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daylight saving time vaxt-e nur anduzi, vaqt-e ~ Fr.: heure d'été A system of adjusting the official local time in some countries in order to provide a better match between the hours of daylight and the active hours of work and school. The "saved" daylight is spent on evening activities which get more daylight, rather than being "wasted" while people sleep past dawn. Although known also as summer time, it includes the spring season and nearly half of autumn. → day; → light; saving, from save, from O.Fr. sauver, from L.L. salvare "to secure," from L. salvus "safe," PIE *solwos, from base *sol- "whole" (cf. O.Pers. haruva-, Av. haurva- "whole, intact," Mod.Pers. har "every, all; any," Skt. sarva- "whole, entire," Gk. holos "whole"); → time. Vaxt, written vaqt |
daytime ruzhangâm (#) Fr.: temps de jour The time interval when the Sun is above the horizon for a given position. Ruzhangâm, from ruz→ day + hangâm "time, hour, season," Mid.Pers. hangâm "time, epoch, season," Av. ham-gam- "to meet together," from ham- "together," → com- + gam- "to come; to go," jamaiti "goes," O.Pers. gam- "to come; to go," Mod.Pers. âmadan "to come," Skt. gamati "goes," Gk. bainein "to go, walk, step," L. venire "to come," Tocharian A käm- "to come," O.H.G. queman "to come," E. come; PIE root *gwem- "to go, come." |
DB gap gâf-e DB Fr.: lacune DB The observed absence of helium white dwarfs with temperatures between 45,000 and 27,000 K. A possible explanation lies in the chemical evolution of white dwarfs. Four processes can change the structure of a white dwarf: gravitational settling, interstellar medium accretion, mass loss, and subsurface convective mixing. → DB white dwarf; → gap. |
DB white dwarf sefid kutule-ye DB Fr.: naine blanche DB A → white dwarf whose spectrum shows strong He I in the absence of hydrogen or metal lines. |
DC white dwarf sefid kutule-ye DC Fr.: naine blanche DC A → white dwarf showing a continuous spectrum with no readily apparent lines. |
de Broglie equation hamugeš-e de Broglie Fr.: équation de de Broglie According to the → de Broglie hypothesis,
which has been verified by experiments, every → particle
of matter, whatever its nature, has a characteristic → wavelength
associated with its wavelike quantum aspect. The de Broglie equation gives the
equivalent wavelength of a moving particle: λ = h/mv, where h is
→ Planck's constant, m the mass of the particle,
and v its velocity. Named after Louis Victor de Broglie (1892-1987), French physicist, creator of a new field in physics, wave mechanics, who won the Nobel prize in physics in 1929. → equation |
de Broglie hypothesis engâre-ye de Broglie Fr.: hypothèse de de Broglie The suggestion by Louis de Broglie in 1924 whereby if → electromagnetic waves possess particle properties (→ particle nature), then it might be reasonable to suppose that material particles, such as → electrons, should possess wave properties (→ wave nature). The de Broglie hypothesis was based on the intuitive feeling that nature seems to have strong attachment to symmetry. In other words, if radiation has particle-like properties, then material particles should possess wave-like properties. At the time no direct experimental evidence was present for the validity of this suggestion. The first confirmation of de Broglie's hypothesis was provided by the → Davisson-Germer experiment. See also → wave-particle duality; |
de Broglie wavelength mowjtul-e de Broglie Fr.: longueur d'onde de Broglie The wavelength of the wave associated with a → particle as given by the → de Broglie equation. |
de Sitter Universe giti-ye de Sitter Fr.: Univers de de Sitter A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (Ω_{M} = 0) or radiation (Ω_{R} = 0), is → Euclidean (k = 0), but has a → cosmological constant (Ω_{Λ} > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe. After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe. |
de Vaucouleurs law qânun-e de Vaucouleurs Fr.: loi de Vaucouleurs A mathematical expression describing the → surface brightness profile of a typical → elliptical galaxy, whereby the projected luminosity distribution varies as exp(-R)^{1/4}. → de Vaucouleurs radius; → law. |
de Vaucouleurs radius šo'â'-e de Vaucouleurs Fr.: rayon de Vaucouleurs An → isophotal radius of a galaxy, where the → surface brightness in the B band falls to 25 mag arcsec^{-2}. After the French-born American astronomer Gérard de Vaucouleurs (1918-1995); → radius. |
de Vaucouleurs system râžmân-e de Vaucouleurs Fr.: système de Vaucouleurs A widely used scheme for classifying galaxies which is an extension
to the → Hubble sequence
Hubble sequence. De Vaucouleurs introduced a more elaborate
classification system for spiral galaxies (→ spiral galaxy)
based on three morphological
characteristics: → de Vaucouleurs radius; → system. |
de- vâ- (#), foru- (#) Fr.: de- A prefix occurring in loanwords from L. to indicate: M.E., from O.Fr. de-, des-, partly from L. de- "from, down, away," and partly from L. → dis-. Prefix vâ- denoting "reversal, opposition; separation; repetition; open;
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." |
de-excitation foru-angizeš Fr.: désexcitation Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus. From → de- + → excitation. |
de-orbit vâmadâridan Fr.: désorbiter To cause a spacecraft to leave its operational orbit to enter a descent phase or to change course. |
de-orbiting vâmadâreš Fr.: désorbitage Of a spacecraft, the act or process of departing from an operational orbit. → de-orbit. Noun form of → de-orbit. |
de-projection vâ-farâšâneš Fr.: déprojection A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images. → de-; → projection. |
de-redden vâsorxidan Fr.: dérougir To correct the intensity or flux of a spectral line by eliminating the → reddening effect of → interstellar dust. De-redden, from → de- + redden verb of → red. Vâsorxidan, from vâ-, → de-, + sorxidan verb of sorx, → red. |
de-reddened magnitude borz-e vâsorxidé Fr.: magnitude dérougie A magnitude which has been corrected for the interstellar reddening. De-reddened, p.p. of → de-redden; → magnitude. Borz, → magnitude; vâsorxidé p.p. of vâsorxidan, → de-redden. |
de-reddening vâsorxeš Fr.: dérougissement The process that de-reddens. The state of being de-reddened. Verbal noun of → de-redden. |
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