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

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.

A word or group of words representing the person or thing upon which the action of a verb is performed or toward which it is directed. In English, generally coming after the verb, without a preposition. In He saw it the pronoun it is the direct object of saw (Dictionary.com).

→ direct; → object.

In → graph theory, an edge where endpoints are distinguished; one is the head and the other is the tail. A directed edge is specified as an ordered pair of → vertices, u, v and is denoted by (u, v) or u→ v.

→ directed; → edge.

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").

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.

A substance, such as mud or dust, that soils someone or something (OxfordDictionaries.com)

Metathesis of M.E. drit, drytt "mud, dirt, dung," from O.N. drit, cognate with O.E. dritan.

Cerk "dirt, filth."

Covered or marked with an unclean substance. → dirty ice, → dirty iceball model.

→ dirt + suffix -y.

Interstellar ice grains with graphite, silicates, or other chemical compounds adsorbed on their surfaces.

→ dirty; → ice.

A model for a → cometary nucleus proposed by Fred Whipple (1950-51), according to which the nucleus is a solid body (a few kilometers across) made up of various → ices (→ frozen water, → methane, → ammonia, → carbon dioxide, and → hydrogen cyanide) in which → dust is embedded. Dust particles are liberated when the ices vaporize as the → comet approaches the → Sun, and they get blown away by → solar radiation pressure, often forming impressive, gently curved → dust tails.

→ dirty; → ice; → ball; → model.

A prefix meaning "lack of, not" (e.g. dishonest); "do the opposite of" (e.g. disallow); "apart, away" (e.g. discard); before f, dif-; before some consonants (b, d, l, m, n, r, s, v, and sometimes g and j) di-.

From O.Fr. des-, from L. dis- "apart," from PIE *dis- "apart, asunder" (cf. O.E. te-, O.S. ti-, O.H.G. ze-, Ger. zer-). The PIE root is a secondary form of *dwis- and is thus related to L. → bis "twice."

Vâ- prefix denoting "separation; repetition; open; reversal, opposition; off; away," variant of bâz-, from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] "away, from;" Av. apa- [pref.] "away, from," apaš [adv.] "toward the back;" cf. Skt. ápāñc "situated behind."

Computers: To cancel the assignment of a particular resource to a user.

→ dis- + → allocate.

The act of disallocating or the state of being disallocated.

→ dis- + → allocate.

The act or state of disapproving; a condemnatory feeling, look, or utterance.

→ dis-; → approval.

To withhold approval from; decline to sanction.

→ dis-; → approve.

The removal or transference of an electric charge, as by the conversion of chemical energy to electrical energy.
The sudden and momentary equalization of a difference of potential, as between two terminals.

From M.E. deschargen, from O.Fr. deschargier "unload," from L.L. discarricare, from → dis- "do the opposite of" + carricare "to load a cart," from carrus "cart."

Vâbâr, from vâ- reversal prefix, → de- + bâr→ charge.

The quality or state of being → disciplinary.

→ disciplinary; → -ity.