An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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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.

lunation
  مَهایند   
mahâyand

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.

Lyot division
  شکاف ِ لی‌یو   
šekâf-e Liyot (#)

Fr.: division de Lyot   

In Saturn's rings, the gap between rings B and C.

Named after Bernard Lyot (1897-1952), French astronomer who discovered the division. He was also a distinguished solar observer and invented (1930) the → coronagraph; → division.

Mach cone
  مخروط ِ ماخ   
maxrut-e Mach

Fr.: cône de Mach   

The cone that confines the pressure disturbance created by a → supersonic object moving in a → compressible medium.

Mach number; → cone.

macronova
  درشت-نو‌اختر   
dorošt-now-axtar

Fr.: macronova   

A stellar → explosion with energies between those of a → nova and a → supernova and observationally distinguished by being brighter than a typical nova (MV ~ -8 mag) but fainter than a typical supernova (MV ~ -19 mag) (Kulkarni 2005; arXiv:astro-ph/0510256).

macro-; → nova.

magnetic advection
  پهنبز ِ مغناتیسی   
pahnbaz-e meqnâtisi

Fr.: advection magnétique   

The transport of the magnetic field by a fluid. It is given by the term ∇ x (v x B) in the → induction equation.

magnetic; → advection.

magnetic connectivity
  هابندندگی ِ مغناتیسی   
hâbandandegi-ye meqnâtisi

Fr.: connectivité magnétique   

Of magnetic field lines, the condition for them to be connected or the process whereby they become connected or connective.

magnetic;→ connectivity.

magnetic constant
  پایای ِ مغناتیسی   
pâyâ-ye meqnâtisi (#)

Fr.: constante magnétique   

A physical constant relating mechanical and electromagnetic units of measurement. It has the value of 4π × 10-7 henry per meter. Also called the permeability of free space, or → absolute permeability.

magnetic; → constant.

magnetic convection
  همبز ِ مغناتیسی   
hambaz-e meqnâtisi

Fr.: convection magnétique   

Thermal → convection modified by the presence of magnetic fields.

magnetic; → convection.

magnetic declination
  واکیلش ِ مغناتیسی   
vâkileš-e meqnâtisi

Fr.: déclinaison magnétique   

In terrestrial magnetism, the difference between → true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point,→ magnetic pole).

magnetic; → declination.

magnetic diffusion
  پخش ِ مغناتیسی   
paxš-e meqnâtisi

Fr.: diffusion magnétique   

The process whereby the magnetic field tends to diffuse across the plasma and to smooth out any local inhomogeneities under the influence of a finite resistance in the plasma. For a stationary plasma the → induction equation becomes a pure → diffusion equation: ∂B/∂t = Dm2B, where Dm = (μ0σ0)-1 is the → magnetic diffusivity.

magnetic; → diffusion.

magnetic inclination
  درکیل ِ مغناتیسی   
darkil-e meqnâtisi

Fr.: inclinaison magnétique   

Same as → magnetic dip or → dip.

magnetic; → inclination.

magnetic induction
  درهازش ِ مغناتیسی   
darhâzeš-e meqnâtisi

Fr.: induction magnétique   

1) Same as → magnetic flux density.
2) The production of a magnetic field in a piece of un-magnetized iron or other → ferromagnetic substance when a magnet is brought near it. The magnet causes the individual particles of iron, which act like tiny magnets, to line up so that the sample as a whole becomes magnetized.

magnetic; → induction.

magnetic monopole
  تک‌قطبه‌ی ِ مغناتیسی   
takqotbe-ye meqnâtisi (#)

Fr.: monopôle magnétique   

A hypothetical particle that carries a single → magnetic pole, in contrast to magnets which are north-south pole pairs. These massive particles (billions of times heavier than the → proton) are required by grand unified theories(→ GUTs) to explain the actual matter content of the Universe, particularly the dominance of matter upon → antimatter. However, their existence contradicts → Gauss's law for magnetism.

magnetic; → monopole.

magnetic monopole problem
  پراسه‌ی ِ تک‌قطبه‌ی ِ مغناتیسی   
parâse-ye takqotbe-ye meqnâtisi

Fr.: problème du monopôle magnétique   

A problem concerning the compatibility of grand unified theories (→ GUTs) with standard cosmology. If standard cosmology was combined with grand unified theories, far too many → magnetic monopoles would have been produced in the early Universe. The → inflation hypothesis aims at explaining the observed scarcity of monopoles. The inflation has deceased their density by a huge factor.

magnetic; → monopole; → problem.

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