An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Same as → constant of aberration.

→ aberration; → constant.

A constant quantity in → equations which takes various values but which remains unaffected by the changes in the values of the → variables of the equation. Most → differential equations have more than one → solution. In general, the number of arbitrary constants of an ordinary differential equation is given by the → order of the highest → derivative.

→ arbitrary; → constant.

A precisely measured fundamental quantity in astronomy, such as the → solar parallax, the → constant of aberration, and the → obliquity of the ecliptic.

→ astronomical; → constant.

The number of units in one → mole of any → substance (defined as its → molecular weight in grams), equal to 6.022 140 857 × 10²³. The units may be electrons, atoms, ions, or molecules, depending on the nature of the substance.

Named after Amedeo Avogadro (1776-1856), whose law allowed other physicists to calculate Avogadro's number; → number.

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.

→ Besselian; → star; → constant.

→ Boltzmann's constant.

→ Boltzmann's constant.

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^-16erg K^-1.

Named after the Austrian physicist Ludwig Boltzmann (1844-1906), who made important contributions to the theory of statistical mechanics; → constant.

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."

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 - e²)^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.

Same as → constant of aberration.

→ constant; → annual; → aberration.

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.

→ gravitational constant.

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.
2) Quantum mechanics: An → observable that remains constant in time. As an example, the energy is a constant of the motion of all systems whose → Hamiltonian does not depend explicitly upon time.

→ constant; → motion.

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.

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 (10¹⁸ GeV). The theoretical vacuum → energy density is (10¹⁸ GeV)⁴ = (10²⁷ eV)⁴ = 10¹¹² 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.

In nuclear physics, a constant that indicates a measure of how strongly two particles interact.

→ coupling; → constant.

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.

→ curvature; → parameter.

A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N₀e^-kt, where N₀ 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.

→ decay; → constant.

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

→ Dirac; → constant.

→ Dirac constant.

→ Dirac; → constant.