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

A theoretical object that is simultaneously a perfect → absorber (it does not reflect any radiation) and a perfect → emitter of → radiation in all → wavelengths and whose radiation is governed solely by its → temperature. Blackbody radiation cannot be explained by → classical physics. The study of its characteristics has, therefore, played an important role in the development of → quantum mechanics. A blackbody can be realized in the form of a cavity with highly absorbing internal walls and a small aperture. Any ray entering through the aperture can leave the cavity only after repeated reflection from the walls. When the aperture is sufficiently small, therefore, the cavity will absorb practically all the radiation incident on the aperture, and so the surface of the aperture will be a black body. The light within the cavity will always interact and exchange energy with the material particles of the walls and any other material particles present. This interaction will eventually → thermalize the radiation within the cavity, producing a → blackbody spectrum, represented by a → blackbody curve.
See also → blackbody photosphere; → blackbody radiation; → Planck's blackbody formula; → Planck's radiation law; → Rayleigh-Jeans law; → Stefan-Boltzmann law; → thermalization; → Wien's displacement law.

→ black; → body.

The characteristic way in which the → intensity of → radiation emitted by a → blackbody varies with its → frequency (or → wavelength), as described by → Planck's radiation law. Also referred to as the → Planck curve. The exact form of the curve depends only on the object's → temperature. The wavelength at which the emitted intensity is highest is an indication of the temperature of the radiating object. As the temperature of the blackbody increases, the peak wavelength decreases (→ Wien's displacement law) and the total energy being radiated (the area under the curve) increases rapidly (→ Stefan-Boltzmann law).

→ blackbody; → curve.

The → blackbody surface of the → Universe defined at a → redshift of about z ≥ 2 × 10⁶. This is distinct from the → last scattering surface, in other words the → cosmic microwave background radiation (CMBR), which refers to z = 1100. Prior to the epoch of the blackbody photosphere the distortions from the → Big Bang are exponentially suppressed.

→ blackbody; → atmosphere.

The radiation emitted by a blackbody at a given → temperature. The → distribution of radiation with → wavelength is given by → Planck's blackbody formula or → Planck's radiation law.

→ blackbody; → radiation.

A curve displaying → blackbody radiation intensity versus the wavelength for a given temperature, according to → Planck's blackbody formula. It is an asymmetrical curve with a sharp rise on the short wavelength side and a much more gradually sloping long-wavelength tale. Same as → Planck spectrum.

→ blackbody; → spectrum.

The temperature at which a blackbody would emit the same radiation per unit area as that emitted by a given body at a given temperature.

→ blackbody; → temperature.

1) The physical structure of a human being or animal, including the bones, flesh, and organs.
2) Any material object characterized by its physical properties.

Body, from O.E. bodig "trunk, chest," related to O.H.G. botah, of unknown origin.

1) Tan, from Mid.Pers. tan "body, person;" O.Pers. tanūš "body," tanūm [acc.sg.] "(to) oneself;" Av. tanū- "body, person, self," tanūm [acc.sg.]; cf. Skt. tanūš- "body, self;" PIE base *ten-uh- "body."
2) Jesm, from Ar. jism "body, corps."

→ astronomical object.

→ celestial; → body.

A → rigid body whose displacement is prevented because it is under → constraint or a body that can move only in certain directions. Contrasted with → free body.

Constrained, p.p. of → constrain; → body.

1) To give a concrete form or body to an idea or quality.
2) To cause to become a body or part of a body.

From en- "in" + → body

Tanigândan, from Mid.Pers. tanig "bodily, corporal," from tan "body," → if and only if, + -ig, → -ik, + -ândan suffix of transitive verbs.

A → rigid body not constrained with other bodies and which from any given position can be displaced in any direction in space. Opposite of → constrained body.

→ free; → body.

A hypothetical body which emits radiation at each wavelength in a constant ratio, less than unity, to that emitted by a black body at the same temperature.

→ gray; → body.

→ astronomical object.

→ heaven; → -ly; → body.

The mathematical problem of solving the equations of motions of any number of bodies which interact gravitationally. More specifically, to find their positions and velocities at any point in the future or the past, given their present positions, masses, and velocities.

Many, from M.E. mani, meni, O.E. monig, manig; → body; → problem.

The mathematical problem of studying the behavior (e.g., velocities, positions) of any number of objects moving under their mutual gravitational attraction for any time in the past or future. Same as the → many-body problem.

→ body; → problem.

A celestial body that causes a perturbation in the orbit of another body.

Agent noun of → perturb; → body.

A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: B_v = (2hν³ / c²)[exp(hν / kT) - 1]^-1 where h is Planck's constant and ν is the frequency.

→ Planck; → blackbody; → formula.

The body that is being orbited; such as the → Sun in the → solar system. As regards → multiple star systems, the most massive, or → primary star. See also: → secondary body.

→ primary; → body.

A special case of the → three-body problem in which the → mass of one of the bodies is negligible compared to that of the two others. If the relative motion of the two massive components is a circle, the situation is referred to as the → circular restricted three-body problem. An example would be a space probe moving in the → gravitational fields of the → Earth and the → Moon, which revolve very nearly in circles about their common → center of mass.

→ restrict; → three; → body; → problem.

Mechanics: A system of many particles whose positions relative to one another remain fixed.

→ rigid; → body.