An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 471 Search : ity
bâyestegi (#)

Fr.: nécessité   

1) The fact of being necessary or indispensable.
2) Something necessary or indispensable.

necessary; → -ity.

neutral density filter
  پالایه‌ی ِ چگالی ِ نتار   
pâlâye-ye cagâli-ye natâr

Fr.: filtre neutre   

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

neutral; → density; → filter.

Newton-Maxwell incompatibility
  ناسازگاری ِ نیوتن-ماکسول   
nâsâzgâri-ye Newton-Maxwell

Fr.: incompatibilité entre Newton et Maxwell   

The incompatibility between → Galilean relativity and Mawxell's theory of → electromagnetism. Maxwell demonstrated that electrical and magnetic fields propagate as waves in space. The propagation speed of these waves in a vacuum is given by the expression c = (ε00)-0.5, where ε0 is the electric → permittivity and μ0 is the magnetic → permeability, both → physical constants. Maxwell noticed that this value corresponds exactly to the → speed of light in vacuum. This implies, however, that the speed of light must also be a universal constant, just as are the electrical and the magnetic field constants! The problem is that → Maxwell's equations do not relate this velocity to an absolute background and specify no → reference frame against which it is measured. If we accept that the principle of relativity not only applies to mechanics, then it must also be true that Maxwell's equations apply in any → inertial frame, with the same values for the universal constants. Therefore, the speed of light should be independent of the movement of its source. This, however, contradicts the vector addition of velocities, which is a verified principle within → Newtonian mechanics. Einstein was bold enough to conclude that the principle of Newtonian relativity and Maxwell's theory of electromagnetism are incompatible! In other words, the → Galilean transformation and the → Newtonian relativity principle based on this transformation were wrong. There exists, therefore, a new relativity principle, → Einsteinian relativity, for both mechanics and electrodynamics that is based on the → Lorentz transformation.

Newton; → Maxwell; → incompatibility.

Newtonian principle of relativity
  پروز ِ بازانیگی ِ نیوتن   
parvaz-e bâzânigi-ye Newton

Fr.: principe de relativité de Newton   

The Newton's equations of motion, if they hold in any → reference frame, they are valid also in any other reference frame moving with uniform velocity relative to the first.

Newtonian; → principle; → relativity.

Newtonian relativity
  بازانیگی ِ نیوتنی   
bâzânigi-ye Newtoni

Fr.: relativité newtonienne   

The laws of physics are unchanged under → Galilean transformation. This implies that no mechanical experiment can detect any intrinsic diff between two → inertial frames. Same as → Galilean relativity.

Newton; → relativity.

nonlinear instability
  ناپایداری ِ ناخطی   
nâpâydâri-ye nâxatti

Fr.: instabilité non-linéaire   

The instability of a physical or mathematical system that arises from the nonlinear nature of relevant variables and their interactions within the system.

nonlinear; → instability.


Fr.: non-linéarité   

The property, condition, or state of being → nonlinear.

non-; → linearity.


Fr.: normalité   

General: The fact or condition of being normal.
Chemistry: Measure of the number of gram-equivalent weights of a compound per liter of solution.

From → normal + → -ity.

nuclear density
  چگالی ِ هسته‌ای   
cagâli-ye haste-yi (#)

Fr.: densité nucléaire   

The density of an atomic nucleus (about 1014 g/cm3).

nuclear; → density.

number density
  چگالی ِ عددی   
cagâli-ye adadi

Fr.: densité nmérique   

Number of a particular type of object found in each unit volume.

number; → density.

objective probability
  شوانایی ِ بر‌آختی   
šavânâi-ye barâxti

Fr.: probabilité objective   

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

objective; → probability.


Fr.: objectivité   

1) The state or quality of being → objective. Opposite of → subjectivity.
2) Ability to view things objectively.
3) External reality.

objective + → -ity.


Fr.: obliquité   

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.

State noun of → oblique.

obliquity of the ecliptic
  یکوری ِ هورپه   
yekvari-ye hurpeh

Fr.: obliquité de l'écliptique   

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.

obliquity; → ecliptic.

tiregi (#)

Fr.: obscurité   

The state or quality of being obscure.

obscure + → -ity.


Fr.: observabilité   

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.

From → observable + → -ity.

kederi (#)

Fr.: opacité   

1) General: The state or quality of being opaque.
2) A measure of the absorption of photons on their way from the stellar center to the surface. Opacity depends upon the frequency of the radiation, the density, the chemical composition, and the thermodynamic state of the gas. For a given density, the hotter the gas the lower the opacity, since the gas absorbs less readily, as described by → Kramers' law. Conversely, the cooler the gas the higher the opacity. See also → stellar pulsation, → kappa mechanism, → valve mechanism, → partial ionization zone.

From Fr. opacité, from L. opacitatem (nom. opacitas) "shade, shadiness," from opacus "shaded, dark, opaque."

Kederi, from keder "opaque," from Ar. kader + -i suffix forming nouns from adjectives.

  آپاریدنیگی، آپارش‌پذیری   
âpâridanigi, âpârešpaziri

Fr.: opérabilité   

The capability of being put into use, operation, or practice.

operable; → -ity.


Fr.: opportunité   

1) An appropriate or favorable time or → occasion.
2) A situation or condition favorable for attainment of a goal (

opportune + → -ity.

optical activity
  ژیرندگیِ نوری   
žirandegi-ye nuri

Fr.: activité optique   

The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).

optical; → activity.

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