An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 558
Tropic of Cancer
  هورگرد ِ خرچنگ   
Hurgard-e Xarcang

Fr.: Tropique du Cancer   

A parallel of latitude on the Earth, 23°26' north of the equator, where the Sun is directly overhead on the northern → summer solstice (around the 21st June each year), because the Sun reaches its most northerly declination. Some 3,000 years ago, this occurred when the Sun was in the → Zodiac constellation → Cancer, hence the name. However, → precession has resulted in a shift of the position of the Sun so that it is now in the constellation → Gemini on the summer solstice.

tropic; → Cancer.

Tropic of Capricorn
  هورگرد ِ وهیگ   
Hurgard-e Vahig

Fr.: Tropique du Capricorne   

A parallel of latitude on the Earth, 23°26' south of the equator, where the Sun is directly overhead on the southern → summer solstice (around the 21st December each year), because the Sun reaches its most southerly declination. Some 3,000 years ago, this occurred when Sun was in the → Zodiac constellation → Cancer, hence the name. However, → precession has resulted in a shift of the position of the Sun so that it is now in the constellation → Sagittarius on the northern → winter solstice.

tropic; → Capricorn.


Fr.: tropique   

1) Geography: Pertaining to, characteristic of, occurring in, or inhabiting the → tropics.
2) Astro.: A misnomer for → solar, as in → tropical year, or → vernal equinox, as in → tropical month.

tropic + → -al.

tropical month
  ماه ِ هموگانی   
mâh-e hamugâni

Fr.: mois tropique   

The average period of the revolution of the Moon about the Earth with respect to the → vernal equinox, a period of 27.321 582 days (27d 7h 43m 4.7s).

tropical; → month.

Mâh, → month; hamugâni, of or pertaining to hamugân, → equinox.

tropical year
  سال ِ هورگردی   
sâl-e hurgardi

Fr.: année tropique   

The interval during which the Sun's mean longitude, referred to the mean equinox of date, increases by 360 degrees. Its mean length for the epoch J2000.0 is 365.24217879 real solar days (approximately 365.2422 days). This concept of tropical year, adopted by the International Astronomical Union at its General Assembly in Dublin, September 1955, has often been confounded with the → vernal-equinox year. In fact the mean period between two successive true vernal equinoxes is different from the tropical year. This period, which is equal to 365.24236460 solar days (about 365.2424 days), is the real mean length of the year in the Iranian calendar. The difference between the two year lengths is due to the fact that the Earth's orbital velocity around the Sun is not uniform, since the orbit is an ellipse. At the perihelion of its orbit the Earth is closest to the Sun, and therefore moves faster than average, while at aphelion, when it is farthest away from the Sun, it moves slower. Therefore the interval between two successive vernal equinoxes is not the same as the period between two successive summer solstices. In fact the tropical year does not depend on a specific origin for the annual apparent motion of the Sun. For detailed discussion see: A concise review of the Iranian calendar.

tropical; → year.


Fr.: tropo-   

A combining form meaning "turn, change."

From Gk. tropos "turn," trope "a turning," from trepein "to turn;" cognate with Pers. sabok and zab, → trivial.

Gašt "turning," past stem of gaštan, gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- "to turn, bend").

gaštmarz (#)

Fr.: tropopause   

The boundary between the → troposphere and → stratosphere. Its height, varying with latitude and seasons, is from 8 km at poles, to 18 km at equator; it is higher during winter time. The atmospheric temperature decreases from ground upward until the tropopause. Then it increases in stratosphere because of the absorption of solar ultraviolet radiation.

tropo- + pause "break, cessation, stop," from M.Fr. pause, from L. pausa "a halt, stop, cessation," from Gk. pausis "stopping, ceasing," from pauein "to stop, to cause to cease."

From gašt, → tropo-, + marz "frontier, border, boundary" (Mid.Pers. marz "boundary;" Av. marəza- "border, district," marəz- "to rub, wipe;" Mod.Pers. parmâs "contact, touching" (→ contact), mâl-, mâlidan "to rub;" PIE base *merg- "boundary, border;" cf. L. margo "edge" (Fr. marge "margin"); P.Gmc. *marko; Ger. Mark; E. mark, margin).


Fr.: troposphère   

The lower part of the Earth's atmosphere in which temperature decreases with height, except for local areas of → temperature inversion.

tropo- + → sphere.

Trouton's rule
  رزن ِ تروتون   
razan-e Trouton

Fr.: règle de Trouton   

The ratio of the → molar heat of vaporization of any liquid to its → boiling point is a constant, about 88 joules per mole per Kelvin. The rule is equivalent to the statement that the → entropy of vaporization is constant. It is not always followed, especially by liquids such as water in which hydrogen bonding occurs between the molecules.

Named after Frederick Thomas Trouton (1863-1922), an Irish physicist; → rule.

Trouton-Noble experiment
  آزمایش ِ تروتون-نوبل   
âzmâyeš-e Trouton-Noble

Fr.: expérience de Trouton-Noble   

An experiment first carried out in 1903 to reveal the absolute motion of the Earth with respect to the → ether. The experiment consists of detecting a torque on a charged parallel-plate → capacitor that was suspended so that its plates were vertical. Since the Earth moves around the Sun, the moving charges were expected to produce magnetic fields, and the resulting torque should tend to turn the capacitor bringing its plates parallel to its velocity. No such effect was observed, and the absence of the torque supports the theory of → special relativity.

Named after Frederick T. Trouton (1863-1922) and Henry R. Noble; → experiment.

râstin (#)

Fr.: vrai   

Being in accordance with the actual state or conditions; e.g. → true anomaly; → true equinox; → true horizon; → true north; → true Sun.

M.E. trewe (adj. and adv.); O.E. triewe, treowe "trustworthy" (cf. O.Fris. triuwi, Du. getrouw, O.H.G. gatriuwu, Ger. treu, O.N. tryggr, Goth. triggws "faithful, trusty").

Râstin, from râst "right, true; just, upright, straight;" Mid.Pers. râst "true, straight, direct;" Soghdian rəšt "right;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Av. rāz- "to direct, put in line, set," razan- "order;" cf. Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" M.E.; O.E. reht, riht; cf. O.H.G. reht, Ger. recht, O.N. rettr, Goth. raihts; PIE base *reg- "move in a straight line," hence, "to direct, rule."

true anomaly
  ناسانی ِ راستین   
nâsâni-ye râstin

Fr.: anomalie vraie   

One of the standard → orbital elements, which is the angle measured at the → focus nearest the → periapsis of an elliptical orbit, between the periapsis and the → radius vector from the focus to the orbiting body.

true; → anomaly.

true celestial equator
  هموگار ِ آسمانی ِ راستین   
hamugâr-e âsmâni-ye râstin

Fr.: équateur céleste vrai   

The → celestial equator of date, which is the → great circle on the → celestial sphere perpendicular to the instantaneous axis of rotation of the Earth. Its interaction with the → ecliptic defines the → vernal equinox of date and the → autumnal equinox of date.

true; → celestial; → equator.

true celestial pole
  قطب ِ آسمانی ِ راستین   
qotb-e âsmâni-ye râstin

Fr.: pôle céleste vrai   

The direction of the Earth's instantaneous rotation pole. It differs from the pole due to the short time-scale (days or decades) variations called → nutation.

true; → celestial; → pole.

true equator
  هموگار ِ راستین   
hamugâr-e râstin

Fr.: équateur vrai   

Same as → true celestial equator.

true; → equtor.

true equinox
  هموگان ِ راستین   
hamugân-e râstin

Fr.: équinoxe vrai   

The intersection of the → ecliptic with the → true celestial equator for a given epoch. It is derived from the → mean equinox accounting for the → nutation.

true; → equinox.

true horizon
  افق ِ راستین   
ofoq-e râstin

Fr.: horizon vrai   

A large circle of the → celestial sphere whose plane is perpendicular to the radius of the Earth through the point. Same as → astronomical horizon. The → visible horizon usually lies lower than the true horizon. See also → dip of the horizon.

true; → horizon.

true north
  هودر ِ راستین   
hudar-e râstin

Fr.: nord vrai   

The geographic north defined by the rotational pole of the Earth, as opposed to magnetic north defined by the geomagnetic north pole.

true; → north.

true position
  نهش ِ راستین   
neheš-e râstin

Fr.: position vraie   

The coordinates of an object for a given date, with respect to the true equator and the true equinoxes for the instant of time in question.

true; → position.

true sidereal time
  زمان ِ اختری ِ راستین   
zamân-e axtari-ye râstin

Fr.: temps sidéral vrai   

The → sidereal time with respect to the → true equinox.

true; → sidereal; → time.

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