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.

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.

An oscillatory motion in which each point is repeatedly displaced at equal time intervals.

→ periodic + → wave.

Same as → periodic.

→ periodic; → -al.

A variable → supergiant star with typical periods of the order of 10 to 100 days and amplitudes less than a few tenths of a magnitude. PVSGs are thought to be pulsating → g modes, caused by a density inversion, arising from an → opacity bump, most likely from Fe, H, and/or He.

→ periodical; → -ly; → variable; → supergiant.

A state or condition characterized by regular repetition in time or space.

→ periodic + → -ity.

In a dynamical system, a form of motion that is regular but never exactly repeating. Quasiperiodic motion appears when the system contains two or more incommensurate frequencies.

→ quasi-; → periodic; → motion.