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

A blue, → main sequence star of → apparent visual magnitude 2.44 and → spectral type A0 Ve located in → Ursa Major. Other designations: Phecda; Phekda; Phegda; Phekha; Phacd.

Phad, from Ar. al-Fakhidh (ad-Dubb) (الفخذ‌الدب) "the thigh (of the Bear)".

Faxez, from Ar., as above.

A hypothetical → planet which once was postulated to have existed between the orbits of → Mars and → Jupiter and its destruction supposedly led to the formation of the → asteroid belt. The idea of such a hypothetical planet was first put forward by the German astronomer Heinrich Wilhelm Olbers (1758-1840).

In Greek mythology Phaeton was the sun god Helios. Phaeton tried to drive his father's solar chariot but crashed after almost setting fire to the whole earth.

The tube or cavity, with its surrounding membrane and muscles, that connects the mouth and nasal passages with the esophagus (Dictionary.com). → throat.

From Gk pharynx (genitive pharyngos) "windpipe, throat."

Halq, loan from Ar.

1) A particular stage or point in a course, development, or graph varying cyclically; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Phase is measured like an angle, when a complete cycle is equivalent to a phase of 360° (or 2π radians), or, sometimes, as a number between 0 and 1. Two or more waves of the same frequency are → in phase when their maxima and minima take place at the same moments. Otherwise, they are said to be → out of phase or that they have a → phase difference.
2) A state in which matter can exist, depending on temperature and pressure, e.g. the → solid, → liquid, → gaseous, and → plasma states.
3) A recurring form of the → Moon or a → planet seen in the sky. → lunar phase, → phases of Venus.
4) In a → binary star system, → orbital phase.

Mod.L. phases, plural of phasis, from Gk. phasis "appearance," from stem of phainein "to show, to make appear."

1) Fâz, loanword from Fr., as above.
2) Simâ "face, aspect, resemblance."

1) Physics: Of a → periodic wave, the number of suitable units of angular measure between a point on the wave and a reference point.
2) Astro.: For an object in the solar system, the angle "Sun-object-Earth" that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun.
3) More generally, the angle between star light incident onto a related revolving object and the light reflected from the object to the observer (Earth).

→ phase; → angle.

1) Astro.: A curve describing the → brightness of a reflecting → natural satellite as a function of its → phase angle.
2) Math.: A plot of the solution to a set of equations of motion in a phase space as a function of time.

→ phase; → curve.

The ratio of the phase shift of a sinusoidal signal in transmission through a system to the frequency of the signal.

→ phase; → delay.

A graph showing the equilibrium relationships between phases (such as vapor-liquid, liquid-solid) of a chemical compound, mixture of compounds, or solution.

→ phase; → diagram.

The difference of phase (usually expressed as a time or an angle) between two periodic quantities which vary sinusoidally and have the same frequency.

→ phase; → difference.

The condition of temperature and pressure under which different phases (e.g. gas, liquid, and solid) of a substance coexist.

→ phase; → equilibrium.

The variation in brightness of a target as the phase angle (the angle between Sun and observer as seen from the target) varies between 0° and 180°. The directional distribution of reflected (or scattered) radiation. The phase angle is the supplement of the scattering angle (the angle between the incident ray and the emerging ray); in other words, the sum of the phase angle and the scattering angle is always 180° (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ phase; → function.

1) General: Same as → phase difference.
2) Cepheids: The observed phase difference between luminosity and velocity in classical (radially pulsating) → Cepheids. On the basis of adiabatic pulsation theory, one would expect the maximum luminosity to occur when the radius of the star is minimal. This means that the maximum outward velocity would be one quarter period out of phase with the maximum velocity. However, in the observations the maximum luminosity and maximum outward velocity are nearly in phase. This effect is due to the → kappa mechanism which is responsible for driving the → pulsations. The pulsations in Cepheids are excited by the helium → partial ionization zone, He⁺↔ He⁺⁺, which is located below the He ↔ He⁺ and H ↔ H⁺ zones. These latter two regions are too shallow to contribute significantly to the driving of the fundamental modes of Cepheids; so their only effect is to introduce a phase shift.

→ phase; lag, possibly from a Scandinavian source; cf. Norw. lagga "go slowly."

Degarsâni, → difference; fâz→ phase.

In electronics, a technique of adjusting the phase of an oscillator signal so that it will follow the phase of a reference signal.

→ phase; lock, from O.E. loc "bolt, fastening, enclosure;" cf. O.N. lok "fastening, lock," Goth. usluks "opening," O.H.G. loh "dungeon," Ger. Loch "opening, hole," Du. luck "shutter, trapdoor."

Fâz, → phase; bast "fastening, lock," from bastan, from Mid.Pers. bastan/vastan "to bind, shut," Av./O.Pers. band- "to bind, fetter," banda- "band, tie," Skt. bandh- "to bind, tie, fasten," PIE *bhendh- "to bind," cf. Ger. binden, E. bind, → band.

Modulation in which the phase angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous magnitude of the modulating wave.

→ phase; → modulation.

An angular shift in phase by 180°.

→ phase; → reversal.

Any change in the phase of a periodic quantity or in the phase difference between two or more periodic quantities.

→ phase; → shift.

Of a dynamical system, a six-dimensional space consisting of the set of values that the position and velocity can take together (x, y, z, v_x, v_y, v_z). → velocity space.

→ phase; → space.

A measure of the relative phase in the image as function of frequency. It is the phase component of the → optical transfer function. A relative phase change of 180°, for example, results in an image with the black and white areas reversed.

→ phase; → transfer; → function.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation. A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

→ phase; → transition.

The speed at which any fixed phase (individual wave) in a → wave packet travels. It is expressed as v_ph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

→ phase; → velocity.