21-centimeter line xatt-e 21-sântimetr (#) Fr.: raie 21-cm The → emission line of → neutral hydrogen in interstellar clouds. It lies in the radio spectrum at wavelength of about 21 cm, frequency 1420 MHz. Its existence was predicted by Hendrik van de Hulst in 1944 and discovered by Harold Ewen and Edward Purcell in 1951. Twenty-one (21); → twenty + → one; → centimeter; → line. |
absolute time zamân-e avast Fr.: temps absolu A universal time supposed to be the same for all observers at any place in the Universe. Absolute time is one of the foundations of → Newtonian mechanics, but it fails to account for physical phenomena in → reference frames with relative motion. Its abandoning was one of the starting points of → special relativity. See also → absolute space; → space-time. |
accretion time zamân-e farbâl, direš-e ~ Fr.: temps d'accrétion The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate. |
altimeter farâzsanj (#), farâzyâb (#) Fr.: altimètre An instrument which determines the altitude of an object with respect to a fixed level, such as sea level. L. altus "high" + → -meter. Farâzsanj, from farâz "above, over, aloft" + -sanj, → -meter; farâzyâb, from farâz + yâb "finder," from yâbidan "to find, discover, obtain." |
altimetry farâzsanji (#), farâzyâbi (#) Fr.: altimétrie The measurement of heights in the atmosphere (altitude) by an → altimeter. |
apparent sidereal time zamân-e axtari-ye padidâr Fr.: temps sidéral apparent The → mean sidereal time corrected for the → nutation and shift in the obliquity of the ecliptic that occurs as a result of the Moon's gravitational effect. Apparent sidereal time differs from mean sidereal time in that the → true vernal equinox point is used. |
apparent solar time zamân-e xoršidi-ye padidâr Fr.: temps solaire apparent The time based on the motion of the → apparent Sun and kept by dividing the day into 24 equal hours. |
arrival time zamân-e raseš Fr.: temps d'arrivée The precise time at which the gamma burst photons hit a detector. Measuring the time difference between the arrival time of the photons at different telescopes separated by known distances permits to determine the burst direction. Arrival, n. from arrive + → -al. Arrive, from O.Fr. ariver "to come to land," from V.L. *arripare "to touch the shore," from L. ad ripam "to the shore," from → ad "to" + ripa "shore;" → time. Zamân, → time; raseš n. from rasidan "to arrive," Mid.Pers. rasitan, O.Pers./Av. rasa- present stem of ar- "to move, go or come toward," cf. Skt. ar-, rcchati. |
atomic time zamân-e atomi (#) Fr.: temps atomique Time measured using atomic clocks. |
Barycentric Coordinate Time (TCB) zamân-e hamârâ-ye gerânigâhi Fr.: temps-coordonnée barycentrique (TCB) A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB). → barycenter; → coordinate; → time. |
Barycentric Dynamical Time (TDB) zamân-e tavânik-e gerânigâhi Fr.: temps dynamique barycentrique (TDB) A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB). → barycenter; → dynamical; → time. |
centimeter (cm) sântimetr (#) Fr.: centimètre A unit of length in the → metric system, equal to one-hundredth of a meter, which is the current unit of length in the → International System of Units (SI). |
civil time zamân-e šârin Fr.: temps civil |
coherence time zamân hamdusi Fr.: temps de cohérence The time over which a propagating electromagnetic wave may be considered coherent. The coherence time of an interferometer is the interval during which the fringe phase remains stable. |
continuous-time quantum walk puyeš-e kuântomi bâ zamân-e peyvasté Fr.: marche quantique à temps continu A → quantum walk taking place entirely in the position space. Continuous-time quantum walk was introduced by E. Farhi & S. Gutmann (1998, Phys. Rev. A 58, 915). → continuous; → time; → quantum; → walk. |
cooling time zamân-e sardeš Fr.: temps de refroidissement 1) The time in which a → white dwarf cools to half its
temperature. It depends on the composition, the mass, and the actual luminosity
at some point in time. Cooling time is given by the relation:
t = 8.8 × 10^{6} (12/A) (M)^{5/7} (μ/2)^{-2/7}
(L)^{-5/7} in years, where M and L are mass and luminosity in
solar units, A the mean → atomic mass, and μ the
→ mean molecular weight (Iben & Tutukov, 1984, ApJ 282, 615).
See also → Mestel theory;
→ white dwarf crystallization. |
coordinate time zamân-e hamârâ Fr.: temps-coordonnée In relativity, the proper time in the specified reference frame. Because of time dilation, this may differ from the time experienced by any participant in the events being considered. It is the time basis (or coordinate) to be used in the theory of motions referred to this system. → coordinate; → time. |
Coordinated Universal Time (UTC) zamân-e jahâni-ye hamrârâsté Fr.: temps universel coordonné An international high-precision time standard based on the Greenwich Mean Time and adjusted to compensate for divergence from atomic time. It is based on the non-uniform rotation of the Earth (UT1) and the perfectly uniform international atomic time (TAI). UTC differs from TAI by the total number of → leap seconds, so that UT1-UTC stays smaller than 0.9 sec in absolute value. → coordinate; → universal; → time. |
crossing time zamân-e gozar (#) Fr.: temps de traversée A concept used for checking the stability of a group of mass such as a cluster of galaxies or a stellar cluster. The crossing time is given by t_{c} = R/V, where R is the average projected radial distance of group members from the center of mass and V the Gaussian dispersion in internal velocity. Zamân, → time; gozar "crossing," infinitive gozaštan, Mid.Pers. vitartan "to pass," Sogdian wytr "to go," O.Pers./Av. vitar- "to put across," from prefix vi- "away, apart," + tar- "to cross over" (Skt. tr- "to cross over," tarati "crosses," cf. Av. taro, Skt. tirah "through, beyond," L. trans "beyond," Ger. durch, E. through; PIE base *tr- "through"). |
curvature of space-time xamidegi-ye fazâ-zamân (#) Fr.: courbure de l'espace-temps According to → general relativity, → space-time is curved by the presence of → matter. The curvature is described in terms of → Riemann's geometry. In → cosmological models three types of curvature are considered: positive (spherical, → closed Universe), zero (Euclidean, → flat Universe), and negative (hyperbolic, → open Universe). See also → curvature constant. → curvature; → space-time. |