A small tail-like structure on a comet that, unlike most comet tails, seems to point toward the Sun. This rare event is an optical illusion due to larger dust particles left along the comet's orbit. And typically occurs when the Earth crosses the plane of the comet's orbit. It seen when the observer is in the plane of the cometary orbit.
Pâddom, from pâd-, → anti-, + dom "tail."
1) Of the nature of or involving → antithesis.
Fr.: Machine pneumatique
The Air Pump. A faint constellation in the southern hemisphere, at about alpha 10h, delta -35 deg. Abbreviation: Ant; genitive form: Antliae.
L. antlia "pump," from Gk. antlia "ship's hold, bilge water," from antlos.
Tolombé "pump," from Turkish tulumba "pump," from It. tromba.
Fr.: équation d'Antoine
A mathematical expression, derived from the → Clausius-Clapeyron equation, of the relation between the vapor pressure and the temperature of pure substances. It shows that the logarithm of vapor pressure is linearly dependent on the reciprocal of → absolute temperature.
Named after Louis Charles Antoine (1825-?), a French marine engineer, who derived the equation; → equation.
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.