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

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.

The brightest star in the constellation → Cetus; a → red supergiant (K0 III) of visual magnitude 2.04.

Diphda, from Ar. zafda' (ضفدع) "frog." It is also designated as Deneb Kaitos, from zanab al-qaytusذنب القیطس "tail of Cetus."

Mid.Pers. wazaγ, vak; Av. vazaγa- "frog," → tadpole orbit.

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.

A pathological condition of vision in which a single object appears double because the eyes are not focusing properly. Same as → double vision.

From L. diplo- "double, in pairs," from Gk., combining form of diplos "twofold" + -opia, from Gk. -opia, from ops "eye."

Dobini, from do→ two + bini "vision, seeing," from bin "to see; seer" (present stem of didan; Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see").

Of or relating to a → dipole.

→ dipole; → -ar.

A → magnetic field produced by a system possessing a net magnetic → dipole moment.

→ dipolar; → magnetic; → field.

1) A combination of two electrically or magnetically charged particles of opposite signs, which are separated by a very small distance.
2) Chemistry: A molecule containing both positively and negatively charged groups.

→ di- "two, twice, double," + → pole.

A form of anistropy in the temperature of the → cosmic microwave background radiation, appearing as one hot pole and one cold pole, caused by our motion with respect to the cosmic background radiation. The temperature variations, amounting to 1 part in 1000, yield a velocity of about 600 km/sec for our Galaxy with respect to the background. → cosmic microwave background anisotropy.

→ dipole; → anisotropy.

One of the simplest kinds of antenna which is connected at the center to a radio-frequency feed line for transmitting or receiving radio frequency energy. It differs from the dish antenna in that it consists of many separate antennas that collect energy by feeding all their weak individual signals into one common receiving set.

→ dipole; → antenna.

1) The product of the strength of either of the charges in an → electric dipole and the distance separating the two charges. It is expressed in → coulomb meters. Dipole moment is a → vector quantity. Its direction is defined as toward the positive charge. In chemistry dipole moment is a quantitative measure of polarity in a molecule; the unit is the → debye.
2) The product of the strength of either of the poles in a → magnetic dipole and the distance separating the two poles. Dipole moment is a vector quantity. Its direction is defined as toward the magnetic north pole.

→ dipole; → moment.

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.

Paul A. M. Dirac (1902-1984), English theoretical → physicist, one of the founders of → quantum mechanics and → quantum electrodynamics, Nobel Prize for Physics in 1933. → Dirac annihilation, → Dirac equation, → Dirac function, → Dirac's constant, → Fermi-Dirac statistics .

Same as → pair annihilation.

→ Dirac; → annihilation.

The equation that describes the behavior of an electron in a way that combines the requirements of quantum mechanics with the requirements of relativity.

→ 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

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

→ Dirac; → constant.

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.