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

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.

From → equi-; → potential; → surface.

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

From M.F. from M.L. æquivalentia, from L. æquivalent-, → equivalent.

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.

→ equivalence; → principle.

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

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

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).

→ 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).

→ 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).

→ 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.

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.

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."

1) General: A period of time marked by a distinctive character, events, etc.
2) A system of chronological notation reckoned from a given date.
3) Geology: A subdivision of geologic time that is longer than a period but shorter than an eon. Precambrian, Paleozoic, Mesozoic, and Cenozoic are the eras of the time scale from oldest to youngest.

From L.L. æra, era "fixed date, era, epoch from which time is reckoned," probably identical with L. æra "counters used for calculation," plural of aes "brass, money," from PIE *aus- "gold" (cf. Av. aiiah- "metal," aiianhaēna- "made of metal;" Skt. áyas- "metal;" O.H.G. ēr "ore;" O.E. ora "ore, unworked metal;" Ger. ehern "brazen").

Dowrân, from Ar. daur "age, time; revolution."

A simple way of calculating the Earth's → circumference using two sticks and two theorems of the → Euclidean geometry. Eratosthenes calculated the length of a → meridian arc by measuring the shadow cast by a vertical → gnomon at noon on the → summer solstice. In Cyene (→ tropic of Cancer), no shadow is cast whereas in Alexandria, further north, the shadow is cast at an angle of 1/50 of 360° (measured using a → scaphe), or 7.2°, from the vertical. The circumference is therefore equal to 50 times the distance between the two cities. The distance from Syene to Alexandria was 5,000 stadia, which when multiplied by 50 gives the measure for the Earth's circumference, 250,000 stadia. Estimating the accuracy of this result is not easy because the unit of stadium is not uniquely defined in the ancient world. The most likely reconstruction puts Eratosthenes' stadium in the range 155-185m, implying an error of about 3% below or 15% above the true value. The modern value for the equatorial circumference of the Earth is 40,075 km. As scholars have pointed out, Eratosthenes' experiment was marred by several errors: Syene is not on the Tropic of cancer, it is not on the same meridian as Alexandria, and the distance between the two cities is less than he estimated. But the errors tended to cancel each other out, so his estimate was relatively accurate. See also: → Mamun's method, → Biruni's method.

Eratosthenes (c. 276-194 B.C.), Gk. mathematician, astronomer, and geographer. He studied in Athens and later became a librarian in Alexandria. His treatise On the Measuring of the Earth is lost. The account of his experiment has been preserved in Cleomedes (probably first century A.D.). See also → sieve of Eratosthenes; → experiment.

The → CGS unit of → energy; the → work done by a → force of 1 → dyne acting over a distance of 1 → centimeter. 1 erg = 10^-7 → joules = 6.242 × 10¹¹ → electron-volts.

From Gk. ergon "work," from PIE base *werg- "to work" (cf. Av. varəz- "to work, do, perform, exercise;" Mod.Pers. varz-, varzidan "to labor, exercise, practise;" Arm. gorc "work;" Lith. verziu "tie, fasten, squeeze," vargas "need, distress;" Goth. waurkjan; O.E. wyrcan "work," wrecan "to drive, hunt, pursue").

The property of a dynamical system such that in an interval of sufficient duration, it will return to states that are closely similar to previous ones.

From → erg + Gk. (h)od(os) "way, road" + → ic.

The study of the relationship between people and their working environment, in particular its effect on a person's efficiency. Ergonomics is applied in designing equipment and office systems to maximize productivity by reducing discomfort and fatigue of people in their workplace.

From Gk. ergon "work," → erg, + -nomics, → -nomy, → -ics.

Varzdâtik, from varz "work, " cognate with Gk. ergon, → erg, + dâtik "law, rule," → -nomy.

The region between the → event horizon and the → stationary limit of a rotating → Kerr black hole. It is possible for a particle falling inside the ergosphere to break into two parts, one of which will fall into the black hole and the other will come out.

→ erg + → sphere.

The River. An extensive constellation in the southern hemisphere that takes windings between 1h20 and 5h 10m right ascension, 0° to 58° south declination. Despite its size, there are not many bright stars in this constellation. Notable are → Achernar and ε Eri, a dwarf star of magnitude 4.6 and type K2, which is just 10.7 light years away. Abbreviation: Eri; genitive: Eridani.

From Gk. Eridanos, a river god, a son of Oceanus and Tethys, and father of Zeuxippe. A purely mythical river which may have been named Eridanos ("Early Burnt") from the story of Phaethon, the boy who attempted to drive the chariot of the sun, and fell flaming into the waters of this mythical river.

Rud, → river.

A → dwarf planet which is a → trans-Neptunian object (TNO) with an orbital → eccentricity of 0.44, an → inclination of 44 degrees and a surface composition very similar to that of → Pluto. It orbits the Sun as far as twice Pluto's distance from the Sun. → Occultation observations carried out in 2010 were used to measure the size of Eris accurately. Eris's newly determined diameter is 2326±12 km. The observations show that Eris is an almost perfect twin of Pluto in size. They also reveal a very reflective surface, with an → albedo of 0.96, suggesting that it is uniformly covered in a thin layer of ice, probably a frozen atmosphere (Sicardy et al. 2011, Nature 478, 493). Like Pluto, Eris has a moon, which has been officially named by the → International Astronomical Union as (136199) Eris I (→ Dysnomia). The informal names of Eris were Xena and 2003 UB313.

Named after Eris the Gk. goddess of chaos and strife. She created a quarrel among goddesses that led to the Trojan War.

Geology: The act or state of eroding; state of being eroded.
A general term applied to the wearing away and movement of earth materials by the action of water, glaciers, winds, gravity, etc.

From M.Fr. erosion, from L. erosionem (nom. erosio), from erodere "to gnaw away," from → ex- "away" + rodere "to gnaw, eat away" (cf. Fr. and E. animal rat).

Farsâyeš, from far- intensive prefix "much, abundant" (Mid.Pers. fra- "forward, before; much; around;" O.Pers. fra- "forward, forth;" Av. frā, fərā-, fra- "forward, forth; excessive;" cf. Skt. prá- "before; forward, in fron;" Gk. pro "before, in front of;" L. pro "on behalf of, in place of, before, for;" PIE *pro-) + verbal noun of sâyidan, variants sâbidan "to bruise, file," pasâvidan "to touch;" Khotanese sauy- "to rub;" Sogdian ps'w- "to touch;" Proto-Iranian *sau- "to rub."

Same as → Gamma Cephei.

From Ar. ar-râ'i (الراعی) "the shepherd." Name approved (2015) by the International Astronomical Union's Working Group on Star Names.

1) A deviation from accuracy or correctness; a mistake.
2) Physics: The difference between the calculated or observed value and the true value of a quantity. See also:
→ absolute error, → bias error, → calibration error, → cascade error, → error bar, → fitting error, → instrument error, → observational error, → probable error (PE), → random error, → relative error, → root-mean-square error, → sampling error, → standard error, → systematic error, → Type I error, → Type II error.

From O.Fr. erreur, from L. errorem (nom. error) "a wandering, straying, mistake," from errare "to wander."

Irang, from Mid.Pers. êrang "error, mistake;" xatâ, from Ar.

On a graph displaying the results of a measurement, the dash used to indicate the confidence range of the value attributed to a quantity.

→ error; bar, from O.Fr. barre, from V.L. *barra "bar, barrier," or perhaps from Gaulish *barro "summit."

Band "that which closes, shuts, blocks," from bastan, band- "to shut, bind," from Mid.Pers. bastan/vastan "to bind, shut," Av./O.Pers. band- "to bind, fetter," banda- "band, tie" (cf. Skt. bandh- "to bind, tie, fasten," PIE *bhendh- "to bind;" Ger. binden; E. bind); → error.