Fr.: accélération instantanée
The → acceleration of a particle at time t defined by a = lim Δv/Δt = dv/dt. It is the limiting value of Δv/Δt at time t as both Δv and Δt approach zero.
In calculus the operation which consists of finding the areas enclosed by curves through the summations of infinitesimals. The inverse process of differentiation.
Verbal noun of → integrate.
Fr.: temps d'intégration
The time during which a detector integrates the incoming photons.
Fr.: nombre irrationnel
A → real number which cannot be exactly expressed as a ratio a/b of two integers. Irrational numbers have decimal expansions that neither terminate nor become periodic. Every → transcendental number is irrational. The most famous irrational number is √ 2.
Fr.: rapport isotopique
The relative abundances of two isotopes of a given chemical element, such as D/H (deuterium/hydrogen), (carbon) 12C/13C, and (uranium) 235U/238U.
A computational process involving a succession of approximations, which consists of repeating the operation by inputting the outcome of each preceding operation to improve the final result until a desired accuracy is achieved. Compare → repetition.
Verbal noun of → iterate.
halâzân, roxgard (#)
Small oscillations of a → celestial body about its mean position. The term is used mainly to mean the Moon's libration caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time. This results from three kinds of libration working in combination: → libration in longitude, → libration in latitude, and → diurnal libration. See also: → geometrical libration, → physical libration.
L. libration- "a balancing."
Halâzân "to and fro motion, oscillation," literally
"a swing: a seat suspended by ropes on which a person may sit for swinging,"
from Gilaki halâcin "a swing," Ilâmi harazân
"a swing," variants (Dehxodâ) holucin, holu "a swing,"
probably from Proto-Ir. *harz- "to send, to set."
libration in latitude
Fr.: libration en latitude
A tiny oscillating motion of the Moon arising from the fact that the Moon's axis is slightly inclined relative to the Earth's. More specifically, the Moon's polar axis is tilted nearly 7° with respect to the plane of its orbit around Earth. Hence for half of each orbit we see slightly more of the north pole when its tipped toward us, and for the other half we see slightly more of its south pole. Libration in latitude displaces the mean center of the Moon north-south by between 6°.5 and 6°.9.
libration in longitude
Fr.: libration en longitude
A tiny oscillating motion of the → Moon arising from the fact that the Moon's orbit is not a precise circle but rather an → ellipse. Therefore, Moon is sometimes a little closer to the Earth than at other times, and as a result its → orbital velocity varies a bit. Since the Moon's rotation on its own axis is more regular, the difference appears as a slight east-west oscillation. Libration in longitude is the most significant kind of libration. It varies between about 4°.5 and 8°.1 because of gravitational perturbations in the Moon's orbit caused by the Sun.
Fr.: accélération linéaire
Fr.: accelération magnetocentrifuge
The acceleration exerted on the plasma particles according to the → magnetocentrifugal model.
Fr.: rapport masse-luminosité
The ratio of the mass of a system, expressed in solar masses, to its visual luminosity, expressed in solar luminosities. The Milky Way Galaxy has a mass-luminosity ratio in its inner regions of about 10, whereas a rich cluster of galaxies such as the Coma Cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter.
mužeš, kuc (#)
1) The process or act of migrating; a migratory movement.
Fr.:rapport de mélange
Mass of water vapor per mass of dry air; expressed as grams per kilogram. → humidity
Fr.: concentration molaire
Of a gas included in the composition of a → gas mixture, the ratio of the number of moles of this gas to the total number of moles of all the gases in the mixture. Same as mole fraction and mole-fraction concentration.
Fr.: vibration moléculaire
The dynamical motion of chemically bound atoms which constantly change their position with each other. The vibration of molecules is treated within → quantum theory. Therefore, the energy of molecular vibration can only take → discrete values. To a first approximation, molecular vibrations can be approximated as → simple harmonic oscillator assigned to each mode.
National Aeronautics and Space Administration (NASA)
Sâzmân-e Fazânavardi-ye Âmrikâ
Fr.: NASA, Administration nationale de l'aéronautique et de l'espace
A federal agency of the United States government founded in 1958 for civil aeronautical research and space exploration, superseding the National Advisory Committee for Aeronautics (NACA). Its goals include improving human understanding of the universe, the solar system, and Earth and establishing a permanent human presence in space. NASA is headquarted at Washington, D.C., and operates several research, development, and test facilities, as follows alphabetically: 1) Ames Research Center; 2) Dryden Flight Research Facility at Edwards, California, used for flight testing and as a landing site for the Space Shuttle; 3) Glenn Research Center at Cleveland, Ohio, concerned with aircraft and rocket propulsion; 4) Goddard Space Flight Center; 5) Jet Propulsion Laboratory; 6) Johnson Space center; 7) Kennedy Space Center; 8) Langley Research Center at Hampton, Virginia, which carries out research in aeronautics and space technology; 9) Marshall Space Flight Center; 10) the Space Telescope Science Institute; 11) Stennis Space Center, near Bay St Louis, Mississippi, for testing rocket engines; and 12) Wallops Flight Facility on Wallops Island, Virginia, which manages NASA's sounding rocket and scientific balloon programs.
→ national; → aeronautics; → space; administration, verbal noun of administer, from M.E. amynistre, from O.Fr. aministrer, from L. administrare "to serve, carry out, manage," from → ad- "to" + ministrare "to serve," from minister "servant, priest's assistant," from minus, minor "less," hence "subordinate," + comparative suffix *-teros.
1) The act of obscuring.
1) General: An act or instance, process, or manner of functioning or operating.
Verbal noun of → operate
Pertaining to a process or series of actions for achieving a result.
Adj. of → operation.