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

The number of complete rotations per unit time, expressed in radian per second: ω = 2πν, where ν is the frequency (cycles per second).

→ angular; → frequency.

Of a telescope, the smallest angle betwwen two → point sources that produces distinct images. It depends on both the wavelength at which observations are made and on the diameter of the telescope. Same as → spatial resolution.

→ angular; → resolution.

→ annual motion.

→ annual; → apparent; → motion.

The high redshift of a quasar which is seemingly physically associated with a galaxy of low redshift.

→ anomalous; → redshift.

A → red supergiant star (→ spectral type M1 Ib) in the constellation → Scorpius, lying about 500 → light-years (170 (+35/-25) → parsecs) from Earth. It has a dwarf massive companion (B3 V), which is a → radio source.

Antares, in Gk. "rival of Mars," from Gk. → anti + Ares "the Gk. god of War, called Mars by the Romans." The comparison with the planet Mars is because they are both red in color and have the same brightness.

Každom-del "the heart of the Scorpion," from každom "scorpion" + del, → heart. Qalb-ol-'Aqrab "the heart of the Scorpion," from Ar. Qalb "heart" + 'aqrab "scorpion".

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.

Statistics: The correlation coefficient of two random variables X and Y is in general defined as the ratio of the Cov(X,Y) to the two standard deviations of X and Y. It varies between 1 and -1 corresponding to complete correlation or anticorrelation.

Anticorrelation, from → anti- + → correlation.

Pâdhambâzâneš, from pâd-, → anti-, + hambâzâneš, → correlation.

Rays of → sunlight that appear to converge at the → antisolar point. Like → crepuscular rays, they are parallel beams of sunlight from holes in the clouds, and their apparently odd directions are a perspective effect.

→ anti-; → crepuscular rays.

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.

General: Open to view, visible; appearing as actual to the eye or mind.
In astronomy, observed.

O.Fr. aparant, from L. apparentem, pr.p. of apparere "to appear," from ad- "to" + perere "to come forth, be visible".

Padidâr "appearing, manifest," from padid "in sight, evident, clear," from Mid.Pers. patdit, from O.Pers./Av. patiy-/paiti "toward, against, back" (cp. Skt. prati- "near, toward, against, in return," Gk. proti, pros "face to face with, toward, in addition to") + O.Pers./Av. di-/dâ(y)- "to see" (Skt. dhi- "to think"), Mod.Pers. didan "to see".

The angular diameter of a celestial body expressed in degrees, minutes, and seconds of arc, or in radians.

→ apparent; → diameter.

The angular distance between two celestial bodies (e.g. the components of a binary star system), expressed in degrees, minutes and seconds of arc.

→ apparent; → distance.

The angular diameter of the circle of light that the eye sees through an eyepiece.

→ apparent; → field.

The circle determined by the intersection of the heavens with a → cone whose → vertex is the → eye, and whose elements are tangent to lines of the Earth's surface. Same as → visible horizon. Assuming that there is no → atmospheric refraction, apparent horizon coincides with → geometric horizon. See also → sea horizon.

→ apparent; → horizon.

A measure of a star's observed brightness (opposed to → absolute magnitude); symbol m. It depends on the star's → intrinsic brightness, its distance from the observer, and the amount of → interstellar absorption. The brightest star → Sirius has an apparent magnitude of -1.46, while the weakest stars visible with the naked eye in the most favorable observation conditions have magnitudes of about +6.5. The stars of magnitudes less than +23 are measured by professional observatories, whereas those of magnitudes less than +30 by a telescope such as the → Hubble Space Telescope (M.S.: SDE).

→ apparent; → magnitude.