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

→ magnetic permeability.

→ absolute; → permeability.

Also called → thermodynamic temperature, the value of a → temperature in the → Kelvin scale. It is is equal to the temperature on the → Celsius scale -273.15 °C.

→ absolute; → temperature.

The temperature gradient defining the → radiative equilibrium condition in a region. It is expressed as: dT/dr = (1 - 1/ γ)((T / P)(dP / dr), where T and P are temperature and pressure, dT / dr and dP / dr temperature and pressure gradients respectively, and γ = C_P / C_V. For radiative equilibrium to be stable against → convection, the actual temperature gradient must be less than the adiabatic temperature gradient, i.e. |dT /dr|_rad < |dT /dr|_ad. See also → Schwarzschild's criterion.

→ adiabatic; → temperature; → gradient.

The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due mainly to gravitational interactions with other planets. The perihelion of the planet Mercury advances about 9'.6 per century. The bulk of the advance was accounted by perturbations from other planets. However, a remaining small advance, by 43'' per century, was eventually explained as an effect predicted by Einstein's theory of → general relativity. In the case of close binary stars, the advance of pericenter may additionally be caused by mass transfer and the stars' distorted (elliptical) shapes. Advance of perihelion (or pericenter) is also known as → apsidal motion.

Advance, from O.Fr. avancer "move forward," from V.L. *abantiare, from L.L. abante "from before," from ab- "from" + ante "before," PIE *ant- "front, forehead;" → perihelion.

Pišraft "advance," from piš "forward; in front; before," Mid.Pers. peš + raft "going; walk, travel," from raftan "to go."

A variable star in the constellation → Perseus, which was the first eclipsing binary discovered. Its brightness varies between 2.2 and 3.5 magnitudes. Lying at a distance of about 82 → light-years, it consists of at least three components. The brightest component (A) is of spectral type B8 V, and the second one (B) a K type giant. The components A and B turn around each other with a period of about 68.8 hours.

Algol, from Ar. Ra's al-ghul "the ghoul's head".

The → SI unit of → electric current; symbol A. It is defined by taking the fixed numerical value of the → elementary charge, e, to be 1.602 176 634 × 10^-19 when expressed in the unit → coulomb (C), which is equal to A s, where the → second (s) is defined in terms of Δν_Cs.

Named after the French physicist and mathematician André-Marie Ampère (1775-1836), one of the pioneers in studying electricity, who laid the foundation of electromagnetic theory.

One of the basic relations between → electricity and → magnetism, stating quantitatively the relation of a → magnetic field to the → electric current or changing electric field that produces it. Ampere's law states that the line integral of the magnetic field around an arbitrarily chosen path is proportional to the net electric current enclosed by the path. Also known as Ampère's theorem, Ampère's circuital law.

→ ampere; → law.

The rate of change of the angles of emergence θ of various wavelengths from a dispersing prism: dθ/dλ.

→ angular; → dispersion.

In → quantum field theory, the operator that lowers → eigenstates one → energy level, contrarily to the → creation operator.

→ annihilation; → operator.

The phenomenon whereby the → refractive index of light in a medium changes rapidly with wavelength in the vicinity of an → absorption band. Hence the → dispersion curve of the substance shows marked deviations from → Cauchy's equation, in contrast with the behavior of → normal dispersion. On the shorter λ side of the absorption band the refractive index falls off more rapidly than required by Cauchy's equation representing values of n for visible light. On the long λ side of the absorption band the index is very high, decreasing at first rapidly and then more slowly as one goes beyond the absorption band.

→ anomalous; → dispersion.

In radio astronomy, a measure of the power absorbed by the antenna. In an ideal, loss-free radio telescope, the antenna temperature is equal to the brightness temperature if the intensity of the received radiation is constant within the main lobe. → antenna; → temperature.

antenna; → temperature.

A system in which the → damping is great enough to prevent oscillation.

Aperiodic, from → a- + periodic; + → damping.

The diameter of the → primary mirror in a → reflecting telescope, the → objective lens in a → refracting telescope, the → dish of a → radio telescope, or the → entrance pupil of an instrument such as → spectrograph or → photometer.

From L. apertura, from apertus, p.p. of aperire "to open, uncover," from PIE *ap-wer-yo- from *ap- "off, away" + base *wer- "to cover".

Ddahâné "an opening," from dahân, → mouth.

The ratio of the → effective aperture of a radio telescope to the true aperture.

→ aperture; → efficiency.

Photometry using a diaphragm to isolate a small sky area, either directly with a focal-plane diaphragm, or with an image processing system.

→ aperture; → photometry.

The ratio of the effective diameter of a lens or mirror to its focal length.

→ aperture; → ratio.

The diaphragm that limits the diameter of the axial light bundle allowed to pass through a lens.

→ aperture; → stop.

The method of combining the signals received by several smaller telescopes distributed over a very large area or baseline to provide the high angular resolution of a much large telescope.

→ aperture; → synthesis.

In psychology of education, the fundamental process in acquiring knowledge, and the part played by existing knowledge.

From Fr. aperception, from N.L. apperceptionem, from ap- variant of → ad- before p + → perception.

From prefix bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + âgerteš, → perception.

The angular distance between the → ascending node of an object orbiting a → primary and its periapsis measured from the primary. Argument of periapsis is measured in the → orbital plane in the direction of motion. It is one of the → orbital elements. See also → argument of perigee, → argument of perihelion.

→ argument; → perigee.