crescent Moon visibility
diyâri-ye helâl-e mâh
Fr.: visibilité du croissant lunaire
The first sighting of the → New Moon after its → conjunction with the Sun. Although the date and time of each New Moon can be computed exactly, the visibility of the lunar → crescent as a function of the → Moon's age depends upon many factors and cannot be predicted with certainty. The sighting within one day of New Moon is usually difficult. The crescent at this time is quite thin, has a low surface brightness, and can easily be lost in the → twilight. Generally, the lunar crescent will become visible to suitably-located, experienced observers with good sky conditions about one day after New Moon. However, the time that the crescent actually becomes visible varies from one month to another. The visibility depends on sky conditions and the location, experience, and preparation of the observer. Ignoring atmospheric conditions, the size and brightness of the lunar crescent depend on the → elongation which in turn depends on several factors: 1) The Moon's elongation at New Moon (the elongation of the Moon at New Moon is not necessarily 0). 2) The speed of the Moon in its elliptical orbit. 3) The distance of the Moon, and 4) The observer's location (parallax). The combined effect of the first three factors gives geocentric elongation of the Moon from the Sun at an age of one day which can vary between about 10 and 15 degrees. This large range of possible elongations in the one-day-old Moon is critical (US Naval Observatory).
Fr.: densité critique
1) Cosmology: The average density of matter in the Universe
that would be needed to eventually halt the
→ cosmic expansion.
In a spatially → flat Universe,
the critical density is expressed by
ρc = (3c2/8πG)Ht2,
where c is the → speed of light,
G is the → gravitational constant, and
Ht the → Hubble parameter.
The critical density is currently 9.3 × 10-30g cm-3,
about 6 hydrogen atoms per cubic meter
(for H0 = 70 km s-1 Mpc-1).
Fr.: métallicité critique
The → metallicity of a → star-forming → molecular cloud when → cooling → rates by → metals dominate the → gravitational → heating during → protostellar collapse. The minimum → Jeans mass achieved by gravitational → fragmentation depends on the presence/absence of → coolants in the cloud. Since cooling rate in metal lines is more efficient than in primordial molecular lines (H2 and HD), metals favor fragmentation in gas and formation of → low-mass stars.
Fr.: vitesse critique
1) A state of molecular structure in some resins attributed to the existence of solid
crystals with a definite geometric form.
Fr.: densité de courant
The electric current per unit of cross-sectional area perpendicular to the direction of current flow. It is a vector quantity and represented by symbol J. Electric current density is usually expressed in amperes per square meter.
degree of stability
Fr.: degré de stabilité
1) izad (#); 2) izadgân
Fr.: 1) dieu, déesse; 2) divinité
1) A god or goddess.
M.E. deite, from O.Fr., from L.L. deitat- (nominative deitas), from L. dei- (combining form of deus "god") + -tat- "-ty," formed after L. divinitas "divinity."
Izad "god;" from Mid.Pers. yazêt "god; angel," izišn "performance of the religious rites, worship," yašt "worship, religious ceremony," yaštan "to venerate, to perform the religious ceremony;" O.Pers. yad- "to worship;" Av. yaz- "to sacrifice, worship, venerate," yazata- "deities," yasna- "religious rite" (Mod.Pers. jašn "feast"); Proto-Ir. *iaz- "to sacrifice, worship, venerate."
The amount of any quantity per unit volume. The mass density is the
mass per unit volume. The energy density is the energy per unit
volume; particle density is the number of particles per unit volume.
Noun form of → dense.
Fr.: cuspide de densité
A localized increase in number of → stellar black holes near a → supermassive black hole predicted by models of galactic → stellar dynamics (Bahcall, Wolf, 1976, ApJ, 209, 214). Same as → stellar cusp.
Fr.: fluctuations de densité
In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies.
density of an element
Fr.: densité d'élément
The number of units of mass of the → chemical element that are present in a certain volume of a medium. The density of an element depends on temperature and pressure. The element Osmium has the highest known density: 22.61 g/cc; in comparison gold is 19.32 g/cc and lead 11.35 g/cc.
Fr.: paramètre de densité
One of the four terms that describe an arranged version of the
→ Friedmann equations. They are all time dependent.
Fr.: profile de densité
mowj-e cagâli (#)
Fr.: onde de densité
A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.
density wave theory
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.
density-bounded H II region
nâhiye-ye H II-ye cagâli karânmand
Fr.: bornée par la densité
An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.
A performance criterion for an electronic detector, reciprocal of the corresponding → noise-equivalent power (D = 1/NEP).
Of a mathematical function, the quality of being → differentiable.
Noun from → differentiable.
paxšandegi, hamgar-e paxš
Fr.: coefficient de diffusion
1) The ability to permit or undergo diffusion.