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

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.

The inner → Saturn's rings, with a width of 7,500 km, lying before the → C ring, at 66,900 km from the center of Saturn.

→ ring.

A hydrodynamical paradox arising from the neglect of → viscosity in the → steady flow of a fluid around a submerged solid body. According to this paradox, the submerged body would offer no resistance to the flow of an → inviscid fluid and the pressure on the surface of the body would be symmetrically distributed about the body. This paradox may be traced to the neglect of the viscous forces, which are indirectly responsible for fluid resistance by modifying the velocity field close to a solid body (Meteorology Glossary, American Meteorological Society).

→ d'Alembert's principle; → paradox.

The statement that a moving body can be brought to a → static equilibrium by applying an imaginary inertia force of the same magnitude as that of the accelerating force but in the opposite direction. More specifically, when a body of mass m is moving with a uniform acceleration a under the action of an external force F, we can write: F = m . a, according to Newton's second law. This equation can also be written as: F - ma = 0. Therefore, by applying the force -ma, the body will be considered in equilibrium as the sum of all forces acting on it is zero. Such equilibrium is called → dynamic equilibrium. Owing to this principle, dynamical problems can be treated as if they were statical.

Named after the French mathematician and philosopher Jean le Rond d'Alembert (1717-1783), who introduced the principle in his Traité de dynamique (1743).

→ d'Alembert's principle.

→ d'Alembert's principle; → Lagrangian.

A second order, → partial differential operator in space-time, defined as: ▫² = ∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²)∂²/∂t², or ▫² = ∇² - (1/c²)(∂²/∂t²), where ∇² is the → Laplacian and c is the → speed of light. This operator is the square of the → four-dimensional operator  ▫, which is Lorentz invariant.

→ d'Alembert's principle; → operator.

An → ionization front of → H II regions whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

D referring to a dense gas; → type; → ionization; → front.

The → neutral → helium  → spectral line at 5876 Å.

D3, because of confusion with the sodium → D lines. When Joseph N. Lockyer first observed this line in the solar spectrum at the eclipse of 1868, helium was not yet isolated on Earth. Initially, this line was thought to be the third member of the D1 and D2 line family of sodium which lie in the same yellow part of the spectrum; → line.

A → white dwarf whose spectrum shows the → Balmer lines of hydrogen only, with no helium or metals.

D short for → dwarf; A a convention; → white.

A 40-year period of unusually low → solar activity, from about 1790 to 1830. See also the → Maunder minimum.

Named after John Dalton (1766-1844), British meteorologist; → minimum.

Of an oscillating system, the progressive decrease with time in the amplitude of the oscillation, due to friction (internal or external) or other resistance.

M.E. damp "black damp, a poison gas occurring in a mine," from Mid.Du. or Mid. Low German; akin to O.H.G. damph "vapor."

Mirâyi, noun from mirâ "getting extinguished, going out, expiring, dying," from mordan, present tense stem mir- "to die," Mid.Pers. murdan "to die," O.Pers. mrt- "to die," amriyta "dies," martiya- "(mortal) man" (Mod.Pers. mard "man"), Av. mərəta- "died," Skt. mar- "to die," mrti- "death;" PIE base *mor-/*mr- "to die." Cognates in other IE languages: Gk. emorten "died," ambrotos "immortal," L. morior "I die," mortuus "dead" (Fr. mourir "to die," mort "dead"), Arm. merani- "to die," O.C.S. mrutvu "dead," O.Ir. marb, Welsh marw "died," O.E. morþ "murder," Lith. mirtis "man."

1a) A successive group of rhythmical steps or bodily motions, or both, usually executed to music.
1b) An act or round of dancing.
2a) To move one's feet or body, or both, rhythmically in a pattern of steps, especially to the accompaniment of music.
2b) To leap, skip, etc., as from excitement or emotion; move nimbly or quickly (Dictionary.com).

M.E. da(u)ncen "to dance," from O.Fr. dancier of unknown origin, perhaps related to O.H.G. *dansjan "to lead (someone) to a dance."

Vašt, variant of gašt, gardidan, gel, gelidan "to turn," → revolve, cf. Eastern Gilâni gilâr "dance."

1) A person who dances.
2) A person who dances professionally, as on the stage (Dictionary.com).

→ dance; → -er.

Vaštâr, from vašt "dnace," + agent noun suffix -âr, as in parastâr, padidâr; vaštande, vaštgar with agent noun suffixes, -ande and -gar, → -or.

1) Liability or exposure to harm or injury; risk; peril.
2) An instance or cause of peril; menace (Dictionary.com).

M.E. daunger, from O.Fr. dangier "power, power to harm, authority, control," alteration of dongier, from V.L. *dominarium "power of a lord," from L. dominus "lord, master," → domain.

Xatar "danger," loan from Ar.

A modern unportable astrolabe which is used for high precision measuring of stellar and geographical coordinates. The instrument uses the simultaneous observations of two images of the same star, one of the images formed directly by the lower face of a prism and the other by the light rays reflected first from a mercury bath and then by the upper face of the prism. The images coincide when the zenithal distance of the star attains a prefixed value (Gauss method of equal altitudes, → almucantar). Apart from astrometry, the Danjon astrolabe was used for studying the Earth's rotation and is currently used for solar radius measurements.

After André Danjon (1890-1967), French astronomer, who developed the instrument at the Strasbourg Observatory before the Second World War and at the Paris Observatory in 1948. The concept of prism astrolabe was initially invented by the French Auguste Claude (1858-1938) around 1900 and was later modified in collaboration with Ludovic Driencourt (1861-1940); → astrolabe.

A scale to evaluate as exactly as possible the darkening degree of a total → lunar eclipse. The five steps of the scale run from 0 (extremely dark, invisible Moon) to 4 (extremely bright, the eclipse having a very weak effect on the Moon's visibility). The darkening at a lunar eclipse is determined to a great extent by the transparency of the terrestrial atmosphere, which is affected by clouds and the dust from the volcanic eruptions (M.S.: SDE).

Named after André Danjon, who set up the scale, → Danjon astrolabe; → scale.

1) To be courageous enough to try to do something.
2) A challenge, especially to prove courage.

M.E. durren, from O.E. durran "be bold enough, have courage" (to do something); cf. O.Norse dearr, O.H.G. giturran, Gothic gadaursan, from PIE root *dhers- "bold" source also of O.Pers. darš-, as below.

Daršidan, from O.Pers. darš- "to dare," Av. darš- (prefixed *upa- in upadarəš- "to dare"); cf. Khotanese darv- "to dare;" Baluci durrit/durr- "to take courage;" Skt. dhars "to venture;" Gk. thrasus "bold;" Goth. ga-daursan "to venture;" E. "to dare;" PIE *d^hers- "to attack, venture, dare" (Cheung 2007).

Taking or willing to take risks; audaciousness.

→ dare; → -ing.

Having very little or no light.

M.E. derk, O.E. deorc, from P.Gmc. *derkaz.

Târik, Mid.Pers. târig "dark," târ "darkness," Av. taθra- "darkness," taθrya- "dark," cf. Skt. támisrâ- "darkness, dark night," L. tenebrae "darkness," Hittite taš(u)uant- "blind," O.H.G. demar "twilight."

The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.

→ dark; → adaptation.