An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 3105 Search : on
Lorentz resonance
  باز‌آوایی ِ لورنتز   
bâzâvâyi-ye Lorentz

Fr.: résonance de Lorentz   

A repeated electromagnetic force on an electrically charged ring particle, nudging the particle in the same direction and at the same point in its orbit. Lorentz resonances are especially important for tiny ring particles whose charge-to-mass ratio is high and whose orbit periods are a simple integer fraction of the rotational period of the planet's magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).

Lorentz; → resonance.

Lorentz transformation
  ترادیس ِ لورنتز   
tarâdis-e Lorentz

Fr.: transformation de Lorentz   

A set of linear equations that expresses the time and space coordinates of one → reference frame in terms of those of another one when one frame moves at a constant velocity with respect to the other. In general, the Lorentz transformation allows a change of the origin of a coordinate system, a rotation around the origin, a reversal of spatial or temporal direction, and a uniform movement along a spatial axis. If the system S'(x',y',z',t') moves at the velocity v with respect to S(x,y,z,t) in the positive direction of the x-axis, the Lorentz transformations will be: x' = γ(x - vt), y' = y, z' = z, t' = γ [t - (vx/c2)], where c is the → velocity of light and γ = [1 - (v/c)2]-1/2. For the special case of velocities much less than c, the Lorentz transformation reduces to → Galilean transformation.

Lorentz; → transformation.

low resolution
keh vâgošud

Fr.: faible résolution   

The quality of an instrument that lacks sufficient resolution for a specific observation. This is a relative quality, but presently a resolution below about 1 arcsecond.

low; → resolution.

low-ionization line
  خط ِ کم‌یونش   
xatt-e kamyoneš (#)

Fr.: raie de faible ionisation   

A spectral line arising from a transition between atomic levels with an ionization potential below approximately 15 electron-volts.

low; → ionization; → line.

low-ionization nuclear emission-line region
  ناحیه‌ی ِ هسته‌ای با خط ِ گسیلی ِ کم‌یونش   
nâhiye-ye hasteyi bâ xatt-e gosili-ye kamyoneš (#)

Fr.: Noyau de galaxie à raies d'émission de faible ionisation   

Same as → LINER.

low; → ionization; → nuclear; → emission; → line; → region.

low-metallicity environment
  پرگیر ِ کم‌فلز   
pargir-e kamfelez

Fr.: environnement faible en métaux   

A medium in which chemical elements have abundances smaller than the solar values.

low; → metallicity; → environment.

lower culmination
  بالست ِ زیرین   
bâlest-e zirin

Fr.: culmination inférieure   

The instant of culmination when the star passes between the pole and the horizon, having an hour angle of 12h. Lower culmination for non-circumpolar objects occur below the horizon and is thus unobservable. Same as → inferior culmination. See also → upper culmination.

lower; → culmination.

luminosity function
  کریا‌ی ِ تابندگی   
karyâ-ye tâbandegi

Fr.: fonction de luminosité   

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

luminosity; → function.

luminosity-size relation
  بازانش ِ تابندگی-اندازه   
bâzâneš-e tâbandegi-andâze

Fr.: relation luminosité-taille   

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

luminosity; → size; → relation.

lunar formation
  دیسش ِ مانگ   
diseš-e Mâng

Fr.: formation de la Lune   

See → Moon formation.

lunar; → formation.

lunar horizon glow
  فروغ ِ افق ِ ماه   
foruq-e ofoq-e mâh

Fr.: éclat de l'horizon lunaire   

A very bright crescent of light glowing on the lunar horizon at → sunset or just before → sunrise. It has been suggested that → lunar dust is transported electrically high into sky, allowing sunlight to scatter and create glows. On the day side of the → Moon, solar → ultraviolet radiation is strong enough to kick → electrons from → dust grains in the lunar soil. Removal of electrons, which have a negative electric charge, leaves the dust with a positive electric charge. Since like charges repel, the positively charged dust particles get pushed away from each other, and the only direction not blocked by more dust is up. In the 1960s, Surveyor probes filmed a glowing cloud floating just above the lunar surface during sunrise. Later, Apollo 17 astronaut Gene Cernan, while orbiting the Moon, recorded a similar phenomenon at the sharp line where lunar day meets night, called the → terminator.

lunar; → horizon; → glow.

lunar mansion
  منزل ِ ماه   
manzel-e mâh (#)

Fr.: maison lunaire   

One of the 28 divisions of the sky, identified by the prominent stars in them, that the Moon passes through during its monthly cycle, as used in ancient Chinese, Hindu, and Arab astronomy/astrology.

From O.Fr. mansion, from L. mansionem (nom. mansio) "a staying, a remaining, night quarters, station," from manere "to stay, abide" (Fr. maison, ménage; E. manor, mansion, permanent); cf. Pers. mân "house, home," mândan "to remain, stay, relinquish, leave;" Mid.Pers. mândan "to remain, stay;" O.Pers. mān- "to remain, dwell;" Av. man- "to remain, dwell; to wait;" Gk. menein "to remain;" PIE base *men- "to remain, wait for."

Manzel, from Ar. "dwelling, habitation, mansion."

lunar month
  ماه ِ مانگی   
mâh-e mângi

Fr.: mois lunaire   

The average time between successive new or full moons. Also called → synodic month, → lunation.

lunar; → month.

lunar recession
  دورشد ِ ماه   
duršd-e mâh

Fr.: éloignement de la lune   

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet. The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon's gravity pulls on the bulge and slows down the → Earth's rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon's orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth's own rotation rate will match the Moon's orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon's own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon's average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon's distance will increase so much that it will be to far away to produce total eclipses of the Sun.

lunar; → recession.

lunar rotation
  چرخش ِ مانگ   
carxeš-e mâng

Fr.: rotation de la Lune   

The Moon's motion around its axis, which takes place in 27.321 661 days (→ sidereal month). Since the Moon and the Earth are → tidally locked our satellite has a → synchronous rotation. This means that it rotates once on its axis in the same length of time it takes to revolve around Earth. That is why the Moon always shows the same face to us. However, over time we can see up to 59 percent of the lunar surface because the Moon does not orbit at a constant speed (→ libration in longitude) and its axis is not perpendicular to its orbit (→ libration in latitude). The Moon also creates tides in Earth oceans. As the Earth rotates, the rising and falling sea waters bring about friction within the liquid itself and between the water and solid Earth. This removes energy from Earth's rotation and causes it to spin more slowly. As a result, days are getting longer, at about 2 milliseconds per century. On the other hand, since the → angular momentum of the → Earth-Moon system must be conserved, the Moon gradually moves away from the Earth. This, in turn, requires its orbital period to increase and, because the Moon is tidally locked to Earth, to spin more slowly.

lunar; → rotation.


Fr.: lunaison   

The interval of a complete lunar cycle, between one new Moon and the next, that is 29 days, 12 hours, 44 minutes, and 2.8 seconds. or 29.5306 days. → synodic month.

M.E. lunacyon, from M.L. lunation-.

Mahâyand, literally "coming, arrival of the Moon," from mâhmoon + âyand "coming, arrival," present stem of âmadan "to come"; O.Pers. aitiy "goes;" Av. ay- "to go, to come," aēiti "goes;" Skt. e- "to come near," eti "arrival;" Gk ion " going," neut. pr.p. of ienai "to go;" L. ire "to go;" Goth. iddja "went," Lith. eiti "to go;" Rus. idti "to go;" from PIE base *ei- "to go, to walk."

lunisolar precession
  پیشایان ِ مانگی-خورشیدی   
pišâyân-e mângi-xorši

Fr.: précession lunisolaire   

precession of the equator.

From luni-, from → lunar, + → solar; → precession.

Lyman continuum
  پیوستار ِ لایمن   
peyvastâr-e Lyman (#)

Fr.: continuum de Lyman   

A continuous range of wavelengths in the spectrum of hydrogen at wavelengths less than the → Lyman limit. The Lyman continuum results from transitions between the → ground state of hydrogen and → excited states in which the single electron is freed from the atom by photons having an energy of 13.6 eV or higher.

Lyman; → continuum.

Lyman continuum escape
  گریز ِ پیوستار ِ لایمن   
goriz-e peyvastâr-e Lyman

Fr.: échappement du continuum de Lyman   

The process whereby → Lyman continuum photons produced by → massive stars escape from a galaxy without being absorbed by interstellar material. Some observations indicate that the Lyman continuum escape fraction evolves with → redshift.

Lyman; → continuum; → escape.

Lyman-Werner photon
  فوتون ِ لایمن-ورنر   
foton-e Lyman-Werner

Fr.: photon de Lyman-Werner   

An → ultraviolet photon with an energy between 11.2 and 13.6 eV, corresponding to the energy range in which the Lyman and Werner absorption bands of → molecular hydrogen (H2) are found (→ Lyman band, → Werner band). The first generation of stars produces a background of Lyman-Werner radiation which can → photodissociate molecular hydrogen, the key → cooling agent in metal free gas below 104 K. In doing so, the Lyman-Werner radiation field delays the collapse of gaseous clouds, and thus star formation. After more massive → dark matter clouds are assembled, atomic line cooling becomes effective and H2 can begin to shield itself from Lyman-Werner radiation.

Lyman; → Werner band; → photon.

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