aberration constant pâyâ-ye birâheš Fr.: constante d'aberration Same as → constant of aberration. → aberration; → constant. |
astronomical constant pâyâ-ye axtaršenâsik, ~ axtaršenâxti (#) Fr.: constante astronomique A precisely measured fundamental quantity in astronomy, such as the → solar parallax, the → constant of aberration, and the → obliquity of the ecliptic. → astronomical; → constant. |
Besselian star constant pâyâ-ye axtari-ye Besseli Fr.: constante stellaire besselienne Any of the eight quantities denoted by a, b, c, d (for → right ascension) and a', b', c', d' (for → declination) used in conjunction with → Besselian day numbers for the reduction of star's → mean catalog place. |
Boltzmann constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann |
Boltzmann's constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann The physical constant, noted by k, relating the mean → kinetic energy of → molecules in an → ideal gas to their → absolute temperature. It is given by the ratio of the → gas constant to → Avogadro's number. Its value is about 1.380 x 10^{-16}erg K^{-1}. Named after the Austrian physicist Ludwig Boltzmann (1844-1906), who made important contributions to the theory of statistical mechanics; → constant. |
constant pâyâ (#) Fr.: constante A quantity that does not change during a particular process. L. constantem "standing firm, stable," pr.p. of constare, from → com- "together" + stare "to stand;" PIE base *sta- "to stand;" cf. O.Pers./Av. sta- "to stand, stand still; set," Pers. istâdan "to stand," Skt. sthâ- "to stand," Gk. histemi "put, place, weigh," stasis "a standing still," pâyâ verbal adj./noun from pâyidan "to stand firm, to be constant, steady, fixed," Mid.Pers. pâyitan, pâtan, pây- "to protect; wait, stand," Sogdian p'y "to protect, watch over," O.Pers./Av. pâ(y)- "to protect, keep" pâtar- "protector, watcher," cf. Skt. pâ- "to protect, keep," pâti "protects," Gk. poimen "shepherd," poma "lid, cover," L. pastor "shepherd," panis "bread;" PIE base *pa- "to protect, guard, pasture, feed." |
constant of aberration pâyâ-ye birâheš Fr.: constante d'aberration The maximum amount of the apparent yearly displacement of a star, resulting from the → aberration of starlight. The value of the constant of aberration, κ, at J2000.0 is 20".49552. κ = (v/c) csc 1", where v is the average speed of the Earth about the Sun and c is the → speed of light in vacuum. The Earth's speed is given by: v = 2πa / [P(1 - e^{2})^{1/2}], where a is the → semi-major axis of the Earth's orbit, e is the → eccentricity of the Earth's orbit, and P is the → sidereal period of the Earth. Same as → constant of annual aberration. See also → constant of diurnal aberration. → constant; → aberration. |
constant of annual aberration pâyâ-ye birâheš sâlâné Fr.: constante d'aberration annuelle Same as → constant of aberration. → constant; → annual; → aberration. |
constant of diurnal aberration pâyâ-ye birâheš ruzâné Fr.: constante d'aberration diurne The quantity 0".3200 ρ cos φ', where ρ is the geocentric distance of the observer measured in units of → equatorial radius the Earth and φ' is the observer's → geocentric latitude. The numerical part is equal to 2πa csc1" / (cP), where a is the equatorial radius of the Earth, P is its → sidereal period of rotation, and c is the → speed of light in vacuum. → constant; → diurnal; → aberration. |
constant of gravitation pâyâ-ye gerâneši (#) Fr.: constante de la gravitation |
constant of the motion pâyâ-ye jonbeš Fr.: constante de mouvement 1) Classical mechanics: A variable X whose total rate of change dX/dt
along the path of a → dynamical system is zero. In other words,
a function of an object's position, velocity, or both that does not change even as the
object moves. For example, the total energy of a
→ simple harmonic oscillator is a constant of the motion. |
cosmological constant pâyâ-ye keyhânšenâsik, ~ keyhânšenâxti Fr.: constante cosmologique A term introduced by Einstein into his gravitational → field equations in order to allow a solution corresponding to a → static Universe. The cosmological constant is physically interpreted as due to the → vacuum energy of quantized fields. See also → dark energy. → cosmological; → constant. |
cosmological constant problem parâse-ye pâyâ-ye keyhânšenâxti Fr.: problème de la constante cosmologique The impressive discrepancy of about 120 orders of magnitude between the theoretical value of the → cosmological constant and its observed value. → Quantum field theory interprets the cosmological constant as the density of the → vacuum energy. This density can be derived from the maximum energy at which the theory is valid, i.e. the → Planck energy scale (10^{18} GeV). The theoretical vacuum → energy density is (10^{18} GeV)^{4} = (10^{27} eV)^{4} = 10^{112} erg cm^{-3}. On the other hand, the observed vacuum energy density is estimated to be about (10^{-3} eV)^{4} = 10^{-8} erg cm^{-3}. There is, therefore, a discrepancy of about 120 orders of magnitude. → cosmological; → constant; → problem. |
coupling constant pâyâ-ye jafsari Fr.: constante de couplage In nuclear physics, a constant that indicates a measure of how strongly two particles interact. |
curvature constant pârâmun-e xamidegi Fr.: paramètre de courbure A parameter occurring in the → Friedmann equations of → general relativity describing the geometry of → space-time. A spatially → open Universe is defined by k = -1, a → closed Universe by k = + 1 and a → flat Universe by k = 0. See also the → Robertson-Walker metric. See also → curvature of space-time. |
decay constant pâyâ-ye tabâhi Fr.: constante de désintégration A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N_{0}e^{-kt}, where N_{0} is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k. |
Dirac's constant pâyâ-ye Dirac Fr.: constante de Dirac The → Planck's constant divided by 2π and denoted ħ, pronounced h-bar. Also called → reduced Planck's constant. |
Einstein's gravitational constant pâyâ-ye gerâneši-ye Einstein (#) Fr.: constante gravitationnelle d'Einstein The coupling constant appearing in → Einstein's field equations, expressed by: κ = 8πG/c^{4}, where G is the Newtonian → gravitational constant and c the → speed of light. → einstein; → gravitational; → constant. |
Fermi constant pâyâ-ye Fermi Fr.: constante de Fermi The → coupling constant associated with the → weak interaction, which gives rise to → beta decay. C_{F} = 1.167 x 10^{-5} GeV^{-2}. |
fine-structure constant pâyâ-ye sâxtâr-e nâzok Fr.: constante de la structure fine A measure of the strength of → interaction between a → charged particle and the → electromagnetic field. It is a → dimensionless number expressed (in → cgs units) by α = e^{2}/ħc, where e is the → electron charge, ħ is the → reduced Planck's constant, and c is the → speed of light. It is approximately equal to 1/137 or 7.3 x 10^{-3}. The smallness of this number is of great importance since it determines the size of → atoms and the → stability of → matter. → fine structure; → constant. |