An Etymological Dictionary of Astronomy and Astrophysics
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A system in which the → damping is great enough to prevent oscillation.

Aperiodic, from → a- + periodic; + → damping.

A period of 76 years after which the new and full moons would return to the same day of the solar year. This was intended as an improvement of the → Metonic cycle because the 6940 days of the Metonic cycle exceeded 19 years by about a quarter of a day, and exceeded 235 → lunations by a larger amount of time.

Named after Calippus of Cyzicus (about 370-300 BC), a Greek astronomer and mathematician.

A period during which a newly constructed observing instrument is used for test.

→ commissioning; → period.

An algorithm for detecting and characterizing periodic signals in unevenly-sampled data. The Lomb-Scargle periodogram has a particularly wide use within the astronomy community. This method allows efficient computation of a Fourier-like → power spectrum estimator from such unevenly-sampled data, resulting in an intuitive means of determining the period of oscillation (see VanderPlas, 2017, astro-ph/1703.09824 and references therein).

Named after Lomb, N. R. 1976, Ap&SS 39, 447 and Scargle, J. D. 1982, ApJ 263, 835; → periodogram.

A comet with orbital period of more than 200 years. → short-period comet; → periodic comet.

→ long; → period; → comet.

A type of → variable star in which variations in brightness occur over long time-scales of months or years. The term generally refers to → Mira variable types.

→ long; → period; → variable.

Same as → sidereal month.

→ lunar; → sidereal; → orbital; → period.

Of a body or system, the period of → free oscillation.

→ natural; → period.

The time interval between two successive passages of an object through the same point in its orbit around another object.

→ orbital; → period .

Physics: The duration of one complete cycle of an oscillation; the reciprocal of the frequency.
Astro.: The amount of time an astronomical body takes to complete an orbit.

From M.E. periode, from M.Fr., from M.L. periodus "recurring portion, cycle," from L. periodus "a complete sentence," from Gk. periodos "rounded sentence, cycle, circuit, period of time," literally "going around," from → peri- "around" + hodos "way, journey;" cognate with L. cedere "to go, yield."

Dowré, from dowr, from Ar. daur "age, time; revolution."

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 10³⁸ erg s^-1. Period derivative is a very important parameter for the determination of the pulsar age.

→ period; → derivative.

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.

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.

→ period; → mean; → density; → relation.

Recurring at regular intervals of time.

Adjective of → period.

A comet with a period of less than 200 years. Also called short-period comet.

→ periodic; → comet.

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; → function.

Any motion that recurs in identical forms at equal intervals of time.

→ periodic; → motion.

Arrangement of the → chemical elements in the → periodic table.

→ periodic; → system.

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.
The periodic table was introduced by Dmitri Ivanovich Mendeleev (1834-1907) in 1869, who originally arranged them in order of their → atomic weights. Using the table, it was possible for Mendeleev to correct some of the atomic weights (e.g. that of beryllium) and to predict the properties of a number of elements yet to be discovered (e.g. gallium, scandium, and germanium). The British physicist Frederick Soddy (1877-1956) showed that the loss of an → alpha particle reduces the nuclear charge by two and hence lowers the atomic number by two and the position of the element in the periodic table by two groups.

→ periodic; → table.

In perturbation theory used in celestial mechanics, a term that indicates a bounded disturbance which recurs regularly. → secular term.

→ periodic; → term.