The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to nightfall. There are three types of twilight: → astronomical twilight, → civil twilight, and → nautical twilight. They are divided on the basis of the → solar depression angle.
M.E., cognate with Du. tweelicht, Ger. zwielicht, from twi- a combining form meaning "two, twice," but it appears to refer to "half" light, rather than the fact that twilight occurs twice a day + → light.
A shining with intermittent light, as the twinkling of the stars. → scintillation.
M.E. twinklen (v.), O.E. twinclian, frequentative of twincan "to wink, blink;" related to M.H.G. zwinken, Ger. zwinkern.
Cešmak "twinke," from cešm, → eye, + -ak relation suffix.
Fr.: paradoxe des jumeaux
A thought experiment in special relativity, according to which if one of a pair of twins (A) remains on Earth, and the other (B) travels at a speed near the speed of light, B will be younger than A upon returning to Earth. In fact there is no paradox, because the two perspectives, A and B's, are actually not completely symmetric. There is no fixed time difference between the events, and different observers experience different intervals of time between the same two events. In fact, B returns younger than A because only B travels in a non-inertial (accelerating) reference frame. From A's point of view, B experiences time dilation, but from B's point of view the distance traveled is shortened because of length contraction. If B leaves in the year 2000 and returns in 2020, for A 20 years have elapsed. For B it depends on his travel speed. If has has moved as fast as 86% of the speed of light for him 10 years have passed. If his speed has been 99.5% of the speed of light the travel duration for him has been 2 years. This effect has been verified experimentally by measurements with atomic clocks.
Twin M.E.; O.E. twinn; cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung; → paradox.
Pârâdaxš, → paradox; hamzâdhâ, plural of hamzâd "twin," literally "born together," from ham- "together" → syn- + zâd "born," from zâdan "to bring forth, give birth" (Mid.Pers. zâtan; Av. zan- "to bear, give birth to a child, be born," infinitive zazāite, zāta- "born;" cf. Skt. janati "begets, bears," janitár "progenitor, father;" Gk. genetor "progenitor;" L. gignere "to beget," nasci "to be born," as above, PIE base *gen- "to give birth, beget").
M.E., from O.E. twa, feminine and neuter form of twegen "two," from P.Gmc. *twai (cf. O.S., O.Fris. twene, twa, O.N. tveir tvau, Du. twee, O.H.G. zwene, zwo, Ger. zwei, Goth. twai), cognate with Pers. do, as below.
Do "two," from Mid.Pers. do; Av. dva-; cf. Skt. dvi-; Gk. duo; L. duo (Fr. deux), di; Lith. dvi; O.C.S. duva; PIE base *duwo.
Two Micron All Sky Survey (2MASS)
An astronomical → survey conducted from 1997 to 2001 of the entire sky in near-infrared J, H, and K bands (wavelengths 1.25, 1.65, and 2.17 microns respectively). The aim was to detect and characterize point sources brighter than about 1 → mJy in each band, with → signal-to-noise ratio greater than 10, using a pixel size of 2".0. Two automated 1.3-m telescopes were used, one at Mt. Hopkins, AZ, and one at CTIO, Chile. 2MASS is currently producing the following data products: 1) A digital atlas of the sky comprising approximately 4 million 8' × 16' images, having about 4" spatial resolution in each of the wavelength bands. 2) A point source catalog containing accurate positions and fluxes for 300 million stars and other unresolved objects. 3) An extended source catalog containing positions and total magnitudes for more than 1,000,000 galaxies and nebulae.
parâse-ye do jesm
Fr.: problème à deux corps
In classical mechanics, the study concerned with the dynamics of an isolated system of two particles subject only to the Newtonian gravitational force between them. The problem can be separated into two single-particle problems with the following solutions. The equation of the → center of mass is governed by the equation of the same form as that for a single particle. Moreover, the motion of either particle, with respect to the other as origin, is the same as the motion with respect to a fixed origin of a single particle of → reduced mass acted on by the same internal force. See also → three-body problem, → n-body problem.
râžmân-e do jesm
Fr.: système à deux corps
nemudâr-e do rang
Fr.: diagramme deux couleurs
A graph on which two color indices such as B-V and U-B are plotted, one along each axis, for a sample of stars or other objects, such as stars.
Fr.: écoulement bi-dimensionnel
A flow whose parameters are functions of time and two space coordinates (x and y) only. There is no variation in the z direction and therefore the same → streamline pattern could at any instant be found in all planes in the fluid perpendicular to the z direction (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).
Fr.: émission à deux photons
The simultaneous emission of two photons whose sum of energies is equal to that of a single electron transition. The energy of each individual photon of the pair is not fixed, so that the spectrum of two-photon emission is continuous from the wavelength of that transition to infinity. In practice, there is a peak in wavelength distribution of the emitted photons. Two-photon emission is studied atomic physics with application in astrophysics, as it contributes to the continuum radiation from → planetary nebulae. It was recently observed in condensed matter and specifically in → semiconductors.
Fr.: TY CrA
A multiple stellar system embedded in a → reflection nebula (NGC 6726/6727) in the → Corona Australis star forming region. The three stars are within 1.5 → astronomical units of each other and lie at a distance of 129 pc. Two of its components form a → double-line eclipsing binary with an orbital period of almost 3 days. The primary is a → main sequence star (mass 3.16 solar masses, effective temperature 12,000 K) and the secondary a → pre-main sequence star (mass 1.64 solar masses, effective temperature 4,900 K) with an age of a few million years. A third spectroscopic component is in a wide orbit around the eclipsing pair. A fourth star is located at a projected distance of about 0.3 arcsecond (40 AU) from the compact tertiary system.
Fr.: étoile de Tycho
A → supernova of Type Ia in the constellation → Cassiopeia, which Tycho Brahe observed in November 1572. At its peak it was as bright as Venus and was visible in the daytime, reaching a magnitude of about -4. It is now visible as a → supernova remnant about 20 light-years across at a distance of about 7,500 light-years. It is associated with faint emission in the optical and X-rays, but is a strong radio source. Other designations: SN 1572, 3C 10, B Cas, 2U 0022+63.
Named after the Danish astronomer Tycho Brahe (1546-1601), who described the event; → star.
Fr.: modèle tychonique
A hybrid of → geocentric and → heliocentric systems in which the Earth remains at the center, with the → Sun orbiting the Earth, but with all the other planets revolving about the Sun. Moreover, the fixed stars were located in a crystalline shell centered on the Earth.
Related to the Danish astronomer Tycho Brahe (1546-1601), who imagined the model; → model.
Thin brass disk inserted into the → mater of a → planispheric astrolabe. It carries the grid of → altazimuth coordinates shown in polar → stereographic projection for the → latitude of the specific observation locality. In addition to the main → vertical circles and the main → almucantars, the tympanum reproduces some elements of the → celestial sphere - the → celestial equator, the → tropic of Cancer, and the → tropic of Capricorn, as well as the hour lines for determining the unequal hours of day or night respectively from the Sun's position on the ecliptic or from the altitude of a star marked on the → rete (online museo galileo, VirtualMuseum).
From L. tympanum "drum," from Gk. tympanon "a drum, panel of a door," from root of typtein "to beat, strike."
Hojré, from Ar. hujrah "chamber."
Fr.: effet Tyndall
The observation whereby when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. The effect is most commonly known as the → Rayleigh scattering.
Names for John Tyndall (1820-1893), who discovered the effect in 1859.
A group of items that have strongly marked and readily defined similarities.
M.E., from M.Fr., from L. typus "figure, image, form, kind," from Gk. typos "dent, impression, mark, figure, original form," from root of typtein "to strike, beat."
Guné "species; color; form; manner, kind;" Mid.Pers. gônak "kind, species;" Av. gaona- "color; body hair;" PIE base *góur- "(animal) body hair," genitive form *gunós.
Type 1 Seyfert (Sy 1)
kahkešân-e Seyfert-e guné-ye 1
Fr.: galaxie Seyfer de type 1
A type of → Seyfert galaxy showing both broad and narrow spectral emission lines. The widths of the broad lines indicates velocities around 1000 km s-1. Sy 1 galaxies are also very bright sources of UV and X-ray emission.
Type 2 Seyfert (Sy 2)
kahkešân-e Seyfert-e guné-ye 2
Fr.: galaxie Seyfer de type 2
A type of → Seyfert galaxy showing only narrow emission lines and weak UV and X-ray emissions but strong IR emission.
Type I burst
belk-e gune-ye I
Fr.: sursaut de type I
A burst of → X-rays observed toward → low-mass X-ray binary (LMXB)s. It is characterized by a sharp increase in → luminosity, which lasts 1-10 s, followed by the peak and a slow decrease, which can last from ~ 10s to 100s. Observationally, X-ray bursts manifest as a bright peak of emission on top of the persistent emission powered by → accretion. See also → Type II burst.
Type I error
irang-e gune-ye I
Fr.: erreur de type I