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 angle created by the observer's line of sight to the → apparent horizon and t he → true horizon. Neglecting the → atmospheric refraction, dip of the horizon can be expressed by θ (radians) = (2h/R)^1/2, where h is the observer's height and R the Earth's radius. An an example, for a height of 1.5m above the sea, and R = 6.4 x 10⁶ m, the dip angle is about 0.00068 radians, or 0.039 degrees, about 2.3 minutes of arc, quite appreciable by the eye. See also → distance to the horizon. Same as → dip angle.

→ dip; → horizon.

Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).

From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos "having two sounds," from → di- "double" + phthongos "sound, voice."

Dovâké, from do "two, → di-" + vâké, vâk, → voice.

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

→ dipole; → radiation.

An → isotope of → helium that consists of two → protons, without any → neutrons. It is extremely → unstable.

→ di-; → proton.

Same as → pair annihilation.

→ Dirac; → annihilation.

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

→ Dirac; → constant.

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

→ Dirac constant.

→ Dirac; → constant.

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.