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

An equation expressing a relationship between an → independent variable, x, an unknown → function, y = f(x), and its → derivatives. The general form of a differential equation is: F(x, y, y', y'', ..., y⁽ⁿ⁾) = 0, or F(x,y, dy/dx, d²y/dx², ..., dⁿy/dxⁿ) = 0. See also: → ordinary differential equation; → partial differential equation; → linear differential equation; → exact differential equation; → first-order differential equation; → homogeneous linear differential equation; → nonhomogeneous linear differential equation; → differential equation with separated variables; → differential equation with separable variables.

→ differential; → equation.

A → differential equation of the form: M₁(x) N₁(y) dx + M₂(x) N₂(y) dy = 0, which can be reduced to a → differential equation with separated variables.

→ differential; → equation; → separate; → variable.

A → differentail equation that can be transformed into the form: M(x)dx + N(x)dy = 0.

→ differential; → equation; → separate; → variable.

The study of curved spaces using differential calculus.

→ differential; → geometry.

A device that is commonly used to measure the → seeing at optical astronomical sites. The DIMM delivers an estimate of the → Fried parameter based on measuring the variance of the differential image motion in two small apertures, usually cut out in a single larger telescope pupil by a mask. The DIMM concept was introduced by Stock & Keller (1960, in Stars and Stellar Systems, Vol. 1, ed. G. P. Kuiper & B. M. Middlehurst, p. 138), whereas its modern implementation was first described by Sarazin & Roddier (1990, A&A 227, 294).

→ differential; → image; → motion; → monitor.

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.

1) Of a single body (such as a star or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
2) Of a galaxy, the orbiting of stars nearer the center faster than those at the edge.

→ differential; → rotation.

A system characterized by → differential rotation. In such a system the → angular velocity decreases as the distance from the rotation center increases.

→ differential; → rotating; → system.

General: To perceive or show the difference in or between.
Math.: To obtain the differential or the derivative of.

M.L. differentiatus "distinguished," p.p. of differentiare.

Degarsânidan, verbal form of → difference.

A description of a planet's interior which is composed of a rocky, dense inner core and a less dense outer crust.

Differentiated, p.p. of → differentiate; → interior.

A meteorite that has distinctly separated stone, metal, and glass. It is derived from a differentiated parent body and hence not primitive. The parent body accreted surrounding material until it was large enough to start melting in the middle. The denser metals sank to the center and the stones and glasses floated to the top. A differentiated meteorite made completely of metal comes from the center of a parent meteoroid which was broken apart. → undifferenciated meteorite.

Differentiated, p.p. of → differentiate; → meteorite.

1) Math.: The operation of finding the → derivative of a function.
2) Astro.: Process by which an originally homogeneous planetary or asteroidal body is separated into regions of different composition, such as core, mantle, and crust.

Verbal noun of → differentiate.

Reflection of light from a rough or granular surface, which takes place in all directions due to the microscopic irregularities of the interface; opposed to → specular reflection.

→ diffuse; → reflection.

A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.

→ local; → -ize; → diffusion; → region.

1a) Proceeding in a straight course or line without deviation or interruption.
1b) Math.: Varying in the same manner as another quantity, especially increasing if another quantity increases or decreasing if it decreases.
1c) Astro.: Designating west-to-east motion of a planet in the same direction as the Sun's movement against the stars.
2) To administer; manage; supervise; to serve as a director in the performance or production of.

From L. directus "straight," p.p. of dirigere "set straight," from → dis- "apart" + regere "to guide;" cognate with Pers. râst, as explained below.

Râst "right, straight" (râšt in afrâštan); Mid.Pers. râst; O.Pers. rāsta- "straight, true," rās- "to be right, straight, true," rād- "to prepare;" Av. rāz- "to direct, put in line, set," razišta- "straightest, most correct," erezu- "correct, straight," rāzayeiti "directs," razan- "order;" Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" Ger. recht; E. right; PIE base *reg- "right, just; to move in a straight line."
Sarrâst, from sar here an intensive element meaning "side," originally "head" (Mid.Pers. sar; Av. sarah- "head;" Skt. siras- "head;" Gk. kara "head," keras "horn;" Mod.Pers. sarun "horn;" L. cerebrum "brain;" PIE *ker- "head, horn") + râst, as above.
Râštidan, infinitive from râšt, variant râst "direct."

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.

An → electric current which flows in one direction only and which is substantially constant in magnitude. Virtually all electronic and computer hardware needs direct current to function. → Alternating current can be converted to direct current by means of a power supply consisting of a → transformer.

→ direct; → current.

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.