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pericynthion pirâmâh Fr.: périlune The point in the orbit of a satellite around the Moon closest to the Moon; opposite of → apocynthion. |
perigalacticon pirâkahkešân Fr.: périgalacticon The point in an object's orbit around a galaxy when the object lies closest to the galactic center; opposite of → apogalacticon. |
perigee full Moon pormâng-e pirâzam Fr.: pleine lune de périgée The → full Moon when our natural satellite is at its closest approach to the Earth. Perigee full Moons are as much as 14% larger and 30% brighter than → apogee full Moons. Also called perigee-syzygy full Moon, super full Moon, and → supermoon. The Supermoon on November 14, 2016, was the closest (356,523 km) a Full Moon has been to Earth since January 26, 1948. The next time a Full Moon is even closer to Earth (356,448 km) will be on November 25, 2034. |
perigee-syzygy full Moon pormâng-e pirâhur-yujân Fr.: lune de périgéé-syzygie |
perihelion pirâhur Fr.: périhélie The nearest point to the Sun in an orbit around the Sun; opposite of → aphelion. Perihelion, from L. perihelium, from → peri- + helios "sun," cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun; PIE *sawel- "sun". Pirâhur, from pirâ-, → peri-, + hur "sun," as above; |
perihelion advance pišraft-e pirâhur Fr.: avance du périhélie |
perihelion distance apest-e pirâhuri Fr.: distance au périhélie The distance between the → Sun and an → object in orbit around it when they are at their closest approach. → perihelion; → distance. |
perihelion precession pišâyân-e pirâhuri Fr.: précession du périhélie → perihelion; → precession. |
period-luminosity relation bâzâneš-e dowré-tâbandegi Fr.: relation période-luminosité A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud. Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe. → period; → luminosity; → relation. |
period-mean density relation bâzâneš-e dowré-cagâli-ye miyângin Fr.: relation période-densité moyenne A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)^{1/2}, where ρ is the mean density, γ the ratio of → specific heats (C_{p}/C_{v}), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs. |
periodic function karyâ-ye dowreyi Fr.: fonction périodique A function f(x) if for all x, f(x + P) = f(x), where P is a positive constant. The least value of P > 0 is called the period of f(x). |
periodic motion jonbeš-e dowreyi Fr.: mouvement périodique Any motion that recurs in identical forms at equal intervals of time. |
peripheral response pâsox-e pirâbari Fr.: réponse périphérique In a charge-coupled device, the detection of charge collected by the transport register rather than by the image-sensing elements. → peripheral; → response. |
peripheral vision did-e pirâbari Fr.: vision périphérique In optics, the ability to see over large angles of view. → peripheral; → vision. |
permission parzâmeš Fr.: permission The act of permitting. Authorization granted to do something. |
permitted transition gozareš-e parzâmidé Fr.: transition permise A transition between two quantum mechanical states that does not violate the quantum mechanical selection rules. → permitted; → transition. |
permutation permuteš, jâygašt Fr.: permutation Math.: A rearrangement of the elements of a set in a particular order. The number of permutations of n objects is equal to n! (→ factorial n). For example, there are 24 permutations of letters A, B, C, and D (4! = 1 × 2 × 3 × 4). The number of permutations of n objects taken r at a time is denoted by ^{n}P_{r} and equals n! / (n - r)!. For example, the number of permutation of A, B, C, and D taken two at a time is 12. If n objects are of k different kinds, with r_{1} alike of one kind, permutations of n objects equals n! / r_{1}! r_{2}! ... r_{k}!, where r_{1} + r_{2} + ... r_{k} = n. Verbal noun of → permute. Permuteš, verbal noun of → permute. |
perpetual motion jonbeš-e hamišegi Fr.: mouvement perpétuel The motion of a hypothetical machine which, once set in motion, will go on for ever without any losses due to → friction or other forms of → dissipation of energy and without receiving any external energy. |
person tanum Fr.: personne 1) A human being, whether man, woman, or child as distinguished from an animal or a thing. M.E. persone, from O.Fr. persone "human being," from L. persona "human being," originally "character in a drama, mask," possibly borrowed from Etruscan phersu "mask," from Gk. prosopa "face; mask" + -na a suffix. Tanum, from O.Pers. and Av., related to Mod./Mid.Pers. tan "body, person," O.Pers. tanūš "body," tanūm [acc.sg.] "(to) onself;" Av. tanū- "body, person, self," tanūm [acc.sg.]; cf. Skt. tanūš- "body, self;" PIE base *ten-uh- "body." |
personage tanumsâ Fr.: personnage 1) A person of distinction or importance. M.E. "form, appearance, stature, figure, air, and the like, of a person," from O.Fr. personage "size, stature; a dignitary," from M.L. personagium, from persona, → person. Tanumsâ, from tanum, → person, + -sâ, contraction of -âsâ, suffix of "form, type, similarity." |
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