational ways.
   Philo.: The intellectual ability to apprehend the truth cognitively, either immediately in intuition, or by means of a process of inference. See also → wisdom.
   

4. (v.) To think or argue in a logical manner; to form conclusions, judgments, or inferences from facts or premises.

Etymology (EN): M.E. resoun, reisun (n.), from O.Fr. reisun, reson, raison, from L. rationem (nom. ratio) “reckoning, understanding, motive, cause,” from ratus, p.p. of reor, reri “to reckon, think;” cognate with Pers. râ, rây “reason,” as below; O.C.S. raditi “to take thought, attend to,” O.Ir. im-radim “to deliberate, consider.”

Etymology (PE): Râyan, from rây + -an. The first component rây, râ “because of, by reason of,” in barây-e “because of,” cerâ “for what reason, why?,” irâ “for this reason, therefore,” zirâ “because, on account of;” also rây “opinion, consult;” râymand “reasonable, wise;” râyzan “a person whom one consults; wise;” Mid.Pers. râd, rây “because of, for the sake of, on behalf of;” O.Pers. rā “reason, cause,” in rādiy “for this reason;” Av. rādah- “generosity, care, generous,” rāsta- “to do right, to fit;” cf. Skt. rādh- “to succeed, be successful;” Gk. arithmos “number, amount;” L. ratio, as above; PIE base *rē-, *rə-. The second component -an a suffix as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.

1. A basis or cause, as for some belief, action, fact, event, etc.
   

2. A statement presented in justification or explanation of a belief or action.
   

3. The mental powers concerned with forming conclusions, judgments, or → inferences especially in orderly rational ways.
   Philo.: The intellectual ability to apprehend the truth cognitively, either immediately in intuition, or by means of a process of inference. See also → wisdom.
   

4. (v.) To think or argue in a logical manner; to form conclusions, judgments, or inferences from facts or premises.

Etymology (EN): M.E. resoun, reisun (n.), from O.Fr. reisun, reson, raison, from L. rationem (nom. ratio) “reckoning, understanding, motive, cause,” from ratus, p.p. of reor, reri “to reckon, think;” cognate with Pers. râ, rây “reason,” as below; O.C.S. raditi “to take thought, attend to,” O.Ir. im-radim “to deliberate, consider.”

Etymology (PE): Râyan, from rây + -an. The first component rây, râ “because of, by reason of,” in barây-e “because of,” cerâ “for what reason, why?,” irâ “for this reason, therefore,” zirâ “because, on account of;” also rây “opinion, consult;” râymand “reasonable, wise;” râyzan “a person whom one consults; wise;” Mid.Pers. râd, rây “because of, for the sake of, on behalf of;” O.Pers. rā “reason, cause,” in rādiy “for this reason;” Av. rādah- “generosity, care, generous,” rāsta- “to do right, to fit;” cf. Skt. rādh- “to succeed, be successful;” Gk. arithmos “number, amount;” L. ratio, as above; PIE base *rē-, *rə-. The second component -an a suffix as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.

Being in accordance with reason.

See also: → reason; → -able.

Being in accordance with reason.

See also: → reason; → -able.

A process of thinking involving → inference,
or of solving problems by employing general principles.

See also: Verbal noun of → reason.

A process of thinking involving → inference,
or of solving problems by employing general principles.

See also: Verbal noun of → reason.

A temperature scale in which the → freezing point and the → boiling point of → water are set to 0 and 80 degrees respectively. See also → Celsius scale, → Fahrenheit scale, → Kelvin scale, → Rankine scale.

See also: Named after René Antoine Ferchault de Réaumur (1683-1757), who first proposed it in 1730; → scale.

A temperature scale in which the → freezing point and the → boiling point of → water are set to 0 and 80 degrees respectively. See also → Celsius scale, → Fahrenheit scale, → Kelvin scale, → Rankine scale.

See also: Named after René Antoine Ferchault de Réaumur (1683-1757), who first proposed it in 1730; → scale.

1. To bound or spring back from force of impact.
   

2. The act of rebounding; recoil.

Etymology (EN): M.E., from O.Fr. rebondir “leap back, resound; push back,” from → re- “back” + bondir “to leap, bound.”

Etymology (PE): Vâjahidan, from vâ-, → re-,

• jahidan “to leap, → jump.”

1. To bound or spring back from force of impact.
   

2. The act of rebounding; recoil.

Etymology (EN): M.E., from O.Fr. rebondir “leap back, resound; push back,” from → re- “back” + bondir “to leap, bound.”

Etymology (PE): Vâjahidan, from vâ-, → re-,

• jahidan “to leap, → jump.”

A galaxy whose distance increases from other galaxies due to the global expansion of the Universe.

Etymology (EN): Receding, verbal adj. of recede, M.E., from M.Fr. receder, from L. recedere “to go back, withdraw,” from → re- “back”

• cedere “to go;” → galaxy.

Etymology (PE): Kahkašân, → galaxy; dur šavandé “receding,” from dur, → far, + šavandé agent noun of šodan “to go, to pass; to become, to be, to be doing;” Mid.Pers. šudan, šaw- “to go;” Av. š(ii)auu-, šiyav- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro.”

A galaxy whose distance increases from other galaxies due to the global expansion of the Universe.

Etymology (EN): Receding, verbal adj. of recede, M.E., from M.Fr. receder, from L. recedere “to go back, withdraw,” from → re- “back”

• cedere “to go;” → galaxy.

Etymology (PE): Kahkašân, → galaxy; dur šavandé “receding,” from dur, → far, + šavandé agent noun of šodan “to go, to pass; to become, to be, to be doing;” Mid.Pers. šudan, šaw- “to go;” Av. š(ii)auu-, šiyav- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro.”

An electronic device that amplifies, detects, and gives a measure of the intensity of radio signals.

Etymology (EN): M.E. receven, from O.Fr. recoivre, from L. recipere from → re- + -cipere, combining form of capere “to take, hold, seize,” PIE base *kap- “to grasp” (cf. Skt. kapati “two handfuls,” Gk. kaptein “to swallow,” O.Ir. cacht “servant-girl,” lit. “captive,” 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): Girandé, agent noun of gereftan “to take, seize, hold;” Mid.Pers. griftan, gir- “to take, hold, restrain;” O.Pers./Av. grab- “to take, seize,”
cf. Skt. grah-, grabh- “to seize, take,” graha- “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE *ghrebh- “to seize.”

An electronic device that amplifies, detects, and gives a measure of the intensity of radio signals.

Etymology (EN): M.E. receven, from O.Fr. recoivre, from L. recipere from → re- + -cipere, combining form of capere “to take, hold, seize,” PIE base *kap- “to grasp” (cf. Skt. kapati “two handfuls,” Gk. kaptein “to swallow,” O.Ir. cacht “servant-girl,” lit. “captive,” 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): Girandé, agent noun of gereftan “to take, seize, hold;” Mid.Pers. griftan, gir- “to take, hold, restrain;” O.Pers./Av. grab- “to take, seize,”
cf. Skt. grah-, grabh- “to seize, take,” graha- “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE *ghrebh- “to seize.”

The unwanted signal affecting a receiver.

See also: → receiver; → noise.

The unwanted signal affecting a receiver.

See also: → receiver; → noise.

1. Of, belonging to, or occurring at a time immediately before the present.
   

2. Geology: Of, belonging to, or denoting the Holocene Epoch.

Etymology (EN): From L. recentem (nominative recens) “fresh, new, young,” from → re- + PIE root *ken- “fresh, new, young;” cf. Av. kainika-, kanya- “young girl;” Mod.Pers. kaniz(ak) “maid, virigin, girl; a female slave;” Mid.Pers. kanig, kanizag “girl, maid;”
Gk. kainos “new;” Skt. kanina- “young;” Old Irish cetu- “first;” O.C.S. koni “beginning.”

Etymology (PE): Tarné, from Tabari tarné “fresh, new, young, recent” (tarné mâr “fresh mother,” i.e. “animal that has just given birth,” tarné guk “recently born calf”), related to Pers. tar “fresh, young; wet,” tarké “sprout;” Mid.Pers. tarr “fresh, moist;” Proto-Ir. *tarna-; cf. Av. tauruna- “young, fresh; young boy, lad;” Skt. táruna- “young, fresh;” Gk. teren “delicate, weak.”

1. Of, belonging to, or occurring at a time immediately before the present.
   

2. Geology: Of, belonging to, or denoting the Holocene Epoch.

Etymology (EN): From L. recentem (nominative recens) “fresh, new, young,” from → re- + PIE root *ken- “fresh, new, young;” cf. Av. kainika-, kanya- “young girl;” Mod.Pers. kaniz(ak) “maid, virigin, girl; a female slave;” Mid.Pers. kanig, kanizag “girl, maid;”
Gk. kainos “new;” Skt. kanina- “young;” Old Irish cetu- “first;” O.C.S. koni “beginning.”

Etymology (PE): Tarné, from Tabari tarné “fresh, new, young, recent” (tarné mâr “fresh mother,” i.e. “animal that has just given birth,” tarné guk “recently born calf”), related to Pers. tar “fresh, young; wet,” tarké “sprout;” Mid.Pers. tarr “fresh, moist;” Proto-Ir. *tarna-; cf. Av. tauruna- “young, fresh; young boy, lad;” Skt. táruna- “young, fresh;” Gk. teren “delicate, weak.”

In the recent past.

See also: → recent + -ly a suffix forming adverbs from adjectives.

In the recent past.

See also: → recent + -ly a suffix forming adverbs from adjectives.

1. The property of having happened or appeared not long ago.
   

2. A time immediately before the present.

See also: → recent; → -ness.

1. The property of having happened or appeared not long ago.
   

2. A time immediately before the present.

See also: → recent; → -ness.

1. The act of receding, going back, or withdrawing.
   

2. Economics: A period of an economic contraction, sometimes limited in scope or duration (Dictionary.com).

Etymology (EN): From Fr. récession “a going backward, a withdrawing,” from L. recessionem “a going back,” noun of action from p.p. stem of recedere “to go back, fall back; withdraw, retire,” from → re- “back” + cedere “to go,” → process.

Etymology (PE): Duršod, from dur, → remote, + šod “going,” past stem of šodan “to go, to become,” → change. Vâraveš, from vâ-, → re-, + raveš verbal noun of raftan “to go,” → method.

1. The act of receding, going back, or withdrawing.
   

2. Economics: A period of an economic contraction, sometimes limited in scope or duration (Dictionary.com).

Etymology (EN): From Fr. récession “a going backward, a withdrawing,” from L. recessionem “a going back,” noun of action from p.p. stem of recedere “to go back, fall back; withdraw, retire,” from → re- “back” + cedere “to go,” → process.

Etymology (PE): Duršod, from dur, → remote, + šod “going,” past stem of šodan “to go, to become,” → change. Vâraveš, from vâ-, → re-, + raveš verbal noun of raftan “to go,” → method.

The velocity with which an object moves away from another object or a reference point.

See also: → recession; → velocity.

The velocity with which an object moves away from another object or a reference point.

See also: → recession; → velocity.

1. General: Any theorem that expresses various reciprocal relations for the behavior of some physical systems, in which input and output can be interchanged without altering the response of the system to a given excitation.
   

2. In classical electromagnetism, the theorem stating that the current in a detector divided by the voltage at the source remains constant when source and detector are interchanged, as long as the frequency and all the impedances are left unchanged.

Etymology (EN): Reciprocity, from L. reciproc(us) “returning the same way, alternating” + → -ity; → theorem.

Etymology (PE): Farbin, → theorem; dosuyegi, quality noun of dosuyé nuanced term of dosu “two-sided,” from do, → two,

• su “direction, side,” from Mid.Pers. sôk “direction, side.”

1. General: Any theorem that expresses various reciprocal relations for the behavior of some physical systems, in which input and output can be interchanged without altering the response of the system to a given excitation.
   

2. In classical electromagnetism, the theorem stating that the current in a detector divided by the voltage at the source remains constant when source and detector are interchanged, as long as the frequency and all the impedances are left unchanged.

Etymology (EN): Reciprocity, from L. reciproc(us) “returning the same way, alternating” + → -ity; → theorem.

Etymology (PE): Farbin, → theorem; dosuyegi, quality noun of dosuyé nuanced term of dosu “two-sided,” from do, → two,

• su “direction, side,” from Mid.Pers. sôk “direction, side.”

An act of recognizing or the state of being recognized.

See also: Verbal noun of → recognize.

An act of recognizing or the state of being recognized.

See also: Verbal noun of → recognize.

1. To identify as something or someone previously seen, known, etc.
   

2. To identify from knowledge of appearance or characteristics (Dictionary.com).

See also: → re-; → cognition; → -ize.

1. To identify as something or someone previously seen, known, etc.
   

2. To identify from knowledge of appearance or characteristics (Dictionary.com).

See also: → re-; → cognition; → -ize.

1. To draw back; to rebound or fly back.
   

2. An act of recoiling. → Compton recoil.

Etymology (EN): M.E. recoilen, reculen, from O.Fr. reculer “to go back, recede, retreat,” from V.L. *reculare, from L. → re- “back” + culus “backside.”

Etymology (PE): Paszadan, from pas-, → back-, + zadan “to strike, beat, dash against,” from Mid.Pers. zatan, žatan, O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”), Skt. han- “to strike, beat” (hantar- “smiter, killer”), cf. Gk. theinein “to strike,” L. fendere “to strike, push,” Gmc *gundjo “war, battle;” PIE *g^when- “to strike, kill.”

1. To draw back; to rebound or fly back.
   

2. An act of recoiling. → Compton recoil.

Etymology (EN): M.E. recoilen, reculen, from O.Fr. reculer “to go back, recede, retreat,” from V.L. *reculare, from L. → re- “back” + culus “backside.”

Etymology (PE): Paszadan, from pas-, → back-, + zadan “to strike, beat, dash against,” from Mid.Pers. zatan, žatan, O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”), Skt. han- “to strike, beat” (hantar- “smiter, killer”), cf. Gk. theinein “to strike,” L. fendere “to strike, push,” Gmc *gundjo “war, battle;” PIE *g^when- “to strike, kill.”

1. The process or result of combining again.
   
2. The capture of an electron by a positive ion. It is the inverse process to → ionization.

See also: Verbal noun of → recombine.

1. The process or result of combining again.
   
2. The capture of an electron by a positive ion. It is the inverse process to → ionization.

See also: Verbal noun of → recombine.

A measure of the specific rate at which oppositely charged ions join to form neutral particles. It is given by the rate at which those ions recombine, divided by the product of the densities of the two species involved.

See also: → recombination; → coefficient.

A measure of the specific rate at which oppositely charged ions join to form neutral particles. It is given by the rate at which those ions recombine, divided by the product of the densities of the two species involved.

See also: → recombination; → coefficient.

A recombination radiation that is continuous over a range of frequencies. Same as → continuum emission.

See also: → recombination; → continuum.

A recombination radiation that is continuous over a range of frequencies. Same as → continuum emission.

See also: → recombination; → continuum.

Same as → recombination era.

See also: → recombination; → epoch.

Same as → recombination era.

See also: → recombination; → epoch.

The era some 380,000 years after the → Big Bang (at a → redshift of about 1,100),
when the Universe had cooled sufficiently so that protons and electrons combined to form → neutral hydrogen in a process called → recombination. The temperature was about 3,000 K and the ionization fraction low enough for Universe to become transparent to light. Consequently matter and radiation decouple from one another because no further → scattering of the radiation occurs. The observation of the → cosmic microwave background radiation provides a means of studying the Universe at the recombination era.
Also called recombination epoch and
→ decoupling era.

See also: → recombination; → era.

The era some 380,000 years after the → Big Bang (at a → redshift of about 1,100),
when the Universe had cooled sufficiently so that protons and electrons combined to form → neutral hydrogen in a process called → recombination. The temperature was about 3,000 K and the ionization fraction low enough for Universe to become transparent to light. Consequently matter and radiation decouple from one another because no further → scattering of the radiation occurs. The observation of the → cosmic microwave background radiation provides a means of studying the Universe at the recombination era.
Also called recombination epoch and
→ decoupling era.

See also: → recombination; → era.

An → emission line in a spectrum produced in an → H II region when a free electron combines
with an ionized atom to form a neutral atom or an ion of lower → ionization stage. Same as → free-bound emission.

See also: → recombination; → line.

An → emission line in a spectrum produced in an → H II region when a free electron combines
with an ionized atom to form a neutral atom or an ion of lower → ionization stage. Same as → free-bound emission.

See also: → recombination; → line.

Radiation produced when a free electron in a plasma is captured by an ionized atom.

See also: → recombination; → radiation.

Radiation produced when a free electron in a plasma is captured by an ionized atom.

See also: → recombination; → radiation.

In → H II regions the rate at which free electrons recombine with → ionized hydrogen atoms (protons).

See also: → recombination; → rate.

In → H II regions the rate at which free electrons recombine with → ionized hydrogen atoms (protons).

See also: → recombination; → rate.

The time period necessary for a cloud of atomic hydrogen to be → ionized by the ultraviolet photons of a central
→ massive stars.

See also: → recombination; → time.

The time period necessary for a cloud of atomic hydrogen to be → ionized by the ultraviolet photons of a central
→ massive stars.

See also: → recombination; → time.

To combine or put together again. To undergo or cause recombination.

See also: → re- + → combine.

To combine or put together again. To undergo or cause recombination.

See also: → re- + → combine.

1. To cause (a person) to accept or be resigned to something not desired.
   

   2. To win over to friendliness; cause to become amicable.
      

   3. To compose or settle (a quarrel, dispute, etc.); to bring into agreement or harmony; make compatible or consistent (Dictionary.com).

See also: Infinitive of → reconciliation.

1. To cause (a person) to accept or be resigned to something not desired.
   

   2. To win over to friendliness; cause to become amicable.
      

   3. To compose or settle (a quarrel, dispute, etc.); to bring into agreement or harmony; make compatible or consistent (Dictionary.com).

See also: Infinitive of → reconciliation.

1. An act of reconciling, as when former enemies agree to an amicable truce.
   

   2. The state of being reconciled, as when someone becomes resigned to something not desired.
      

   3. The process of making consistent or compatible (Dictionary.com).

Etymology (EN): From O.Fr. reconciliacion and directly from L. reconciliationem noun of action from p.p. stem of reconciliare. “to bring together again; regain,” from → re- “again” + concilare “make friendly”

Etymology (PE): šti, from Mid.Pers. âštêh “peace;” Av. āšti- “peace, treaty of peace; agreement,” āxšta– “pacified, appeased.”

1. An act of reconciling, as when former enemies agree to an amicable truce.
   

   2. The state of being reconciled, as when someone becomes resigned to something not desired.
      

   3. The process of making consistent or compatible (Dictionary.com).

Etymology (EN): From O.Fr. reconciliacion and directly from L. reconciliationem noun of action from p.p. stem of reconciliare. “to bring together again; regain,” from → re- “again” + concilare “make friendly”

Etymology (PE): šti, from Mid.Pers. âštêh “peace;” Av. āšti- “peace, treaty of peace; agreement,” āxšta– “pacified, appeased.”

1a) An act of recording.

1b) The state of being recorded, as in writing.

1c) Something on which sound or images have been recorded for subsequent reproduction, as a grooved disk that is played on a phonograph or an optical disk for recording sound (audiodisk) or images (videodisk).

2a) To set down in writing or the like, as for the purpose of preserving evidence.

2b) To set down or → register in some permanent form, as on a seismograph.

2c) To set down, register, or fix by characteristic marks, incisions, magnetism, etc., for the purpose of reproduction by a phonograph or magnetic reproducer; to make a recording of.

Etymology (EN): From M.E. recorden “to repeat, to report,” from O.Fr. recorder “to get by heart,” from L. recordari “to call to mind, remember,” from → re-
“back, again” + cor “heart” (as the metaphoric seat of memory, as in learn by heart).

Etymology (PE): Vâgat literally “to take, seize, to take back,” cf. Ger. aufnehmen “to record,” from nehmen “to take;” E. “take down” “to note down;” Šahmirzâdi vagatan “to take,” Lâhijâni vitan, “to take, seize,” Aftari veytu “to take,” Delijâni bitan “to take,” Tâleši gate, Târi gata/ger, Sorxeyi gil, all variants of gereftan, → concept.

1a) An act of recording.

1b) The state of being recorded, as in writing.

1c) Something on which sound or images have been recorded for subsequent reproduction, as a grooved disk that is played on a phonograph or an optical disk for recording sound (audiodisk) or images (videodisk).

2a) To set down in writing or the like, as for the purpose of preserving evidence.

2b) To set down or → register in some permanent form, as on a seismograph.

2c) To set down, register, or fix by characteristic marks, incisions, magnetism, etc., for the purpose of reproduction by a phonograph or magnetic reproducer; to make a recording of.

Etymology (EN): From M.E. recorden “to repeat, to report,” from O.Fr. recorder “to get by heart,” from L. recordari “to call to mind, remember,” from → re-
“back, again” + cor “heart” (as the metaphoric seat of memory, as in learn by heart).

Etymology (PE): Vâgat literally “to take, seize, to take back,” cf. Ger. aufnehmen “to record,” from nehmen “to take;” E. “take down” “to note down;” Šahmirzâdi vagatan “to take,” Lâhijâni vitan, “to take, seize,” Aftari veytu “to take,” Delijâni bitan “to take,” Tâleši gate, Târi gata/ger, Sorxeyi gil, all variants of gereftan, → concept.

A → quadrilateral all of whose angles are → right angles.

Etymology (EN): M.Fr. rectangle, from M.L. rectangulum “a triangle having a right angle,” from rect-, combining form of rectus→ right + angulum, → angle.

Etymology (PE): Râstgušé, from râst, → right, + gušé “corner, angle;” Mid.Pers. gôšak “corner.”

A → quadrilateral all of whose angles are → right angles.

Etymology (EN): M.Fr. rectangle, from M.L. rectangulum “a triangle having a right angle,” from rect-, combining form of rectus→ right + angulum, → angle.

Etymology (PE): Râstgušé, from râst, → right, + gušé “corner, angle;” Mid.Pers. gôšak “corner.”

Having the base or section in the form of a rectangle. Shaped like a rectangle.

See also: Adj. of → rectangle.

Having the base or section in the form of a rectangle. Shaped like a rectangle.

See also: Adj. of → rectangle.

A → window function that is constant inside a specified interval.

See also: → rectangular; → window.

A → window function that is constant inside a specified interval.

See also: → rectangular; → window.

1. Math.: The process of determining the length of a curve.
   
2. Physics: Conversion of an alternating into a direct current.
   
3. Astro.: The process of rectifying a spectrum.

See also: Verbal noun of → rectify.

1. Math.: The process of determining the length of a curve.
   
2. Physics: Conversion of an alternating into a direct current.
   
3. Astro.: The process of rectifying a spectrum.

See also: Verbal noun of → rectify.

A spectrum after correction for detector’s → baseline.

See also: Rectified p.p. of → rectify; → spectrum.

A spectrum after correction for detector’s → baseline.

See also: Rectified p.p. of → rectify; → spectrum.

1. General: A person or thing that rectifies.
   
2. Physics: A device for transforming an
   → alternating current into a → direct current.

See also: Agent noun of → rectify.

1. General: A person or thing that rectifies.
   
2. Physics: A device for transforming an
   → alternating current into a → direct current.

See also: Agent noun of → rectify.

1. General: To make, put, or set right; to correct.
   
2. Electricity: To convert an → alternating current into a → direct current.
   
3. Chem.: To refine a substance, especially by repeated distillation or sublimation.

Etymology (EN): M.E. rectifien, from O.Fr. rectifier “to make straight,” from L.L. rectificare “make right,” from L. rectus “straight,” cognate with Pers. râst, as below, → right + root of
facere “to make, do, perform” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cf. Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Râstidan, literally “to make straight, right,” infinitive from râst cognate with L. rectus “straight,” → right.

1. General: To make, put, or set right; to correct.
   
2. Electricity: To convert an → alternating current into a → direct current.
   
3. Chem.: To refine a substance, especially by repeated distillation or sublimation.

Etymology (EN): M.E. rectifien, from O.Fr. rectifier “to make straight,” from L.L. rectificare “make right,” from L. rectus “straight,” cognate with Pers. râst, as below, → right + root of
facere “to make, do, perform” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cf. Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Râstidan, literally “to make straight, right,” infinitive from râst cognate with L. rectus “straight,” → right.

In a straight line; consisting of straight lines. → curvilinear

Etymology (EN): From recti- combining form of rectus, → right,

• → line + -ar, variant of the adjective-forming suffix → -al.

Etymology (PE): Râst-xatt, from râst, → right, + xatt→ line.

In a straight line; consisting of straight lines. → curvilinear

Etymology (EN): From recti- combining form of rectus, → right,

• → line + -ar, variant of the adjective-forming suffix → -al.

Etymology (PE): Râst-xatt, from râst, → right, + xatt→ line.

The motion of light in the first approximation, as evidenced from the formation of shadows and other every day experience. However, → diffraction

See also: → rectilinear; → propagation; → light.

The motion of light in the first approximation, as evidenced from the formation of shadows and other every day experience. However, → diffraction

See also: → rectilinear; → propagation; → light.

An optical system that is corrected for → distortion and → spherical aberration and therefore forms the image of a straight line as a straight line.

See also: → rectilinear; → system.

An optical system that is corrected for → distortion and → spherical aberration and therefore forms the image of a straight line as a straight line.

See also: → rectilinear; → system.

To occur again, as an event, experience, etc.

Etymology (EN): From L. recurrere “to return, run back,” → re- + currere “to run,”
→ current.

Etymology (PE): Bâzâmadan “to come back, return,” from bâz, → re-,

• âmadan “to come, arrive, become” (present stem ây-);
  Av. ay- “to go, to come,” aēiti “goes;” O.Pers. aitiy “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went;” Lith. eiti “to go;” Rus. idti “to go.”

To occur again, as an event, experience, etc.

Etymology (EN): From L. recurrere “to return, run back,” → re- + currere “to run,”
→ current.

Etymology (PE): Bâzâmadan “to come back, return,” from bâz, → re-,

• âmadan “to come, arrive, become” (present stem ây-);
  Av. ay- “to go, to come,” aēiti “goes;” O.Pers. aitiy “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went;” Lith. eiti “to go;” Rus. idti “to go.”

1. An act or instance of recurring.
   

2. Return to a previous condition, habit, subject, etc. (Dictionary.com). → recurrence relation.

See also: Verbal noun of → recur.

1. An act or instance of recurring.
   

2. Return to a previous condition, habit, subject, etc. (Dictionary.com). → recurrence relation.

See also: Verbal noun of → recur.

A → sequence based on a → rule that gives the next → term as a → function of the previous term(s). For example, the sequence 3, 9, 21, 45,… can be represented by the recurrence relation u_n+1 = 2u_n + 3, where u₁ = 3 and n ≥ 1.

See also: → recurrence; → relation.

A → sequence based on a → rule that gives the next → term as a → function of the previous term(s). For example, the sequence 3, 9, 21, 45,… can be represented by the recurrence relation u_n+1 = 2u_n + 3, where u₁ = 3 and n ≥ 1.

See also: → recurrence; → relation.

Occurring or appearing again, especially repeatedly or periodically (Dictionary.com). → recurrence nova.

See also: Verbal adj. from → recur.

Occurring or appearing again, especially repeatedly or periodically (Dictionary.com). → recurrence nova.

See also: Verbal adj. from → recur.

A → cataclysmic variable star that undergoes → outbursts similar to those found in novae.

See also: → recurrent; → nova.

A → cataclysmic variable star that undergoes → outbursts similar to those found in novae.

See also: → recurrent; → nova.

1. A running backward, return.
   

2. Math.: A process in which objects are defined by the repeated application of a rule or algorithm.

Etymology (EN): From L. recursionem (nominative recursio); → recurrent.

Etymology (PE): Bâzâneš, verbal noun of bâzâmadan, → recur.

1. A running backward, return.
   

2. Math.: A process in which objects are defined by the repeated application of a rule or algorithm.

Etymology (EN): From L. recursionem (nominative recursio); → recurrent.

Etymology (PE): Bâzâneš, verbal noun of bâzâmadan, → recur.

1. Pertaining to or using a rule or procedure that can be applied repeatedly.
   

2. Math.: Pertaining to or using the process of → recursion.

See also: Adjective from → recursion.

1. Pertaining to or using a rule or procedure that can be applied repeatedly.
   

2. Math.: Pertaining to or using the process of → recursion.

See also: Adjective from → recursion.

Math.: A definition of a function from which values of the same function can be calculated in a finite number of steps. In mathematical logic and computer science, a recursive definition is used to define an object in terms of itself. An example is the → factorial: n! = n*(n-1)!

See also: → recursive; → definition.

Math.: A definition of a function from which values of the same function can be calculated in a finite number of steps. In mathematical logic and computer science, a recursive definition is used to define an object in terms of itself. An example is the → factorial: n! = n*(n-1)!

See also: → recursive; → definition.

1. To treat or process used material so that it can be used again.
   

2. To repeat or pass through a process again. → recycled pulsar.

See also: → re-; → cycle.

1. To treat or process used material so that it can be used again.
   

2. To repeat or pass through a process again. → recycled pulsar.

See also: → re-; → cycle.

A → pulsar of abnormally low magnetic field and short period. The short period suggests that the pulasr is young, while the low field suggests a very old pulsar. According to theoretical models, a pulsar at some point in its evolution
stops functioning as a pulsar. If it resides in a binary system (→ binary pulsar), its magnetic field decays in the interval when the companion evolves and fills its → Roche lobe. The dead pulsar is eventually spun up to life when → mass transfer from the companion begins. The pulsar gains → angular momentum from the infalling gas and increases its spin rate as more gas falls onto it. → Millisecond pulsars that spin hundreds of times per second are thought to be the result of such a transfer (see, e.g., G. Srinivasan, 2010, New Astronomy Reviews 54, 93, and references therein).

See also: → recycle; → pulsar.

A → pulsar of abnormally low magnetic field and short period. The short period suggests that the pulasr is young, while the low field suggests a very old pulsar. According to theoretical models, a pulsar at some point in its evolution
stops functioning as a pulsar. If it resides in a binary system (→ binary pulsar), its magnetic field decays in the interval when the companion evolves and fills its → Roche lobe. The dead pulsar is eventually spun up to life when → mass transfer from the companion begins. The pulsar gains → angular momentum from the infalling gas and increases its spin rate as more gas falls onto it. → Millisecond pulsars that spin hundreds of times per second are thought to be the result of such a transfer (see, e.g., G. Srinivasan, 2010, New Astronomy Reviews 54, 93, and references therein).

See also: → recycle; → pulsar.

That part of the electromagnetic spectrum, with a wavelength between 610 and 780 nm, that produces the impression of a variety of colors resembling that of blood.

Etymology (EN): M.E. red, from O.E. read (cf. Dan. rød, M.Du. root, Ger. rot), from PIE base *reudh- (cf. Av. raoidita- “red, reddish;” Skt. rudhirá- “red, bloody;” L. ruber “red;”
Gk. erythros “red”).

Etymology (PE): Sorx “red” (variants Tâleši sər, Kurd. sûr),
from Mid.Pers. suxr “red;” O.Pers. θuxra-; Av. suxra- “red, of fire-color;” Ossetic syrx, surx “red;” cf. Skt. śukrá- “clear, pure, bright.”

That part of the electromagnetic spectrum, with a wavelength between 610 and 780 nm, that produces the impression of a variety of colors resembling that of blood.

Etymology (EN): M.E. red, from O.E. read (cf. Dan. rød, M.Du. root, Ger. rot), from PIE base *reudh- (cf. Av. raoidita- “red, reddish;” Skt. rudhirá- “red, bloody;” L. ruber “red;”
Gk. erythros “red”).

Etymology (PE): Sorx “red” (variants Tâleši sər, Kurd. sûr),
from Mid.Pers. suxr “red;” O.Pers. θuxra-; Av. suxra- “red, of fire-color;” Ossetic syrx, surx “red;” cf. Skt. śukrá- “clear, pure, bright.”

A concentration, on the → horizontal branch, of → red giant stars that roughly have the same intrinsic brightness. These core → helium burning stars are the metal rich equivalents of the better known → horizontal branch stars. Theoretical models predict that their absolute luminosity only weakly depends on their age and chemical composition.

See also: → red; → clump.

A concentration, on the → horizontal branch, of → red giant stars that roughly have the same intrinsic brightness. These core → helium burning stars are the metal rich equivalents of the better known → horizontal branch stars. Theoretical models predict that their absolute luminosity only weakly depends on their age and chemical composition.

See also: → red; → clump.

A star on the → horizontal branch which results from the evolution of a → red giant with an initial mass of ~ 1 Msun.

See also: → red; → clump; → star.

A star on the → horizontal branch which results from the evolution of a → red giant with an initial mass of ~ 1 Msun.

See also: → red; → clump; → star.

A small, cool, very faint, main sequence star whose surface temperature is under about 3500 K. Red dwarfs generally have masses of less than one-third that of the Sun. In the neighbourhood of the Sun the majority of stars are red dwarfs.

See also: → red; → dwarf.

A small, cool, very faint, main sequence star whose surface temperature is under about 3500 K. Red dwarfs generally have masses of less than one-third that of the Sun. In the neighbourhood of the Sun the majority of stars are red dwarfs.

See also: → red; → dwarf.

A certain star of spectral type K or later that occupies the upper right portion of the → H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a → luminosity up to 1000 times greater than → main sequence stars of the same → spectral type. Red giants belong to the → luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (→ Schönberg-Chandrasekhar limit). The
→ hydrostatic equilibrium is no longer maintained, and
the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. → helium flash. Prominent bright red giants in the night sky include → Aldebaran
and → Arcturus.

See also: → red; → giant.

A certain star of spectral type K or later that occupies the upper right portion of the → H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a → luminosity up to 1000 times greater than → main sequence stars of the same → spectral type. Red giants belong to the → luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (→ Schönberg-Chandrasekhar limit). The
→ hydrostatic equilibrium is no longer maintained, and
the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. → helium flash. Prominent bright red giants in the night sky include → Aldebaran
and → Arcturus.

See also: → red; → giant.

The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the → main-sequence turnoff and the → helium flash.

See also: → red giant; → branch.

The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the → main-sequence turnoff and the → helium flash.

See also: → red giant; → branch.

Same as → red horizontal branch star.

See also: → red; → horizontal; → branch; → star.

Same as → red horizontal branch star.

See also: → red; → horizontal; → branch; → star.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

See also: → red; → horizontal; → branch; → star.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

See also: → red; → horizontal; → branch; → star.

Unwanted secondary window in a filter band pass, on the red side of the main window.

Etymology (EN): → red; leak, from M.E leken, from O.N. leka “to drip, leak;” akin to Du. lek, Ger. lech “leaky,” O.E. leccan “to moisten.”

Etymology (PE): Našt “leak, leakage,” of unknown origin; sorx, → red.

Unwanted secondary window in a filter band pass, on the red side of the main window.

Etymology (EN): → red; leak, from M.E leken, from O.N. leka “to drip, leak;” akin to Du. lek, Ger. lech “leaky,” O.E. leccan “to moisten.”

Etymology (PE): Našt “leak, leakage,” of unknown origin; sorx, → red.

See → Great Red Spot, on Jupiter.

See also: → red; → spot.

See → Great Red Spot, on Jupiter.

See also: → red; → spot.

A supergiant star with spectral type K or M. Red supergiants are the
largest stars in the Universe, but not necessarily the most massive.
Betelgeuse and Antares are the best known examples of a red supergiant.

See also: → red; → supergiant.

A supergiant star with spectral type K or M. Red supergiants are the
largest stars in the Universe, but not necessarily the most massive.
Betelgeuse and Antares are the best known examples of a red supergiant.

See also: → red; → supergiant.

A member of a class of exploding stars that are more luminous than → novae but not as luminous as → supernovae. Moreover, their outburst → light curves have multiple peaks. One of the most characteristic features of red transients is that after exploding they cool down to → late-type → M star and develop circumstellar material rich in molecules and dust. Some of the members of the red transients in our Galaxy are V838V, OGLE-2002-BLG-360, V4332 Sgr, and V1309 Sco.

See also: → red; → transient.

A member of a class of exploding stars that are more luminous than → novae but not as luminous as → supernovae. Moreover, their outburst → light curves have multiple peaks. One of the most characteristic features of red transients is that after exploding they cool down to → late-type → M star and develop circumstellar material rich in molecules and dust. Some of the members of the red transients in our Galaxy are V838V, OGLE-2002-BLG-360, V4332 Sgr, and V1309 Sco.

See also: → red; → transient.

Of a spectral line profile, the → line wing with wavelengths longer than that of the emission or absorption peak.

See also: → red; → wing.

Of a spectral line profile, the → line wing with wavelengths longer than that of the emission or absorption peak.

See also: → red; → wing.

A rise in a planet’s surface → reflectivity between red → absorbance and → near-infrared reflection due to → vegetation. The red-edge is one of the possible signs of life on distant → habitable  → exoplanets.

Its presence is attributed to the chlorophyll molecule and leaf structure. The leaves of land plants reflect sunlight much more efficiently long-ward of this edge than they do in the visible.

Although the red-edge position for Earth’s vegetation is fixed at around 700-760 nm, that for exoplanets may not necessarily be the same (Takizawa et al., 2017, Nature Scientific Reports 7, Article number: 7561).

See also: → red; → edge.

A rise in a planet’s surface → reflectivity between red → absorbance and → near-infrared reflection due to → vegetation. The red-edge is one of the possible signs of life on distant → habitable  → exoplanets.

Its presence is attributed to the chlorophyll molecule and leaf structure. The leaves of land plants reflect sunlight much more efficiently long-ward of this edge than they do in the visible.

Although the red-edge position for Earth’s vegetation is fixed at around 700-760 nm, that for exoplanets may not necessarily be the same (Takizawa et al., 2017, Nature Scientific Reports 7, Article number: 7561).

See also: → red; → edge.

1. (v.intr.) Of a spectral line, to reduce in intensity due to absorption by interstellar dust grains.
   
2. (v.tr.) Of interstellar dust, to absorb the light that passes through it.

See also: Infinitives from → red.

1. (v.intr.) Of a spectral line, to reduce in intensity due to absorption by interstellar dust grains.
   
2. (v.tr.) Of interstellar dust, to absorb the light that passes through it.

See also: Infinitives from → red.

A star whose light has undergone → reddening.

See also: Reddened, p.p. of → redden; → star.

A star whose light has undergone → reddening.

See also: Reddened, p.p. of → redden; → star.

The process by which light from an astronomical object grows red as it travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted.

See also: Verbal noun of → redden.

The process by which light from an astronomical object grows red as it travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted.

See also: Verbal noun of → redden.

A dimensionless quantity determined from the comparison of the observed → Balmer decrements with respect to the theoretical values for given physical conditions of electron temperature and density. The reddening coefficient at Hβ is defined as

c(Hβ) = log (I(Hβ)/F(Hβ)), where I(Hβ) and F(Hβ) are → de-reddened and reddened Hβ fluxes respectively. Also called logarithmic extinction.

See also: → reddening; → coefficient.

A dimensionless quantity determined from the comparison of the observed → Balmer decrements with respect to the theoretical values for given physical conditions of electron temperature and density. The reddening coefficient at Hβ is defined as

c(Hβ) = log (I(Hβ)/F(Hβ)), where I(Hβ) and F(Hβ) are → de-reddened and reddened Hβ fluxes respectively. Also called logarithmic extinction.

See also: → reddening; → coefficient.

The normalized interstellar extinction at a given wavelength. It is defined by f(λ) = A(λ)/A(Hβ) - 1, where A(λ) is the extinction at the given wavelength and A(Hβ) the extinction at Hβ, with f(Hβ) = 0. It is used to → de-redden observed fluxes:

I(λ)/I(Hβ) = F(λ)/F(Hβ).10^c(Hβ).f(λ),

where I represents the flux in the absence of extinction and F the observed flux affected by extinction, c(Hβ) being the → reddening coefficient.

See also: → reddening; → function.

The normalized interstellar extinction at a given wavelength. It is defined by f(λ) = A(λ)/A(Hβ) - 1, where A(λ) is the extinction at the given wavelength and A(Hβ) the extinction at Hβ, with f(Hβ) = 0. It is used to → de-redden observed fluxes:

I(λ)/I(Hβ) = F(λ)/F(Hβ).10^c(Hβ).f(λ),

where I represents the flux in the absence of extinction and F the observed flux affected by extinction, c(Hβ) being the → reddening coefficient.

See also: → reddening; → function.

A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio:
R_V = A_V/E(B-V). The typical value found for the reddening parameter in the Milky Way is R_V ~ 3.1, but it is known to vary from one
line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with R_V → ∞.

See also: → reddening; → parameter.

A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio:
R_V = A_V/E(B-V). The typical value found for the reddening parameter in the Milky Way is R_V ~ 3.1, but it is known to vary from one
line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with R_V → ∞.

See also: → reddening; → parameter.

A vector indicating the direction in which interstellar reddening moves the position of a star in a multi-dimensional space of color indices.

See also: → reddening; → vector.

A vector indicating the direction in which interstellar reddening moves the position of a star in a multi-dimensional space of color indices.

See also: → reddening; → vector.

A shift in the lines of an object’s spectrum toward longer wavelengths. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving. Redshift is expressed by z = Δλ/λ = v/c, where λ is the wavelength, Δλ the wavelength shift, v the velocity of the source relative to the observer, and c the → speed of light. When v approaches c, redshift is expressed by the → relativistic formula

z = ((1 +v/c)/(1 - v/c))^&frac12 - 1.

See also: → red; → shift.

A shift in the lines of an object’s spectrum toward longer wavelengths. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving. Redshift is expressed by z = Δλ/λ = v/c, where λ is the wavelength, Δλ the wavelength shift, v the velocity of the source relative to the observer, and c the → speed of light. When v approaches c, redshift is expressed by the → relativistic formula

z = ((1 +v/c)/(1 - v/c))^&frac12 - 1.

See also: → red; → shift.

The space corresponding to → redshift measurements, as contrasted with real space. See also → redshift space distortion

See also: → redshift; → space.

The space corresponding to → redshift measurements, as contrasted with real space. See also → redshift space distortion

See also: → redshift; → space.

The distortion observed in → redshift space of → galaxy clusters caused by peculiar velocities of the members (→ peculiar velocity). In a perfectly homogeneous → Friedmann-Lemaitre Universe the redshifts would accurately measure radial distances from the observer, and the mapping from real space to redshift space would simply be an identity. In an inhomogeneous Universe the peculiar velocities associated with any inhomogeneous structure will introduce a distortion in this mapping (N. Kaiser, 1987, MNRAS 227, 1). See also: → fingers of God, → Kaiser effect.

See also: → redshift; → space; → distortion;.

The distortion observed in → redshift space of → galaxy clusters caused by peculiar velocities of the members (→ peculiar velocity). In a perfectly homogeneous → Friedmann-Lemaitre Universe the redshifts would accurately measure radial distances from the observer, and the mapping from real space to redshift space would simply be an identity. In an inhomogeneous Universe the peculiar velocities associated with any inhomogeneous structure will introduce a distortion in this mapping (N. Kaiser, 1987, MNRAS 227, 1). See also: → fingers of God, → Kaiser effect.

See also: → redshift; → space; → distortion;.

A survey of a large region of the sky to measure the redshifts of all the galaxies down to a certain limiting magnitude.

See also: → redshift; → survey.

A survey of a large region of the sky to measure the redshifts of all the galaxies down to a certain limiting magnitude.

See also: → redshift; → survey.

The correlation, first established by E. Hubble, between the cosmological recession velocities of galaxies and their distances.

See also: → redshift; → distance;
→ relation.

The correlation, first established by E. Hubble, between the cosmological recession velocities of galaxies and their distances.

See also: → redshift; → distance;
→ relation.

A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.

See also: → red; → shift; → line.

A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.

See also: → red; → shift; → line.

1. To bring to a certain state, condition, arrangement, etc.
   Math.: To change the denomination or form, but not the value, of (a fraction, polynomial, etc.).
   Chem.: To add an electron to.
   
2. To bring down to a smaller extent, size, amount, number, etc.; synonymous with
   → decrease.

Etymology (EN): M.E. reducen “to lead back,” from
O.Fr. reducer, from L. reducere, from → re- “back” + ducere “to bring, to lead.”

Etymology (PE): From bâz-, → re- + Mid.Pers. hâxtan, hâzidan “to lead, guide, persuade,” Av. hak-, hacaiti “to attach oneself to, to join,” cf. Skt. sacate “accompanies, follows,” Gk. hepesthai “to follow,” L. sequi “to follow;” PIE *sekw- “to follow.”
Kâstan, → decrease.

1. To bring to a certain state, condition, arrangement, etc.
   Math.: To change the denomination or form, but not the value, of (a fraction, polynomial, etc.).
   Chem.: To add an electron to.
   
2. To bring down to a smaller extent, size, amount, number, etc.; synonymous with
   → decrease.

Etymology (EN): M.E. reducen “to lead back,” from
O.Fr. reducer, from L. reducere, from → re- “back” + ducere “to bring, to lead.”

Etymology (PE): From bâz-, → re- + Mid.Pers. hâxtan, hâzidan “to lead, guide, persuade,” Av. hak-, hacaiti “to attach oneself to, to join,” cf. Skt. sacate “accompanies, follows,” Gk. hepesthai “to follow,” L. sequi “to follow;” PIE *sekw- “to follow.”
Kâstan, → decrease.

The “effective” → inertial mass appearing in the → two-body problem of → Newtonian mechanics. The reduced mass is a quantity which allows the two-body problem to be solved as if it were a one-body problem. For the masses m₁ and m₂, it is given by the ratio μ = m₁m₂ / (m₁ + m₂). The value of μ is generally smaller than m₁ and m₂. The larger the difference between the two masses, the closer μ will be to the smaller mass. If the particles are of equal mass, μ is half the mass of either.

See also: → reduce; → mass.

The “effective” → inertial mass appearing in the → two-body problem of → Newtonian mechanics. The reduced mass is a quantity which allows the two-body problem to be solved as if it were a one-body problem. For the masses m₁ and m₂, it is given by the ratio μ = m₁m₂ / (m₁ + m₂). The value of μ is generally smaller than m₁ and m₂. The larger the difference between the two masses, the closer μ will be to the smaller mass. If the particles are of equal mass, μ is half the mass of either.

See also: → reduce; → mass.

The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called the → Dirac constant.

See also: Reduced, p.p. of → reduce; → Planck’s constant.

The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called the → Dirac constant.

See also: Reduced, p.p. of → reduce; → Planck’s constant.

→ reduced Planck constant.

See also: → reduced Planck constant.

→ reduced Planck constant.

See also: → reduced Planck constant.

A substance which removes → oxygen from, or adds → hydrogen, to another substance. In the more general sense, one which loses electrons. Also called → reductant.

See also: → reduce; → -ing; → agent.

A substance which removes → oxygen from, or adds → hydrogen, to another substance. In the more general sense, one which loses electrons. Also called → reductant.

See also: → reduce; → -ing; → agent.

1. An atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidating gasses or vapours. Usually nitrogen or hydrogen gas is used in order to produce specific effects, e.g. on ceramic wares being fired.
   

2. An atmosphere of a planet or moon which has a high hydrogen content, either in the form of free hydrogen or hydrogen-containing compounds, such as methane or ammonia. The early atmosphere of Earth is thought to be reducing, dominated by carbon dioxide.

See also: Reducing verbal adj. of → reduce; → atmosphere.

1. An atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidating gasses or vapours. Usually nitrogen or hydrogen gas is used in order to produce specific effects, e.g. on ceramic wares being fired.
   

2. An atmosphere of a planet or moon which has a high hydrogen content, either in the form of free hydrogen or hydrogen-containing compounds, such as methane or ammonia. The early atmosphere of Earth is thought to be reducing, dominated by carbon dioxide.

See also: Reducing verbal adj. of → reduce; → atmosphere.

Same as → reducing agent.

See also: Agent noun from → reduce.

Same as → reducing agent.

See also: Agent noun from → reduce.

Logic, Math.: A method of → reasoning in which one assumes some statement to be → true and from that → assumption proceeds to deduce a logical → absurdity and hence to a conclusion that the original assumption must have been → false.

See also: L. reductio ad absurdum “reduction to absurdity,” → reduction; → absurd.

Logic, Math.: A method of → reasoning in which one assumes some statement to be → true and from that → assumption proceeds to deduce a logical → absurdity and hence to a conclusion that the original assumption must have been → false.

See also: L. reductio ad absurdum “reduction to absurdity,” → reduction; → absurd.

1. In → data processing, the transformation of data from a “raw” form to some usable form.
   

2. Chem.: The removal of → oxygen from a substance, or the addition of → hydrogen to it. The term is also used more generally to include any reaction in which an atom gains → electrons.

See also: Verbal noun of → reduce.

1. In → data processing, the transformation of data from a “raw” form to some usable form.
   

2. Chem.: The removal of → oxygen from a substance, or the addition of → hydrogen to it. The term is also used more generally to include any reaction in which an atom gains → electrons.

See also: Verbal noun of → reduce.

The fact of repeating or duplicity.
That part of the data content that can be dropped without any significant loss of the original information.

Etymology (EN): From L. redundantia “an overflowing, excess,” from redundare “to flow back, overflow, be excessive,” from
→ re- “again” + undare “rise in waves,” from unda “a wave.”

Etymology (PE): Afzun-âyi “redundancy, superabounding,” from afzun “more, greater; more ample,” from afzudan “to add, increase” (Mid.Pers. abzudan “to increase, grow;” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “press forward;” Av. gav- “to hasten, drive;” Sk. jav- “to press forward, impel quickly, excite,” javate “hastens”) + âyi verbal noun of ây- present stem of âmadan “to come, arrive, become”
(Av. ay- “to go, to come,” aēiti “goes;” O.Pers. aitiy “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go”).

The fact of repeating or duplicity.
That part of the data content that can be dropped without any significant loss of the original information.

Etymology (EN): From L. redundantia “an overflowing, excess,” from redundare “to flow back, overflow, be excessive,” from
→ re- “again” + undare “rise in waves,” from unda “a wave.”

Etymology (PE): Afzun-âyi “redundancy, superabounding,” from afzun “more, greater; more ample,” from afzudan “to add, increase” (Mid.Pers. abzudan “to increase, grow;” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “press forward;” Av. gav- “to hasten, drive;” Sk. jav- “to press forward, impel quickly, excite,” javate “hastens”) + âyi verbal noun of ây- present stem of âmadan “to come, arrive, become”
(Av. ay- “to go, to come,” aēiti “goes;” O.Pers. aitiy “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go”).

The → Sachs-Wolfe effect in which the calculations are extended to nonlinear mass concentrations. In the non-linear regime of large-scale → structure formation the → gravitational potential changes with time, and photons climb out of a → potential well slightly different from the one that they fell into. Therefore, nonlinear density fluctuations produce extra evolution of the potentials against the background expansion. On large scales, the nonlinear contribution to the full ISW effect is expected to be dominated by the linear ISW effect in a Universe with → cosmological constant (Seljak, 1996, ApJ 460, 549).

See also: Martin J. Rees (1942-) & Dennis W. Sciama (1926-1999), 1968, Nature 217, 511; → effect.

The → Sachs-Wolfe effect in which the calculations are extended to nonlinear mass concentrations. In the non-linear regime of large-scale → structure formation the → gravitational potential changes with time, and photons climb out of a → potential well slightly different from the one that they fell into. Therefore, nonlinear density fluctuations produce extra evolution of the potentials against the background expansion. On large scales, the nonlinear contribution to the full ISW effect is expected to be dominated by the linear ISW effect in a Universe with → cosmological constant (Seljak, 1996, ApJ 460, 549).

See also: Martin J. Rees (1942-) & Dennis W. Sciama (1926-1999), 1968, Nature 217, 511; → effect.

1. To direct for information or anything required.
   

2. To hand over or submit for information, consideration, decision, etc.
   

3. To have relation; relate.
   

4. To make reference or allusion (Dictionary.com).

Etymology (EN): M.E. referren, from L. referre “to bring back,” from → re- “back” + ferre “carry, bear,” cognate with Pers. bordan “to carry, bear,” as below.

Etymology (PE): Bâzbordan, literally “to bring back,” from bâz “back,” → re- + bordan “to carry, bear” (Mid.Pers. burdan, O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”).

1. To direct for information or anything required.
   

2. To hand over or submit for information, consideration, decision, etc.
   

3. To have relation; relate.
   

4. To make reference or allusion (Dictionary.com).

Etymology (EN): M.E. referren, from L. referre “to bring back,” from → re- “back” + ferre “carry, bear,” cognate with Pers. bordan “to carry, bear,” as below.

Etymology (PE): Bâzbordan, literally “to bring back,” from bâz “back,” → re- + bordan “to carry, bear” (Mid.Pers. burdan, O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”).

1. An act or instance of referring.
   

2. A book, article, or passage to which one is directed.

See also: Verbal noun of → refer.

1. An act or instance of referring.
   

2. A book, article, or passage to which one is directed.

See also: Verbal noun of → refer.

A mathematically defined surface that approximates the Earth’s shape, which is basically a sphere “flattened” at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level. It is the first-order definition of the non-spherical shape of the Earth as an ellipsoid of revolution. To first order, it accounts for over 90% of the → geoid.

See also: → reference; → ellipsoid.

A mathematically defined surface that approximates the Earth’s shape, which is basically a sphere “flattened” at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level. It is the first-order definition of the non-spherical shape of the Earth as an ellipsoid of revolution. To first order, it accounts for over 90% of the → geoid.

See also: → reference; → ellipsoid.

A set of axes to which positions and motions in a system can be referred. Same as → frame of reference.

See also: → reference; → frame.

A set of axes to which positions and motions in a system can be referred. Same as → frame of reference.

See also: → reference; → frame.

An astronomical source in a field used as a reference for the detection of another object, astrometry, etc.

See also: → reference; → source.

An astronomical source in a field used as a reference for the detection of another object, astrometry, etc.

See also: → reference; → source.

1. To bring to a fine or a pure state; free from impurities.
   

2. To make more fine, subtle, or precise (Dictionary.com).

Etymology (EN): → re-; → fine.

Etymology (PE): 1) Pâludan “to purify, starin, → filter.”

2. Nâzokidan, from nâzok “tender, delicate, subtle, neat,” → fine.

1. To bring to a fine or a pure state; free from impurities.
   

2. To make more fine, subtle, or precise (Dictionary.com).

Etymology (EN): → re-; → fine.

Etymology (PE): 1) Pâludan “to purify, starin, → filter.”

2. Nâzokidan, from nâzok “tender, delicate, subtle, neat,” → fine.

1. Fineness or elegance of feeling, taste, manners, language, etc.
   

2. The act or process of refining. the quality or state of being refined.
   

3. An improved, higher, or extreme form of something.
   Math.: A → set whose elements include the elements of a given set (Dictionary.com).

See also: Verbal noun of → refine.

1. Fineness or elegance of feeling, taste, manners, language, etc.
   

2. The act or process of refining. the quality or state of being refined.
   

3. An improved, higher, or extreme form of something.
   Math.: A → set whose elements include the elements of a given set (Dictionary.com).

See also: Verbal noun of → refine.

Any of a series of features occurring in the → light curve of → dwarf novae and → Soft X-ray Transient (SXT)s during → outburst decay. Reflares appear when the surface density Σ behind the cooling front is high enough to reach Σ_max. At the radius at which this happens, the disk becomes thermally unstable and a new heating front develops. This front propagates outward like an inside-out outburst, reheating the disk until Σ(R) ≤ Σ_min,
when cooling can resume. The density in the cold region is depleted as matter is accreted during this process, and the following reflare occurs at smaller radii and have lower amplitudes (G. Dubus et al., 2001, A&A 373, 251).

See also: → re-; → flare.

Any of a series of features occurring in the → light curve of → dwarf novae and → Soft X-ray Transient (SXT)s during → outburst decay. Reflares appear when the surface density Σ behind the cooling front is high enough to reach Σ_max. At the radius at which this happens, the disk becomes thermally unstable and a new heating front develops. This front propagates outward like an inside-out outburst, reheating the disk until Σ(R) ≤ Σ_min,
when cooling can resume. The density in the cold region is depleted as matter is accreted during this process, and the following reflare occurs at smaller radii and have lower amplitudes (G. Dubus et al., 2001, A&A 373, 251).

See also: → re-; → flare.

To throw or bend back from a surface, specially light, sound, or heat.

Etymology (EN): M.E. reflecten, from L. reflectere “to bend back,” from → re- “back” + flectere “to bend.”

Etymology (PE): Bâztâbidan, from bâz-, → re- + tâbidan, variants tâftan “to shine,” tafsidan “to become hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian t’b “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “; to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

To throw or bend back from a surface, specially light, sound, or heat.

Etymology (EN): M.E. reflecten, from L. reflectere “to bend back,” from → re- “back” + flectere “to bend.”

Etymology (PE): Bâztâbidan, from bâz-, → re- + tâbidan, variants tâftan “to shine,” tafsidan “to become hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian t’b “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “; to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

1. Same as → reflection factor.
   
2. Computers: In optical character recognition, the relative value of the ink color compared to the background.

Etymology (EN): From → reflect + -ance

Etymology (PE): Bâztâbâyi, verbal noun from adj./agent noun bâztâbâ “reflecting.”

1. Same as → reflection factor.
   
2. Computers: In optical character recognition, the relative value of the ink color compared to the background.

Etymology (EN): From → reflect + -ance

Etymology (PE): Bâztâbâyi, verbal noun from adj./agent noun bâztâbâ “reflecting.”

A → light ray that is reflected from a surface.

See also: → reflect; → ray.

A → light ray that is reflected from a surface.

See also: → reflect; → ray.

An instrument for measuring angular distances, based on the same principle as the → octant, but with a full circular limb divided into 720°. It was invented in 1752 by the German astronomer Johann Tobias Mayer (1723-1762) to improve on the octant which often gave wrong results because of incorrect graduations. The instrument consisted of an index arm and a small telescope, both pivoted centrally. In practice, the index arm is first set to zero, and the telescope rotated until the two images of a star are seen in coincidence (the one directly, the other by double reflection). Then the index arm is freed, and rotated until the other object is seen in coincidence after double reflection. The angle has now been measured, but the double operation is repeated several times, and the final angle divided by the number of repetitions to find a mean value. Hence, the instrument was sometimes called a “repeating circle.” The reflecting circle had little success because it was heavy and uncomfortable to use. Its improved form is called → Borda circle.

See also: → reflection; → circle.

An instrument for measuring angular distances, based on the same principle as the → octant, but with a full circular limb divided into 720°. It was invented in 1752 by the German astronomer Johann Tobias Mayer (1723-1762) to improve on the octant which often gave wrong results because of incorrect graduations. The instrument consisted of an index arm and a small telescope, both pivoted centrally. In practice, the index arm is first set to zero, and the telescope rotated until the two images of a star are seen in coincidence (the one directly, the other by double reflection). Then the index arm is freed, and rotated until the other object is seen in coincidence after double reflection. The angle has now been measured, but the double operation is repeated several times, and the final angle divided by the number of repetitions to find a mean value. Hence, the instrument was sometimes called a “repeating circle.” The reflecting circle had little success because it was heavy and uncomfortable to use. Its improved form is called → Borda circle.

See also: → reflection; → circle.

A telescope in which the image is produced by reflection of light by a concave mirror.

See also: Reflecting, verbal adj. of → reflect; → telescope.

A telescope in which the image is produced by reflection of light by a concave mirror.

See also: Reflecting, verbal adj. of → reflect; → telescope.

The return of radiation after striking a surface, without change in wavelength. If the surface is smooth, reflection is regular, otherwise it is diffuse.
→ diffuse reflection; → specular reflection.

See also: Verbal noun of → reflect.

The return of radiation after striking a surface, without change in wavelength. If the surface is smooth, reflection is regular, otherwise it is diffuse.
→ diffuse reflection; → specular reflection.

See also: Verbal noun of → reflect.

The ratio given by the → amplitude (or energy) of a reflected wave divided by the amplitude (or energy) of the incident wave.

See also: → reflection; → coefficient.

The ratio given by the → amplitude (or energy) of a reflected wave divided by the amplitude (or energy) of the incident wave.

See also: → reflection; → coefficient.

The ratio of total flux that is reflected from a surface to the incident flux. Also called reflectance, reflectivity.

See also: → reflection; → factor.

The ratio of total flux that is reflected from a surface to the incident flux. Also called reflectance, reflectivity.

See also: → reflection; → factor.

A type of nebula that is visible from its reflection of starlight. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color.

See also: → reflection; → nebula.

A type of nebula that is visible from its reflection of starlight. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color.

See also: → reflection; → nebula.

That reflects; reflecting. Of or pertaining to reflection.

Etymology (EN): From → reflect + -ive a suffix of adjectives and nouns of adjectival origin.

Etymology (PE): Bâztâbi adj. of bâztâb, → reflection.

That reflects; reflecting. Of or pertaining to reflection.

Etymology (EN): From → reflect + -ive a suffix of adjectives and nouns of adjectival origin.

Etymology (PE): Bâztâbi adj. of bâztâb, → reflection.

Same as → reflectance; → reflection factor.

See also: From → reflective + → -ity.

Same as → reflectance; → reflection factor.

See also: From → reflective + → -ity.

1. A → reflecting telescope.
   
2. A type of conducting surface or material used to → reflect radiant energy.

See also: From reflect, → reflection, + → -or.

1. A → reflecting telescope.
   
2. A type of conducting surface or material used to → reflect radiant energy.

See also: From reflect, → reflection, + → -or.

1. (n.) The improvement, amendment, or reorganization of something that is considered to be wrong, ineffective, or unsatisfactory; e.g. calendar reform.
   

2. (v.tr.) To change and improve by correction of faults, alteration, substitution, abolition, etc.

Etymology (EN): From M.E. reformen, from M.Fr. reformer, from O.Fr., from L. reformare “to form again, change, alter,” from → re- “again” + formare “to form,” from forma “form, mold, shape, case,” origin unknown.

Etymology (PE): 1) Bâzdisi, from bâz- “again,” → re-, + dis, disé “form, appearance,” (variants -diz, -diš (tandis “body form, like a body; effigy; statute;” mâhdis “moon-like;” šabdiz “night color; a horse of dark rusty color;” andiš- “to think, contemplate”); Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Av. daēs- “to show,” daēsa- “sign, omen;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”)

• -i noun suffix.
  

2. Bâzdisidan infinitive from bâzdisi, as above.

1. (n.) The improvement, amendment, or reorganization of something that is considered to be wrong, ineffective, or unsatisfactory; e.g. calendar reform.
   

2. (v.tr.) To change and improve by correction of faults, alteration, substitution, abolition, etc.

Etymology (EN): From M.E. reformen, from M.Fr. reformer, from O.Fr., from L. reformare “to form again, change, alter,” from → re- “again” + formare “to form,” from forma “form, mold, shape, case,” origin unknown.

Etymology (PE): 1) Bâzdisi, from bâz- “again,” → re-, + dis, disé “form, appearance,” (variants -diz, -diš (tandis “body form, like a body; effigy; statute;” mâhdis “moon-like;” šabdiz “night color; a horse of dark rusty color;” andiš- “to think, contemplate”); Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Av. daēs- “to show,” daēsa- “sign, omen;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”)

• -i noun suffix.
  

2. Bâzdisidan infinitive from bâzdisi, as above.

To alter the course of a wave of light, sound, etc. when the wave crosses the boundary between this medium and another through which it travels at a different speed.

Etymology (EN): From L. refractus, p.p. of refringere, → refraction.

Etymology (PE): Verb of šekast, → refraction.

To alter the course of a wave of light, sound, etc. when the wave crosses the boundary between this medium and another through which it travels at a different speed.

Etymology (EN): From L. refractus, p.p. of refringere, → refraction.

Etymology (PE): Verb of šekast, → refraction.

A → light ray that undergoes a change of velocity and direction, as a result of interaction with the material medium in which it travels.

See also: → reflect; → ray.

A → light ray that undergoes a change of velocity and direction, as a result of interaction with the material medium in which it travels.

See also: → reflect; → ray.

1. The action or result of the verb → refract.
   
2. Participial adjective of → refract.

See also: From → refract + → -ing.

1. The action or result of the verb → refract.
   
2. Participial adjective of → refract.

See also: From → refract + → -ing.

A prism that is used as a dispersing element in a spectrograph.

See also: → refracting; → prism.

A prism that is used as a dispersing element in a spectrograph.

See also: → refracting; → prism.

A telescope in which an image is formed by the refraction of light through a lens or lens system.

See also: → refracting; → telescope.

A telescope in which an image is formed by the refraction of light through a lens or lens system.

See also: → refracting; → telescope.

1. Optics: The change of direction which a beam of light undergoes as it enters a medium of different → refractive index.
   
2. Acoustics: The change in direction in sound waves on reaching the boundary between two media.
   
3. Seismology: The bending of a seismic wave as it enters a material of different density.

Etymology (EN): From L.L. refractionem (nominative refractio) “a breaking up,” from L. refractus, p.p. of refringere “to break up,” from re- “back” + combining form of frangere “to break,” from PIE base *bhreg- “to break” (cf. Goth. brikan, O.E. brecan “to break;” Lith. brasketi
“crash, crack”).

Etymology (PE): Šekast, past stem of šekastan “to break, split;” Mid.Pers. škastan “to break;” Av. scind-, scand “to beak, cleave;”
Proto-Iranian *skand- “to break, cleave;” PIE sken- “to cut off.”

1. Optics: The change of direction which a beam of light undergoes as it enters a medium of different → refractive index.
   
2. Acoustics: The change in direction in sound waves on reaching the boundary between two media.
   
3. Seismology: The bending of a seismic wave as it enters a material of different density.

Etymology (EN): From L.L. refractionem (nominative refractio) “a breaking up,” from L. refractus, p.p. of refringere “to break up,” from re- “back” + combining form of frangere “to break,” from PIE base *bhreg- “to break” (cf. Goth. brikan, O.E. brecan “to break;” Lith. brasketi
“crash, crack”).

Etymology (PE): Šekast, past stem of šekastan “to break, split;” Mid.Pers. škastan “to break;” Av. scind-, scand “to beak, cleave;”
Proto-Iranian *skand- “to break, cleave;” PIE sken- “to cut off.”

Of any optical medium, the ratio of the → speed of light in vacuum (c) to that in the medium (v): n = c/v. The refractive index for vacuum, by definition, is 1. The refractive index of air is 1.00029 at standard temperature (25 °C) and pressure (1 atm). The refractive index of a medium depends on the wavelength of refracted wave. With light waves, n increases as the wavelength decreases. → Snell’s law can be used to derive n.
Same as → index of refraction.

See also: Refractive, pertaining to → refraction; → index.

Of any optical medium, the ratio of the → speed of light in vacuum (c) to that in the medium (v): n = c/v. The refractive index for vacuum, by definition, is 1. The refractive index of air is 1.00029 at standard temperature (25 °C) and pressure (1 atm). The refractive index of a medium depends on the wavelength of refracted wave. With light waves, n increases as the wavelength decreases. → Snell’s law can be used to derive n.
Same as → index of refraction.

See also: Refractive, pertaining to → refraction; → index.

Same as → refracting telescope.

See also: Agent noun of refract, → refraction.

Same as → refracting telescope.

See also: Agent noun of refract, → refraction.

A substance that has a high → melting point. It is therefore resistant to treatment and not easily melted or worked. The opposite of → volatile.