An Etymological Dictionary of Astronomy and Astrophysics

A method used to determine the size of certain types of pulsating stars, including Cepheids, from their magnitude variations (photometry) and the corresponding radial velocities (spectroscopy).

Etymology (EN): Baade, from Walter (Wilhelm Heinrich) Baade (1893-1960), German/American astronomer, who made important contributions to the research on variable stars; Wesselink, from Adriaan J. Wesselink (1909-1995), Dutch/American astronomer,
the originator of the method. → method.

A method used to determine the size of certain types of pulsating stars, including Cepheids, from their magnitude variations (photometry) and the corresponding radial velocities (spectroscopy).

Etymology (EN): Baade, from Walter (Wilhelm Heinrich) Baade (1893-1960), German/American astronomer, who made important contributions to the research on variable stars; Wesselink, from Adriaan J. Wesselink (1909-1995), Dutch/American astronomer,
the originator of the method. → method.

An area of the sky with relatively low amounts of  → interstellar dust along the → line of sight, occurring toward the  → constellation of  → Sagittarius, near the → globular cluster NGC 6522. The window, through which stars in the → Galactic bulge are visible, lies 3.9 degrees south of the → Galactic center, corresponding to a line of sight that passes within 1,800 → light-years of the → Milky Way’s core. It is named after Walter Baade, who used it to observe → RR Lyrae stars  in the → Galactic bulge region. 

See also: → Baade-Wesselink method; → window.

An area of the sky with relatively low amounts of  → interstellar dust along the → line of sight, occurring toward the  → constellation of  → Sagittarius, near the → globular cluster NGC 6522. The window, through which stars in the → Galactic bulge are visible, lies 3.9 degrees south of the → Galactic center, corresponding to a line of sight that passes within 1,800 → light-years of the → Milky Way’s core. It is named after Walter Baade, who used it to observe → RR Lyrae stars  in the → Galactic bulge region. 

See also: → Baade-Wesselink method; → window.

A crystal device made of two → quartz  → prisms
with equal acute angles assembled in a rhombus that is used in analyzing → polarized light. The wedge-shaped prisms are placed against each other and can be displaced along their plane of contact by a → micrometer. Thus they form a parallel plate of variable thickness allowing the optical retardation to be adjusted.

See also: Jacques Babinet (1794-1872), French physicist; → compensator.

A crystal device made of two → quartz  → prisms
with equal acute angles assembled in a rhombus that is used in analyzing → polarized light. The wedge-shaped prisms are placed against each other and can be displaced along their plane of contact by a → micrometer. Thus they form a parallel plate of variable thickness allowing the optical retardation to be adjusted.

See also: Jacques Babinet (1794-1872), French physicist; → compensator.

One of several points on the sky where the degree of → linear polarization is zero in skylight. See also → neutral point; → Arago point, → Brewster point.

See also: → Babinet compensator; → point.

One of several points on the sky where the degree of → linear polarization is zero in skylight. See also → neutral point; → Arago point, → Brewster point.

See also: → Babinet compensator; → point.

The → diffraction pattern for an → aperture is the same as the diffraction pattern for its → complementary aperture.

See also: → Babinet compensator; → principle.

The → diffraction pattern for an → aperture is the same as the diffraction pattern for its → complementary aperture.

See also: → Babinet compensator; → principle.

1. General: Related to or located at the back “the side or surface opposite the front or face.”
   

2. The side of the → planispheric astrolabe
   opposite the hollow of the → mater, → tympanum, and → rete. At its center is hinged the → alidade, whose rim slides on a set of graduated circular scales engraved on the mater. The number of scales varies from one instrument to another. On this face the user can perform observations and read the data needed to set the front side of the instrument in the proper position (online museo galileo, VirtualMuseum).

Etymology (EN): Back, from M.E., from O.E. bæc; akin to O.H.G. bah “back.”

Etymology (PE): Pas-, from pas “behind” (e.g.: pas-e pardé “behind the curtain”); Mid.Pers. pas “behind, before, after;” O.Pers. pasā “after;” Av. pasca “behind (of space); then, afterwards (of time);” cf.
Skt. paścā “behind, after, later;” L. post “behind, in the rear; after, afterwards;” O.C.S. po “behind, after;” Lith. pas “at, by;”
PIE *pos-, *posko-.
Pošt-, from pošt, variant of pas.

1. General: Related to or located at the back “the side or surface opposite the front or face.”
   

2. The side of the → planispheric astrolabe
   opposite the hollow of the → mater, → tympanum, and → rete. At its center is hinged the → alidade, whose rim slides on a set of graduated circular scales engraved on the mater. The number of scales varies from one instrument to another. On this face the user can perform observations and read the data needed to set the front side of the instrument in the proper position (online museo galileo, VirtualMuseum).

Etymology (EN): Back, from M.E., from O.E. bæc; akin to O.H.G. bah “back.”

Etymology (PE): Pas-, from pas “behind” (e.g.: pas-e pardé “behind the curtain”); Mid.Pers. pas “behind, before, after;” O.Pers. pasā “after;” Av. pasca “behind (of space); then, afterwards (of time);” cf.
Skt. paścā “behind, after, later;” L. post “behind, in the rear; after, afterwards;” O.C.S. po “behind, after;” Lith. pas “at, by;”
PIE *pos-, *posko-.
Pošt-, from pošt, variant of pas.

In a radiotelescope, the unit forming the end part of the reception chain. It generally consists of a spectrometer and performs frequency analysis of the signals. → front-end.

Etymology (EN): Back-end, from → back + end, from
M.E., O.E. ende (cf. Du. einde, O.H.G. enti “top, forehead, end,” Ger. ende, Goth. andeis “end”), originally “the opposite side,” from PIE *antjo “end, boundary,” from base *anta-/*anti- “opposite, in front of, before.”

Etymology (PE): Pas-tah, from pas, → back, + tah “end,” Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to Gk. tenagos “bottom, swamp,” Latvian tigas«i>*tingas < *tenegos “depth,” PIE *tenegos “water bottom.”

In a radiotelescope, the unit forming the end part of the reception chain. It generally consists of a spectrometer and performs frequency analysis of the signals. → front-end.

Etymology (EN): Back-end, from → back + end, from
M.E., O.E. ende (cf. Du. einde, O.H.G. enti “top, forehead, end,” Ger. ende, Goth. andeis “end”), originally “the opposite side,” from PIE *antjo “end, boundary,” from base *anta-/*anti- “opposite, in front of, before.”

Etymology (PE): Pas-tah, from pas, → back, + tah “end,” Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to Gk. tenagos “bottom, swamp,” Latvian tigas«i>*tingas < *tenegos “depth,” PIE *tenegos “water bottom.”

General: That part of a view or scene that serves as a setting for the main objects, persons, etc.
Astro.: The part of an observed field which is not physically related to the objects of interest and lies behind them.

Etymology (EN): Background, from → back + ground, from M.E., from O.E. grund; akin to O.H.G. grunt “ground.”

Etymology (PE): Paszaminé, from pas-, → back, + zaminé “ground,” from zamin “ground,” → earth.

See also: → foreground.

General: That part of a view or scene that serves as a setting for the main objects, persons, etc.
Astro.: The part of an observed field which is not physically related to the objects of interest and lies behind them.

Etymology (EN): Background, from → back + ground, from M.E., from O.E. grund; akin to O.H.G. grunt “ground.”

Etymology (PE): Paszaminé, from pas-, → back, + zaminé “ground,” from zamin “ground,” → earth.

See also: → foreground.

An unwanted signal in a system which is producing or recording a signal.
For instance, a randomly fluctuating signal superimposed on the signal from a cosmic radio source.

See also: → background; → noise.

An unwanted signal in a system which is producing or recording a signal.
For instance, a randomly fluctuating signal superimposed on the signal from a cosmic radio source.

See also: → background; → noise.

The isotropic residual microwave radiation in space left from the primordial → Big Bang. Same as
→ cosmic microwave background (CMB) radiation.

See also: → background; → radiation.

The isotropic residual microwave radiation in space left from the primordial → Big Bang. Same as
→ cosmic microwave background (CMB) radiation.

See also: → background; → radiation.

1. (v.tr.) To deflect photons or particles in a direction opposite to their initial path.
   
2. Same as → backscattering.
   
3. The radiation or particles so deflected.

See also: From → back + → scattering.

1. (v.tr.) To deflect photons or particles in a direction opposite to their initial path.
   
2. Same as → backscattering.
   
3. The radiation or particles so deflected.

See also: From → back + → scattering.

The light that has undergone → backscattering.

See also: → backscatter; → backscattering.

The light that has undergone → backscattering.

See also: → backscatter; → backscattering.

Scattering of radiation or particles through angles greater than 90° with respect to the original direction of motion.

See also: → scattering.

Scattering of radiation or particles through angles greater than 90° with respect to the original direction of motion.

See also: → scattering.

A copy of computer files that is stored separately from the original in order to protect against loss of data.

Etymology (EN): Backup “substitute, support,” from → back + up.

Etymology (PE): Poštvân “prop, support, help” from pošt,
→ back, + -vân suffix denting protection, variant of -bân.

A copy of computer files that is stored separately from the original in order to protect against loss of data.

Etymology (EN): Backup “substitute, support,” from → back + up.

Etymology (PE): Poštvân “prop, support, help” from pošt,
→ back, + -vân suffix denting protection, variant of -bân.

An auxiliary observing program to be carried out at telescope in case the atmospheric conditions make the main program unfeasible.

Etymology (EN): Backup, from → back + up;
→ program.

Etymology (PE): Barnâmé, → program; yadaki “reserve, substitute,” from yadak “a led horse.”

An auxiliary observing program to be carried out at telescope in case the atmospheric conditions make the main program unfeasible.

Etymology (EN): Backup, from → back + up;
→ program.

Etymology (PE): Barnâmé, → program; yadaki “reserve, substitute,” from yadak “a led horse.”

1. Toward the back or rear.
   

2. Directed toward the back or past.

See also: → back; → -ward.

1. Toward the back or rear.
   

2. Directed toward the back or past.

See also: → back; → -ward.

A sort of → greenhouse effect in → stellar atmospheres where the deeper layers heat up due to overlying → opacity. The presence of numerous → bound-bound opacities of → metals amplifies the → scattering of → photons, in particular their → backscattering, forcing the → temperature to increase in order to conserve the radiation flux and the transport of energy from the interior to the outer parts of the atmosphere.

See also: → back; → warming; → effect.

A sort of → greenhouse effect in → stellar atmospheres where the deeper layers heat up due to overlying → opacity. The presence of numerous → bound-bound opacities of → metals amplifies the → scattering of → photons, in particular their → backscattering, forcing the → temperature to increase in order to conserve the radiation flux and the transport of energy from the interior to the outer parts of the atmosphere.

See also: → back; → warming; → effect.

Column of a → CCD detector that does not correctly read out charge.

Etymology (EN): Bad, from M.E. badde, but the origin of the word is not clear; → column.

Etymology (PE): Sotun, → column; bad, from Mid.Pers. wad, maybe from Old Iranian *vata- “small;” cf. Scythian bata- “small, bad,” Sogdian wtγy “suffer, sorrow.”

Column of a → CCD detector that does not correctly read out charge.

Etymology (EN): Bad, from M.E. badde, but the origin of the word is not clear; → column.

Etymology (PE): Sotun, → column; bad, from Mid.Pers. wad, maybe from Old Iranian *vata- “small;” cf. Scythian bata- “small, bad,” Sogdian wtγy “suffer, sorrow.”

A phenomenon that occurs during a total eclipse of the Sun. Just prior to and after totality, sunlight shines through the lunar valleys on the Moon’s limb, causing the dark face of the Moon to appear to be surrounded by a shining “necklace of pearls”.

Etymology (EN): Baily, from Francis Baily (1774-1844), English amateur astronomer, who discovered the phenomenon during the solar eclipse of 1836. Beads “a necklace of beads or pearls; a rosary,” from bead “a small, often round piece of material, such as glass, plastic, or wood, that is pierced for stringing or threading,” from M.E. bede “rosary bead,” from O.E. bed, bedu, gebed “prayer;” PIE *gwhedh- “to ask, pray”.

Etymology (PE): Mohrehâ “beads,” from mohré “a kind of small shell resembling pearls; glass or coral beads,” cf. Khotanese mrâhe, may be related to morvârid, → pearl,

• -hâ suffix of plurals.

A phenomenon that occurs during a total eclipse of the Sun. Just prior to and after totality, sunlight shines through the lunar valleys on the Moon’s limb, causing the dark face of the Moon to appear to be surrounded by a shining “necklace of pearls”.

Etymology (EN): Baily, from Francis Baily (1774-1844), English amateur astronomer, who discovered the phenomenon during the solar eclipse of 1836. Beads “a necklace of beads or pearls; a rosary,” from bead “a small, often round piece of material, such as glass, plastic, or wood, that is pierced for stringing or threading,” from M.E. bede “rosary bead,” from O.E. bed, bedu, gebed “prayer;” PIE *gwhedh- “to ask, pray”.

Etymology (PE): Mohrehâ “beads,” from mohré “a kind of small shell resembling pearls; glass or coral beads,” cf. Khotanese mrâhe, may be related to morvârid, → pearl,

• -hâ suffix of plurals.

Something edible placed on a hook or in a trap
to attract fish or other animals as prey.

Etymology (EN): M.E., from O.Norse beita “food,” O.E. bat “food,” literally “to cause to bite.”

Etymology (PE): Cašté “bait,” related to câšni “taste,” cašidan, caš- “to taste,” câšt “breakfast;” Mid.Pers. câšt “meal,” câšnig “taste;” cf. Skt. cas- “to eat;” Proto-Ir. caš- “to eat, to drink; to drip.”

Something edible placed on a hook or in a trap
to attract fish or other animals as prey.

Etymology (EN): M.E., from O.Norse beita “food,” O.E. bat “food,” literally “to cause to bite.”

Etymology (PE): Cašté “bait,” related to câšni “taste,” cašidan, caš- “to taste,” câšt “breakfast;” Mid.Pers. câšt “meal,” câšnig “taste;” cf. Skt. cas- “to eat;” Proto-Ir. caš- “to eat, to drink; to drip.”

1a) A weighing device.

1b) The constellation → Libra.

2. A state of equilibrium.

Etymology (EN): M.E balaunce, from O.Fr. balance “balance, scales for weighing,” from M.L. bilancia, from L.L. bilanx, from L. (libra) bilanx “(scale) having two pans,” possibly from L. bis “twice” + lanx “dish, plate, scale of a balance.”

Etymology (PE): Tarâzu, → Libra.

1a) A weighing device.

1b) The constellation → Libra.

2. A state of equilibrium.

Etymology (EN): M.E balaunce, from O.Fr. balance “balance, scales for weighing,” from M.L. bilancia, from L.L. bilanx, from L. (libra) bilanx “(scale) having two pans,” possibly from L. bis “twice” + lanx “dish, plate, scale of a balance.”

Etymology (PE): Tarâzu, → Libra.

Having little or no hair on the scalp. → bald patch.

Etymology (EN): Bald from M.E. ball(e)d; Celtic bal “white patch, blaze;” Gk. phalios “having a white spot;” L. fulica “coot;” → patch.

Etymology (PE): Kal “bald,” Mid.Pers. gar “bald;”
Av. kaurva- “bald;” cf. Skt. kulva- “bald, thin-haired;” L. calvus “naked, bald.”
Kacal may be related to kal.
Tâs, variant taz “bald;” Tabari tisâ, Saraxsi, Lâsgardi, Sangesari tusâ “empty;” related to tohi “empty;” Mid.Pers. tuhig; Av. taoš- “to become empty,” pres. tusa-, caus. taošaya-, tusən “they lose their posture;” cf. Skt. tuccha-, tucchya- “empty;” L. tesqua, tesca “deserted place;” Russ. tošcij “hollow;” PIE base *teus- “to empty.”

Having little or no hair on the scalp. → bald patch.

Etymology (EN): Bald from M.E. ball(e)d; Celtic bal “white patch, blaze;” Gk. phalios “having a white spot;” L. fulica “coot;” → patch.

Etymology (PE): Kal “bald,” Mid.Pers. gar “bald;”
Av. kaurva- “bald;” cf. Skt. kulva- “bald, thin-haired;” L. calvus “naked, bald.”
Kacal may be related to kal.
Tâs, variant taz “bald;” Tabari tisâ, Saraxsi, Lâsgardi, Sangesari tusâ “empty;” related to tohi “empty;” Mid.Pers. tuhig; Av. taoš- “to become empty,” pres. tusa-, caus. taošaya-, tusən “they lose their posture;” cf. Skt. tuccha-, tucchya- “empty;” L. tesqua, tesca “deserted place;” Russ. tošcij “hollow;” PIE base *teus- “to empty.”

The location on the surface of the → Sun where → coronal  → magnetic field lines become tangent to the → photosphere. Bald patches play an important role in solar → magnetohydrodynamics.

See also: → bald, such called because of visual reference to a haircut (Titov et al. 1993, A&A 276, 564); → patch.

The location on the surface of the → Sun where → coronal  → magnetic field lines become tangent to the → photosphere. Bald patches play an important role in solar → magnetohydrodynamics.

See also: → bald, such called because of visual reference to a haircut (Titov et al. 1993, A&A 276, 564); → patch.

A set of nebular → emission line diagrams used to distinguish the ionization mechanism of → nebular gas. The most famous version consists of [N II]λ6584/Hα versus [OIII] λ5007/Hβ. The next two more commonly used BPT diagnostics are [S II] λλ6717,6731/Hα versus [O III] λ5007/Hβ and [O I] λ6300/Hα versus [O III]λ5007/Hβ. These diagrams use strong, optical lines of close proximity in the ratios to limit → reddening and → spectrophotometric effects. They are able to clearly distinguish different classes of → ionization, for example → LINERs from normal → H II regions and → active galactic nuclei.

See also: Baldwin, J. A., Phillips, M. M., Terlevich, R., 1981 PASP 93, 5; → diagram.

A set of nebular → emission line diagrams used to distinguish the ionization mechanism of → nebular gas. The most famous version consists of [N II]λ6584/Hα versus [OIII] λ5007/Hβ. The next two more commonly used BPT diagnostics are [S II] λλ6717,6731/Hα versus [O III] λ5007/Hβ and [O I] λ6300/Hα versus [O III]λ5007/Hβ. These diagrams use strong, optical lines of close proximity in the ratios to limit → reddening and → spectrophotometric effects. They are able to clearly distinguish different classes of → ionization, for example → LINERs from normal → H II regions and → active galactic nuclei.

See also: Baldwin, J. A., Phillips, M. M., Terlevich, R., 1981 PASP 93, 5; → diagram.

A spherical or approximately spherical body, either solid or hollow.

Etymology (EN): From M.E. bal, balle, from O.Fr.; cf. O.H.G. ballo, Ger. Ball; PIE root *bhel- “to blow, swell.”

Etymology (PE): Tup “ball,” initially “clmup, aggregation, parcel, group” (tup tup “many”); Tabari tupa “compressed, assembled,” tuppi “round;” Kurd. top “ball,” topâl “round;”
guy, → globe.

A spherical or approximately spherical body, either solid or hollow.

Etymology (EN): From M.E. bal, balle, from O.Fr.; cf. O.H.G. ballo, Ger. Ball; PIE root *bhel- “to blow, swell.”

Etymology (PE): Tup “ball,” initially “clmup, aggregation, parcel, group” (tup tup “many”); Tabari tupa “compressed, assembled,” tuppi “round;” Kurd. top “ball,” topâl “round;”
guy, → globe.

A rare form of lightning occurring as a bright red globe observed floating or moving through the atmosphere close to the ground. It usually is seen shortly before or after, or during, a → thunderstorm. Its duration varies from a few seconds to a few minutes. See also → Saint Elmo’s fire.

See also: → ball; → lightning.

A rare form of lightning occurring as a bright red globe observed floating or moving through the atmosphere close to the ground. It usually is seen shortly before or after, or during, a → thunderstorm. Its duration varies from a few seconds to a few minutes. See also → Saint Elmo’s fire.

See also: → ball; → lightning.

Of or relating to → ballistics.

Of or relating to → ballistics.

A missile that after being launched and guided in the early part of its flight, travels unpowered in a ballistic trajectory.

See also: → ballistic; → missile.

A missile that after being launched and guided in the early part of its flight, travels unpowered in a ballistic trajectory.

See also: → ballistic; → missile.

Transfer of microbes and biochemical compounds from a planet to another due to meteoric impacts. Debris being knocked off a planet like Mars can reach escape velocity and enter the atmosphere of another planet with passenger micro-organisms intact.

See also: → ballistic; → panspermia.

Transfer of microbes and biochemical compounds from a planet to another due to meteoric impacts. Debris being knocked off a planet like Mars can reach escape velocity and enter the atmosphere of another planet with passenger micro-organisms intact.

See also: → ballistic; → panspermia.

A curved path followed by an unpowered object that is being acted upon only by gravitational forces and the friction of the medium through which it moves.

See also: → ballistic; → trajectory.

A curved path followed by an unpowered object that is being acted upon only by gravitational forces and the friction of the medium through which it moves.

See also: → ballistic; → trajectory.

Audible disturbance or wave caused by the compression of air ahead of a projectile in flight.

See also: → ballistic; → wave.

Audible disturbance or wave caused by the compression of air ahead of a projectile in flight.

See also: → ballistic; → wave.

The science of the motion and behavior of → projectiles. The study of the functioning of firearms.

Etymology (EN): From L. ballista “ancient military machine for hurling stones,” from Gk. ballistes, from ballein “to throw,” from PIE *g^welH₁- “to throw;” cf. Pers. garzin “arrow;” Av. niγr- “to throw down;” Khotanese (+ *abi-, *ui-) bīr- “to throw, sow;” Proto-Iranian *garH- “to throw.”

Etymology (PE): Partâbik, from partâb “a throw, an arrow that flies far,” partâbidan “to throw,” + -ik, → -ics;
partâbšenâsi, from partâb + -šenâsi,
→ -logy.

The science of the motion and behavior of → projectiles. The study of the functioning of firearms.

Etymology (EN): From L. ballista “ancient military machine for hurling stones,” from Gk. ballistes, from ballein “to throw,” from PIE *g^welH₁- “to throw;” cf. Pers. garzin “arrow;” Av. niγr- “to throw down;” Khotanese (+ *abi-, *ui-) bīr- “to throw, sow;” Proto-Iranian *garH- “to throw.”

Etymology (PE): Partâbik, from partâb “a throw, an arrow that flies far,” partâbidan “to throw,” + -ik, → -ics;
partâbšenâsi, from partâb + -šenâsi,
→ -logy.

A branch of modern astronomy in which balloons are used to carry telescopes and instruments to high altitudes (up to 50 km) for observation.

Etymology (EN): Balloon, from Fr. ballon, from It. dialectal ballone, augmentative of balla, ball, from P.Gmc. *ball-, from PIE *bhel- “to blow, swell”. → astronomy.

Etymology (PE): Axtaršenâsi, → astronomy; bâlon, from Fr. ballon.

A branch of modern astronomy in which balloons are used to carry telescopes and instruments to high altitudes (up to 50 km) for observation.

Etymology (EN): Balloon, from Fr. ballon, from It. dialectal ballone, augmentative of balla, ball, from P.Gmc. *ball-, from PIE *bhel- “to blow, swell”. → astronomy.

Etymology (PE): Axtaršenâsi, → astronomy; bâlon, from Fr. ballon.

A remotely guided or automatic telescope carried to high altitudes by a balloon.

Etymology (EN): → balloon astronomy; borne “a past participle of bear," from O.E. beran “bear, bring, wear,” from P.Gmc. *beranan (O.H.G. beran, Goth. bairan “to carry”), from PIE root *bher-; “to carry;” compare with Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother,” Mod.Pers. bordan “to carry,” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry.” → telescope.

Etymology (PE): → balloon astronomy. Bord in bâlon-bord “borne, carried,” from Mod.Pers. bordan “to bear, carry,” as explained above. Durbin, → telescope.

A remotely guided or automatic telescope carried to high altitudes by a balloon.

Etymology (EN): → balloon astronomy; borne “a past participle of bear," from O.E. beran “bear, bring, wear,” from P.Gmc. *beranan (O.H.G. beran, Goth. bairan “to carry”), from PIE root *bher-; “to carry;” compare with Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother,” Mod.Pers. bordan “to carry,” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry.” → telescope.

Etymology (PE): → balloon astronomy. Bord in bâlon-bord “borne, carried,” from Mod.Pers. bordan “to bear, carry,” as explained above. Durbin, → telescope.

From Johann Jakob Balmer (1825-1898),
Swiss mathematician and physicist, who explained the visible spectral lines of the hydrogen spectrum in 1885.

From Johann Jakob Balmer (1825-1898),
Swiss mathematician and physicist, who explained the visible spectral lines of the hydrogen spectrum in 1885.

A continuous range of wavelengths in the Balmer spectrum of hydrogen corresponding to transitions between the energy levels n = 2 and n = ∞.

See also: → Balmer; → continuum.

A continuous range of wavelengths in the Balmer spectrum of hydrogen corresponding to transitions between the energy levels n = 2 and n = ∞.

See also: → Balmer; → continuum.

The intensity ratio among the couple of relatively adjacent → Balmer lines, for example Hα/Hβ and Hβ/Hγ, which have well-known theoretical values. They are used to determine the → interstellar extinction.

See also: → Balmer; → decrement.

The intensity ratio among the couple of relatively adjacent → Balmer lines, for example Hα/Hβ and Hβ/Hγ, which have well-known theoretical values. They are used to determine the → interstellar extinction.

See also: → Balmer; → decrement.

An abrupt decrease in the intensity of the continuum at the limit of the → Balmer series of hydrogen (at about 3650 Å), caused by the energy absorbed when electrons originally in the second → energy level are ionized. Same as → Balmer jump.

See also: → Balmer; → discontinuity.

An abrupt decrease in the intensity of the continuum at the limit of the → Balmer series of hydrogen (at about 3650 Å), caused by the energy absorbed when electrons originally in the second → energy level are ionized. Same as → Balmer jump.

See also: → Balmer; → discontinuity.

A special solution of the mathematical formula which represents the wavelengths of the various spectral series of hydrogen in which the lower energy level is n = 2.

See also: → Balmer; → formula.

A special solution of the mathematical formula which represents the wavelengths of the various spectral series of hydrogen in which the lower energy level is n = 2.

See also: → Balmer; → formula.

Same as → Balmer discontinuity.

See also: → Balmer; → jump.

Same as → Balmer discontinuity.

See also: → Balmer; → jump.

The wavelength in the blue end of the → Balmer series, at 3646 Å, near which the separation between successive lines decreases and approaches a → continuum.

See also: → Balmer; → limit.

The wavelength in the blue end of the → Balmer series, at 3646 Å, near which the separation between successive lines decreases and approaches a → continuum.

See also: → Balmer; → limit.

The → spectral lines making up the → Balmer series.

See also: → Balmer; → line.

The → spectral lines making up the → Balmer series.

See also: → Balmer; → line.

A series of hydrogen → spectral lines (Hα, Hβ, Hγ, and others) that lies in the visible portion of the spectrum and results when electrons from upper → energy levels (n > 2) undergo → transition to n = 2.
Historically, Balmer emission lines (mainly Hα) from ionized nebulae were first observed by O. Struve and C. T. Elvey (1938, ApJ 88, 364). This was an important indication of the existence of hydrogen, in the ionized state, in the → interstellar medium.

See also: → Balmer; → series.

A series of hydrogen → spectral lines (Hα, Hβ, Hγ, and others) that lies in the visible portion of the spectrum and results when electrons from upper → energy levels (n > 2) undergo → transition to n = 2.
Historically, Balmer emission lines (mainly Hα) from ionized nebulae were first observed by O. Struve and C. T. Elvey (1938, ApJ 88, 364). This was an important indication of the existence of hydrogen, in the ionized state, in the → interstellar medium.

See also: → Balmer; → series.

General:1) A strip serving to encircle and bind one object or to hold a number of objects together. 2) A strip or stripe that contrasts with something else in color, texture, or material.
Physics: 1) A specific range of wavelengths or frequencies of electromagnetic radiation.
2) Closely packed spectral lines that appear to form a continuous group. → absorption band;
→ emission band.

Etymology (EN): From M.E. bende, O.E. bend, from O.Fr. bande, bende, P.Gmc. *bindan, from PIE *bendh- “to bind” (cf. Goth bandi “that which binds;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” bandhah “a tying, bandage.”

Etymology (PE): Bând, adoption from E. band, which is cognate and synonymous with Pers. band, present tense stem of bastan “to bind, shut,” Mid.Pers. bastan, band, Av./O.Pers. band-, as explained above. See also → strip.

General:1) A strip serving to encircle and bind one object or to hold a number of objects together. 2) A strip or stripe that contrasts with something else in color, texture, or material.
Physics: 1) A specific range of wavelengths or frequencies of electromagnetic radiation.
2) Closely packed spectral lines that appear to form a continuous group. → absorption band;
→ emission band.

Etymology (EN): From M.E. bende, O.E. bend, from O.Fr. bande, bende, P.Gmc. *bindan, from PIE *bendh- “to bind” (cf. Goth bandi “that which binds;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” bandhah “a tying, bandage.”

Etymology (PE): Bând, adoption from E. band, which is cognate and synonymous with Pers. band, present tense stem of bastan “to bind, shut,” Mid.Pers. bastan, band, Av./O.Pers. band-, as explained above. See also → strip.

A location on the spectrogram of a molecule at which the lines of a band stack.

Etymology (EN): Band head, from → band + head, from O.E. heafod “top of the body,” also “upper end of a slope,” also “chief person, leader,” from P.Gmc. *khaubuthan, from PIE *kauput- “head” (cf. Skt. kaput-, L. caput “head,” Lori kapu “head,” kapulek “skull, middle of the head”).

Etymology (PE): Bândsar, from → bând + sar “head,” soru, sorun “horn,” karnâ “a trumpet-like wind instrument” (originally made from animal horns), variant sornâ “a wind instrument;” Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;” P.Gmc. *khurnaz (E. horn; Ger. Horn, Du. horen), from PIE *ker- “head, horn.”

A location on the spectrogram of a molecule at which the lines of a band stack.

Etymology (EN): Band head, from → band + head, from O.E. heafod “top of the body,” also “upper end of a slope,” also “chief person, leader,” from P.Gmc. *khaubuthan, from PIE *kauput- “head” (cf. Skt. kaput-, L. caput “head,” Lori kapu “head,” kapulek “skull, middle of the head”).

Etymology (PE): Bândsar, from → bând + sar “head,” soru, sorun “horn,” karnâ “a trumpet-like wind instrument” (originally made from animal horns), variant sornâ “a wind instrument;” Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;” P.Gmc. *khurnaz (E. horn; Ger. Horn, Du. horen), from PIE *ker- “head, horn.”

A spectrum which consists of a number of bands each having one sharp edge. Each band is composed of a large number of closely spaced emission or absorption lines. Band spectra are typical of molecules. Bands produced by titanium oxide, zirconium oxide, and carbon compounds are characteristic of low temperature stars.

Etymology (EN): Band spectrum, from → band + → spectrum.

Etymology (PE): Binâb-e bândi, from binâb, → spectrum + bândi, relating to bând, → band.

A spectrum which consists of a number of bands each having one sharp edge. Each band is composed of a large number of closely spaced emission or absorption lines. Band spectra are typical of molecules. Bands produced by titanium oxide, zirconium oxide, and carbon compounds are characteristic of low temperature stars.

Etymology (EN): Band spectrum, from → band + → spectrum.

Etymology (PE): Binâb-e bândi, from binâb, → spectrum + bândi, relating to bând, → band.

A range of frequencies that can pass through a filter such as one in an electrical circuit.

Etymology (EN): From → band + pass, from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step.”

Etymology (PE): Gozar-bând, from gozar “passage, transit, passing,” gozaštan “to pass, cross, transit,” from Mid.Pers. vitârtan + bând, → band.

A range of frequencies that can pass through a filter such as one in an electrical circuit.

Etymology (EN): From → band + pass, from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step.”

Etymology (PE): Gozar-bând, from gozar “passage, transit, passing,” gozaštan “to pass, cross, transit,” from Mid.Pers. vitârtan + bând, → band.

An electric filter that transmits a known band of frequencies but suppresses unwanted frequencies above and below this band.

See also: → bandpass; → filter.

An electric filter that transmits a known band of frequencies but suppresses unwanted frequencies above and below this band.

See also: → bandpass; → filter.

The portion of the electromagnetic spectrum that is permitted to pass through an electronic device, such as a radio telescope detector. The term refers to either a wavelength interval or a frequency interval.

Etymology (EN): Bandwidth, from → band + → width.

Etymology (PE): Bândpahnâ, from bând, → band, + pahnâ, → width, from pahn “wide,”
→ broad.

The portion of the electromagnetic spectrum that is permitted to pass through an electronic device, such as a radio telescope detector. The term refers to either a wavelength interval or a frequency interval.

Etymology (EN): Bandwidth, from → band + → width.

Etymology (PE): Bândpahnâ, from bând, → band, + pahnâ, → width, from pahn “wide,”
→ broad.

1. A unit of → pressure, not belonging to the → International System (SI), equivalent to 10⁶→ dynes per cm² and 0.987 → atmospheres.
   

2. → stellar bar, → galactic bar, → barred spiral galaxy.

Etymology (EN): 1) From Gk. baros “weight,” cf. Skt guru, L. gravis; PIE *gwere- “heavy;” cf. Pers. bâr “weight,” gerân “heavy,” L. brutus “heavy, dull, stupid, brutish,” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

2. From O.Fr. barre, from V.L. *barra “bar, barrier,” or perhaps from Gaulish *barro “summit.”

Etymology (PE): 1) Loan from Fr., as above.

2. Mile, from Ar. mil “any long and narrow piece of metal,” + -é noun suffix.

1. A unit of → pressure, not belonging to the → International System (SI), equivalent to 10⁶→ dynes per cm² and 0.987 → atmospheres.
   

2. → stellar bar, → galactic bar, → barred spiral galaxy.

Etymology (EN): 1) From Gk. baros “weight,” cf. Skt guru, L. gravis; PIE *gwere- “heavy;” cf. Pers. bâr “weight,” gerân “heavy,” L. brutus “heavy, dull, stupid, brutish,” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

2. From O.Fr. barre, from V.L. *barra “bar, barrier,” or perhaps from Gaulish *barro “summit.”

Etymology (PE): 1) Loan from Fr., as above.

2. Mile, from Ar. mil “any long and narrow piece of metal,” + -é noun suffix.

A whitish, malleable, metallic → chemical element; symbol Ba. → Atomic number 56; → atomic weight 137.33; → melting point 725°C; → boiling point 1,640°C; → specific gravity 3.5 at 20°C. Barium was discovered by the Swedish pharmacist and chemist Carl Wilhelm Scheele in 1774. It was first isolated by the British chemist Humphry Davy in 1808.

See also: From Mod.L., from Gk. barys “heavy,” from the mineral barytes “heavy spar” (BaSO₄),
in which the element was discovered; cognate with Pers. bâr “weight,” → bar.

A whitish, malleable, metallic → chemical element; symbol Ba. → Atomic number 56; → atomic weight 137.33; → melting point 725°C; → boiling point 1,640°C; → specific gravity 3.5 at 20°C. Barium was discovered by the Swedish pharmacist and chemist Carl Wilhelm Scheele in 1774. It was first isolated by the British chemist Humphry Davy in 1808.

See also: From Mod.L., from Gk. barys “heavy,” from the mineral barytes “heavy spar” (BaSO₄),
in which the element was discovered; cognate with Pers. bâr “weight,” → bar.

A type of star, usually G or K → giants, whose spectra show unusually strong absorption lines of → barium, → strontium, and other → s-process elements.

See also: → barium; → star.

A type of star, usually G or K → giants, whose spectra show unusually strong absorption lines of → barium, → strontium, and other → s-process elements.

See also: → barium; → star.

The external covering on the trunks, boughs, and branches of trees.

Etymology (EN): M.E., from O.Norse börkr “bark.”

Etymology (PE): Kâlun, from Mâzandarâni kâlun “bark,” variants (Dâmqân) kul “bark,” (Tâti) lo “bark,” (Yazd, Mâzandarân) kol “bark,” (Nâin) kuluz “egg shell,” (Aftar) cokola “egg shell, pistaschio shell,” pukel, → shell, keler, → scalp, probably related to (Khotan Sacca) karastra- “fur garment,” (Waxi) kurust “bark of tree,” from PIE root *(s)ker- “to cut off,” from which are derived L. cortex “bark,” corium “thick skin,” scortum “hide,” and Persian carm “leather.”

The external covering on the trunks, boughs, and branches of trees.

Etymology (EN): M.E., from O.Norse börkr “bark.”

Etymology (PE): Kâlun, from Mâzandarâni kâlun “bark,” variants (Dâmqân) kul “bark,” (Tâti) lo “bark,” (Yazd, Mâzandarân) kol “bark,” (Nâin) kuluz “egg shell,” (Aftar) cokola “egg shell, pistaschio shell,” pukel, → shell, keler, → scalp, probably related to (Khotan Sacca) karastra- “fur garment,” (Waxi) kurust “bark of tree,” from PIE root *(s)ker- “to cut off,” from which are derived L. cortex “bark,” corium “thick skin,” scortum “hide,” and Persian carm “leather.”

A → negative lens placed in a telescope between the → objective and the → ocular. Its diverging action reduces the convergence of the light cone, forming a larger image at a slightly greater distance.

See also: Peter Barlow (1776-1862), English physicist; → lens.

A → negative lens placed in a telescope between the → objective and the → ocular. Its diverging action reduces the convergence of the light cone, forming a larger image at a slightly greater distance.

See also: Peter Barlow (1776-1862), English physicist; → lens.

In nuclear physics, unit of area for measuring the cross-sections of nuclei. 1 barn equals 10^-24 sq. cm.

Etymology (EN): Barn, from O.E. bereærn “barn,” lit. “barley house,” from bere “barley” + aern “house.” The use of barn in nuclear physics comes from the fact that the term denotes also “an unexpectedly large quantity of something.” It seems that when physicists were first studying nuclear interactions, they found out that the interaction probabilities, or cross-sections, were far more larger than expected; the nuclei were `as big as a barn'.

In nuclear physics, unit of area for measuring the cross-sections of nuclei. 1 barn equals 10^-24 sq. cm.

Etymology (EN): Barn, from O.E. bereærn “barn,” lit. “barley house,” from bere “barley” + aern “house.” The use of barn in nuclear physics comes from the fact that the term denotes also “an unexpectedly large quantity of something.” It seems that when physicists were first studying nuclear interactions, they found out that the interaction probabilities, or cross-sections, were far more larger than expected; the nuclei were `as big as a barn'.

From Edward Emerson Barnard (1857-1923) American astronomer who made several obserational discoveries.

From Edward Emerson Barnard (1857-1923) American astronomer who made several obserational discoveries.

A very faint nebular shell of huge size enveloping the central portion of Orion.

Etymology (EN): Named after → Barnard, who discovered the loop in 1895; → loop.

A very faint nebular shell of huge size enveloping the central portion of Orion.

Etymology (EN): Named after → Barnard, who discovered the loop in 1895; → loop.

A → red dwarf in the constellation → Ophiuchus discovered in 1916 by E.E. Barnard, that until 1968 had the largest → proper motion of any star. It moves on the sky 10.3 arcseconds per year, which means that it travels the equivalent of a lunar diameter every 180 years. It is the second nearest star system to the Sun.

Etymology (EN): In honor of → Barnard; → star.

A → red dwarf in the constellation → Ophiuchus discovered in 1916 by E.E. Barnard, that until 1968 had the largest → proper motion of any star. It moves on the sky 10.3 arcseconds per year, which means that it travels the equivalent of a lunar diameter every 180 years. It is the second nearest star system to the Sun.

Etymology (EN): In honor of → Barnard; → star.

A prefix meaning → pressure used in the formation of compound words, such as → baroclinic, → barometer, → barotropic.

Etymology (EN): Baro- combining form of Gk. baros “weight;” cognate with Pers. bâr “weight,” gerân “heavy;” cf. Skt. guru, L. gravis; PIE *gwere- “heavy;”
L. brutus “heavy, dull, stupid, brutish;” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

Etymology (PE): Fešâr-, → pressure.

A prefix meaning → pressure used in the formation of compound words, such as → baroclinic, → barometer, → barotropic.

Etymology (EN): Baro- combining form of Gk. baros “weight;” cognate with Pers. bâr “weight,” gerân “heavy;” cf. Skt. guru, L. gravis; PIE *gwere- “heavy;”
L. brutus “heavy, dull, stupid, brutish;” Skt. bhara- “burden, load,” bharati “he carries;” PIE *bher- “carry, give birth.”

Etymology (PE): Fešâr-, → pressure.

Of, pertaining to, or characterized by → baroclinicity. Sometimes called → barocline.

See also: → baro-; → -cline; → -ic.

Of, pertaining to, or characterized by → baroclinicity. Sometimes called → barocline.

See also: → baro-; → -cline; → -ic.

1. A type of instability occurring within a rapidly → rotating star where non-axisymmetric motions can separate surfaces of constant pressure from → equipotential surfaces.
   

2. A hydrodynamic instability associated with a baroclinic layer of the atmosphere. It arises from temperature variation along the pressure surfaces. Baroclinic instability is associated with the vertical → shear of the mean flow, which is related to the horizontal temperature gradient by the thermal wind equation. Instabilities in a baroclinic region grow by converting potential energy associated with the mean horizontal temperature gradient into kinetic energy through ascending warm air and descending cold air (Rasmussen & Turner (eds.), Polar Lows, Cambridge Univ. Press, 2003).

See also: → baroclinic; → instability.

1. A type of instability occurring within a rapidly → rotating star where non-axisymmetric motions can separate surfaces of constant pressure from → equipotential surfaces.
   

2. A hydrodynamic instability associated with a baroclinic layer of the atmosphere. It arises from temperature variation along the pressure surfaces. Baroclinic instability is associated with the vertical → shear of the mean flow, which is related to the horizontal temperature gradient by the thermal wind equation. Instabilities in a baroclinic region grow by converting potential energy associated with the mean horizontal temperature gradient into kinetic energy through ascending warm air and descending cold air (Rasmussen & Turner (eds.), Polar Lows, Cambridge Univ. Press, 2003).

See also: → baroclinic; → instability.

The state of stratification in a fluid in which surfaces of constant pressure do not coincide with those of constant density, but intersect. Where baroclinicity is zero, the fluid is → barotropic. Same as baroclinity.

See also: → baroclinic; → -ity.

The state of stratification in a fluid in which surfaces of constant pressure do not coincide with those of constant density, but intersect. Where baroclinicity is zero, the fluid is → barotropic. Same as baroclinity.

See also: → baroclinic; → -ity.

Instrument for measuring the atmospheric pressure. It is used in determining height above sea level and predicting changes in weather.

See also: → baro- + → -meter.

Instrument for measuring the atmospheric pressure. It is used in determining height above sea level and predicting changes in weather.

See also: → baro- + → -meter.

A law which describes the vertical pressure distribution in the lower parts of Earth’s atmosphere. The atmospheric pressure decreases exponentially from any reference surface as the altitude increases.

See also: → barometer; → law.

A law which describes the vertical pressure distribution in the lower parts of Earth’s atmosphere. The atmospheric pressure decreases exponentially from any reference surface as the altitude increases.

See also: → barometer; → law.

In a fluid, conditions where surfaces of constant pressure
are parallel to surfaces of constant temperature. This state is equivalent to zero
→ baroclinicity.

See also: → baro-; → -tropic.

In a fluid, conditions where surfaces of constant pressure
are parallel to surfaces of constant temperature. This state is equivalent to zero
→ baroclinicity.

See also: → baro-; → -tropic.

A gas whose density is a function solely of pressure.

See also: → barotropic; → gas.

A gas whose density is a function solely of pressure.

See also: → barotropic; → gas.

A hydrodynamical instability that arises when the horizontal → shear gradient becomes very large. Barotropic instabilities grow by extracting kinetic energy from the mean flow field.

See also: → barotropic; → instability.

A hydrodynamical instability that arises when the horizontal → shear gradient becomes very large. Barotropic instabilities grow by extracting kinetic energy from the mean flow field.

See also: → barotropic; → instability.

A state of a fluid in which the surfaces of constant density coincide with surfaces of constant pressure (isobaric).

See also: → barotropic gas.

A state of a fluid in which the surfaces of constant density coincide with surfaces of constant pressure (isobaric).

See also: → barotropic gas.

Having a bar like structure.

See also: → bar.

Having a bar like structure.

See also: → bar.

A transitional class of object between the classic spiral galaxies and true irregular systems. The → Large Magellanic Cloud, the nearest and best studied example of the class, is, contrary to popular opinion, not an irregular galaxy. The LMC and other members of the SBm class have definite structural signatures. They are generally dominated by a pronounced asymmetric bar – one that is offset from the optical center of the galaxy – with a nascent spiral arm emanating from one end. As is the case with irregular galaxies, the optical centers of SBm type systems are not particularly special places. Disk systems later than Sc characteristically lack a central stellar concentration in addition to having weak spiral structure; this is true of SBm-type galaxies. SBm galaxies are typically very active in their star formation activity, often containing a large star-forming complex situated at one end of the bar. Beyond these general trends there is a tremendous amount of dispersion in physical properties within the SBm class, particularly in the strength of the spiral structure. At one extreme are the “one-armed” spirals such as NGC 3664 and NGC 4027 which are dominated by single, looping spiral arm. On the other hand NGC 4861 shows little evidence of spiral structure and it is dominated by a large star-forming complex at one end of its bar. The class smoothly leads to the Barred Magellanic irregulars (IBm) which show no indication of spiral structure

(Wilcots et al. 1996, AJ 111, 1575).

See also: → Magellanic; → spiral; → galaxy.

A transitional class of object between the classic spiral galaxies and true irregular systems. The → Large Magellanic Cloud, the nearest and best studied example of the class, is, contrary to popular opinion, not an irregular galaxy. The LMC and other members of the SBm class have definite structural signatures. They are generally dominated by a pronounced asymmetric bar – one that is offset from the optical center of the galaxy – with a nascent spiral arm emanating from one end. As is the case with irregular galaxies, the optical centers of SBm type systems are not particularly special places. Disk systems later than Sc characteristically lack a central stellar concentration in addition to having weak spiral structure; this is true of SBm-type galaxies. SBm galaxies are typically very active in their star formation activity, often containing a large star-forming complex situated at one end of the bar. Beyond these general trends there is a tremendous amount of dispersion in physical properties within the SBm class, particularly in the strength of the spiral structure. At one extreme are the “one-armed” spirals such as NGC 3664 and NGC 4027 which are dominated by single, looping spiral arm. On the other hand NGC 4861 shows little evidence of spiral structure and it is dominated by a large star-forming complex at one end of its bar. The class smoothly leads to the Barred Magellanic irregulars (IBm) which show no indication of spiral structure

(Wilcots et al. 1996, AJ 111, 1575).

See also: → Magellanic; → spiral; → galaxy.

A → spiral galaxy that exhibits a bar-shaped structure in its nucleus. → galactic bar.

See also: → barred; → spiral; → galaxy.

A → spiral galaxy that exhibits a bar-shaped structure in its nucleus. → galactic bar.

See also: → barred; → spiral; → galaxy.

A defect in an optical system in which magnification decreases with distance from the optical axis,
whereby the image of a square appears barrel-shaped. Opposite of → pincushion distortion.

Etymology (EN): Barrel, M.E. barel, from O.Fr. baril; → distortion.

Etymology (PE): Cowlegi, → distortion; celiki, relating to celik “barrel”.

A defect in an optical system in which magnification decreases with distance from the optical axis,
whereby the image of a square appears barrel-shaped. Opposite of → pincushion distortion.

Etymology (EN): Barrel, M.E. barel, from O.Fr. baril; → distortion.

Etymology (PE): Cowlegi, → distortion; celiki, relating to celik “barrel”.

General: Anything that prevents passage or blocks.
Physics: 1) A region where the potential energy is greater than the total energy of a particle, whereby the particle cannot go through.
2) The depletion layer of a P-N junction in a diode.

Etymology (EN): O.F. barrière “obstacle,” from V.L. *barraria, from *barra “bar, barrier.”

Etymology (PE): Varqé, from varq “a mound, a dam” + -é nuance suffix. Varq is
probably related to Av. vâra- “barrage,” vara- “enclosure,” var- “castle,” Mid.Pers. var “enclosure,” from Av. root var- “to cover, to conceal;” variants: barq (Torbat Heydariyei), valgâ (štiyâni), var (Qomi); cf. Skt. vatra- “a dike, a dam,"varana- “rampart, wall,” from vr- “to obstruct, close, cover, hide; to choose.”

General: Anything that prevents passage or blocks.
Physics: 1) A region where the potential energy is greater than the total energy of a particle, whereby the particle cannot go through.
2) The depletion layer of a P-N junction in a diode.

Etymology (EN): O.F. barrière “obstacle,” from V.L. *barraria, from *barra “bar, barrier.”

Etymology (PE): Varqé, from varq “a mound, a dam” + -é nuance suffix. Varq is
probably related to Av. vâra- “barrage,” vara- “enclosure,” var- “castle,” Mid.Pers. var “enclosure,” from Av. root var- “to cover, to conceal;” variants: barq (Torbat Heydariyei), valgâ (štiyâni), var (Qomi); cf. Skt. vatra- “a dike, a dam,"varana- “rampart, wall,” from vr- “to obstruct, close, cover, hide; to choose.”

Same as → Meteor Crater.

See also: Names after Daniel Barringer (1860-1929), American geologist, who bought the Crater in 1903, convinced that it was made by a huge → meteorite; → crater.

Same as → Meteor Crater.

See also: Names after Daniel Barringer (1860-1929), American geologist, who bought the Crater in 1903, convinced that it was made by a huge → meteorite; → crater.

The center of mass of a system of bodies.

Etymology (EN): From Gk. barus “heavy,” → bar, +
→ center.

Etymology (PE): Gerânigâh, from gerâni “weight;” cognate with Gk. barus, → bar, + gâh “place.”

The center of mass of a system of bodies.

Etymology (EN): From Gk. barus “heavy,” → bar, +
→ center.

Etymology (PE): Gerânigâh, from gerâni “weight;” cognate with Gk. barus, → bar, + gâh “place.”

A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

Etymology (EN): → barycenter; → coordinate; → time.

A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

Etymology (EN): → barycenter; → coordinate; → time.

A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB).

See also: → barycenter; → dynamical; → time.

A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB).

See also: → barycenter; → dynamical; → time.

The → Julian Date referenced to the → barycenter of the → solar system. The BJD is more precise than the → Heliocentric Julian Day because the Sun is not stationary. It moves due to the → gravitational attraction of Jupiter and the other planets.

See also: → barycentric; → Julian Date.

The → Julian Date referenced to the → barycenter of the → solar system. The BJD is more precise than the → Heliocentric Julian Day because the Sun is not stationary. It moves due to the → gravitational attraction of Jupiter and the other planets.

See also: → barycentric; → Julian Date.

The hypothetical mechanism of creating the → baryon asymmetry in the → Universe. Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology. → Sakharov conditions.

See also: From baryo-, from → baryon + → -genesis.

The hypothetical mechanism of creating the → baryon asymmetry in the → Universe. Universe. Explaining the observed matter asymmetry is an important open question in physical cosmology. → Sakharov conditions.

See also: From baryo-, from → baryon + → -genesis.

Any of the class of the heaviest → subatomic particles that includes → protons, → neutrons, as well as a number of short-lived particles whose decay products include protons. Baryons obey the → Fermi-Dirac statistics. They form a subclass of the → hadrons and
are further subdivided into → nucleons and → hyperons.

See also: Gk. barys “heavy” + → -on, from “fermion.”

Any of the class of the heaviest → subatomic particles that includes → protons, → neutrons, as well as a number of short-lived particles whose decay products include protons. Baryons obey the → Fermi-Dirac statistics. They form a subclass of the → hadrons and
are further subdivided into → nucleons and → hyperons.

See also: Gk. barys “heavy” + → -on, from “fermion.”

In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1Mpc (h being the → Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148).

See also: → baryon; → acoustic; → oscillation.

In cosmology, one of a series of peaks and troughs that are present in the power spectrum of matter fluctuations after the → recombination era, and on large scales. At the time of the Big Bang, and for about 380,000 years afterwards, Universe was ionized and photons and baryons were tightly coupled. Acoustic oscillations arose from perturbations in the primordial plasma due to the competition between gravitational attraction and gas+photons pressure. After the epoch of recombination, these oscillations froze and imprinted their signatures in both the → CMB and matter distribution. In the case of the photons, the acoustic mode history is manifested as the high-contrast Doppler peaks in the temperature anisotropies. As for baryons, they were in a similar state, and when mixed with the non-oscillating → cold dark matter perturbations, they left a small residual imprint in the clustering of matter on very large scales, ~100 h^-1Mpc (h being the → Hubble constant in units of 100 km s^-1 Mpc^-1). The phenomenon of BAOs, recently discovered using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology. Like → Type Ia supernovae, BAOs provide a → standard candle for determining cosmic distances. The measurement of BAOs is therefore a powerful new technique for probing how → dark energy has affected the expansion of the Universe (see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401, 2148).

See also: → baryon; → acoustic; → oscillation.

The observation that in the present → Universe there is → matter but not much → antimatter. Observations do not show the presence of galaxies made of antimatter, nor gamma rays are observed that would be produced if large entities of antimatter would undergo → annihilation with matter. However, the → early Universe could have been baryon symmetric, and for some reason the matter excess has been generated, through some process called → baryogenesis. → Sakharov conditions.

See also: → baryon; → asymmetry.

The observation that in the present → Universe there is → matter but not much → antimatter. Observations do not show the presence of galaxies made of antimatter, nor gamma rays are observed that would be produced if large entities of antimatter would undergo → annihilation with matter. However, the → early Universe could have been baryon symmetric, and for some reason the matter excess has been generated, through some process called → baryogenesis. → Sakharov conditions.

See also: → baryon; → asymmetry.

1. The difference between the total number of → baryons and
   the total number of → antibaryons in a system of → subatomic particles. It is a measure of → baryon asymmetry and is defined by the quantity η = (n_b - n_b^-)/n_γ, called the → baryon-photon ratio, where n_b is the → comoving number density of baryons, n_b^- is the number of antibaryons, and n_γ is that of photons. The value of η for the → cosmic microwave background radiation (CMBR) has been very well determined by the → WMAP satellite to be η = (6.14 ± 0.25) x 10^-10. The baryon number is assumed to be constant. The photons created in stars amount to only a small fraction, less than 1%, of those in the CMBR.
   

2. A property of an → elementary particle represented by a
   → quantum number. It is
   equal to +1 for a baryon and -1 for an antibaryon.
   → Bosons, → leptons, and → mesons have a baryon number B = 0. → Quarks and → antiquarks have baryon numbers of B = +1/3 and -1/3, respectively. The baryon number is → conserved in all observed types of particle-particle interaction.

See also: → baryon; → number.

1. The difference between the total number of → baryons and
   the total number of → antibaryons in a system of → subatomic particles. It is a measure of → baryon asymmetry and is defined by the quantity η = (n_b - n_b^-)/n_γ, called the → baryon-photon ratio, where n_b is the → comoving number density of baryons, n_b^- is the number of antibaryons, and n_γ is that of photons. The value of η for the → cosmic microwave background radiation (CMBR) has been very well determined by the → WMAP satellite to be η = (6.14 ± 0.25) x 10^-10. The baryon number is assumed to be constant. The photons created in stars amount to only a small fraction, less than 1%, of those in the CMBR.
   

2. A property of an → elementary particle represented by a
   → quantum number. It is
   equal to +1 for a baryon and -1 for an antibaryon.
   → Bosons, → leptons, and → mesons have a baryon number B = 0. → Quarks and → antiquarks have baryon numbers of B = +1/3 and -1/3, respectively. The baryon number is → conserved in all observed types of particle-particle interaction.

See also: → baryon; → number.

The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature  T is aT⁴, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_γ = aT⁴/kT = 3.7 x 10² photons cm^-3, where a = 7.6 x 10^-15 erg cm^-3 K^-4 (→ radiation density constant)
and k = 1.38 x 10^-16 erg K^-1 (→ Boltzmann’s constant). The number density of baryons can be expressed by ρ_m/m_p, where ρ_m is the mass density of the Universe and m_p is the mass of the → proton (1.66 x 10^-24 g). → CMB measurements show that the baryonic mean density is ρ_m = 4.2 x 10^-31 g cm^-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_b/n_γ = 2 x 10^-7/3.7 x 10² ~ 5 x 10^-10. In other words, for each baryon in the Universe there is 10¹⁰ photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.

See also: → baryon; → photon; → ratio.

The → baryon number compared with the number of photons in the → Universe. The baryon-photon ratio can be estimated in a simple way. The → energy density associated with → blackbody radiation of → temperature  T is aT⁴, and the mean energy per photon is ~kT. Therefore, the number density of blackbody photons for T = 2.7 K is: n_γ = aT⁴/kT = 3.7 x 10² photons cm^-3, where a = 7.6 x 10^-15 erg cm^-3 K^-4 (→ radiation density constant)
and k = 1.38 x 10^-16 erg K^-1 (→ Boltzmann’s constant). The number density of baryons can be expressed by ρ_m/m_p, where ρ_m is the mass density of the Universe and m_p is the mass of the → proton (1.66 x 10^-24 g). → CMB measurements show that the baryonic mean density is ρ_m = 4.2 x 10^-31 g cm^-3 (roughly 5% of the → critical density). This leads to the value of ~ 2 x 10^-7 for the number density of baryons. Thus, the baryon/photon ratio is approximately equal to η = n_b/n_γ = 2 x 10^-7/3.7 x 10² ~ 5 x 10^-10. In other words, for each baryon in the Universe there is 10¹⁰ photons. This estimate is in agreement with the precise value of the baryon-photon ratio 6.14 x 10^-10 derived with the → WMAP. Since the photon number and the baryon number are conserved, the baryon-photon ratio stays constant as the Universe expands.

See also: → baryon; → photon; → ratio.

→ Dark matter made up of → baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is → non-baryonic.
The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.

See also: → baryonic; → dark; → matter.

→ Dark matter made up of → baryons that are not luminous enough to produce any detectable radiation. It is generally believed that most dark matter is → non-baryonic.
The baryonic dark matter could reside in a number of forms, including cold gas and compact objects.

See also: → baryonic; → dark; → matter.

Ordinary matter composed of → baryons, i.e. → protons and → neutrons, as distinct from → non-baryonic, exotic forms.

See also: Adj. of → baryon; → matter.

Ordinary matter composed of → baryons, i.e. → protons and → neutrons, as distinct from → non-baryonic, exotic forms.

See also: Adj. of → baryon; → matter.

A dark fine-grained → igneous rock typically composed of → plagioclase with → pyroxene and → olivine and often displaying a columnar structure.

See also: From L.L. basaltes, misspelling of L. basanites “very hard stone,” from Gk. basanites, from basanos “touchstone,” from Egyptian baban “a stone used by the Egyptians as a touchstone of gold.”

A dark fine-grained → igneous rock typically composed of → plagioclase with → pyroxene and → olivine and often displaying a columnar structure.

See also: From L.L. basaltes, misspelling of L. basanites “very hard stone,” from Gk. basanites, from basanos “touchstone,” from Egyptian baban “a stone used by the Egyptians as a touchstone of gold.”

1. The bottom support of anything; a fundamental principle or groundwork.
   

2. Geometry: The line or surface on which a figure is assumed to stand.
   

3. Arithmetic: The number which, raised to various powers, forms the main counting units of a system. Thus 10 is the base of the decimal system.
   

4. Logarithm: The number a in the equation N = a^x. The base of common logarithm is 10.
   

5. A centre of operations or supply, such as a → database.
   

6. Chemistry: A substance that reacts with an acid to yield a salt and water.

Etymology (EN): M.E., from O.Fr. bas, from L. basis “foundation,” from Gk. basis “step, pedestal,” from bainein “to step.”

Etymology (PE): Pâyé “base,” from pâ, pây “foot,” from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos; L. pes; PIE *pod-/*ped-.
Pâygâh, from pâyé + gâh “place” (O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Bâz, loan from Fr., as above.

1. The bottom support of anything; a fundamental principle or groundwork.
   

2. Geometry: The line or surface on which a figure is assumed to stand.
   

3. Arithmetic: The number which, raised to various powers, forms the main counting units of a system. Thus 10 is the base of the decimal system.
   

4. Logarithm: The number a in the equation N = a^x. The base of common logarithm is 10.
   

5. A centre of operations or supply, such as a → database.
   

6. Chemistry: A substance that reacts with an acid to yield a salt and water.

Etymology (EN): M.E., from O.Fr. bas, from L. basis “foundation,” from Gk. basis “step, pedestal,” from bainein “to step.”

Etymology (PE): Pâyé “base,” from pâ, pây “foot,” from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos; L. pes; PIE *pod-/*ped-.
Pâygâh, from pâyé + gâh “place” (O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Bâz, loan from Fr., as above.

1. In radio interferometry, the separation between the electrical, or phase centers of two interferometer elements.
   

2. In spectroscopy, the contribution to a spectrum from phenomena
   that are not of astronomical interest, such as frequency dependent properties of the receivers, or by astronomical sources other than
   those under study.

See also: → base; → line.

1. In radio interferometry, the separation between the electrical, or phase centers of two interferometer elements.
   

2. In spectroscopy, the contribution to a spectrum from phenomena
   that are not of astronomical interest, such as frequency dependent properties of the receivers, or by astronomical sources other than
   those under study.

See also: → base; → line.

Chemistry: Of or denoting or of the nature of or containing a → base. Same as → alkaline.

See also: → base; → -ic.

Chemistry: Of or denoting or of the nature of or containing a → base. Same as → alkaline.

See also: → base; → -ic.

A large impact crater on a planet or moon, typically several hundred kilometers across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs.

Etymology (EN): From O.Fr. bacin, from V.L. *baccinum, from L. bacca “water vessel,” perhaps originally Gaulish.

Etymology (PE): Howzé, from howz “pond, a large reservoir of water” (from Ar. hauz) + -é noun suffix.

A large impact crater on a planet or moon, typically several hundred kilometers across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs.

Etymology (EN): From O.Fr. bacin, from V.L. *baccinum, from L. bacca “water vessel,” perhaps originally Gaulish.

Etymology (PE): Howzé, from howz “pond, a large reservoir of water” (from Ar. hauz) + -é noun suffix.

A combination of → cells connected together so as to produce useful electrical energy.

Etymology (EN): M.Fr. batterie “a grouping of artillery pieces for tactical purposes,” from O.Fr. baterie “beatng, thrashing, assault,” from battre “to beat,” from L. battuere “to beat.”

Etymology (PE): Bâtri, loanword from Fr., as above.

A combination of → cells connected together so as to produce useful electrical energy.

Etymology (EN): M.Fr. batterie “a grouping of artillery pieces for tactical purposes,” from O.Fr. baterie “beatng, thrashing, assault,” from battre “to beat,” from L. battuere “to beat.”

Etymology (PE): Bâtri, loanword from Fr., as above.

A body of water forming an indentation of the shoreline, larger than a cove but smaller than a → gulf (Dictionary.com).

Etymology (EN): M.E. baye, from M.Fr. baie, from L.L. bâia, perhaps ultimately from Iberian bahia.

Etymology (PE): Bâhé, loan from Sp. bahia.

A body of water forming an indentation of the shoreline, larger than a cove but smaller than a → gulf (Dictionary.com).

Etymology (EN): M.E. baye, from M.Fr. baie, from L.L. bâia, perhaps ultimately from Iberian bahia.

Etymology (PE): Bâhé, loan from Sp. bahia.

A stellar designation system in which a specific star is identified by a Greek letter, followed by the genitive form of its hosting
→ constellation’s Latin name. For example, Alpha Eridani, Delta Cephei, Lambda Bootis. The Greek alphabet has only 24 letters. In case a single constellation contained a larger number of stars, Bayer amended with Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer did not go beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include, for Vela: a Vel (Velorum), z Vel, A Vel, Q Vel; for Scorpius: d Sco (Scorpii), A Sco; for Leo: b Leo (Leonis), o Leo, A Leo, → c Orionis.

Compare with the → Flamsteed designation.

See also: First introduced by Johann Bayer (1572-1625) in his atlas
Uranometria, published in 1603 at Augsburg, Germany; → designation.

A stellar designation system in which a specific star is identified by a Greek letter, followed by the genitive form of its hosting
→ constellation’s Latin name. For example, Alpha Eridani, Delta Cephei, Lambda Bootis. The Greek alphabet has only 24 letters. In case a single constellation contained a larger number of stars, Bayer amended with Latin letters: upper case A, followed by lower case b through z (omitting j and v), for a total of another 24 letters. Bayer did not go beyond z, but later astronomers added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include, for Vela: a Vel (Velorum), z Vel, A Vel, Q Vel; for Scorpius: d Sco (Scorpii), A Sco; for Leo: b Leo (Leonis), o Leo, A Leo, → c Orionis.

Compare with the → Flamsteed designation.

See also: First introduced by Johann Bayer (1572-1625) in his atlas
Uranometria, published in 1603 at Augsburg, Germany; → designation.

A theorem in probability theory concerned with determining the → conditional probability of an event when another event has occurred.
Bayes’ theorem allows revision of the original probability with new information. Its simplest form is: P(A|B) = P(B|A) P(A)/P(B), where P(A): independent probability of A, also called prior probability; P(B): independent probability of B; P(B|A): conditional probability of B given A has occurred; P(A|B): conditional probability of A given B has occurred, also called posterior probability. Same as Bayes’ rule.

See also: Named after its proponent, the British mathematician Reverend Thomas Bayes (1702-1761). However, Bayes did not publish the theorem during his lifetime; instead, it was presented two years after his death to the Royal Society of London.

A theorem in probability theory concerned with determining the → conditional probability of an event when another event has occurred.
Bayes’ theorem allows revision of the original probability with new information. Its simplest form is: P(A|B) = P(B|A) P(A)/P(B), where P(A): independent probability of A, also called prior probability; P(B): independent probability of B; P(B|A): conditional probability of B given A has occurred; P(A|B): conditional probability of A given B has occurred, also called posterior probability. Same as Bayes’ rule.

See also: Named after its proponent, the British mathematician Reverend Thomas Bayes (1702-1761). However, Bayes did not publish the theorem during his lifetime; instead, it was presented two years after his death to the Royal Society of London.

Being, relating to, or denoting statistical methods based on → Bayes’ theorem.

See also: Referring to → Bayes’ theorem.

Being, relating to, or denoting statistical methods based on → Bayes’ theorem.

See also: Referring to → Bayes’ theorem.

An approach to → statistical analysis in which → unknowns to be estimated have a prior → probability distribution which combined with the information from data produces a posterior probability distribution for the target quantities.

See also: → Bayesian; → inference.

An approach to → statistical analysis in which → unknowns to be estimated have a prior → probability distribution which combined with the information from data produces a posterior probability distribution for the target quantities.

See also: → Bayesian; → inference.

A mathematical framework described by the prior distribution of a random parameter and by the likelihood of the observations. In this framework, all information on the random parameter based on the observations is included in the posterior distribution
which can be obtained using → Bayes’ theorem (see, e.g., Andrieu et al., 2001, “An Introduction to Monte Carlo Methods for Bayesian Data Analysis,” in Nonlinear Dynamics and Statistics, ed. A. I. Mees, Boston: Birkhäuser).

See also: Bayesian; → model.

A mathematical framework described by the prior distribution of a random parameter and by the likelihood of the observations. In this framework, all information on the random parameter based on the observations is included in the posterior distribution
which can be obtained using → Bayes’ theorem (see, e.g., Andrieu et al., 2001, “An Introduction to Monte Carlo Methods for Bayesian Data Analysis,” in Nonlinear Dynamics and Statistics, ed. A. I. Mees, Boston: Birkhäuser).

See also: Bayesian; → model.

An approach to model selection in which one bases inference on an average of all possible models instead of a single best model. The BMA is largely used in various branches of knowledge to properly account for model uncertainty in performing predictions.

See also: → Bayesian; → model; → average.

An approach to model selection in which one bases inference on an average of all possible models instead of a single best model. The BMA is largely used in various branches of knowledge to properly account for model uncertainty in performing predictions.

See also: → Bayesian; → model; → average.