An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

→ 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.

→ atmospheric; → turbulence.

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

→ 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.

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.

→ 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.

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.

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.

→ atomic; → clock.

→ element diffusion.

→ 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.

→ 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."

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).

→ 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 .

→ atomic; → heat.

Same as → neutral hydrogen or → H I.

→ 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.

→ 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.

→ 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.

→ 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.

→ 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.

→ atomic; → number.

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

→ atomic; → proposition.

Time measured using atomic clocks.

→ atomic; → number.