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

The energy density of a radiation field divided by the equilibrium value for the same color temperature.

→ dilution; → factor.

1) Math.: Independent extension in a given direction; a property of space.
2) Physics: → physical dimension.

From L. dimensionem (nom. dimensio), from stem of dimetri "to measure out," from → dis- + metri "to measure."

Vâmun, from vâ-, → dis-, + mun, variant mân "measure" (as in Pers. terms pirâmun "perimeter," âzmun "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl"), from O.Pers./Av. mā(y)- "to measure;" PIE base *me- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum.

Of or pertaining to → dimension.

→ dimension; → -al.

A technique used in physics based on the fact that the various terms in a physical equation must have identical → dimensional formulae if the equation is to be true for all consistent systems of unit. Its main uses are:
a) To test the probable correctness of an equation between physical quantities.
b) To provide a safe method of changing the units in a physical quantity.
c) To solve partially a physical probable whose direct solution cannot be achieved by normal methods.

→ dimensional; → analysis.

Symbolic representation of the definition of a physical quantity obtained from its units of measurement. For example, with M = mass, L = length, T = time, area = L², velocity = LT^-1, energy = ML²T^-2. → dimensional analysis.

→ dimensional; → formula.

A physical quantity or number lacking units.

→ dimension; → -less.

A quantity without an associated → physical dimension. Dimensionless quantities are defined as the ratio of two quantities with the same dimension. The magnitude of such quantities is independent of the system of units used. A dimensionless quantity is not always a ratio; for instance, the number of people in a room is a dimensionless quantity. Examples include the → Alfven Mach number, → Ekman number, → Froude number, → Mach number, → Prandtl number, → Rayleigh number, → Reynolds number, → Richardson number, → Rossby number, → Toomre parameter. See also → large number.

→ dimension; → quantity.

The fourth largest moon of Saturn and the second densest after Titan. Its diameter is 1,120 km and its orbit 377,400 km from Saturn. It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.

Discovered in 1684 by Jean-Dominique Cassini, Italian born French astronomer (1625-1712). In Gk. mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).

The electromagnetic radiation produced by an oscillating → electric dipole or → magnetic dipole.

→ dipole; → radiation.

Same as → pair annihilation.

→ Dirac; → annihilation.

The equation that describes the behavior of an → electron in a way that combines the requirements of → quantum mechanics with the requirements of → special relativity. The Dirac equation predicted the existence of antimatter

→ Dirac; → equation.

A function of x defined as being zero for all values of x other than x = x₀ and having the definite integral from x = -∞ to x = +∞ equal to unity.

→ Dirac; → function

A correlation between two variables such that as one variable becomes large, the other also becomes large, and vice versa. The correlation coefficient is between 0 and +1. Also called positive correlation.

→ direct; → correlation.

The motion of a solar system body from West to East across the sky against the background stars. It is the "normal" direction of motion within the solar system. For rotating or orbiting solar system objects it is anti-clockwise as seen from above the solar system in the direction of the North Pole. The same as → prograde motion. See also → retrograde motion.

→ direct; → motion.

1) A position to which motion or another position is referred.
2) The act or an instance of directing.

M.E. direccioun, from M.Fr., from L. direction-, stem of directio "arranging in line, straightening," → direct.

1) Râstâ, from direct→ direct + -â dimension suffix; su, from Mid.Pers. sôk "side."
2) Râštâri, noun from râštâr→ director + suffix -i.

An angle made by a given vector and a coordinate axis.
The angle between the North and a chosen direction.

→ direct; → angle.

One of the following conditions for a → Fourier series to converge:
1) The function f(x) is defined and single valued, except possibly at a finite number of points in the interval -π, +π.
2) f(x) has a period of 2π.
3) f(x) and f'(x) are → piecewise continuous functions on -π, +π.
Then, the Fourier series converges to:
(a) f(x) if x is a point of continuity.
(b) (f(x + 0) + f(x - 0))/2, if x is a point of discontinuity.

Named after Peter Gustav Lejeune Dirichlet (1805-1859), German mathematician who made valuable contributions to → number theory, → analysis, and → mechanics; → condition.

The act of disallocating or the state of being disallocated.

→ dis- + → allocate.

The rapid, systematic changes in the absorption parts of the → P Cygni profiles of the ultraviolet → resonance lines (Si IV, C IV, and N V) observed in a majority of massive → hot stars. DACs are typically seen to accelerate to the → blue wing of the profile over a few days, becoming narrower as they approach the → terminal velocity.

→ discrete; → absorption; → component.

A transition between two quantum-mechanical energy levels. See also → discrete spectrum.

→ discrete; → transition.