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

The angle between the direction in which a ray is refracted and the normal to the refracting surface.

→ angle; → refraction.

The → angular → displacement of a point on the → celestial sphere due to the Earth's → atmospheric refraction.

→ astronomical; → refraction.

The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth's atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.

→ atmospheric; → refraction.

A fraction written as a/b where a and b are → positive  → integers, as opposed to a → decimal fraction; for example, 5/7. Common fractions are sometimes also called → vulgar fractions.

→ common; → fraction.

A fraction in which the → numerator or → denominator, or both, contain fractions. For example (3/5)/(6/7). Also called → compound fraction.

→ complex; → fraction.

Same as → complex fraction.

→ compound; → fraction.

In mathematics, a fraction whose numerator is an integer and whose denominator is an integer plus a fraction whose numerator is an integer and whose denominator is an integer plus a fraction and so on.

→ continued; → fraction.

A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.

→ decimal; → fraction.

The difference between the deuterium (D)/hydrogen (H) → abundance → ratio in an object with respect to that representing a standard or mean value for that type of objects. Same as → isotope fractionation of deuterium. In the gas phase chemistry many of the D fractionation reactions produce an excess of D atoms relative to → hydrogen atoms. Deuterium fractionation in → interstellar cloud cores, → protostars, and → Solar System bodies is frequently used to infer important aspects of their physical and chemical histories. For example, the → deuterium enhancement in the Earth's sea water, with respect to the cosmic abundance, has been interpreted as being due to → enrichment by → comet-like → planetesimals colliding with the young Earth.

→ deuterium; → fractionation.

A problem encountered in astronomical spectroscopy, which consists of a loss of light from some wavelengths due to → atmospheric dispersion. In simple terms, differential refraction means that at nonzero → zenith distances an object cannot be simultaneously placed at the same position within a → slit at all wavelengths. This problem becomes more important for increasing → airmass, larger → spectral range, and smaller → slitwidths. To remedy this drawback, the slit should always be oriented along a direction perpendicular to the horizon, since differential refraction occurs in that direction.

→ differential; → refraction.

A wave property of light which allows it to curl around obstacles whose size is about that of the wavelength of the light. As a → wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form a → diffraction pattern.
Related terms: → diffusion; → dispersion; → distribution; → scatter; → scattering.

From Fr. diffraction, from Mod.L. diffractionem, from L. diffrac-, stem of diffringere "break in pieces," from → dis- "apart" + frangere "to break."

Parâš "dispersion, scattering," variant of pâš, pâšidan, → dispersion.

An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.

→ diffraction; → grating.

A series of concentric rings of dark and light color produced by interference.

→ diffraction; → pattern.

Olgu, loanword from Turkish; parâš→ diffraction.

One of several light rays emanating from a bright light source in images taken with → reflecting telescopes. They are artifacts caused by light diffracting around the support or → spider vanes of the → secondary mirror.

→ diffraction; → spike.

The quality of an → optical system that is capable of producing images with angular resolution as small as the theoretical limit of the → Airy disk.

→ diffraction; limited, adj. of → limit.

Karânmand "bounded, limited," from karân→ boundary + -mand possession suffix; parâš→ diffraction.

Formation of two refracted rays of light from a single incident ray; property of certain crystals, notably calcite.

→ double; → refraction.

A diffraction phenomenon resulting from the passage of electrons through matter, analogous to the diffraction of visible light. This phenomenon is the main evidence for the existence of waves associated with elementary particles; → de Broglie wavelength.

→ electron; → diffraction.

A rational number of the form a/b where a is called the numerator and b is called the denominator.

From L.L. fractionem (nom. fractio) "a breaking in pieces," from frangere "to break," from PIE base *bhreg- "to break" (cf. Goth. brikan, O.E. brecan "to break;" Lith. brasketi "crash, crack").

Barxé, from barx "lot, portion," variant bahr, from Mid.Pers. bahr "lot, share, portion," Av. baxəδra- "portion."

1) Math.: Pertaining to fractions; constituting a fraction.
2) Chemistry: Of or relating to any process by which parts of a mixture are separated by exploiting differences in their physical properties, such as their boiling points, solubility, or other characteristics.

→ fraction; → -al.

The portion of the 4π → steradians of the sky that a radiotelescope can observe from a given location on Earth over a 24-hour time interval.

→ fractional; → sky; → coverage.