An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The number of complete rotations per unit time, expressed in radian per second: ω = 2πν, where ν is the frequency (cycles per second).

→ angular; → frequency.

One of the frequencies that results from the combination of two waves of slightly different frquencies. A beat frequency is equal to the absolute value of the difference between the two frequencies. An unknown frequency can be determined by beating it with a reference frequency. More specifically, when the two frequencies are superimposed, the phase difference will change with time and wave interference alternate between constructive and destructive. The alterations of intensity brings about a beat frequency.

→ beat; → frequency.

The fraction of stars that have at least one → companion. It is at least 50%. The binary fraction appears to increase with increasing → primary star mass, at least among the more massive stars: the → O stars and → B stars have a companion frequency of at least 70%, while for the → G stars the binary frequency is around 50% and the → M stars may have an even lower binary frequency of around 30-40%. Brown dwarfs are rare as companions to lower-main-sequence stars, although brown-dwarf binaries appear not to be rare. An increase in binary frequency with mass would be expected if most stars form in → multiple systems that disintegrate, since the more massive stars would then preferentially remain in binaries while the less massive ones would preferentially be ejected as single stars (see Richard B. Larson, 2001, in IAU Symposium 200, p. 93 and references therein).

→ binary; → frequency.

The frequency at which an air parcel will oscillate when subjected to an infinitesimal perturbation in a stably stratified atmosphere. For a medium with a continuous density gradient, it is expressed by the formula: N² = -(g/ρ)∂ρ/∂z , where g is the → gravitational acceleration, ρ is density, and z geometric height. The stability condition is N > 0. It is also sometimes referred to as the buoyancy frequency. The higher the value of N the more stable the flow.

Named aster David Brunt (1886-1965), British meteorologist (1927, Q.J.R.Met.Soc. 53, 30) and Vilho Väisälä (1889-1969), Finnish meteorologist (1925, Soc. Sci. Fenn. Commental. Phys. Math. 2 (19), 19); → frequency.

Same as the → Brunt-Vaisala frequency.

→ buoyancy; → frequency.

A quantity defined as f = 2ω.sinθ, where ω is the Earth's → angular velocity, 2π/T, T is the rotation period of the Earth (→ sidereal day), and θ is the → latitude. Also called the → Coriolis parameter. This frequency occurs often in oceanographic studies. If an → iceberg is floating in a frictionless sea, and is given a push and allowed to move freely, it will travel in a circle of radius U/f, where U is the initial speed imparted by the push. This circle is called an inertial circle.

→ Coriolis effect; → frequency.

The frequency with which a → non-relativistic particle of charge q and mass m turns in a cyclotron with → uniform magnetic field B. The equality of the → centripetal force to the → Lorentz force leads to the expression: by: f_cycl = qB/2πm. For → relativistic case, the frequency is smaller and is called → synchrotron frequency.

→ cyclotron; → frequency.

In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort's constants. In the solar neighborhood the epicyclic frequency is about 32 km s^-1 kpc^-1.

→ epicyclic; → frequency.

The number of times per second that the frame is scanned in television. Also known as picture frequency.

→ frame; → frequency.

The number of complete oscillations per unit time of a vibrating system. The reciprocal of the → period, T.

From L. frequentia "assembly, multitude, crowd."

Basâmad, from bas "many, much" (Mid.Pers. vas "many, much;" O.Pers. vasiy "at will, greatly, utterly;" Av. varəmi "I wish," vasô, vasə "at one's pleasure or will," from vas- "to will, desire, wish") + âmad past stem of âmadan "to occur, to come, to become" (Mid.Pers. âmatan; O.Pers. gam- "to come; to go," Av. gam- "to come; to go," jamaiti "goes;" Proto-Iranian *āgmatani; Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE root *gwem- "to go, come").
Feregi, from feré "much, more; increase; priority," farâ(vân) "abundant," Mid.Pers. frêh "more, much;" O.Pers. fra- "before, forth;" Av. frā, fərrā "forth, forward;" PIE base *pro-; cf. L. pro "on behalf of, in place of, before, for;" Gk. pro- "before, in front of."

A range of frequencies that is continuous between two specified limits, selected from a more extended range of frequencies.

→ frequency; → band.

An undesired progressive change in an oscillator's frequency with time.

→ frequency; → drift.

The change in the frequency of a wave motion due to the → Doppler effect.

→ frequency; → shift.

A graphical display of the intensity of radiation or energy versus frequency.

→ frequency; → spectrum.

A mode of observation in radio astronomy in which the telescope remains at a fixed position and data is accumulated while the center of the receiver passband is switched between signal and offset frequencies. Data accumulated at the offset frequency is subtracted from the signal frequency data before storage as a frequency switched scan. → beam switching.

→ frequency; → switching.

Deriving the → wavelength of an undulatory phenomenon from its → frequency, and vice versa.
1) For → electromagnetic waves: λ = c / f, where λ is the wavelength, c is the → speed of light in → meters per second and f the frequency in → hertz. It can be written as: λ (m) = 2.998 × 10⁸ / f (Hz).
2) For → sound waves: λ = C / f, where C is the → sound speed. For air at temperature 0°C, λ (m) = 332 / f (Hz).

→ frequency; → wavelength; → conversion.

The lowest frequency in a complex wave.

→ fundamental; → frequency.

The frequency with which an electron or other charged particle executes spiral gyrations in moving across a magnetic field.

→ gyro-; → frequency.

In a → superheterodyne receiver, a frequency resulting from the combination of the received modulated → carrier frequency and the → local oscillator frequency.

→ intermediate; → frequency.

The frequency of precession of a charged particle describing a circular motion in a plane perpendicular to the magnetic induction in a uniform magnetic field.

Named after Joseph Larmor (1857-1942), an Irish physicist, the first to calculate the rate at which energy is radiated by an accelerated electron, and the first to explain the splitting of spectrum lines by a magnetic field; → frequency.