Grammar: A word opposite in meaning to another.
That part of the Moon's shadow that extends beyond the → umbra. It is similar to the → penumbra in that the Sun is only partially blocked by the Moon. From within the antumbra, the Sun appears larger than the Moon which is seen in complete silhouette. An → annular eclipse is seen when an observer passes through the antumbra (F. Espenak, NASA).
Fr.: étoile Ap
A star of spectral type A in which lines of ionized metals and → rare-earth elements are abnormally enhanced. Such stars have unusually strong magnetic fields, thousands of times stronger than the Sun's typical surface field. Ap stars are generally slow rotators because of magnetic braking.
The point at which a binary star is furthest from its companion.
Apastron, from Gk. ap-, → apo- + astron "star."
Fr.: amortissement apériodique
A system in which the → damping is great enough to prevent oscillation.
The diameter of the main mirror in a reflecting telescope, the objective lens in a refracting telescope, the dish of a radio telescope, or the entry of an instrument such as spectrograph, 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," from Mid.Pers./Mod.Pers. zafar, Av. zafar, zafan "mouth," compare with Skt. jambha- "set of teeth, mouth, jaws," Ger. Kiefer "mouth".
Fr.: efficacité d'ouverture
The ratio of the → effective aperture of a radio telescope to the true aperture.
Fr.: photométrie d'ouverture
Photometry using a diaphragm to isolate a small sky area, either directly with a focal-plane diaphragm, or with an image processing system.
Fr.: rapport d'ouverture
The ratio of the effective diameter of a lens or mirror to its focal length.
Fr.: diaphragme d'ouverture
The diaphragm that limits the diameter of the axial light bundle allowed to pass through a lens.
Fr.: synthèse d'ouverture
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.
1) General: The highest point or level.
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."
Fr.: distance à l'aphélie
Fr.: lentille aplanétique
→ aplanatism; → lens.
Fr.: système aplanétique
→ 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.
The point on an elliptic orbit at the greatest distance from the principal focus or center of attraction.