An Etymological Dictionary of Astronomy and Astrophysics
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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.

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 .

→ period; + euphonic infix -o-; → -gram.

The interval with which a rotating body precesses. The precession period of the Earth is 25,770 years. For a → spinning top it is given by: T_p = (4π²I)/(mgrT_s), where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top.

→ precession; → period.

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.

The last two million years of geologic time, comprising the Pleistocene and Holocene glacial epochs. Estimates of the date of the beginning of the Quaternary vary between 2.5 and 1.6 million years ago.

Quaternary, from L. quaternarius "consisting of four," from quatern(i) "fourt at a time" + -arius "-ary;" → period.

Dowrân, → period; cahârom "fourth," from cahâr "four" + -om "-th."

The interval of time during which an object turns once about its axis.

→ rotation; → period.

→ rotation period.

→ rotational; → period.

A comet with a period less than 200 years. Same as → periodic comet.

→ short; → period; → comet.

A variable star that has a relatively short period with respect to stars of similar types.

→ short; → period; → variable.

→ sidereal revolution period; → sidereal rotation period.

→ sidereal; → period.

The time taken by a planet or satellite to complete one revolution about its primary with respect to stars. For Earth, same as → sidereal year. Sidereal periods of the solar system planets, interms of the sidereal year, are as follows: Mercury 0.240846 (87.9691 days); Venus 0.615 (225 days); Earth 1 (365.25636 solar days); Mars 1.881; Jupiter 11.86; Satrurn 29.46; Uranus 84.01; and Neptune 164.8. That of the Moon is 0.0748 (27.32 days) and for → Sedna 12050.

→ sidereal; → revolution; → period.

The rotation period of a celestial body with respect to fixed stars. For Earth, same as → sidereal day.

→ sidereal; → rotation; → period.

The interval after which the heliacal rising of the star Sirius occurs at the same time of the year. It is a period of 1,460 Sothic years.

From Fr. sothique, from Gk. Sothis, an Egyptian name of Sirius; → period.

Tištari, of or pertaining to Tištar→ serius; dowré, → period.

For planets, the mean interval of time between two successive → conjunctions with or → oppositions to the Sun. For example, → Mars has a synodic period of 779.9 days from Earth; thus Mars' oppositions occur once roughly 2.135 years. In comparison, the synodic period of → Venus is 583.9 days. If the sideral periods of the two bodies around the third are denoted T₁ and T₂, their synodic period is given by: 1/T_syn = |1/T₁ - 1/T₂|.

→ synodic; → period.

A → Cepheid star of → spectral type A-F with regular pulsation period of 1-3 hours and with small variations in amplitude. This group is also known as δ Scuti stars.

→ ultra-; → short; → period; → Cepheid.

A series of 532 years, arising from the cycles of the Sun and Moon multiplied into one another. It was used by the Western churches for many years, in computing the time of Easter, till the → Gregorian calendar was established.

Named after Victorius (a French clergyman), who invented the period about the middle of the 5th century.