1) Geography: An imaginary line on the Earth's surface joining the north and south
poles at right angles to the equator. See also
→ local meridian,
→ prime meridian.
M.E., from O.Fr. meridien, from L. meridianus "of noon, southern," from meridies "noon, south," from meridie "at noon," altered by dissimilation from *medi die, locative of medius "mid-" + dies "day."
Nimruzân, coined by Pers. astronomer (A.D. 973-1048) in his at-Tafhim, from nim "mid-, half" (Mid.Pers. nêm, nêmag "half;" Av. naēma- "half;" cf. Skt. néma- "half") + ruz, → day, + -ân suffix denoting time and place.
Fr.: angle horaire
Same as hour angle.
Fr.: circle méridien
A telescope with a graduated vertical scale, used to measure the declinations of heavenly bodies and sometimes to determine the time of meridian transits.
Fr.: instrument méridien
An instrument designed to observe objects when they cross the meridian.
Fr.: observation au méridien
The observation of a star when it crosses an observer's meridian.
Fr.: passage au méridien
The moment when a celestial object crosses an observer's meridian. Same as meridian transit.
Of, pertaining to, or resembling a meridian.
From nimruzân, → meridian, + -i adj. suffix.
Fr.: circulation méridien
The mass motion of material within a → rotating star generated by the star's departure from spherical symmetry. For a rotating star in which → centrifugal forces are not negligible, → radiative equilibrium and → hydrostatic equilibrium cannot be satisfied. In this condition energy transfer is accomplished by means of the physical motion of material. According to → von Zeipel theorem, the heating on an → equipotential surface is generally higher in the polar direction than in the equatorial direction, which drives a large scale circulation current rising at the pole and descending at the equator. As a consequence, → mixing of material takes place in the stellar interior; see also → Eddington-Sweet time scale. The meridional circulation plays an important role in the evolution of → massive stars. The circulation current was first suggested by Arthur S. Eddington in 1926 (The Internal Constitution of the Stars, Dover Pub. Inc., New York) and subsequently quantified by P. A. Sweet (1950, MNRAS 110, 548).
Fr.: courant méridien
Meteo.: A flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator; a negative value, flow toward the equator.
meridional magnetic field
meydân-e meqnâtisi-ye nimruzâni
Fr.: champ magnétiquue méridien
In the → solar dynamo model, a magnetic field that points from the north to south or south to north.
One of the stars in the Pleiades with a visual magnitude 4.17. It is a giant of spectral type B6 lying at a distance of about 1400 light-years.
Merope was one of the seven Pleiades, daughters of the Titan Atlas.
Fr.: nombre de Mersenne premier
A → prime number of the form 2p - 1, where p is a prime. As of February 2013, 48 Mersenne primes are known. The largest known prime number is 257,885,161 - 1. Each prime gives rise to an even → perfect number.
Marin Mersenne (1588-1648), French theologian, philosopher, mathematician and music theorist; → prime.
Mesarthim (γ Arietis)
A star of visual magnitude 4.8 lying 204 light-years away in the constellation → Aries. It is in fact a triple star system.
The origin of Mesarthim (or Mesartim) is a matter of controversy.
Some scholars have related it to the Ar. methartim (
Fr.: maille, maillage
1) One of the open spaces between the cords or ropes of a net.
M.E. mesche "open space in a net," apparently from O.E. max "net," earlier mæscre (cf. Dan. maske, Sw. maska, M.Du. maessce, Du. maas "mesh," O.H.G. masca, Ger. Masche "mesh").
Bâncé "aperture, opening, window" in (Kermânšâhi) Kurd., ultimately from Proto-Ir. *banaka-, from *baH- "to shine," cf. Av. bāmya- "light, bright;" Pers. bâm "morning, dawn; splendor, light," Ossetic bon "day," probably related to bezel "opening, aperture," in several dialects of the Fârs province (Lâr, Gerâš, Xonj, Fišvar), → morning.
Meso-, loan from Gk.
A nuclear particle with a mass intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers. Their charge can be positive, negative, or zero.
The layer of the atmosphere located between the → stratosphere and the → ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.
1) General: A piece of information (written, spoken, or by signals).
M.E., from O.Fr. message "message, news, embassy," from M.L. missaticum, from L. missus "a sending away, dispatching," from mittere "to send," → mission.
Fr.: catalogue de Messier
A catalog of more than 100 nebulous-appearing astronomical objects, initially established to avoid confusion with comets. These objects are now well known to be among the brightest and most striking gaseous nebulae, star clusters, and galaxies. The designations of the catalog are still used in identification; e.g. M1 is the Crab Nebula (in Taurus).
In honor of the French astronomer Charles Messier (1730-1817), who compiled the list between 1760 and 1784 in order to avoid confusion with comets; → catalog.