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

In → graph theory, a graph with → directed edges. Also calle → digraph.

→ directed; → graph.

In a → directed graph, a path in which all → edges are oriented in the same direction.

→ directed; → edge.

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.

A person who directs a project, a group, a production.

From → direct + -tor a suffix found in loanwords from L., forming personal agent nouns from verbs.

Râštâr, from râšt- stem of râštidan→ direct + -âr suffix of agent noun (as in parastâr "nurse").

Divergence or disagreement, as between facts, claims, or calculations.

From L. discrepantia, from discrepantem, pr.p. of discrepare "sound differently, differ," from → dis- "apart, off" + crepare "to rattle, crack."

Nâhamxâni, from nâ- "non, un-," → a-, + hamxâni "siniging together," from ham- "together," → com-, + xâni, from xân stem of xândan "to sing; to read; to call," Mid.Pers. xwân- "to resound; to call," Av. x^van- "to sound," Proto-Iranian *huan- "to call;" cf. Skt. svan-, sváranti "to sound, make a sound, sing;" L. sonus "sound," sonare "to sound;" O.E. swinn "music, song," PIE base *suen- "to sound" (Cheung 2007).

Apart or detached from others; separate; distinct. Composed of distinct or discontinuous elements.

From O.Fr. discret, from L. discretus "separated, distinct," from p.p. of discernere "to distinguish," from → dis- "off, away" + cernere "distinguish, separate, sift," → crisis.

Gosasté "broken, broken off," p.p. of gosastan "to break, split," Mid.Pers. wisistan "to break, split," Av. saed- "to split," asista- "unsplit, unharmed," Skt. chid- "to split, break, cut off," Gk. skhizein "to split," L. scindere "to split;" PIE stem *skei- "to split, cut."
Jodâ "separate, distinct," Mid.Pers. jutâk (yutâk), jut (yut).

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.

Data that can only take a small set of particular values, usually whole numbers. For instance, number of children in a family cannot be 2.4, since parents cannot actually have 2.4 children. → continuous data.

→ discrete; → data.

A localized source on the celestial sphere that can be observationally separated at radio wavelengths from its background emission.

→ discrete; → radio; → source.

A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values.

→ discrete; → spectrum.

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

→ discrete; → transition.

A quantity that has certain magnitudes and does not represent a → continuous variable.

→ discrete; → value.

A variable which has only → discrete values and has no in-between values.

→ discrete; → variable.

A → quantum walk involving a probabilistic → operator that changes the direction while leaving the position fixed, and a shift operator that changes the position. Discrete-time quantum walk was introduced by J. Watrous (2001, Journal of Computer and System Sciences 62, 376)

→ discrete; → time; → quantum; → walk.

An accretion process involving an → accretion disk.

→ disk; → accretion.

Lacking → order.

→ disorder + -ed.

Birâyé, from bi- "without," → a-, + râyé, → order.

A parameter used in radio astronomy which describes the amount of dispersion in a radio signal due to its passage through an intervening plasma. It is proportional to the product of the interstellar electron density and the distance to the source.

→ dispersion; → measure.

An equation that describes how the → angular frequency, ω, of a wave depends on its → wave number, k. For the simplest of waves, where the speed of propagation, c, is a constant, ω(k) = ck. If the → phase velocity depends on k, that is for a dispersive medium, the function ω(k) is nonlinear.

→ dispersion; → relation.

In electromagnetism, a quantity which is not a real current (movement of charge), but has the units of current and has an associated magnetic field. The physical meaning of this displacement current is that a changing electric field makes a changing magnetic field.

→ displacement; → current.