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period derivative vâxane-ye dowré Fr.: dérivée de la période The rate at which the rotation period of a → pulsar changes over time. This quantity, dP/dT, can range from as small as 0.05 picoseconds per year (1.5 x 10^{-21} seconds per second) to as large as about 10 milliseconds per year (4.2 x 10^{-10} seconds per second). For the → Crab pulsar, the period derivative is 4.2 x 10^{-13} s s^{-1}, implying a decrease in the star's → rotation energy of about 4.5 x 10^{38} erg s^{-1}. Period derivative is a very important parameter for the determination of the pulsar age. → period; → derivative. |
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 dowreyi (#) Fr.: périodique Recurring at regular intervals of time. Adjective of → period. |
periodic comet dombâledâr-e dowreyi Fr.: comète périodique A comet with a period of less than 200 years. Also called short-period comet. |
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. |
periodic system râžmân-e dowreyi Fr.: système périodique Arrangement of the → chemical elements in the → periodic table. |
periodic table jadval-e dowreyi (#) Fr.: tableau périodique An arrangement of the → chemical elements
in order of their → atomic numbers in such a way
as to demonstrate periodic similarities and trends in physical and chemical properties.
Elements with similar properties are arranged in the same column
(called a group), and elements with the same number of
→ valence electrons, or number of electrons in the outer shell,
are arranged in the same row (called a period).
Under the latest recommendations from IUPAC (the International Union of Pure
and Applied Chemistry), the groups are labelled 1 to 18 from left
to right (1988, Pure and Applied Chemistry 60, 431).
Also called Mendeleev's table. |
periodic term tarm-e dowre-yi Fr.: terme périodique In perturbation theory used in celestial mechanics, a term that indicates a bounded disturbance which recurs regularly. → secular term. |
periodic wave mowj-e dowre-yi Fr.: onde périodique An oscillatory motion in which each point is repeatedly displaced at equal time intervals. |
periodicity dowreigi Fr.: périodicité A state or condition characterized by regular repetition in time or space. |
periodogram dowrenegâšt Fr.: périodogramme A plot for examining frequency-domain data in an equi-spaced → time series. The periodogram is the → Fourier transform of the → autocovariance function. The periodogram method relies on the definition of the → power spectral density . |
peripheral pirâbari Fr.: périphérique Pertaining to, situated in, or constituting the periphery. Adj. of → periphery. |
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. |
periphery pirâbar Fr.: périphérie The external surface or boundary of a body. The circumference or perimeter of any closed figure. From, M.E., from O.Fr. periferie, from L.L. peripheria, from Gk. peripheria "circumference, outer surface," literally "a carrying around," from peripheres "rounded, moving round," peripherein "to carry or move round," from → peri- "round about" + pherein "to carry;" cognate with Pers. bordan "to carry, lead," as below. Pirâbar, from pirâ-, → peri-, + bar present stem of bordan "to carry, lead" (Mid.Pers. burdan, O.Pers./Av. bar- "to bear, carry," barəθre "to bear (infinitive)," Skt. bharati "he carries," Gk. pherein, L. fero "to carry;" PIE base *bher- "to carry"). |
periscope pirâbin, pirânamâ (#) Fr.: périscope An optical instrument for viewing objects which are above the eye-level of the observer, or are placed so that direct vision is blocked. |
permafrost mândbašm Fr.: pergélisol Layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0°C from a few to several thousands of years. It exists where summer heating fails to reach the base of the layer of frozen ground. From perma(nent) + → frost. |
permanent mândegâr (#) Fr.: permanent Lasting or remaining without essential change. Permanent, from M.Fr. permanent, from L. permanentem "remaining," pr.p. of permanere "endure, continue, stay to the end," from per- "through" + manere "stay," cognate with Pers. mândan, as below; → gas. |
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