An Etymological Dictionary of Astronomy and Astrophysics

A loosely bound grouping of O and B stars that typically stretches up to several hundred → light-years and may contain between a dozen and several hundred → O stars
and → B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.

See also: O and B, from spectral types; → association.

A loosely bound grouping of O and B stars that typically stretches up to several hundred → light-years and may contain between a dozen and several hundred → O stars
and → B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.

See also: O and B, from spectral types; → association.

A collective designation for massive O and B stars.

See also: → O star; → B star.

A collective designation for massive O and B stars.

See also: → O star; → B star.

A hot star belonging to one of the two groups of → EHB stars or → pre-WD stars.

See also: → OB star; → subdwarf.

A hot star belonging to one of the two groups of → EHB stars or → pre-WD stars.

See also: → OB star; → subdwarf.

A → carbon (C)-rich → O-type or → B-type star.

See also: → O star; → B star; → carbon; → star.

A → carbon (C)-rich → O-type or → B-type star.

See also: → O star; → B star; → carbon; → star.

The symbol ÷ used to indicate → division. This symbol was first used as a → division sign by Johann Rahn (or Rhonius) (1622-1676) in 1659 in Teutsche Algebra.

Etymology (EN): From Gk. obelus “sharpened stick, spit, pointed pillar,” related to obelisk, originally used in ancient manuscripts to mark passages that were suspected of being corrupted, doubtful, or spurious.

Etymology (PE): → division sign.

The symbol ÷ used to indicate → division. This symbol was first used as a → division sign by Johann Rahn (or Rhonius) (1622-1676) in 1659 in Teutsche Algebra.

Etymology (EN): From Gk. obelus “sharpened stick, spit, pointed pillar,” related to obelisk, originally used in ancient manuscripts to mark passages that were suspected of being corrupted, doubtful, or spurious.

Etymology (PE): → division sign.

The outermost of Uranus’ large satellites and the second largest. It has a diameter of 523 km and orbits 583,420 km from its planet.
Compared to Uranus’ moons Ariel, Titania, and Miranda, Oberon is heavily cratered.
Like all of Uranus’ large moons, Oberon is composed of roughly half ice and half rock. Oberon was discovered by Herschel in 1787.

See also: Oberon is the King of the Fairies and husband of Titania in Shakespeare’s Midsummer-Night’s Dream.

The outermost of Uranus’ large satellites and the second largest. It has a diameter of 523 km and orbits 583,420 km from its planet.
Compared to Uranus’ moons Ariel, Titania, and Miranda, Oberon is heavily cratered.
Like all of Uranus’ large moons, Oberon is composed of roughly half ice and half rock. Oberon was discovered by Herschel in 1787.

See also: Oberon is the King of the Fairies and husband of Titania in Shakespeare’s Midsummer-Night’s Dream.

1. General: Anything that is visible or tangible.
   

2. Physics: A collection of masses, taken to be one.
   

3. Optics: The collection of points that acts as a source of light rays for an optical system.
   

4. Astro.: A general designation for material bodies studied in this science.
   

5. Math.: Anything that could be formally defined, and with which one may do deductive reasoning and mathematical proofs. → mathematical object
   

6. Computer science: A free standing chunk of code that defines the properties of some thing.
   

7. Grammar: In English grammar, a noun, noun phrase, or pronoun that is affected by the action of a verb (a direct object or an indirect object) or that completes the meaning of a preposition (the object of a preposition).
   

8. To raise or state an argument against.

Etymology (EN): From M.L. objectum “thing thrown down or put before” (the mind or sight), neutral of objectus, p.p. of obicere “to present, oppose, cast in the way of,” from ob “against” + jacere “to throw,” from PIE base *ye- “to do” (cf. Gk. iemi, ienai “to send, throw,” Hitt. ijami “I make”).

Etymology (PE): Barâxt “thing drawn against, before” from bar- + âxt. The prefix bar- “on; upon; against; before; at; in,” from Mid.Pers. abar, O.Pers. upariy “above; over, upon, according to,” Av. upairi “above, over” (upairi.zəma- “located above the earth”), cf. Gk. hyper- “over, above,” L. super-, O.H.G. ubir “over;” PIE base *uper “over”.
The second component âxt, contraction of âxté, p.p. of âxtan, variants âhixtan, âhiz- “to draw (a sword),” âhanjidan “to draw up, pull, extract,” Mid.Pers. âhixtan, âhanjitan “to draw out, pull up, extract,” Av. θanj- “to draw, pull, drive;” Proto-Iranian
*θanj-.
Kondâr, literally “action receiver,” from kon- present stem of kardan “to do, make,” → -or, used as action noun (like xor, xâb, foruš),

• dâr agent noun from dâštan “to receive” (Dehxodâ), originally “to have, hold, possess,” → property.

1. General: Anything that is visible or tangible.
   

2. Physics: A collection of masses, taken to be one.
   

3. Optics: The collection of points that acts as a source of light rays for an optical system.
   

4. Astro.: A general designation for material bodies studied in this science.
   

5. Math.: Anything that could be formally defined, and with which one may do deductive reasoning and mathematical proofs. → mathematical object
   

6. Computer science: A free standing chunk of code that defines the properties of some thing.
   

7. Grammar: In English grammar, a noun, noun phrase, or pronoun that is affected by the action of a verb (a direct object or an indirect object) or that completes the meaning of a preposition (the object of a preposition).
   

8. To raise or state an argument against.

Etymology (EN): From M.L. objectum “thing thrown down or put before” (the mind or sight), neutral of objectus, p.p. of obicere “to present, oppose, cast in the way of,” from ob “against” + jacere “to throw,” from PIE base *ye- “to do” (cf. Gk. iemi, ienai “to send, throw,” Hitt. ijami “I make”).

Etymology (PE): Barâxt “thing drawn against, before” from bar- + âxt. The prefix bar- “on; upon; against; before; at; in,” from Mid.Pers. abar, O.Pers. upariy “above; over, upon, according to,” Av. upairi “above, over” (upairi.zəma- “located above the earth”), cf. Gk. hyper- “over, above,” L. super-, O.H.G. ubir “over;” PIE base *uper “over”.
The second component âxt, contraction of âxté, p.p. of âxtan, variants âhixtan, âhiz- “to draw (a sword),” âhanjidan “to draw up, pull, extract,” Mid.Pers. âhixtan, âhanjitan “to draw out, pull up, extract,” Av. θanj- “to draw, pull, drive;” Proto-Iranian
*θanj-.
Kondâr, literally “action receiver,” from kon- present stem of kardan “to do, make,” → -or, used as action noun (like xor, xâb, foruš),

• dâr agent noun from dâštan “to receive” (Dehxodâ), originally “to have, hold, possess,” → property.

The → lens or lenses in the → object end of the body tube of a → microscope, by means of which the rays coming from the object examined are brought to a focus. Same as → objective.

An old term for the → objective lens of a → refracting telescope.

See also: → object; → glass.

The → lens or lenses in the → object end of the body tube of a → microscope, by means of which the rays coming from the object examined are brought to a focus. Same as → objective.

An old term for the → objective lens of a → refracting telescope.

See also: → object; → glass.

Any language described by a → metalanguage. For example, the sentence “In Persian, the word setâré means “star” " is part of a metalanguage (here, English), and the language described (namely Persian) is an object language. Metalanguage and object language may be identical.

See also: → object; → language.

Any language described by a → metalanguage. For example, the sentence “In Persian, the word setâré means “star” " is part of a metalanguage (here, English), and the language described (namely Persian) is an object language. Metalanguage and object language may be identical.

See also: → object; → language.

In an → optical system, the space between the object being viewed and the → entrance pupil. See also → image space.

See also: → object; → space.

In an → optical system, the space between the object being viewed and the → entrance pupil. See also → image space.

See also: → object; → space.

In computer science a programming technique that uses → objects and their interactions to design applications and programs.

Etymology (EN): → object; oriented, p.p. from the verb of → orientation; → programming.

Etymology (PE): Barnâme-sâzi, → programming; barâxt,
→ object; gerâ agent adj. of gerâyidan “to incline toward; to intend; to make for.”
The stem gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

In computer science a programming technique that uses → objects and their interactions to design applications and programs.

Etymology (EN): → object; oriented, p.p. from the verb of → orientation; → programming.

Etymology (PE): Barnâme-sâzi, → programming; barâxt,
→ object; gerâ agent adj. of gerâyidan “to incline toward; to intend; to make for.”
The stem gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

1. The act of objecting.
   

   2. A statement presented in opposition.

See also: Verbal noun of → object.

1. The act of objecting.
   

   2. A statement presented in opposition.

See also: Verbal noun of → object.

1a) Of or pertaining to something that can exist independent of thought or perception as part of reality. Opposite of → subjective.

1b) Free of any bias or prejudice caused by personal feelings.

2. The → lens or combination of lenses nearest the → object in an optical instrument ( → telescope, → microscope, → camera).
   

3. The → primary mirror of a → reflecting telescope.
   

4. Grammar: Relating to nouns, noun phrases, or pronouns that are the objects of verbs or prepositions.

See also: Adjective of → object.

1a) Of or pertaining to something that can exist independent of thought or perception as part of reality. Opposite of → subjective.

1b) Free of any bias or prejudice caused by personal feelings.

2. The → lens or combination of lenses nearest the → object in an optical instrument ( → telescope, → microscope, → camera).
   

3. The → primary mirror of a → reflecting telescope.
   

4. Grammar: Relating to nouns, noun phrases, or pronouns that are the objects of verbs or prepositions.

See also: Adjective of → object.

A diffraction grating placed over the aperture of a telescope in order to produce spectra of all the objects in the field of view.

See also: → objective; → grating.

A diffraction grating placed over the aperture of a telescope in order to produce spectra of all the objects in the field of view.

See also: → objective; → grating.

The lens or system of lenses in a → telescope or → microscope that magnify an object and project a larger → image. The object lens is nearest the object being viewed

See also: → objective; → lens.

The lens or system of lenses in a → telescope or → microscope that magnify an object and project a larger → image. The object lens is nearest the object being viewed

See also: → objective; → lens.

A dispersing prism placed in front of a telescope
objective to produce spectra of all luminous objects in the field of view.

See also: → objective; → prism.

A dispersing prism placed in front of a telescope
objective to produce spectra of all luminous objects in the field of view.

See also: → objective; → prism.

A probability determined by the long-term frequency of an event; in contrast to subjective probability.

See also: → objective; → probability.

A probability determined by the long-term frequency of an event; in contrast to subjective probability.

See also: → objective; → probability.

1. The state or quality of being → objective. Opposite of → subjectivity.
   

2. Ability to view things objectively.
   

3. External reality.

See also: → objective + → -ity.

1. The state or quality of being → objective. Opposite of → subjectivity.
   

2. Ability to view things objectively.
   

3. External reality.

See also: → objective + → -ity.

Adjective meaning flattened, as → oblate spheroid. Opposed to → prolate.

Etymology (EN): From L. ob- “toward” + latus, abstracted from its opposite, prolatus “lengthened.”

Etymology (PE): Pax “oblate,” of unknown etymology.

Adjective meaning flattened, as → oblate spheroid. Opposed to → prolate.

Etymology (EN): From L. ob- “toward” + latus, abstracted from its opposite, prolatus “lengthened.”

Etymology (PE): Pax “oblate,” of unknown etymology.

An ellipsoid produced by rotating an ellipse through 360° about its minor axis. Compare with → prolate spheroid.

See also: → oblate; → spheroid.

An ellipsoid produced by rotating an ellipse through 360° about its minor axis. Compare with → prolate spheroid.

See also: → oblate; → spheroid.

The property possessed by a round body that is flattened at the poles.

See also: Noun from → oblate.

The property possessed by a round body that is flattened at the poles.

See also: Noun from → oblate.

1a) To bind or oblige morally or legally.

1b) To pledge, commit, or bind (funds, property, etc.) to meet an obligation.

2a) Morally or legally bound; obliged; constrained.

2b) Necessary; essential (Dictionary.com).

See also: Back-formation from → obligation.

1a) To bind or oblige morally or legally.

1b) To pledge, commit, or bind (funds, property, etc.) to meet an obligation.

2a) Morally or legally bound; obliged; constrained.

2b) Necessary; essential (Dictionary.com).

See also: Back-formation from → obligation.

1. Something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
   

2. Something that is done or is to be done for such reasons (Dictionary.com).

Etymology (EN): M.E. obligacioun, from O.Fr. obligacion “obligation, duty, responsibility,” from L. obligationem “an engaging or pledging,” literally “a binding,” noun of action from p.p. stem of obligare “to bind, bind up, bandage,” from → ob- “to” + ligare “to bind,” from PIE root *leig- “to bind.”

Etymology (PE): Fariz, from Mid.Pers. frêz “obligation; duty.”

1. Something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
   

2. Something that is done or is to be done for such reasons (Dictionary.com).

Etymology (EN): M.E. obligacioun, from O.Fr. obligacion “obligation, duty, responsibility,” from L. obligationem “an engaging or pledging,” literally “a binding,” noun of action from p.p. stem of obligare “to bind, bind up, bandage,” from → ob- “to” + ligare “to bind,” from PIE root *leig- “to bind.”

Etymology (PE): Fariz, from Mid.Pers. frêz “obligation; duty.”

Required as a matter of obligation; mandatory; incumbent; compulsory.

Etymology (EN): M.E., from L.L. obligatorius “binding,” → obligate, + -tory, from L. -torius, from → -tor + -ius.

Etymology (PE): Farizgun, from fariz, → duty, + -gun “manner, fashion.”

Required as a matter of obligation; mandatory; incumbent; compulsory.

Etymology (EN): M.E., from L.L. obligatorius “binding,” → obligate, + -tory, from L. -torius, from → -tor + -ius.

Etymology (PE): Farizgun, from fariz, → duty, + -gun “manner, fashion.”

Neither perpendicular nor horizontal; having the axis not perpendicular to the base; slanting; sloping.

Etymology (EN): From M.Fr. oblique, from L. obliquus “slanting, sidelong, indirect,” from ob “against” + root of licinus “(of an ox whose corn tips are) bent upward,” from PIE base *lei- “to bend, be movable.”

Etymology (PE): Yekvar, literally “slantingn to a side,” from yek-, → one,

• var “side,” variant
  bar “side; breadth; breast” (Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s“wide, broad”).

Neither perpendicular nor horizontal; having the axis not perpendicular to the base; slanting; sloping.

Etymology (EN): From M.Fr. oblique, from L. obliquus “slanting, sidelong, indirect,” from ob “against” + root of licinus “(of an ox whose corn tips are) bent upward,” from PIE base *lei- “to bend, be movable.”

Etymology (PE): Yekvar, literally “slantingn to a side,” from yek-, → one,

• var “side,” variant
  bar “side; breadth; breast” (Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s“wide, broad”).

A stellar model in which the rotation axis is tilted relative to the magnetic dipole axis. As the star rotates, observable quantities (e.g. the line-of-sight component of the magnetic field, stellar brightness, emission lines) are modulated according to the rotational period. Such a model was first put forward by D. Stibbs (1950, MNRAS, 110, 395).

See also: → oblique; → rotator; → model.

A stellar model in which the rotation axis is tilted relative to the magnetic dipole axis. As the star rotates, observable quantities (e.g. the line-of-sight component of the magnetic field, stellar brightness, emission lines) are modulated according to the rotational period. Such a model was first put forward by D. Stibbs (1950, MNRAS, 110, 395).

See also: → oblique; → rotator; → model.

A → shock wave that is inclined to the flow direction. Depending on the shape of the object and the speed of the → flow, the shock wave may be inclined to the flow direction.

See also: → oblique; → shock.

A → shock wave that is inclined to the flow direction. Depending on the shape of the object and the speed of the → flow, the shock wave may be inclined to the flow direction.

See also: → oblique; → shock.

The celestial sphere when the circles parallel to the equator are cut
obliquely by the horizon plane, which divides them into two unequal parts.
In other words, the sphere when its axis is oblique to the horizon of the place.

See also: → oblique; → sphere.

The celestial sphere when the circles parallel to the equator are cut
obliquely by the horizon plane, which divides them into two unequal parts.
In other words, the sphere when its axis is oblique to the horizon of the place.

See also: → oblique; → sphere.

1. General: The quality or state of being oblique.
   

2. The → angle that an object’s → rotational axis makes with its → orbital axis. In other words, the angle between its → equatorial plane and → orbital plane. Same as → axial tilt.

See also: State noun of → oblique.

1. General: The quality or state of being oblique.
   

2. The → angle that an object’s → rotational axis makes with its → orbital axis. In other words, the angle between its → equatorial plane and → orbital plane. Same as → axial tilt.

See also: State noun of → oblique.

The angle between the Earth’s → equatorial plane and the → ecliptic. Its present value is 23°26'13.5’' (or 23.43708 degrees). The effects of → precession and → nutation cause it to change between extreme values of 22.1 and 24.5 degrees over a period of approximately 41,000 years. It is currently decreasing.

See also: → obliquity; → ecliptic.

The angle between the Earth’s → equatorial plane and the → ecliptic. Its present value is 23°26'13.5’' (or 23.43708 degrees). The effects of → precession and → nutation cause it to change between extreme values of 22.1 and 24.5 degrees over a period of approximately 41,000 years. It is currently decreasing.

See also: → obliquity; → ecliptic.

Elongated, usually from the square or circular form.

Etymology (EN): Late M.E. oblonge, from L. oblongus “rather long,” from ob- a prefix meaning “toward, to, on, over, against”

• longus, → long.

Etymology (PE): Derâzak, from derâz “long,” → length + -ak suffix.

Elongated, usually from the square or circular form.

Etymology (EN): Late M.E. oblonge, from L. oblongus “rather long,” from ob- a prefix meaning “toward, to, on, over, against”

• longus, → long.

Etymology (PE): Derâzak, from derâz “long,” → length + -ak suffix.

A → nitrogen (N)-rich → O-type or → B-type star.

See also: → O star; → B star; → nitrogen; → star.

A → nitrogen (N)-rich → O-type or → B-type star.

See also: → O star; → B star; → nitrogen; → star.

1. Opposition to the spread of knowledge; a policy of withholding knowledge from the general public.
   

   2. A style (as in literature or art) characterized by deliberate vagueness or abstruseness (Merriam-Webster.com).

See also: From Fr. obscurantisme (1819), from obscurant, from obscure, from L. obscurus “dark, dusky,” figuratively “unknown; unintelligible,” from ob- “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” → -ism.

1. Opposition to the spread of knowledge; a policy of withholding knowledge from the general public.
   

   2. A style (as in literature or art) characterized by deliberate vagueness or abstruseness (Merriam-Webster.com).

See also: From Fr. obscurantisme (1819), from obscurant, from obscure, from L. obscurus “dark, dusky,” figuratively “unknown; unintelligible,” from ob- “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” → -ism.

1. The act of obscuring.
   

2. The state of being obscured.

See also: → obscure; → -tion.

1. The act of obscuring.
   

2. The state of being obscured.

See also: → obscure; → -tion.

1a) Lacking in light or illumination; dark; dim; murky.

1b) Indistinct to the sight or any other sense.

1c) Not clear to the understanding.

2. To make dark, dim, indistinct, etc.

Etymology (EN): From O.Fr. obscur “dark, dim, not clear,” from L. obscurus “covered over, dark, obscure,” from ob “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” from which derives also the term → sky.

Etymology (PE): Tiré, from Mid.Pers. târag “dark, turbid,” related to târik “dark,” Mid.Pers. târig “dark,” târ “darkness,” Av. taθra- “darkness,” taθrya- “dark,” cf. Skt. támisrâ- “darkness, dark night,” L. tenebrae “darkness,” Hittite taš(u)uant- “blind,” O.H.G. demar “twilight.”
Tiré kardan compound verb with kardan “to do,” → -ize.

1a) Lacking in light or illumination; dark; dim; murky.

1b) Indistinct to the sight or any other sense.

1c) Not clear to the understanding.

2. To make dark, dim, indistinct, etc.

Etymology (EN): From O.Fr. obscur “dark, dim, not clear,” from L. obscurus “covered over, dark, obscure,” from ob “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” from which derives also the term → sky.

Etymology (PE): Tiré, from Mid.Pers. târag “dark, turbid,” related to târik “dark,” Mid.Pers. târig “dark,” târ “darkness,” Av. taθra- “darkness,” taθrya- “dark,” cf. Skt. támisrâ- “darkness, dark night,” L. tenebrae “darkness,” Hittite taš(u)uant- “blind,” O.H.G. demar “twilight.”
Tiré kardan compound verb with kardan “to do,” → -ize.

A structure of dust and gas postulated to surround the central → black hole in an → active galactic nucleus (AGN). The presence of an obscuring torus allows the unification of the two main types of AGNs containing a → broad-line region (Type I) and a → narrow-line region (Type II), respectively. In this unified model, the two types represent the same sort of object, the appearance of which depends on the viewer’s → line of sight.
The best evidence for this model comes from spectropolarimetry observations of some type II AGNs in which broad → emission lines are seen in → polarized light, as would happen if the broad-line region truly were hidden, and the light were being reflected off the torus and into the viewer’s line of sight.

See also: → obscure; → torus.

A structure of dust and gas postulated to surround the central → black hole in an → active galactic nucleus (AGN). The presence of an obscuring torus allows the unification of the two main types of AGNs containing a → broad-line region (Type I) and a → narrow-line region (Type II), respectively. In this unified model, the two types represent the same sort of object, the appearance of which depends on the viewer’s → line of sight.
The best evidence for this model comes from spectropolarimetry observations of some type II AGNs in which broad → emission lines are seen in → polarized light, as would happen if the broad-line region truly were hidden, and the light were being reflected off the torus and into the viewer’s line of sight.

See also: → obscure; → torus.

The state or quality of being obscure.

See also: → obscure + → -ity.

The state or quality of being obscure.

See also: → obscure + → -ity.

1. General: The character of something that can be observed.
   

2. Astro.: The possibility that an object be observable at a particular time.
   

3. Control theory: A measure for how well internal states of a system can be inferred by knowledge of its external outputs.

See also: From → observable + → -ity.

1. General: The character of something that can be observed.
   

2. Astro.: The possibility that an object be observable at a particular time.
   

3. Control theory: A measure for how well internal states of a system can be inferred by knowledge of its external outputs.

See also: From → observable + → -ity.

1. Capable of being or liable to be observed.
   

2. A quantity that can be measured.
   

3. Quantum mechanics: A → real dynamical variable whose → eigenstates form a → complete set.

See also: From → observe + → -able.

1. Capable of being or liable to be observed.
   

2. A quantity that can be measured.
   

3. Quantum mechanics: A → real dynamical variable whose → eigenstates form a → complete set.

See also: From → observe + → -able.

The extent of the Universe that we can see with the aid of the largest telescopes. Its ultimate boundary is determined by the → cosmic horizon size.

See also: → observable; → universe.

The extent of the Universe that we can see with the aid of the largest telescopes. Its ultimate boundary is determined by the → cosmic horizon size.

See also: → observable; → universe.

1. An act or instance of following a custom, rule, or law.
   

2. A customary practice, rite, or ceremony; a rule governing members of a religious order.
   

3. An act or instance of watching (Merriam-Webster.com).

Etymology (EN): → observe; → -ance.

Etymology (PE): Nepâhdâri, on the model of negâhdâri “preservation, protection,” from nepâhdâr, → observant, + -i.

1. An act or instance of following a custom, rule, or law.
   

2. A customary practice, rite, or ceremony; a rule governing members of a religious order.
   

3. An act or instance of watching (Merriam-Webster.com).

Etymology (EN): → observe; → -ance.

Etymology (PE): Nepâhdâri, on the model of negâhdâri “preservation, protection,” from nepâhdâr, → observant, + -i.

1. Paying strict attention.
   

2. Careful in observing (Merriam-Webster.com).

Etymology (EN): → observe + -ant, a suffix forming adjective, → -ance.

Etymology (PE): Nepâhdâr, from nepâh present stem of nepâhidan, → observe, + dâr preset stem and agent noun of dâštan, dâridan “to have, hold, maintain, possess,” → protected band.

1. Paying strict attention.
   

2. Careful in observing (Merriam-Webster.com).

Etymology (EN): → observe + -ant, a suffix forming adjective, → -ance.

Etymology (PE): Nepâhdâr, from nepâh present stem of nepâhidan, → observe, + dâr preset stem and agent noun of dâštan, dâridan “to have, hold, maintain, possess,” → protected band.

1. Act or instance of observing; → observe.
   

2. Careful watching of an astronomical object or event using appropriate instruments, usually for collecting data.

See also: Verbal noun of → observe.

1. Act or instance of observing; → observe.
   

2. Careful watching of an astronomical object or event using appropriate instruments, usually for collecting data.

See also: Verbal noun of → observe.

Pertaining to, or founded on observation, especially based on observation rather than theory.

See also: Adj. of → observation.

Pertaining to, or founded on observation, especially based on observation rather than theory.

See also: Adj. of → observation.

That part of astrophysics that is mainly concerned with the collection of observational data, in comparison with theoretical astrophysics

See also: → observational; → astrophysics.

That part of astrophysics that is mainly concerned with the collection of observational data, in comparison with theoretical astrophysics

See also: → observational; → astrophysics.

An error in observation arising from systematically favoring brighter or weaker objects or some particular object morphologies; e.g. → Malmquist bias.

See also: → observational; → bias.

An error in observation arising from systematically favoring brighter or weaker objects or some particular object morphologies; e.g. → Malmquist bias.

See also: → observational; → bias.

The application of observational data to the study of the Universe as a whole.

See also: → observational; → cosmology.

The application of observational data to the study of the Universe as a whole.

See also: → observational; → cosmology.

A feature appearing in an observation, which is not intrinsic to the object observed, but is due to the inappropriate method used (e.g. limited angular resolution).

See also: → observational; → effect.

A feature appearing in an observation, which is not intrinsic to the object observed, but is due to the inappropriate method used (e.g. limited angular resolution).

See also: → observational; → effect.

The difference between a measured value of quantity and its true value.

See also: → observational; → error.

The difference between a measured value of quantity and its true value.

See also: → observational; → error.

A place or building equipped for making observations of astronomical, meteorological, or other natural phenomena, especially a place provided
with a telescope for observing astronomical objects.

Etymology (EN): From Fr. observatoire, from
L. observa(re), → observe, + -toire, from L. -torius, from -tor a suffix forming agent nouns + -ius adj. suffix.

Etymology (PE): Nepâhešgâh, from nepâheš, → observation,

• -gâh suffix of place (O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode,” PIE *gwem- “to go, come”).

A place or building equipped for making observations of astronomical, meteorological, or other natural phenomena, especially a place provided
with a telescope for observing astronomical objects.

Etymology (EN): From Fr. observatoire, from
L. observa(re), → observe, + -toire, from L. -torius, from -tor a suffix forming agent nouns + -ius adj. suffix.

Etymology (PE): Nepâhešgâh, from nepâheš, → observation,

• -gâh suffix of place (O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode,” PIE *gwem- “to go, come”).

To watch carefully or note for a scientific or special purpose, e.g. to observe a star (astronomy), to observe the behavior of a patient (medicine, psychology), an animal (ethology, zoology), social groups (sociology), etc.

Etymology (EN): From O.Fr. observer, from L. observare “watch over, look to, attend to, guard,” from ob “over” + servare “to watch, keep safe,” from PIE base *ser- “to protect;” cf. Av. har- “to guard, observe, pay attention to,” haraiti “guards, keeps,” harətar- “protector, watcher,” harəθra- “guarding, protection,” hāra- “watched, guarded,” Mod.Pers. zinhâr “beware!, mind!,” Gk. heros “protector, hero.”

Etymology (PE): Note 1: Observation is the most important basis of empirical sciences. All theories rely on observation, and must finally be supported by observational evidence. Persian, in contrast to European languages, lacks a distinct term that recognizes observation as a conceptual premise of sciences. In astronomy the Ar. rasad (رصد) is currently used, while in classical astronomical texts the Pers. pâyidan (پاییدن) is used mainly by prominent figures like Biruni and Tusi. However, the term rasad is never used for cases outside astronomy. The situation for other sciences is not clear. Often one resorts to the Ar. loanword mošâhedé
“to look at each other,” which lacks ability to form derivatives. Therefore, here we suggest a term that can be used in all fields of knowledge.

Nepâhian, from ne- + pâhidan. The prefix ne- “down; into” (as in negâh “look, glance,” negaristan “to view, look at,” nešastan “to sit down,” nemudan “to display”) from O.Pers./Av. ni- “down, into;” cf.
Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz, Du. neder, Ger. nieder;
PIE *ni- “down, below.”
The second component pâhidan, variants pâyidan, pâsidan “to watch, observe, look steadily;” Mid.Pers. pây- “to protect, guard;” Sogdian p’y “to observe, protect, watch over;” O.Pers. pā- “to protect,” pāta- “protected;” Av. pā- “to protect,” pāti “guards,” nipā(y)- (with ni-) “to watch, observe, guard,” nipātar- “protector, watcher,” nipāθri- “protectress;”
cf. Skt. pā- “to protect, keep,” tanû.pā- “protecting the body,” paś.pā- “shepherd;” Gk. poma “lid, cover,” poimen “shepherd;” L. pascere “to put out to graze,” pastor “shepherd;” Lith. piemuo “shepherd;” PIE base *pā- “to protect, feed.”

Note 2: Since pây-, pâyidan is used for → conserve, we adopt the variant pâh-, pâhidan, which is extant in Lâri and
Gerâši dialects; cf. Gilaki pâstan, pâssan “to guard, pay attention.” In fact the interchange of phonemes h, i and s is not rare in Indo-European languages.

Note 3: Examples of pâyidan “to observe” in astronomy, from Biruni’s al-Tafhim (written c. 1029 A.D.)
p. 313: و بپای ارتفاع آ فتاب را.
p. 64: و بپای تا بدایره اندر آید.

Note 4: As indicated above, nepâh- exists in Av. as nipāy- “to watch, protect.”

To watch carefully or note for a scientific or special purpose, e.g. to observe a star (astronomy), to observe the behavior of a patient (medicine, psychology), an animal (ethology, zoology), social groups (sociology), etc.

Etymology (EN): From O.Fr. observer, from L. observare “watch over, look to, attend to, guard,” from ob “over” + servare “to watch, keep safe,” from PIE base *ser- “to protect;” cf. Av. har- “to guard, observe, pay attention to,” haraiti “guards, keeps,” harətar- “protector, watcher,” harəθra- “guarding, protection,” hāra- “watched, guarded,” Mod.Pers. zinhâr “beware!, mind!,” Gk. heros “protector, hero.”

Etymology (PE): Note 1: Observation is the most important basis of empirical sciences. All theories rely on observation, and must finally be supported by observational evidence. Persian, in contrast to European languages, lacks a distinct term that recognizes observation as a conceptual premise of sciences. In astronomy the Ar. rasad (رصد) is currently used, while in classical astronomical texts the Pers. pâyidan (پاییدن) is used mainly by prominent figures like Biruni and Tusi. However, the term rasad is never used for cases outside astronomy. The situation for other sciences is not clear. Often one resorts to the Ar. loanword mošâhedé
“to look at each other,” which lacks ability to form derivatives. Therefore, here we suggest a term that can be used in all fields of knowledge.

Nepâhian, from ne- + pâhidan. The prefix ne- “down; into” (as in negâh “look, glance,” negaristan “to view, look at,” nešastan “to sit down,” nemudan “to display”) from O.Pers./Av. ni- “down, into;” cf.
Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz, Du. neder, Ger. nieder;
PIE *ni- “down, below.”
The second component pâhidan, variants pâyidan, pâsidan “to watch, observe, look steadily;” Mid.Pers. pây- “to protect, guard;” Sogdian p’y “to observe, protect, watch over;” O.Pers. pā- “to protect,” pāta- “protected;” Av. pā- “to protect,” pāti “guards,” nipā(y)- (with ni-) “to watch, observe, guard,” nipātar- “protector, watcher,” nipāθri- “protectress;”
cf. Skt. pā- “to protect, keep,” tanû.pā- “protecting the body,” paś.pā- “shepherd;” Gk. poma “lid, cover,” poimen “shepherd;” L. pascere “to put out to graze,” pastor “shepherd;” Lith. piemuo “shepherd;” PIE base *pā- “to protect, feed.”

Note 2: Since pây-, pâyidan is used for → conserve, we adopt the variant pâh-, pâhidan, which is extant in Lâri and
Gerâši dialects; cf. Gilaki pâstan, pâssan “to guard, pay attention.” In fact the interchange of phonemes h, i and s is not rare in Indo-European languages.

Note 3: Examples of pâyidan “to observe” in astronomy, from Biruni’s al-Tafhim (written c. 1029 A.D.)
p. 313: و بپای ارتفاع آ فتاب را.
p. 64: و بپای تا بدایره اندر آید.

Note 4: As indicated above, nepâh- exists in Av. as nipāy- “to watch, protect.”

Pertaining to a value which has been measured, in contrast to one which is computed.

See also: Past participle of → observe.

Pertaining to a value which has been measured, in contrast to one which is computed.

See also: Past participle of → observe.

1. Someone or something that observes.
   

2. Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.

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

1. Someone or something that observes.
   

2. Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.

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

A place located either at the top of the tube of a large telescope
from where one observes or at the back of the tube where instruments are
attached to the → Cassegrain focus.

See also: → observer; → cage.

A place located either at the top of the tube of a large telescope
from where one observes or at the back of the tube where instruments are
attached to the → Cassegrain focus.

See also: → observer; → cage.

A written report provided by a visiting astronomer to the observatory describing how his/her observing run has been carried out.

See also: Observing, noun of → observe; → report.

A written report provided by a visiting astronomer to the observatory describing how his/her observing run has been carried out.

See also: Observing, noun of → observe; → report.

A period of telescope time for observations allocated to a science project.

See also: Observing, noun of → observe; → run.

A period of telescope time for observations allocated to a science project.

See also: Observing, noun of → observe; → run.

The long opening in the dome of a telescope through which observation is conducted.

See also: Observing, noun of → observe; → slit.

The long opening in the dome of a telescope through which observation is conducted.

See also: Observing, noun of → observe; → slit.

The state, process, or condition of being or becoming obsolete or outdated and no longer used.

See also: → obsolete; → -escence.

The state, process, or condition of being or becoming obsolete or outdated and no longer used.

See also: → obsolete; → -escence.

Becoming obsolete; passing out of use.

See also: → obsolete; → -escent.

Becoming obsolete; passing out of use.

See also: → obsolete; → -escent.

1a) No longer in general use; fallen into disuse.

1b) Biology: (of a living organism) Reduced to a redument, or completely lacking, as compared with a corresponding part in related organisms.

2. To make obsolete by replacing with something newer or better.

Etymology (EN): From L. obsoletus “grown old, worn-out,” p.p. of obsolescere “fall into disuse, be forgotten about, become tarnished,” which probably is from ob “away” + an expanded form of solere “to be used to, be accustomed.”

Etymology (PE): Pitâr, from Tabari pitâr “worn out,” specifically “rotten tree,” pitə-pât “anything worn out or useless,” from pit, variant of Pers. pud “worn out, decayed,” pusidan/pus- “to rot,” cf. Pashto puda, Wakhi pitk “rotten, foul,” Kurd. (Kurm.) puc “rotten, useless;” Av. pu- “to stink, rot,” akin to Gk. puos, L. pus “pus.”

1a) No longer in general use; fallen into disuse.

1b) Biology: (of a living organism) Reduced to a redument, or completely lacking, as compared with a corresponding part in related organisms.

2. To make obsolete by replacing with something newer or better.

Etymology (EN): From L. obsoletus “grown old, worn-out,” p.p. of obsolescere “fall into disuse, be forgotten about, become tarnished,” which probably is from ob “away” + an expanded form of solere “to be used to, be accustomed.”

Etymology (PE): Pitâr, from Tabari pitâr “worn out,” specifically “rotten tree,” pitə-pât “anything worn out or useless,” from pit, variant of Pers. pud “worn out, decayed,” pusidan/pus- “to rot,” cf. Pashto puda, Wakhi pitk “rotten, foul,” Kurd. (Kurm.) puc “rotten, useless;” Av. pu- “to stink, rot,” akin to Gk. puos, L. pus “pus.”

An angle whose measure is greater than 90° and less than 180°.

Etymology (EN): Obtuse, from M.Fr. obtus (fem. obtuse), from L. obtusus “blunted, dull,” p.p. of obtundere “to beat against, make dull,” from ob “against” + tundere “to beat,” from PIE *(s)tud- “to beat, strike, push, thrust;” → angle.

Etymology (PE): Zâviyé, → angle; bâz “open,” 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.”

An angle whose measure is greater than 90° and less than 180°.

Etymology (EN): Obtuse, from M.Fr. obtus (fem. obtuse), from L. obtusus “blunted, dull,” p.p. of obtundere “to beat against, make dull,” from ob “against” + tundere “to beat,” from PIE *(s)tud- “to beat, strike, push, thrust;” → angle.

Etymology (PE): Zâviyé, → angle; bâz “open,” 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.”