Fr.: tache de diffraction, ~ d'Airy
The bright disk-like image of a point source of light, such as a star, as seen in an optical system with a circular → aperture.
Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, great administrator, who much improved the equipment at Greenwich Observatory. → disk.
Gerdé, → disk; Airy, see above.
Airy transit circle
parhun-e nimruzâni-ye Airy
Fr.: circle méridien d'Airy
A → transit circle that defines the position of the → Greenwich Meridian since the first observation was taken with it in 1851. Airy's transit circle lies at longitude 0°, by definition, and latitude 51° 28' 38'' N.
Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, at the Royal Observatory in Greenwich from 1835 to 1881. Airy transformed the observatory, installing some of the most advanced astronomical apparatus of his day and expanded both staff numbers and their workload; → transit; → circle.
The fraction of the total light or other radiation which falls on a non-luminous body, such as a → planet, → satellite, or → asteroid, and which is reflected by it. Generally, the albedo is equal to the ratio between the light quantity reflected and the light quantity received. The albedo values range between 0.0 (0%), for a perfectly black area, which absorbs all incident light, and 1.0 (100%) for a perfect reflector. The planets or planetary satellites with dense atmospheres have greater albedos than those of transparent atmospheres or of no atmospheres. The albedo can vary from one surface point to another, so that a mean albedo is given for practical purposes. The natural surfaces reflect different light quantities in different directions and the albedo can be expressed in several ways, depending on the way in which the measurement was made: in one direction or, on the average, in all directions (M.S.: SDE). See also → Bond albedo, → geometric albedo.
Albedo, L. "whiteness," from albus "white," from PIE base *albho- "white". Compare with Gk. alphos "white leprosy," O.H.G. albig, O.E. elfet "swan, the white bird". The idea of whiteness derives from the fact that whiter bodies have a higher reflective power, while opaque objects are more absorptive.
Sepidâ, from sepid, →, white, + -â noun-forming prefix from certain adjectives.
Albireo (β Cygni)
Menqâr-e Dajâjé (#), Nok-e Mâkiyân
The second brightest star of the constellation → Cygnus, with a visual magnitude of 3.0. It is a double star of strikingly different colors, with components separated by 35''. The brighter component is a K3 giant while its partner is a main-sequence B9 star. About 380 → light-years away, the two rotate around each other with a period of about 75,000 years. The main component is itself a binary system.
Albireo may be a corruption of the L. phrase ab ireo "from the rainbow," as suggested by some writers on star names. It does not mean "the hen's beak".
Menqâr-e Dajâjé "hen's beak," from Ar. Minqâr
al-Dajâjah, from minqâr "beak" + dajâjah "hen".
An organic compound having a → hydroxyl (-OH) group attached to a carbon atom. Specifically the term is applied to ethyl alcohol or → ethanol (C2H5OH). Alcohol exists abundantly in the → interstellar medium in gaseous state also in the form of → methanol.
The discovery of alcohol is attributed to the Iranian physician and scientist
Mohammad son of Zakariyâ Râzi (864-930 AD, known in Europe as Razes or Rhazes).
He wrote in Ar., which was the scientific language of
that period. However, he himself did not use a specific term for this substance
as far as we know.
Alcohol was first used in medicine about 1250 by two Italian physicians Valis de Furo and
Thaddaeus of Florence. It was not yet called alcohol, but aqua ardens or
aqua vini. The name alcohol, of Arabic origin, was introduced by
the Swiss alchemist and physician Paracelsus (1493-1541) in the
sixteenth century. It is composed of two parts,
al-, a definite article (like "the"), plus a second component the origin of
which is not clear.
A broadly spread explanation for the second component is (kuHl)
A 4th magnitude star lying in the constellation → Ursa Major (also called 80 Ursae Majoris) which forms a visual pair with the brighter star → Mizar (Zeta Ursae Majoris). Alcor is separated by about 11.5 minutes of arc from Mizar. It is a → main sequence star of type A5 with a mass of 1.8 Msun. Recent observations show that Alcor is a → spectroscopic binary, whose → companion has M-band (λ = 4.8 μm) magnitude 8.8 and projected separation 1''.11 (28 AU) from Alcor. The companion is most likely a low-mass (~ 0.3 Msun) active star which is responsible for Alcor's → X-ray emission detected by → ROSAT (LX ~ 1028.3 erg/s). Alcor is a nuclear member of the → Ursa Major star cluster (distance ~ 25 pc, age ~ 0.5 Gyr). The Alcor binary is probably → gravitationally bound to the Mizar star system, making them a → sextuplet with physical separation 0.36 pc, or 74,000 → astronomical units (Mamajek et al., 2010, AJ 139, 919).
Alcor, perhaps from Ar. al-khawr "the low ground."
Sohâ, from Ar. Suhâ.
Alcyone (η Tauri)
Alkuone, Nayyer-e Sorayyâ, Raxšân-e Parvin
In Gk. mythology, a daughter of Aelous who, with her husband, Ceyx, was transformed into a kingfisher.
Nayyer-e Sorayyâ "the bight of the Pleiades," from Ar. nayyir
"luminous" + Thorayyâ "the Pleiades".
Aldebaran (α Tauri)
The brightest star in the constellation → Taurus (visual magnitude about 0.9), Aldebaran is an orange K-type giant that lies 60 → light-years away. It has a faint M2 V companion. It is slowly and irregularly variable.
Ar. Aldebaran "the follower" (of the Pleiades, which rise shortly before it does), from al "the" + dabaran "follower," from dobur "to follow". Gâvcašm "the bull's eye," from Mod.Pers. Gâv "bull, cow" + cašm "eye," corresponding to the alternative Ar. name of the star Ayno 's Sowr.
Any of a class of organic compounds containing the -CH=O group, that is a double-bonded oxygen and hydrogen bonded to the same terminal carbon atom.
From N.L. al(cohol) dehyd(rogenatum) "alcohol deprived of hydrogen."
Alderamin (α Cephei)
Alderamin, from Ar. al dhirâ' al-yamin "right arm" (of Cepheus), from Ar. dhirâ' "arm" + yamin "right".
Zerâ'-e Yamin, from Ar. al dhira al-yamin.
1) The first letter of the Hebrew alphabet (ℵ).
Hebrew and Phoenician letter, from Semitic languages.
Alexander's dark band
navâr-e siyâh-e Aleksânder
Fr.: bande noire d'Alexandre
A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.
Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.
Fr.: Tables alfonsines
A set of tables created in Toledo, under Alfonso X, el sabio, king of Castile and Léon (1252 to 1284) to correct the anomalies in the → Toledan Tables. The starting point of the Alfonsine Tables is January 1, 1252, the year of king's coronation (1 June). The original Spanish version of the tables is lost, but a set of canons (introductory instructions) for planetary tables are extant. They are written by Isaac ben Sid and Judah ben Moses ha-Cohen, two of the most active collaborators of Alfonso X. The Alfonsine Tables were the most widely used astronomical tables in the Middle Ages and had an enormous impact on the development of European astronomy from the 13th to 16th century. They were replaced by Erasmus Reinhold's → Prutenic Tables, based on Copernican models, that were first published in 1551.The Latin version of the Alfonsine Tables first appeared in Paris around 1320, where a revision was undertaken by John of Lignères and John of Murs, accompanied by a number of canons for their use written by John of Saxony. There is a controversy as to the exact relationship of these tables with the work commissioned by the Spanish king.
After the Spanish monarch Alfonso X (1221-1284); → table.
Alfvén Mach number
adad-e Mach-e Alfvéni
Fr.: nombre de Mach alfvénique
The ratio of the flow velocity to the → Alfvén speed in a medium.
Fr.: rayon d'Alfvén
1) In theories of magnetized → accretion disks,
the distance from a non-rotating star where the → free fall
of a spherical accretion flow is stopped, which occurs where the
→ ram pressure of the infalling matter equals the
→ magnetic pressure of the star.
Fr.: vitesse d'Alfvén
The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: vA = B/(μ0.ρ)1/2, where B is the → magnetic field strength, μ0 is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.
Fr.: onde d'Alfvén
A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.
Involving → Alfvén waves.
Fr.: fluctuation alfvénique
Large amplitude fluctuations in the → solar wind with properties resembling those of → Alfvén waves. A fluctuation is said to be Alfvénic if the following relationship between the velocity fluctuations (Δv) and magnetic field fluctuations (ΔB) is satisfied: Δv = ± ΔB/(μ0ρ)1/2. Also called Alfvénicity.