An Etymological Dictionary of Astronomy and Astrophysics

As great as; like or alike in quantity, degree, value.

Etymology (EN): From L. æqualis “uniform, identical, equal,” from æquus “level, even, just,” of unknown origin, + -alis, → -al.

Etymology (PE): Hamug, from Mid.Pers. hamôg “equal, like,” from ham “the same; together; also” (O.Pers./Av. ham-; cf. Skt. sam-; also O.Pers./Av. hama- “one and the same;” Skt. sama-; Gk. homos-; originally identical with PIE numeral *sam- “one,” from *som-)

• suffix -og/-ok/-uk, as in nêrog “force” (from nar “man, male”), nêvakôk “good, nice” (from nêvak “good, beautiful, nice, favorable”), mastôk “drunk” (from mast “drunk, drunken”), câpuk “quick; active,” sapuk “light, brisk.”

1. The state or quality of being equal.
   

2. Math.: A → statement of two → mathematical objects being equal. Like → equations, equalities are written as two mathematical objects connected by the → equality sign.

Etymology (EN): M.E. from L. aequalitat-, stem of aequalitats, → equal

• -ity.

Etymology (PE): Hamugi noun of hamug, → equal.

Same as → equals sign.

See also: → equality; → sign.

The act of making equal or uniform.
Electronics: The reduction of frequency or phase distortion by introducing networks which compensate for the particular type of distortion over the required frequency band.

Etymology (EN): Noun of equalize.

To make equal; to make uniform.

Etymology (EN): From → equal + → -ize.

Etymology (PE): From hamug, → equal + sâz contraction of sâzandé “doer, maker,” from sâxtan, sâzidan “to make, form, fashion, prepare” (Mid.Pers. sâxtan, sâz- “to form, prepare, build, make;” Proto-Iranian *sac- “to fit, be suitable; to prepare”).

Electronics: A device, usually an electric network, designed to correct for unequal attenuation of phase shift in the transmission of signals.

See also: Agent noun from → equalize.

A mathematical symbol (=) that indicates equality of two expressions on each side of the sign. Same as → equality sign. The equals sign appears for the first time in Robert Recorde’s book The Whetstone of Witte published in 1557.
He was a Welsh physician and mathematician.

See also: → equal; → sign.

In Ptolemy’s → geocentric system, an imaginary point near the center of the → deferent but at a position opposite to that of the Earth from the center of the deferent. Ptolemy further supposed that the distance from the Earth to the center of the deferent was equal to the distance from the center of the deferent to the equant. He also claimed that the planet’s deferent and the → epicycle described uniform circular motion around the equant.

Etymology (EN): L. aequant-, s. of aequans, pr.p. of aequare “to make equal.”

Etymology (PE): Falak-e mo’adel (al-masir), literally “the sphere that equalizes (the path),” from Ar. falak “celestial orbit; sphere; heaven,” from Babylonian pulluku

• mo’adel “equalizing” (+ masir “path”).

To put in the form of an equation; to state the equality of or between.

Etymology (EN): L. æquatus, p.p. of æquare “to make equal,” from æquus “equal, level, even.”

Etymology (PE): Infinitive form of hamug, → equal.

A statement asserting the equality of two numbers or two expressions. It consists of two parts, called sides or members of the equation, separated by the Same as → equality sign.

Etymology (EN): From L. æquation- “an equalizing,” noun of → equate.

Etymology (PE): Verbal noun of hamugidan, → equate.

1. Any equation that describes the motion of objects, i.e., variation of velocity, distance covered, acceleration, etc., as a function of time; e.g., V = V₀ + at, S = Vt + (1/2)at².
   

2. For a fluid, a relation, in its most fundamental form, equating the rate of change of momentum of a selected portion of fluid and the sum of all forces acting on that portion of fluid.
   

3. In quantum mechanics, an equation that governs the time variation of the → state of the system. → Schrodinger equation. However, in the Heisenberg formulation of quantum mechanics the equation of motion does not involve the states, which in this case is time independent, but rather the → observables of the system.

See also: → equation; → motion.

In physics and thermodynamics, the equation that describes the relationship between pressure, density, and temperature, e.g. → ideal gas law, → van der Waals equation, → polytropic process, → virial equation of state.

See also: → equation; → state.

In cosmology, a → dimensionless parameter introduced by the → equation of state
representing the ratio of the pressure to the energy density of a fluid, such as the
→ dark energy: w = p/ρ.
The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for
→ non-relativistic matter (present-day → baryons): w = 0, and
for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.

See also: → equation; → state; → parameter.

The difference between → apparent sidereal time and → mean sidereal time. It is due to the nutation of the Earth’s polar axis of rotation about its precessional motion. It ranges from +0.8 to +1.2 seconds. Also known as → nutation in right ascension.

See also: → equation; → equinox.

The difference, due to Earth’s elliptical orbit and variable orbital velocity, between apparent solar time and mean solar time. It varies throughout the year, and slightly from year to year. At present, it reaches extremes of about -14 minutes in February, and about +16 minutes in November. The equation of time is visually illustrated by an → analemma.

See also: → equation; → time.

The great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. → celestial equator.

Etymology (EN): From M.L. æquator “equalizer” (of day and night, as when the sun crosses the equator), from æquare “to make equal, equate” + -tor.

Etymology (PE): Hamugâr, from hamug, → equal + -âr suffix forming agent nous (as in parastâr) or contracted present stem of âvardan “to bring; to cause, to produce.”

An imaginary plane → perpendicular to the → axis of a → sphere dividing the sphere into two congruent parts.

See also: → equator; → plane.

A set of celestial coordinates based on the celestial equator as the primary great circle. The coordinates are → declination and → right ascension.

See also: → equator; → system.

Of, pertaining to, or near an equator, especially the equator of the Earth.

Etymology (EN): From → equator + -ial, variant of → -al.

Etymology (PE): Hamugâri, from hamugâr, → equator,

• adj. suffix -i.

The excess of the equatorial diameter over the polar diameter of a celestial object, such as the Earth or the Moon.

Etymology (EN): → equatorial; bulge, from O.Fr. bouge “leather bag,” from L. bulga “leather bag,” of Gaulish origin.

Etymology (PE): Barâmadegi, from barâmadan “to grow out; to emerge,” from bar- “on, upon, up” (Mid.Pers. abar, O.Pers. upariy “above; over, upon, according to,” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”) + âmadan “to come” (Mid.Pers. âmadan;
O.Pers. gam- “to come; to go;” Av. gam- “to come; to go,” jamaiti “goes;”
cf. 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”).

An astronomical → coordinate system for indicating the positions of → celestial objects on the → celestial sphere. The system consists of two components, → right ascension and → declination. Right ascension is the angle between the → vernal equinox and the point where the → hour circle intersects the → celestial equator. The right ascension is always measured eastward from the vernal equinox, in the units of hours, minutes, and seconds.
Declination is the angle between the celestial equator and the position of the star measured along the star’s hour circle. It is measured in degrees, minutes, and seconds north or south of the celestial equator. By definition, the vernal equinox is located at right ascension 0h and declination 0°. Equatorial coordinates change with time due to the → precession of the Earth’s → rotation axis.

See also: → equatorial; → coordinate; → system.

Celestial coordinates in the → equator system.

See also: → equatorial, → coordinate.

A telescope mounting consisting of a polar axis pointed toward the celestial pole, and a declination axis supporting the instrument at right angles to the polar axis.

See also: → equatorial; → mounting.

The plane containing a celestial object’s equator.

See also: → equatorial; → plane.

Of a planet, the distance from the center to the equator. For Earth it is 6,378.1370 km. Jupiter has an equatorial radius 11.2 times Earth’s value.

See also: → equatorial; → radius.

A slow, dense → stellar wind (high → mass loss rate) emanating from equatorial regions of a → B[e] star. The equatorial and → polar winds are
the two main wind components in B[e] stars. The mechanism suggested to explain this wind morphology is the rotationally induced → bistability mechanism.

See also: → equatorial; → wind.

A prefix meaning “equal,” as in

→ equinox, → equilibrium, → equipartition.

Etymology (EN): M.E., from L. aequi-, combining form representing aequus, → equal.

Etymology (PE): Hamug-, → equal.

Math.: having three axes of the same length. Also equiaxed.
Physics: A crystal exhibiting similar dimensions in all directions.

See also: → equi-; → axis.

A triangle having three equal sides.

Etymology (EN): → equi-, → lateral, → triangle.

Etymology (PE): Sé-pahlu-barâbar, from sé, → three, pahlu, → side, barâbar, → equal.

A state of balance or rest between the forces operating on or within a physical system. → stable equilibrium; → unstable equilibrium; → dynamical equilibrium.

Etymology (EN): From L. æquilibrium, from æquus, → equal + libra “a balance, scale.”

Etymology (PE): Tarâzmandi, noun of tarâzmand “in equilibrium,” from tarâz “level; a level” + possession suffix -mand. The first component
from tarâzu “balance, scales,” Mid.Pers. tarâzên-, taraênidan “to weigh;” Proto-Iranian *tarāz-, from *tarā- “balance, scale” (cf. Skt. tulā- “scales, balance, weight,” from tul- “to weigh, make equal in weight, equal,” tolayati “weighs, balances;” L. tollere “to raise;”
Gk. talanton “balance, weight,” Atlas “the Bearer” of Heaven;" Lith. tiltas “bridge;” PIE base telə- “to lift, weigh”) + Av. az- “to convey, conduct, drive,” azaiti drives" (cf. Skt. aj- “to dive, sling,” ájati “drives,” ajirá- “agile, quick;”
Gk. agein “to lead, guide, drive, carry off;” L. agere “to do, set in motion, drive,” from PIE root *ag- “to drive, move,” → act).

A concept whereby chemical → concentrations among geological or environmental media are at equilibrium, and therefore the partitioning of metals in those media can be predicted based on → partition ratios.

See also: → equilibrium; → partitioning.

The position of an oscillating body at which no net force acts on it.

See also: → equilibrium; → position.

A state in which a → thermodynamic system is in → thermodynamic equilibrium.

See also: → equilibrium; → state.

The date on which the day and night have exactly the same length. Contrary to the widespread statement, the day and night are not equal at the → equinox. The higher the → latitude, the greater the difference. In fact the day and night lengths are equal at the equinox only if the strict theoretical definition is used, according to which sunset and sunrise are the moments when the center of the Sun crosses the → horizon. There are two reasons for this inequality:

1. The Sun is a disk, not a point source. It is about 30 arc minute wide, hence sunrise corresponds to the moment the top of the disk (and not its center) emerges out of the horizon. Similarly, sunset is when the last part of the disk sinks below the horizon. The Sun takes about a minute to move from its center to its edge (the Earth rotates about 1 degree in 4 minutes). This sums to two minutes (a minute for sunset and a minute for sunrise) that adds to 4 minutes in the total difference.
2. The atmosphere acts as a lens, and slightly bends the Sun’s rays because of the → atmospheric refraction. When we look at the setting Sun, the fact is that it was already set.

Unlike the equinox, which is a fixed date all-over the globe, the date

of the equilux is dependent upon the → latitude of the observer. Between the poles and about 20 degrees latitude, it is generally a few days before the → vernal equinox or a few days after the → autumnal equinox .

See also: → equi-; → lux.

Of or relating to an equinox or to the equality of day and night.

See also: Adjective of → equinox.

The great circle of the celestial sphere through the celestial poles and equinoxes; the hour circle of the vernal equinox. → colure.

See also: → equinoctial; → colure.

One of the two points of intersection of the ecliptic and the celestial equator. Same as equinox.

See also: → equinoctial; → point.

1. One of the two points on the → celestial sphere where the → celestial equator intersects the → ecliptic, that is when the apparent → ecliptic longitude of the Sun is 0° or 180°.
   

2. Either of the times at which the center of the Sun’s disk passes through these points. → autumnal equinox; → vernal equinox.
   

At equinox, the length of the day and the night are equal all over the globe. The equinox is not a fixed point; it moves due to → precession and → nutation. If only precession is considered, we deal with the → mean equinox of date. If nutation is also taken into account, then we are concerned with the → true equinox.

Etymology (EN): M.E., from O.Fr. équinoxe, from M.L. equinoxium “equality of night (and day),” from L. æquinoctium, from æquus, “→ equal” + nox “→ night” (gen. noctis). In Gk. isimeria “equal day,” from isos “equal,” → iso-, + hemera “day.”

Etymology (PE): From hamug, → equal, + -ân suffix denoting time and place.

1. General: Equal sharing of the → total energy among all → components of a → system.
   

2. In the → kinetic theory of gases, the → theorem according to which → molecules in → thermal equilibrium have the same average energy (¹/₂ kT) associated with each independent → degree of freedom of their motion.

See also: → equi-; → partition.

An imaginary surface surrounding a body, or group of bodies, over which the gravitational field is of constant strength and, at all points, is directed perpendicular to the surface. For a single star the surface is spherical. In a close binary system the equipotential surface of the components interact to become hourglass-shaped. → Roche lobe;
→ Lagrangian points.

See also: From → equi-; → potential; → surface.

The state or fact of being equivalent; equality in value, force, significance, etc. → covalence.

Etymology (EN): From M.F. from M.L. æquivalentia, from L. æquivalent-,
→ equivalent.

Etymology (PE): Hamug-arzi, noun of hamug-arz, → equivalent.

A fundamental concept of physics, put forward by A. Einstein, that states that gravitational and inertial forces are of a similar nature and indistinguishable. In other words, acceleration due to gravity is equivalent to acceleration due to other forces, and gravitational mass is the same as inertial mass. Same as the → principle of equivalence.

See also: → equivalence; → principle.

Equal in value, measure, force, effect, significance, etc.

Etymology (EN): From L.L. æquivalentem (nominative æquivalens) “equivalent,” p.p. of æquivalere “be equivalent,” from L. æquus, → equal + valere “to be worth; be strong.”

Etymology (PE): Hamug-arz, from hamug-, → equi-,

• arz stem of arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth;” Mid.Pers. arz “value, worth,” arzidan “be worth,”
  arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value.”

A measure of the number of particles passing a given point in a → planetary ring per unit time. It is obtained by multiplying the physical width of the ring by its average → optical depth. For the variable-width eccentric rings of → Uranus, equivalent depth remains almost constant around a given ring (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → equivalent; → depth.

Complete set of points in any given space group which are obtained by performing the symmetry operations of the space group on a single point (x, y, z).

See also: → equivalent; → position.

1. A measure of the → strength of a → spectral line. The equivalent width is the width of a → rectangle centered on a spectral line that, on a plot of → intensity against → wavelength, has the same → area as the line.
   

2. The width-integrated → optical depth of a → planetary ring. For rings with very small optical depths, the equivalent width is very nearly equal to the equivalent depth (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → equivalent; → width.

The Foal. A small, faint constellation in the northern hemisphere, lying between → Delphinus and → Pegasus, at 21h 10m right ascension, 5° north declination. Its brightest star, Kitalpha, has a visual magnitude of 3.9. Abbreviation: Equ; Genitive: Equulei.

Etymology (EN): L. Equuleus “little horse,” diminutive of
equus “horse,” from PIE base *ekwos “horse” (cf. Pers. asb; Av. aspa- “horse;”
Skt. áśva-; Gk. hippos; O.E. eoh; Arm. ēš). The origin of Equuleus is not clear. It is not mentioned in any classical Gk. or Roman myths. The first mention of the constellation was in Ptolemy’s catalog, where it is referred to as Hippou Protome “the bust or upper part of an animal figure.” Some mythologists have associated Equuleus with the foal Celeris, the brother of the winged horse Pegasus, given to Castor by Mercury.

Etymology (PE): Pâré asb “part of a horse,” from pâré “piece, part, portion, fragment” (Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;” cf. L. pars “part, piece, side, share,” portio “share, portion;” Gk. peprotai “it has been granted;” Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble”) + asb “horse,” Mid.Pers. asb; O.Pers. asa- “horse;” Av. aspa- “horse,” aspā- “mare,” āsu.aspa- “unbound horse;” Skt. áśvā- “mare;” cognate with L. equus, as above.
Korré asb, from korré “foal, colt of asb,” as above. Mod.Pers. korré “baby of an animal, colt;” Laki korr “son, boy,” kol “little mare (1-2 years old);” Lori kor “son, boy;” Kurd kur “son, boy;” Malâyeri kora “boy,” korra “colt;” cf. Gk. kouros, koros “boy, child;” Skt. kúla- “race, household; herd, flock, multitude,” svakúla- “one’s own family or race;” Sogd. kur “child.”
Interestingly, the “group, herd” sense is present also in Pers. dialects Tâleši kavla “group, multitude;” Gilaki kowge “group, tribe.”