An Etymological Dictionary of Astronomy and Astrophysics

One of the largest ground-based astronomy projects and a major new facility for world astronomy located on the plain of the → Chajnantor Chilean Andes, San Pedro de Atacama, some 5000 m above sea level. ALMA will initially comprise 66 high precision antennas, with the option to expand in the future. There will be an array of fifty 12 m antennas, acting together as an → interferometer to capture → millimeter and → submillimeter wavelengths of 0.3 to 9.6 mm. It will have reconfigurable baselines ranging from 15 m to 18 km. A compact array of 7 m antenna and few 12 m diameter antennas (ACA) will be used to measure the diffuse emission. Resolutions as fine as 0’’.005 will be achieved at the highest frequencies. Construction of ALMA started in 2003 and will be completed in 2012. The ALMA project is an international collaboration between Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the → European Southern Observatory (ESO). The first 12 m diameter antenna, built by Mitsubishi Electric Corporation for the National Astronomical Observatory of Japan, was handed over to ESO in 2008. It will shortly be joined by North American and European antennas. ALMA will allow astronomers to study the cool Universe, i.e. the molecular gas and tiny dust grains from which stars, planetary systems, galaxies, and even life are formed.

See also: Atacama the name of a desert, west of the Andes mountains in Chile, covering a 1,000 km strip of land on the Pacific coast of South America; → large; → millimeter; → submillimeter; → array.

One of the largest ground-based astronomy projects and a major new facility for world astronomy located on the plain of the → Chajnantor Chilean Andes, San Pedro de Atacama, some 5000 m above sea level. ALMA will initially comprise 66 high precision antennas, with the option to expand in the future. There will be an array of fifty 12 m antennas, acting together as an → interferometer to capture → millimeter and → submillimeter wavelengths of 0.3 to 9.6 mm. It will have reconfigurable baselines ranging from 15 m to 18 km. A compact array of 7 m antenna and few 12 m diameter antennas (ACA) will be used to measure the diffuse emission. Resolutions as fine as 0’’.005 will be achieved at the highest frequencies. Construction of ALMA started in 2003 and will be completed in 2012. The ALMA project is an international collaboration between Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the → European Southern Observatory (ESO). The first 12 m diameter antenna, built by Mitsubishi Electric Corporation for the National Astronomical Observatory of Japan, was handed over to ESO in 2008. It will shortly be joined by North American and European antennas. ALMA will allow astronomers to study the cool Universe, i.e. the molecular gas and tiny dust grains from which stars, planetary systems, galaxies, and even life are formed.

See also: Atacama the name of a desert, west of the Andes mountains in Chile, covering a 1,000 km strip of land on the Pacific coast of South America; → large; → millimeter; → submillimeter; → array.

A member of a class of → near-Earth asteroids with → perihelion distances between 0.983 and 1.0 → astronomical units.
It is estimated that 6% of the total number of NEAs are Atens.

See also: → Aten; → asteroid.

A member of a class of → near-Earth asteroids with → perihelion distances between 0.983 and 1.0 → astronomical units.
It is estimated that 6% of the total number of NEAs are Atens.

See also: → Aten; → asteroid.

1. The doctrine or belief that there is no → God.
   

2. Disbelief in the existence of a supreme being or beings.

See also: → a-; → theism.

1. The doctrine or belief that there is no → God.
   

2. Disbelief in the existence of a supreme being or beings.

See also: → a-; → theism.

Of or pertaining to the Atlantic Ocean.

See also: M.E., from L. Atlanticum (mare) “the Atlantic (ocean),” from Gk. Atlantikos “of Atlas,” adj. of → Atlas, in reference to Mount Atlas in NW Africa. So called because it lay beyond that mountain.

Of or pertaining to the Atlantic Ocean.

See also: M.E., from L. Atlanticum (mare) “the Atlantic (ocean),” from Gk. Atlantikos “of Atlas,” adj. of → Atlas, in reference to Mount Atlas in NW Africa. So called because it lay beyond that mountain.

1. The second of → Saturn’s known satellites. It has a diameter of about 30 km and orbits Saturn between the outer edge of the A ring and the F ring at a mean distance of about 137,600 km. It was discovered by Richard Terrile in 1980 from Voyager 1 photos. Also known as Saturn XV.
   

2. A bound collection of maps often including illustrations and informative texts.
   

3. A blue star of visual magnitude V = 3.63, B - V = -0.09, and spectral type B8 III in the → Pleiades. Other designations: 27 Tauri, HR 1178, HD 23850. It is in fact a → triple system.

See also: In Gk. mythology, Atlas a son of the Titan → Iapetus and the nymph Clymene.
After the Titans revolted and lost a war against Zeus, Atlas was condemned by Zeus to stand forever holding up the heavens. He was identified with the Atlas Mountains in NW Africa.

1. The second of → Saturn’s known satellites. It has a diameter of about 30 km and orbits Saturn between the outer edge of the A ring and the F ring at a mean distance of about 137,600 km. It was discovered by Richard Terrile in 1980 from Voyager 1 photos. Also known as Saturn XV.
   

2. A bound collection of maps often including illustrations and informative texts.
   

3. A blue star of visual magnitude V = 3.63, B - V = -0.09, and spectral type B8 III in the → Pleiades. Other designations: 27 Tauri, HR 1178, HD 23850. It is in fact a → triple system.

See also: In Gk. mythology, Atlas a son of the Titan → Iapetus and the nymph Clymene.
After the Titans revolted and lost a war against Zeus, Atlas was condemned by Zeus to stand forever holding up the heavens. He was identified with the Atlas Mountains in NW Africa.

A combining form meaning “air, vapor,” used in the formation of compound terms. air or vapour

Etymology (EN): From Gk. atmos “vapor.”

Etymology (PE): Atmo-, loan from Gk., as above.
Havâ-, → air.

A combining form meaning “air, vapor,” used in the formation of compound terms. air or vapour

Etymology (EN): From Gk. atmos “vapor.”

Etymology (PE): Atmo-, loan from Gk., as above.
Havâ-, → air.

→ atmophile element.

See also: → atmo-; → phile.

→ atmophile element.

See also: → atmo-; → phile.

In the → Goldschmidt classification,
a → chemical element that is extremely → volatile, i.e., forms a gas or liquid at the surface of the Earth. The atmophile elements are usually concentrated in the terrestrial → atmosphere and → hydrosphere. They are → hydrogen (H), → carbon (C), → nitrogen (N), and → noble gas/qot>es, namely → helium (He), → neion (Ne), → argon (Ar), → krypton (Kr), → xenon (Xe), and → radon (Rn) (Pinti D.L., 2017, Atmophile Elements. In: White W. (eds) Encyclopedia of Geochemistry, Springer).

See also: → atmophile; → element.

In the → Goldschmidt classification,
a → chemical element that is extremely → volatile, i.e., forms a gas or liquid at the surface of the Earth. The atmophile elements are usually concentrated in the terrestrial → atmosphere and → hydrosphere. They are → hydrogen (H), → carbon (C), → nitrogen (N), and → noble gas/qot>es, namely → helium (He), → neion (Ne), → argon (Ar), → krypton (Kr), → xenon (Xe), and → radon (Rn) (Pinti D.L., 2017, Atmophile Elements. In: White W. (eds) Encyclopedia of Geochemistry, Springer).

See also: → atmophile; → element.

1. The gaseous envelope surrounding a star, planet, or moon. Several solar system planets retain considerable atmospheres, due to their strong gravitational force. The gas motions in the planetary atmosphere, as a response to the heating, coupled with the rotation forces, generate the meteorological systems. The planetary satellites → Titan and → Triton also have atmospheres (M.S.: SDE).
   

2. A unit of pressure, called standard atmosphere, which is the pressure of air balanced by a column of mercury 76 cm high with a density of the mercury of 13.595 g/cm³ at normal acceleration of gravity. Such a column applies a pressure equal to its weight to each square cm, or 1.01325 x 10⁶ dynes/cm² = 1.01325 x 10⁵ N/m². Since this pressure is equal to 1.03323 kilograms of force per square centimeter, instead of it use is often made of the technical atmosphere (at), exactly equal to 1 kgf/cm².

Etymology (EN): New L. atmosphaera, from Gk. atmos “vapor” + spharia “sphere.”

Etymology (PE): Havâsepehr, from Mod.Pers. havâ, → air, + sepehr, → sphere. Javv “air, atmosphere,” from Ar. jauw.

1. The gaseous envelope surrounding a star, planet, or moon. Several solar system planets retain considerable atmospheres, due to their strong gravitational force. The gas motions in the planetary atmosphere, as a response to the heating, coupled with the rotation forces, generate the meteorological systems. The planetary satellites → Titan and → Triton also have atmospheres (M.S.: SDE).
   

2. A unit of pressure, called standard atmosphere, which is the pressure of air balanced by a column of mercury 76 cm high with a density of the mercury of 13.595 g/cm³ at normal acceleration of gravity. Such a column applies a pressure equal to its weight to each square cm, or 1.01325 x 10⁶ dynes/cm² = 1.01325 x 10⁵ N/m². Since this pressure is equal to 1.03323 kilograms of force per square centimeter, instead of it use is often made of the technical atmosphere (at), exactly equal to 1 kgf/cm².

Etymology (EN): New L. atmosphaera, from Gk. atmos “vapor” + spharia “sphere.”

Etymology (PE): Havâsepehr, from Mod.Pers. havâ, → air, + sepehr, → sphere. Javv “air, atmosphere,” from Ar. jauw.

Pertaining to or existing in the atmosphere of an astronomical object such as a
planet, moon, or star.

See also: → atmosphere; → -ic.

Pertaining to or existing in the atmosphere of an astronomical object such as a
planet, moon, or star.

See also: → atmosphere; → -ic.

The absorption of → electromagnetic radiation in the → atmosphere mainly by → water vapor, → carbon dioxide, and oxygen. The atmosphere introduces two more limiting factors in → remote sensing:
→ atmospheric scattering and → atmospheric turbulence.

See also: → atmospheric; → absorption.

The absorption of → electromagnetic radiation in the → atmosphere mainly by → water vapor, → carbon dioxide, and oxygen. The atmosphere introduces two more limiting factors in → remote sensing:
→ atmospheric scattering and → atmospheric turbulence.

See also: → atmospheric; → absorption.

The large-scale movements of air around areas of high and low pressure whereby heat is distributed on the surface of the Earth. Atmospheric motion is driven by uneven heating of the planet. The atmosphere (and ocean) → transfers the excess heat from → tropics to → poles. The flow is determined by balance between → pressure gradients and the → Coriolis effect.

See also: → atmospheric; → circulation.

The large-scale movements of air around areas of high and low pressure whereby heat is distributed on the surface of the Earth. Atmospheric motion is driven by uneven heating of the planet. The atmosphere (and ocean) → transfers the excess heat from → tropics to → poles. The flow is determined by balance between → pressure gradients and the → Coriolis effect.

See also: → atmospheric; → circulation.

The splitting of starlight into a spectrum in the atmosphere because the atmosphere acts as a refracting prism. This phenomenon brings about a practical problem for spectroscopic observations using a slit. → differential refraction; → atmospheric refraction.

See also: → atmospheric; → dispersion.

The splitting of starlight into a spectrum in the atmosphere because the atmosphere acts as a refracting prism. This phenomenon brings about a practical problem for spectroscopic observations using a slit. → differential refraction; → atmospheric refraction.

See also: → atmospheric; → dispersion.

The emission of electromagnetic radiation from the atmosphere due to thermal and → non-thermal processes. → Thermal emission comes mainly from
→ water vapor. Non-thermal processes result in emission lines oxygen (optical) and OH (near-IR). Atmospheric emission is a very significant source of noise in astronomical observations. See also → airglow, → aurora.

See also: → atmospheric; → emission.

The emission of electromagnetic radiation from the atmosphere due to thermal and → non-thermal processes. → Thermal emission comes mainly from
→ water vapor. Non-thermal processes result in emission lines oxygen (optical) and OH (near-IR). Atmospheric emission is a very significant source of noise in astronomical observations. See also → airglow, → aurora.

See also: → atmospheric; → emission.

A process by which a planet loses its atmospheric gases to space. There are three main types: 1) → thermal escape, 2) → suprathermal escape (or → nonthermal escape), and 3) → impact erosion. According to models,

the two mechanisms that can most efficiently cause substantial atmospheric loss are hydrodynamic escape and impact erosion (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → atmospheric; → escape.

A process by which a planet loses its atmospheric gases to space. There are three main types: 1) → thermal escape, 2) → suprathermal escape (or → nonthermal escape), and 3) → impact erosion. According to models,

the two mechanisms that can most efficiently cause substantial atmospheric loss are hydrodynamic escape and impact erosion (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → atmospheric; → escape.

The decrease in the intensity of light from a celestial body due to absorption and scattering by Earth’s atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith.

See also: → atmospheric; → extinction.

The decrease in the intensity of light from a celestial body due to absorption and scattering by Earth’s atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith.

See also: → atmospheric; → extinction.

A → subatomic particle produced when → primary cosmic rays, impinge on the Earth’s atmosphere producing a particle cascade, in which secondary particles decay into → muons. In the energy range up to 100 → GeV atmospheric muons come mostly from the decay of secondary → pions:
π^±→ μ^± + anti-ν_μ.

At higher energies, the → kaon contribution to the muon flux become significant, reaching the asymptotic value of 27% at about 10 TeV:

K^±→ μ^± + anti-ν_μ.

See also: → atmospheric; → muon.

A → subatomic particle produced when → primary cosmic rays, impinge on the Earth’s atmosphere producing a particle cascade, in which secondary particles decay into → muons. In the energy range up to 100 → GeV atmospheric muons come mostly from the decay of secondary → pions:
π^±→ μ^± + anti-ν_μ.

At higher energies, the → kaon contribution to the muon flux become significant, reaching the asymptotic value of 27% at about 10 TeV:

K^±→ μ^± + anti-ν_μ.

See also: → atmospheric; → muon.

A neutrino produced in the collision of → cosmic rays
(typically → protons) with nuclei in the → upper atmosphere. This creates a shower of → hadrons, mostly → pions. The pions decay to a → muon and a muon neutrino. The muons decay to an → electron, another muon neutrino, and an electron neutrino.

See also: → atmospheric; → neutrino.

A neutrino produced in the collision of → cosmic rays
(typically → protons) with nuclei in the → upper atmosphere. This creates a shower of → hadrons, mostly → pions. The pions decay to a → muon and a muon neutrino. The muons decay to an → electron, another muon neutrino, and an electron neutrino.

See also: → atmospheric; → neutrino.

Noise in radio wavelengths caused by natural atmospheric processes, mainly lightening discharges in thunderstorms. They can affect radio observations.

See also: → atmospheric; → noise.

Noise in radio wavelengths caused by natural atmospheric processes, mainly lightening discharges in thunderstorms. They can affect radio observations.

See also: → atmospheric; → noise.

The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth’s atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.

See also: → atmospheric; → refraction.

The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth’s atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.

See also: → atmospheric; → refraction.

The → scattering of → electromagnetic radiation by various particles in the Earth’s → atmosphere. The phenomenon is caused by collisions between photons and several scattering agents such as atoms, molecules, → aerosols, and water droplets in clouds. → Rayleigh scattering.

See also: → atmospheric; → scattering.

The → scattering of → electromagnetic radiation by various particles in the Earth’s → atmosphere. The phenomenon is caused by collisions between photons and several scattering agents such as atoms, molecules, → aerosols, and water droplets in clouds. → Rayleigh scattering.

See also: → atmospheric; → scattering.

Random fluctuations of the atmosphere caused by the constant injection of energy into the atmosphere from solar and local sources, changing the temperature and pressure of the air where it is absorbed and leading to fluid instabilities. The development over time of the instabilities gives rise to fluctuations in the density of air, and therefore the → refractive index of the atmosphere. → turbulence; → seeing.

See also: → atmospheric; → turbulence.

Random fluctuations of the atmosphere caused by the constant injection of energy into the atmosphere from solar and local sources, changing the temperature and pressure of the air where it is absorbed and leading to fluid instabilities. The development over time of the instabilities gives rise to fluctuations in the density of air, and therefore the → refractive index of the atmosphere. → turbulence; → seeing.

See also: → atmospheric; → turbulence.

Gaps in → atmospheric absorption, allowing a range of electromagnetic wavelengths to pass through the atmosphere and reach the Earth.

See also: → atmospheric; → window.

Gaps in → atmospheric absorption, allowing a range of electromagnetic wavelengths to pass through the atmosphere and reach the Earth.

See also: → atmospheric; → window.

A coral island or group of coral islands forming a ring that is surrounded by deep ocean water and that encloses a shallow lagoon. Atolls range in diameter from about 1 km to over 100 km and are especially common in the western and central Pacific Ocean. They are believed to form along the fringes of underwater volcanoes. → atoll source.

See also: From atollon, atolon, from Divehi (Indo-Aryan language of the Maldive Islands) atolu “reef.”

A coral island or group of coral islands forming a ring that is surrounded by deep ocean water and that encloses a shallow lagoon. Atolls range in diameter from about 1 km to over 100 km and are especially common in the western and central Pacific Ocean. They are believed to form along the fringes of underwater volcanoes. → atoll source.

See also: From atollon, atolon, from Divehi (Indo-Aryan language of the Maldive Islands) atolu “reef.”

A member of a class of → low-mass X-ray binary systems containing low-magnetic field → neutron stars. They have soft spectra and no pulsations. An example is 4U 1705-44. See also → Z source.

See also: → atoll; the name derives from the fact that on X-ray → color-color diagrams these sources often resemble a band of points at constant hard X-ray color, with “islands” of points appearing on time-scales of weeks and months.

A member of a class of → low-mass X-ray binary systems containing low-magnetic field → neutron stars. They have soft spectra and no pulsations. An example is 4U 1705-44. See also → Z source.

See also: → atoll; the name derives from the fact that on X-ray → color-color diagrams these sources often resemble a band of points at constant hard X-ray color, with “islands” of points appearing on time-scales of weeks and months.

The smallest stable unit forming the basic elements. An atom
consists of positively charged → protons and → neutrons in the nucleus surrounded by negatively charged → electrons.

See also: From L. atomus, from Gk. atomos “uncut,” from → a- “not” + tomos “a cutting,” from temnein “to cut.”

The smallest stable unit forming the basic elements. An atom
consists of positively charged → protons and → neutrons in the nucleus surrounded by negatively charged → electrons.

See also: From L. atomus, from Gk. atomos “uncut,” from → a- “not” + tomos “a cutting,” from temnein “to cut.”

Of or relating to an atom or atoms; of or employing nuclear energy.

See also: Atomic, adj. from → atom + suffix → -ic

Of or relating to an atom or atoms; of or employing nuclear energy.

See also: Atomic, adj. from → atom + suffix → -ic

A modern clock, in which the characteristic frequencies of certain atoms (most commonly chosen cesium 133) are utilized for precision time measurement. → atomic fountain clock.

See also: → atomic; → clock.

A modern clock, in which the characteristic frequencies of certain atoms (most commonly chosen cesium 133) are utilized for precision time measurement. → atomic fountain clock.

See also: → atomic; → clock.

→ element diffusion.

See also: → atomic; → diffusion.

→ element diffusion.

See also: → atomic; → diffusion.

A gaseous ball of atoms, usually → cesium (¹³³Cs), created by the → laser cooling technique and used in an → atomic fountain clock. The ball, typically a few millimeters in diameter and containing some 10⁷ atoms,
can be launched upward against gravity using a → laser beam. The launch velocity is chosen such that the atoms reach a height of about one meter before they turn back and fall down the same path they came up. The motion of the ball resembles that of the water in a pulsed fountain.

Etymology (EN): → atomic; fountain, from M.E. fontayne
from O.Fr. fontaine, from L.L. fontana, noun use of feminine of L. fontanus “of a spring,” from fons “spring of water.”

Etymology (PE): Favvâré, Pers. construction from Ar. faur “boiling, bubbling.”

A gaseous ball of atoms, usually → cesium (¹³³Cs), created by the → laser cooling technique and used in an → atomic fountain clock. The ball, typically a few millimeters in diameter and containing some 10⁷ atoms,
can be launched upward against gravity using a → laser beam. The launch velocity is chosen such that the atoms reach a height of about one meter before they turn back and fall down the same path they came up. The motion of the ball resembles that of the water in a pulsed fountain.

Etymology (EN): → atomic; fountain, from M.E. fontayne
from O.Fr. fontaine, from L.L. fontana, noun use of feminine of L. fontanus “of a spring,” from fons “spring of water.”

Etymology (PE): Favvâré, Pers. construction from Ar. faur “boiling, bubbling.”

An → atomic clock based on the principle of the → atomic fountain. A ball of atoms, usually → cesium (¹³³Cs),
created by the → laser cooling technique,
is trapped in the intersection region of six laser beams. The ball is thrown upward by a laser beam and passes twice through a cavity
where the atoms interact with the → microwave radiation
generated by an → oscillator. The ball reaches the summit of its trajectory (about 1 m above the cooling zone) and then due to gravity falls through the same microwave cavity.
The microwave radiation causes the electrons of the cesium atoms to move between two specific → energy states as they pass through the cavity. The clock is based on a → hyperfine transition (9.192631770 GHz) between two energy states in the electronic → ground state of the atom. The upper hyperfine state can in principle radiate to the lower state by → spontaneous emission, but the process takes a very long time – thousands of years.
Selection and detection of the hyperfine state is performed via → optical pumping and laser induced resonance fluorescence. In a carefully controlled setup, a relative uncertainty of 10 ^-16 can be reached for the cesium clock. This means an accuracy of 1 sec every 300 million years. This fluorescence is measured by a detector. The entire process is repeated until the maximum fluorescence of the cesium atoms is determined. This determination is used to lock the oscillator to the atomic frequency of cesium, which is used to define the SI → second. The first atomic fountain for metrological use was developed at the Paris Observatory (A. Clairon et al. 1996, Proc. 5th Symp. Frequency Standards and Metrology, p. 45).

See also: → atomic fountain; → clock.

An → atomic clock based on the principle of the → atomic fountain. A ball of atoms, usually → cesium (¹³³Cs),
created by the → laser cooling technique,
is trapped in the intersection region of six laser beams. The ball is thrown upward by a laser beam and passes twice through a cavity
where the atoms interact with the → microwave radiation
generated by an → oscillator. The ball reaches the summit of its trajectory (about 1 m above the cooling zone) and then due to gravity falls through the same microwave cavity.
The microwave radiation causes the electrons of the cesium atoms to move between two specific → energy states as they pass through the cavity. The clock is based on a → hyperfine transition (9.192631770 GHz) between two energy states in the electronic → ground state of the atom. The upper hyperfine state can in principle radiate to the lower state by → spontaneous emission, but the process takes a very long time – thousands of years.
Selection and detection of the hyperfine state is performed via → optical pumping and laser induced resonance fluorescence. In a carefully controlled setup, a relative uncertainty of 10 ^-16 can be reached for the cesium clock. This means an accuracy of 1 sec every 300 million years. This fluorescence is measured by a detector. The entire process is repeated until the maximum fluorescence of the cesium atoms is determined. This determination is used to lock the oscillator to the atomic frequency of cesium, which is used to define the SI → second. The first atomic fountain for metrological use was developed at the Paris Observatory (A. Clairon et al. 1996, Proc. 5th Symp. Frequency Standards and Metrology, p. 45).

See also: → atomic fountain; → clock.

The → heat capacity of a → mole of a substance, expresses as: C_a = C.A , where C is the → specific heat and A the → atomic weight .

See also: → atomic; → heat.

The → heat capacity of a → mole of a substance, expresses as: C_a = C.A , where C is the → specific heat and A the → atomic weight .

See also: → atomic; → heat.

Same as → neutral hydrogen or → H I.

See also: → atomic; → hydrogen.

Same as → neutral hydrogen or → H I.

See also: → atomic; → hydrogen.

The mass of a single atom, when the atom is at rest at its lowest energy level (→ ground state). Because a → chemical element may exist as various → isotopes, possessing different numbers of neutrons in their atomic nuclei, atomic mass is calculated for each isotope separately. Atomic mass is most often expressed in unified → atomic mass units, where one unified atomic mass unit is defined as one-twelfth the mass of a single atom of the carbon-12 isotope.

See also: → atomic; → mass.

The mass of a single atom, when the atom is at rest at its lowest energy level (→ ground state). Because a → chemical element may exist as various → isotopes, possessing different numbers of neutrons in their atomic nuclei, atomic mass is calculated for each isotope separately. Atomic mass is most often expressed in unified → atomic mass units, where one unified atomic mass unit is defined as one-twelfth the mass of a single atom of the carbon-12 isotope.

See also: → atomic; → mass.

The total number of → protons and → neutrons in the → nucleus of an → atom (symbol A). For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.

See also: → atomic; → mass; → number.

The total number of → protons and → neutrons in the → nucleus of an → atom (symbol A). For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.

See also: → atomic; → mass; → number.

A unit of mass used for atoms and molecules, equal to 1/12 of the mass of an atom of carbon-12 (including orbital electrons). It is equal to 1.660 33 × 10^-24 g.

See also: → atomic; → mass; → unit.

A unit of mass used for atoms and molecules, equal to 1/12 of the mass of an atom of carbon-12 (including orbital electrons). It is equal to 1.660 33 × 10^-24 g.

See also: → atomic; → mass; → unit.

The central part of the → atom. It is made up of → protons and, in most cases, → neutrons. The nucleus is surrounded by a swarm of fast-moving → electrons. Almost all of the mass (more than 99%) of an atom is contained in the dense nucleus. The number of protons in the nucleus (called → atomic number) determines the type of → chemical element.
Atoms that differ only in the number of neutrons in their nuclei are called → isotopes.

See also: → atomic; → nucleus.

The central part of the → atom. It is made up of → protons and, in most cases, → neutrons. The nucleus is surrounded by a swarm of fast-moving → electrons. Almost all of the mass (more than 99%) of an atom is contained in the dense nucleus. The number of protons in the nucleus (called → atomic number) determines the type of → chemical element.
Atoms that differ only in the number of neutrons in their nuclei are called → isotopes.

See also: → atomic; → nucleus.

The number of → protons in an → atomic nucleus (symbol Z). Same as → Z-number.

The atomic number is written as a subscript to the left of the → chemical element name. For example, the most common isotope of oxygen is shown as ₈¹⁶O, which has 8 → protons and its → mass number (A) is 16.

See also: → atomic; → number.

The number of → protons in an → atomic nucleus (symbol Z). Same as → Z-number.

The atomic number is written as a subscript to the left of the → chemical element name. For example, the most common isotope of oxygen is shown as ₈¹⁶O, which has 8 → protons and its → mass number (A) is 16.

See also: → atomic; → number.

In → propositional logic, a → sentence without any → connectives. See also → molecular proposition.

See also: → atomic; → proposition.

In → propositional logic, a → sentence without any → connectives. See also → molecular proposition.

See also: → atomic; → proposition.

Time measured using atomic clocks.

See also: → atomic; → number.

Time measured using atomic clocks.

See also: → atomic; → number.

A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

See also: → atomic; → transition.

A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

See also: → atomic; → transition.

The volume one → mole of a → chemical element occupies at room temperature. Atomic volume is typically given in cubic centimeters per mole (cc/mol). In other words, atomic volume is the ratio of → atomic mass to the
→ density of an element.

See also: → atomic; → volume. .

The volume one → mole of a → chemical element occupies at room temperature. Atomic volume is typically given in cubic centimeters per mole (cc/mol). In other words, atomic volume is the ratio of → atomic mass to the
→ density of an element.

See also: → atomic; → volume. .

A graph displaying → atomic volumes of → chemical elements against their → atomic masses, first plotted by Lother Meyer (1830-1895). The elements with similar properties occupy the same positions on the graph. In the original curve, Lothar Meyer plotted atomic volumes against → atomic weights.

→ Alkali metals such as Na, K, Rb, and Cs occupy the top position on the graph.

Elements like Be, Mg, Ca, Sr, and Ba occupy the positions on the ascending part of the graph. → Inert gases, except He, occupy the positions on the descending part of the graph.

→ Halogen elements like F, Cl, and Br also occupy the descending part of the graph.

See also: → atomic; → volume; → curve.

A graph displaying → atomic volumes of → chemical elements against their → atomic masses, first plotted by Lother Meyer (1830-1895). The elements with similar properties occupy the same positions on the graph. In the original curve, Lothar Meyer plotted atomic volumes against → atomic weights.

→ Alkali metals such as Na, K, Rb, and Cs occupy the top position on the graph.

Elements like Be, Mg, Ca, Sr, and Ba occupy the positions on the ascending part of the graph. → Inert gases, except He, occupy the positions on the descending part of the graph.

→ Halogen elements like F, Cl, and Br also occupy the descending part of the graph.

See also: → atomic; → volume; → curve.

→ relative atomic mass.

See also: → atomic; → weight.

→ relative atomic mass.

See also: → atomic; → weight.

i) If t₁, t₂, …, t_n are terms and P is a → predicate of arity n, then P(t₁, t₂, …, t_n) is an atomic wff.
ii) If t₁ and t₂ are terms, then (t₁ = t₂) is an atomic wff.

See also: → atomic; → wff.

i) If t₁, t₂, …, t_n are terms and P is a → predicate of arity n, then P(t₁, t₂, …, t_n) is an atomic wff.
ii) If t₁ and t₂ are terms, then (t₁ = t₂) is an atomic wff.

See also: → atomic; → wff.

1a) An aggressive and violent act against a person or place.

1b) Chem.: The beginning of a series of destructive reactions.

2a) To apply aggressive military action against (a place or enemy forces).

2b) Chem.: To begin a destructive reaction by breaking a bond or forming a new bond.

Etymology (EN): From M.Fr. attaquer, from Florentine Italian attaccare (battaglia) “join (battle).”

Etymology (PE): Patâk, from pa-, short for pati- “contrary, opposite” (az in panâh, padid), → against,

• tâk, variants tak, tag, tâz “rush, running, attack,” related to tâxtan “to run; to hasten; to assault,” → flow.

1a) An aggressive and violent act against a person or place.

1b) Chem.: The beginning of a series of destructive reactions.

2a) To apply aggressive military action against (a place or enemy forces).

2b) Chem.: To begin a destructive reaction by breaking a bond or forming a new bond.

Etymology (EN): From M.Fr. attaquer, from Florentine Italian attaccare (battaglia) “join (battle).”

Etymology (PE): Patâk, from pa-, short for pati- “contrary, opposite” (az in panâh, padid), → against,

• tâk, variants tak, tag, tâz “rush, running, attack,” related to tâxtan “to run; to hasten; to assault,” → flow.

1. To pay attention.
   

2. To be present at.

Etymology (EN): M.E. atenden, from O.Fr. atendre “to expect, wait for, pay attention,” from L. attendere “give heed to,” literally “to stretch toward,” from → ad- “to” + tendere “stretch,” → tension.

Etymology (PE): Âtânidan, from prefix â- + tân, from tan-, tanidan “to spin, twist, weave” (cf. tân “thread, warp of a web,” variants
târ “thread, warp, string,” tâl “thread” (Borujerdi dialect), tur “fishing net, net, snare”); Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tantram “loom;” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare;” PIE base *ten- “to stretch.”

1. To pay attention.
   

2. To be present at.

Etymology (EN): M.E. atenden, from O.Fr. atendre “to expect, wait for, pay attention,” from L. attendere “give heed to,” literally “to stretch toward,” from → ad- “to” + tendere “stretch,” → tension.

Etymology (PE): Âtânidan, from prefix â- + tân, from tan-, tanidan “to spin, twist, weave” (cf. tân “thread, warp of a web,” variants
târ “thread, warp, string,” tâl “thread” (Borujerdi dialect), tur “fishing net, net, snare”); Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tantram “loom;” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare;” PIE base *ten- “to stretch.”

1. The act or state of attending.
   

   2. The act or state of going to or being present at a place or event.
      

   3. The persons or number of persons present at a particular place or event.

See also: → attend; → -ance.

1. The act or state of attending.
   

   2. The act or state of going to or being present at a place or event.
      

   3. The persons or number of persons present at a particular place or event.

See also: → attend; → -ance.

A person who is present at a specific time or place.

Etymology (EN): From → attend + suffix -ee

Etymology (PE): Âtângar agent noun from → attend; pârgertandé agent noun from pârgertidan, → participate.

A person who is present at a specific time or place.

Etymology (EN): From → attend + suffix -ee

Etymology (PE): Âtângar agent noun from → attend; pârgertandé agent noun from pârgertidan, → participate.

1. The act or faculty of attending, especially by directing the mind to an object.
   

2. Observant care; consideration (Dictionary.com).

See also: → attend; → -tion.

1. The act or faculty of attending, especially by directing the mind to an object.
   

2. Observant care; consideration (Dictionary.com).

See also: → attend; → -tion.

1. Characterized by or giving attention; observant.
   

2. Thoughtful of others; considerate; polite; courteous (Dictionary.com).

See also: → attend; → -ive.

1. Characterized by or giving attention; observant.
   

2. Thoughtful of others; considerate; polite; courteous (Dictionary.com).

See also: → attend; → -ive.

1. To reduce in force, value, amount, or degree. → attenuation, → attenuation factor.
   

2. To reduce the amplitude of an electrical signal with little or no distortion.

Etymology (EN): L. attenuatus, p.p. of attenuare “to make thin,” from → ad- “to” + tenuare “make thin,” from tenuis “thin;” cf. Gk. tanaos “thin, slender, elongated;” Skt. tanuka-, tanu- “thin;” Av. tan- “to stretch;” Pers. tonok “thin,” as below; O.Ir. tanae “delicate, thin;” O.H.G. dunni “thin.”

Etymology (PE): Tonokidan, from tonok “thin, slender, slight, tender, delicate” + -idan, infinitive suffix. Tonok, from Mid.Pers. tanuk, Av. root tan- “to stretch, extend,” cognate with L. tenuis, as above.

1. To reduce in force, value, amount, or degree. → attenuation, → attenuation factor.
   

2. To reduce the amplitude of an electrical signal with little or no distortion.

Etymology (EN): L. attenuatus, p.p. of attenuare “to make thin,” from → ad- “to” + tenuare “make thin,” from tenuis “thin;” cf. Gk. tanaos “thin, slender, elongated;” Skt. tanuka-, tanu- “thin;” Av. tan- “to stretch;” Pers. tonok “thin,” as below; O.Ir. tanae “delicate, thin;” O.H.G. dunni “thin.”

Etymology (PE): Tonokidan, from tonok “thin, slender, slight, tender, delicate” + -idan, infinitive suffix. Tonok, from Mid.Pers. tanuk, Av. root tan- “to stretch, extend,” cognate with L. tenuis, as above.

The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium.

See also: Verbal noun of → attenuate.

The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium.

See also: Verbal noun of → attenuate.

The fraction of a beam of → X-rays or → gamma rays that is absorbed or scattered per unit thickness of the → absorber.

The linear attenuation coefficient, denoted by the symbol μ, appears in the equation I(x) = I₀e^-μx, where I(x) is the intensity at depth of x cm and I₀ is the original intensity.

See also: → attenuation; → coefficient.

The fraction of a beam of → X-rays or → gamma rays that is absorbed or scattered per unit thickness of the → absorber.

The linear attenuation coefficient, denoted by the symbol μ, appears in the equation I(x) = I₀e^-μx, where I(x) is the intensity at depth of x cm and I₀ is the original intensity.

See also: → attenuation; → coefficient.

The ratio of the radiation intensity after traversing a layer of matter to its intensity before.

See also: → attenuation; → factor.

The ratio of the radiation intensity after traversing a layer of matter to its intensity before.

See also: → attenuation; → factor.

Position of a satellite with respect to the horizon or some other fixed reference plane.

Etymology (EN): Fr., from It. attitudine “disposition, posture,” from L.L. aptitudo “faculty.”

Etymology (PE): Ruykard, noun from ruy kardan “to turn the face toward,” from ruy “face” (Mid.Pers. rôy, rôdh “face,” Av. raoδa- “growth,” in plural “appearance,” from raod- “to grow, sprout, shoot,” cf. Skt. róha- “rising, height”) + kardan “to do, make, perform” (Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,” karma “act, deed;” PIE base k^wer- “to do, to make”).


→ position.

Position of a satellite with respect to the horizon or some other fixed reference plane.

Etymology (EN): Fr., from It. attitudine “disposition, posture,” from L.L. aptitudo “faculty.”

Etymology (PE): Ruykard, noun from ruy kardan “to turn the face toward,” from ruy “face” (Mid.Pers. rôy, rôdh “face,” Av. raoδa- “growth,” in plural “appearance,” from raod- “to grow, sprout, shoot,” cf. Skt. róha- “rising, height”) + kardan “to do, make, perform” (Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,” karma “act, deed;” PIE base k^wer- “to do, to make”).


→ position.

A prefix meaning 10^-18.

Etymology (EN): From Danish or Norwegian atten “eighteen,” from O.N. attjan “eighteen,” from atta “eight” (compare with Gk. okto, L. octo, Skt. astau, Av. ašta-, Mod.Pers. hašt; PIE *okt(u))

• tjan “ten” (compare with Skt. dasa, Av. dasa, Mod.Pers. dah, Gk. deka, L. decem; PIE *dekm).

A prefix meaning 10^-18.

Etymology (EN): From Danish or Norwegian atten “eighteen,” from O.N. attjan “eighteen,” from atta “eight” (compare with Gk. okto, L. octo, Skt. astau, Av. ašta-, Mod.Pers. hašt; PIE *okt(u))

• tjan “ten” (compare with Skt. dasa, Av. dasa, Mod.Pers. dah, Gk. deka, L. decem; PIE *dekm).

To cause to draw near or adhere by physical force.

Etymology (EN): L. attractus, p.p. of attrahere “to draw, to attract,” from ad- “to” + trahere “to pull, draw.”

Etymology (PE): Darkašidan, from dar- “in, into” + kašidan “to draw, attract,” → galaxy.

To cause to draw near or adhere by physical force.

Etymology (EN): L. attractus, p.p. of attrahere “to draw, to attract,” from ad- “to” + trahere “to pull, draw.”

Etymology (PE): Darkašidan, from dar- “in, into” + kašidan “to draw, attract,” → galaxy.

The act or capability of attracting. A physical force (gravitational, electric, magnetic, etc.) exerted by material bodies.

See also: Attraction, n. from → attract.

The act or capability of attracting. A physical force (gravitational, electric, magnetic, etc.) exerted by material bodies.

See also: Attraction, n. from → attract.

Having the quality of attracting.

See also: Verbal adj. from → attract.

Having the quality of attracting.

See also: Verbal adj. from → attract.

A physical force (→ gravitational, → electric, → magnetic, etc.) by which a body attracts another.

See also: → attractive; → force.

A physical force (→ gravitational, → electric, → magnetic, etc.) by which a body attracts another.

See also: → attractive; → force.

The physical body that attracts. → Great Attractor.

See also: → attract; → -or.

The physical body that attracts. → Great Attractor.

See also: → attract; → -or.

1. General: Something attributed as belonging to a person, thing, or group; a quality, characteristic, or property.
   

2. Computer science: A characteristic that describes an → entity.
   

3. To explain as resulting from a specified cause; to regard as caused by something indicated.

Etymology (EN): M.E., from L. attributus, p.p. of attribuere “to assign to, add, bestow;” figuratively “to attribute, ascribe, impute,” from → ad- “to” + tribuere “to pay, assign, give, bestow,” → distribute.

Etymology (PE): Âbâž, âbâžé, from â- strength or nuance prefix + bâž “tribute, toll, impost,” → distribute.

1. General: Something attributed as belonging to a person, thing, or group; a quality, characteristic, or property.
   

2. Computer science: A characteristic that describes an → entity.
   

3. To explain as resulting from a specified cause; to regard as caused by something indicated.

Etymology (EN): M.E., from L. attributus, p.p. of attribuere “to assign to, add, bestow;” figuratively “to attribute, ascribe, impute,” from → ad- “to” + tribuere “to pay, assign, give, bestow,” → distribute.

Etymology (PE): Âbâž, âbâžé, from â- strength or nuance prefix + bâž “tribute, toll, impost,” → distribute.

1. The act of attributing.
   

   2. Something attributed.

See also: Verbal noun of → attribute.

1. The act of attributing.
   

   2. Something attributed.

See also: Verbal noun of → attribute.