# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 6 Search : diameter
 angular diameter   ترامون ِ زاویه‌ای، قطر ِ ~   tarâmun-e zâviye-yi, qotr-e ~Fr.: diamètre angulaire   The apparent diameter of an object in angular measure.→ angular; → diameter. angular diameter distance   اپست ِ زاویه‌ای   apest-e zâviye-yiFr.: distance angulaire   1) The ratio of an object's → linear size (l) to its → angular size (δθ, in → radians), that is DA = l/δθ. It is used to convert observed angular separations into proper separations at the source. 2) In cosmology, a distance defined as the ratio of an object's physical transverse size (l) to its angular size (δθ). It is used to convert angular separations in telescope images into proper separations at the source. The angular diameter distance is defined by: DA = l / δθ. Consider a light source of size l at r = r1 and t = t1 subtending an angle δθ at the origin (r = 0, t = t0). The proper distance between the two ends of the object is related to δθ by: δθ = l / [a(t1). r1], where a(t1) is the → scale factor at the present epoch. Therefore, DA = r1 / (1 + z). The angular diameter distance has the particularity that it does not increase infinitely with z→ ∞. It gets its maximum value at a → redshift of ~ 1 and then decreases for higher z. Therefore, more distant objects appear larger in angular size. This is explained by considering the size of the Universe when the light of the object was emitted. At that time the Universe was smaller and therefore the object occupied a larger fraction of the size of the Universe. In other words, objects appear larger because the entire Universe acts as a → gravitational lense.→ angular; → diameter; → distance. apparent diameter   ترامون ِ پدیدار، قطر ِ ~   tarâmun-e padidâr, qotr-e ~Fr.: diamètre apparent   The angular diameter of a celestial body expressed in degrees, minutes, and seconds of arc, or in radians.→ apparent; → diameter. diameter   ترامون   tarâmun (#)Fr.: diamètre   Any chord passing through the center of a figure. The length of this chord.O.Fr. diamètre, from L. diametrus, from Gk. diametros "diagonal of a circle," from dia- "across, through" + metron "a measure" → meter.Tarâmun, from tarâ- "across, through," → trans- + mun/mân "measure," as in Pers. terms pirâmun "perimeter," âzmun "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl," from O.Pers./Av. mā(y)- "to measure," cf. Skt. mati "measures," matra- "measure," Gk. metron "measure," L. metrum; PIE base *me- "to measure." linear diameter   ترامون ِ خطی   tarâmun-e xattiFr.: diamètre linéaire   The real physical diameter, as opposed to angular diameter.→ linear; → diameter. semidiameter   نیم-ترامون   nim-tarâmunFr.: demi-diamètre   The angle at the observer subtended by the equatorial radius of the Sun, Moon, or a planet.→ semi-; → diameter.