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

The point at which a binary star is furthest from its companion.

Apastron, from Gk. ap-, → apo- + astron "star."

Apâstâr, apâsetâré, from apâ-, → apo-, + stâr, setâré, → star.

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.

1) General: The highest point or level.
2) Mathematics: The highest point of a geometric figure or solid relative to some line or plane.
3) Astro.: Solar apex. The point on the celestial sphere toward which the Sun and the solar system are moving relative to the stars in our vicinity.

L. apex "summit, peak, tip," probably related to apere "to fasten, fix," hence "the tip of anything".

Cakâd "summit of a mountain; top, crown of the head, top of the forehead," from Mid.Pers. cakât "summit," cf. Skt. kakud-, kakuda- "peak, summit," L. cacumen "top, point," cumulus "heap."

The point in the orbit of a planet, or other object in the solar system, which is furthest from the Sun.

Aphelion, from L. aphelium, from Gk. → apo- + helios "sun," cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun; PIE *sawel- "sun".

Apâhur, from Pers. prefix apâ, → apo-, + hur "sun."

The distance between the → Sun and an → object in orbit around it when they are at their farthest approach.

→ perihelion; → distance.

A lens designed so as to minimize both its → spherical aberration and → coma.

→ aplanatism; → lens.

An → optical system that is able to produce an image essentially free from → spherical aberration and → coma. See also the → Abbe sine condition.

→ aplanatism; → system.

Freedom from spherical aberration and coma.

Aplanatism, from aplanatic, from a- "negation prefix" + Gk. plane "wandering," from planasthai "to wander" + -tic adjective-forming suffix.

Nâbirahi, from nâ- "negation prefix" + birah "a devious path; a wanderer, who deviates, errs," + -i noun affix.

Prefix meaning "away from, off; out of; without," ap- before a vowel or h.

From Gk. apo "away from, from;" cf. Av. apâ "away from, from."

Persian apâ- from Av. and O.Pers. apâ "away from, from". Compare with Skt. apa "away, off," L. ab- "from, away," Hittite appa, Gothic af-, Ger. ab-, E. of, off; PIE *apo- "off, away."

The point in an orbit where the moving body lies furthest from the celestial body around which it turns.

Apoapsis, from → apo- + → apsis.

Apâhabâk, from apâ-, → apo-, + habâk, → apsis.

The point on an elliptic orbit at the greatest distance from the principal focus or center of attraction.

Apocenter, from → apo- + → center.

Apâmarkaz, apâkayân, from apâ-, → apo-, + markaz or kayân, → center.

Same as → apochromatic lens.

Corrected for → spherical aberration at two wavelengths or colors and for → chromatic aberration at three wavelengths.

→ apo-; → chromatic.

A lens that is → apochromatic.

→ apochromatic; → lens.