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

If E₁, E₂, ..., E_n are n → mutually exclusive events, then the probability of occurrence of at least one of them is the sum of their individual probabilities: P(E₁ + E₂ + ... + E_n) = P(E₁) + P(E₂) + ... + P(E_n).

→ additive; → law; → probability.

One of the basic relations between → electricity and → magnetism, stating quantitatively the relation of a → magnetic field to the → electric current or changing electric field that produces it. Ampere's law states that the line integral of the magnetic field around an arbitrarily chosen path is proportional to the net electric current enclosed by the path. Also known as Ampère's theorem, Ampère's circuital law.

→ ampere; → law.

In mathematics, the rule that states that the result of two identical operations is independent of the sequence of these operations. For ex., in the addition operation, a + (b + c) = (a + b) + c = a + b + c. Multiplication of numbers is also associative.

→ associative; → law.

A statement according to which equal volumes of different gases contain an equal number of molecules under the same conditions of temperature and pressure.

After Amedeo Avogadro (1776-1856), Italian chemist and physicist, who advanced the hypothesis in 1811.

A law which describes the vertical pressure distribution in the lower parts of Earth's atmosphere. The atmospheric pressure decreases exponentially from any reference surface as the altitude increases.

→ barometer; → law.

The → magnetic field due to → electric current flowing in a long straight conductor is directly proportional to the current and inversely proportional to the distance of the point of observation from the conductor. The law is derivable from → Ampere's law, but was obtained experimentally by the authors.

Named after the French physicists Jean-Baptiste Biot (1774-1862) and Félix Savart (1791-1841); → law.

→ Titius-Bode law.

In a → perfect gas where mass and temperature are kept constant, the volume of the gas will vary inversely with the absolute pressure. The law can be expressed as PV = constant, where P = absolute pressure and V = volume.

After Robert Boyle (1627-1691), an Irish philosopher, chemist, and physicist, and Edme Mariotte (1620-1684), a French physicist and pioneer of neurophysiology, who discovered the law independently, the first one in 1662 and the second one in 1676; → law.

A parallel beam of monochromatic X-rays of wavelength λ, incident on a given set of parallel crystal planes at a grazing angle θ will give rise to a reflected beam whenever: n λ = 2d . sinθ, where n is an integer representing the difference in path length, and d is the perpendicular distance between a pair of adjacent planes.

Named after William Lawrence Bragg (1890-1971), British physicist, who, in collaboration with his father, William Henry Bragg (1862-1942), joint Nobel Prize in Physics 1915, pioneered X-ray analysis and spectrometry; → law.

The amount of the polarization of light reflected from a surface is a maximum when the reflected ray is at right angles to the refracted ray. See also → polarizing angle.

Named after Sir David Brewster (1781-1868), Scottish physicist; → law.

Any of the three empirical laws governing the rotational dynamics of the → Moon:
1) The Moon rotates uniformly about its polar axis with a → rotational period equal to the mean → sidereal period of its orbit about the Earth.
2) The → inclination of the Moon's equator to the → ecliptic is a constant angle approximately 1.5°.
3) The → ascending node of the lunar orbit on the ecliptic coincides with the → descending node of the lunar equator on the ecliptic. This law could also be expressed as: the spin axis and the normals to the ecliptic and orbit plane remain coplanar (Noyelles, 2009, Icarus, 202, 225).

Named after Jean-Dominique Cassini (1625-1712), French astronomer of Italian origin, who established these laws in 1693 (Traité de l'origine et du progrès de l'astronomie), ; → law.

The volume of a fixed mass of any gas increases for each degree rise in temperature by a constant fraction of the volume at 0° C, the pressure being constant throughout.

Named after Jacques Charles (1746-1823), French physicist, who first discovered the law, and who was responsible for the first balloon ascents using hydrogen.

An empirical power-law representing the number of stellar clusters as a function of the number of stars per cluster within an interval. It is expressed as: N(N_*) dN_*∝ N_*^-α dN_*, where N(N_*) is the number of clusters containing N_* stars and dN_* is the interval in star number. It is believed that this relationship applies to a variety of systems, including stellar clusters, globular clusters, H II regions (Oey et al. 2004, AJ 127, 1632).

→ clustering; → law.

A principle holding for the operations of addition and multiplication (in some number domains) that asserts that the consequence of the given operation is not affected by the order in which the terms are considered. Thus x + y = y + x; x . y = y . x.

→ commutative; → law. The first use of the word commutative in this sense is ascribed to the French mathematician François Joseph Servois (1768-1847).

A general statement that a → physical quantity, such as → energy, → mass, → momentum, or → electric charge is unchanged in an → interaction occurring within a → closed system. See also:
→ conservation of charge, → conservation of energy, → conservation of mass, → conservation of mass and energy, → conservation of matter, → conservation of momentum, → conservation of probability, → parity conservation, → conservative field.

→ conservation; → law.

The electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects

→ coulomb; → law.

A mathematical expression describing the → surface brightness profile of a typical → elliptical galaxy, whereby the projected luminosity distribution varies as exp(-R)^1/4.

→ de Vaucouleurs radius; → law.

The → Debye model at → low  → temperatures, where the → specific heat is proportional to the cube of the → absolute temperature, i.e. C_V ∝ T³.

→ debye; → law.

Math.: In multiplication, the principle that permits the multiplier to be applied separately to each term in the multiplicand: x(y + z) = xy + xz. → associative law; → commutative law.

→ distributive; → law.

The product of the → specific heat and → atomic weight of most solid elements at room → temperature is nearly the same. In other words, specific heat is constant for a solid and independent of temperature. Experiment shows that at moderate temperatures this law is satisfied for → crystals with rather simple structure. However, the law fails for crystals with more complex structures. More specifically the law cannot explain the variation of specific heat with temperature. The specific heat drops to zero as the temperature approaches 0 K. This behavior is explained only with the quantum theory. → Debye model.

Named after Pierre L. Dulong (1785-1838) and Alexis T. Petit (1797-1820), French chemists, who proposed the law in 1819. They collaborated in several important investigations, including studies of thermal expansion of gases and of liquids and the specific heats of substances; → law.