An Etymological Dictionary of Astronomy and Astrophysics

Same as → Ap/Bp star.

See also: → Ap/Bp star.

Same as → Ap/Bp star.

See also: → Ap/Bp star.

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.

See also: A for the spectral type, p for → peculiar; → star.

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.

See also: A for the spectral type, p for → peculiar; → star.

A class of → intermediate-mass stars which possess anomalously strong → magnetic fields (about 100-10000 G). Ap/Bp stars typically show → overabundances of → iron peak elements, → rare earths, and → silicon, ranging up to ~2 dex above solar. These magnetic → chemically peculiar stars make up about 5% of the → main sequence A and B population (→ A star, → B star). Ap/Bp stars have predominantly → dipolar magnetic fields. The presence of strong, ordered magnetic fields in some main sequence A and B stars has been known for nearly one-half of a century (Babcock 1947). However the cause of the magnetic field is still a matter of debate.

There are two competing theories: the contemporaneous → dynamo effect, and the → fossil magnetic field theory. Contemporaneous dynamo effect suggests that there is a dynamo effect currently working in the → convective core of the star. The fossil field theory assumes that the magnetic field is a remnant, produced by a dynamo effect operating at an earlier evolutionary phase, or swept up from the → interstellar medium during → star formation (Power et al., 2006, astro-ph/0612557).

See also: A and B represent spectral types and p stands for → peculiar.

A class of → intermediate-mass stars which possess anomalously strong → magnetic fields (about 100-10000 G). Ap/Bp stars typically show → overabundances of → iron peak elements, → rare earths, and → silicon, ranging up to ~2 dex above solar. These magnetic → chemically peculiar stars make up about 5% of the → main sequence A and B population (→ A star, → B star). Ap/Bp stars have predominantly → dipolar magnetic fields. The presence of strong, ordered magnetic fields in some main sequence A and B stars has been known for nearly one-half of a century (Babcock 1947). However the cause of the magnetic field is still a matter of debate.

There are two competing theories: the contemporaneous → dynamo effect, and the → fossil magnetic field theory. Contemporaneous dynamo effect suggests that there is a dynamo effect currently working in the → convective core of the star. The fossil field theory assumes that the magnetic field is a remnant, produced by a dynamo effect operating at an earlier evolutionary phase, or swept up from the → interstellar medium during → star formation (Power et al., 2006, astro-ph/0612557).

See also: A and B represent spectral types and p stands for → peculiar.

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

Etymology (EN): Apastron, from Gk. ap-, → apo- + astron “star.”

Etymology (PE): Apâstâr, apâsetâré, from apâ-, → apo-,

• stâr, setâré, → star.

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

Etymology (EN): Apastron, from Gk. ap-, → apo- + astron “star.”

Etymology (PE): Apâstâr, apâsetâré, from apâ-, → apo-,

• stâr, setâré, → star.

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

See also: Aperiodic, from → a- + periodic; + → damping.

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

See also: 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.

Etymology (EN): 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”.

Etymology (PE): Ddahâné “an opening,” from dahân, → mouth.

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.

Etymology (EN): 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”.

Etymology (PE): Ddahâné “an opening,” from dahân, → mouth.

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

See also: → aperture; → efficiency.

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

See also: → 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.

See also: → aperture; → photometry.

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

See also: → aperture; → photometry.

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

See also: → aperture; → ratio.

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

See also: → aperture; → ratio.

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

See also: → aperture; → stop.

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

See also: → 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.

See also: → aperture; → synthesis.

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.

See also: → 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.

Etymology (EN): L. apex “summit, peak, tip,” probably related to apere
“to fasten, fix,” hence “the tip of anything”.

Etymology (PE): 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.”

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.

Etymology (EN): L. apex “summit, peak, tip,” probably related to apere
“to fasten, fix,” hence “the tip of anything”.

Etymology (PE): 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.

Etymology (EN): 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”.

Etymology (PE): Apâhur, from Pers. prefix apâ, → apo-, + hur “sun.”

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

Etymology (EN): 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”.

Etymology (PE): 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.

See also: → perihelion; → distance.

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

See also: → perihelion; → distance.

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

See also: → aplanatism; → lens.

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

See also: → 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.

See also: → aplanatism; → system.

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

See also: → aplanatism; → system.

Freedom from spherical aberration and coma.

Etymology (EN): Aplanatism, from aplanatic, from a- “negation prefix” + Gk. plane “wandering,” from
planasthai “to wander” + -tic adjective-forming suffix.

Etymology (PE): Nâbirahi, from nâ- “negation prefix” + birah “a devious path; a wanderer, who deviates, errs,” + -i noun affix.

Freedom from spherical aberration and coma.

Etymology (EN): Aplanatism, from aplanatic, from a- “negation prefix” + Gk. plane “wandering,” from
planasthai “to wander” + -tic adjective-forming suffix.

Etymology (PE): 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.

Etymology (EN): From Gk. apo “away from, from;” cf. Av. apâ “away from, from.”

Etymology (PE): 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.”

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

Etymology (EN): From Gk. apo “away from, from;” cf. Av. apâ “away from, from.”

Etymology (PE): 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.

Etymology (EN): Apoapsis, from → apo- + → apsis.

Etymology (PE): Apâhabâk, from apâ-, → apo-, + habâk, → apsis.

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

Etymology (EN): Apoapsis, from → apo- + → apsis.

Etymology (PE): Apâhabâk, from apâ-, → apo-, + habâk, → apsis.

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

Etymology (EN): Apocenter, from → apo- + → center.

Etymology (PE): Apâmarkaz, apâkayân, from apâ-, → apo-,

• markaz or kayân, → center.

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

Etymology (EN): Apocenter, from → apo- + → center.

Etymology (PE): Apâmarkaz, apâkayân, from apâ-, → apo-,

• markaz or kayân, → center.

Same as → apochromatic lens.

Same as → apochromatic lens.

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

See also: → apo-; → chromatic.

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

See also: → apo-; → chromatic.

A lens that is → apochromatic.

See also: → apochromatic; → lens.

A lens that is → apochromatic.

See also: → apochromatic; → lens.

An optical system that is → apochromatic.

See also: → apochromatic; → system.

An optical system that is → apochromatic.

See also: → apochromatic; → system.

The capacity of an optical system to bring three widely separated wavelengths of light into a single focus.

See also: → apochromatic; → -ism.

The capacity of an optical system to bring three widely separated wavelengths of light into a single focus.

See also: → apochromatic; → -ism.

To bring the wavelengths of the spatially separate colors to a common focus.

See also: → apochromat; → -ize.

To bring the wavelengths of the spatially separate colors to a common focus.

See also: → apochromat; → -ize.

The point in a lunar orbit that is farthest from the center of the Lune. Also apolune.

Etymology (EN): Apocynthion, from → apo- “away from, off” + cynthion, from Gk. Cynthia “goddess of the Moon.”

Etymology (PE): Apâmah, apâmâh, from apâ, → apo-,

• mah, mâh, → Moon.

The point in a lunar orbit that is farthest from the center of the Lune. Also apolune.

Etymology (EN): Apocynthion, from → apo- “away from, off” + cynthion, from Gk. Cynthia “goddess of the Moon.”

Etymology (PE): Apâmah, apâmâh, from apâ, → apo-,

• mah, mâh, → Moon.

1. Generally, any process suppressing the secondary maxima of a diffraction pattern, such as the faint rings around the Airy disk of an optical image. This allows the telescope to resolve finer details.
   
2. Over a telescope aperture, the use of a screen that progressively cuts down, from the center to the edge of the aperture, the amount of light transmitted, in order to reduce diffraction.
   
3. A mathematical treatment carried out on data received from an interferometer before the Fourier transformation is calculated to obtain the spectrum.

Etymology (EN): Apodization from v. apodize, → a- “negation prefix” + pod from Gk. podos “foot” (compare with Pers. pâ, see below)

• -tion “noun forming suffix;” literally “removing feet,” i.e. suppressing the secondary maxima at the side of the Airy spot.

Etymology (PE): Pâzodâyi “removing feet,” from pâ “foot,” Mid.Pers. pâd, pây , Av. pad-, Skt. pat, Gk. pos, gen. podos, L. pes, gen. pedis, PIE *pod-/*ped-. Zodâyi, n. from zodudan “to polish, clean,” Mid.Pers. uzdâtan,
Av. uzdâ-, from uz-, → ex-, + dâ- “make, create.”

1. Generally, any process suppressing the secondary maxima of a diffraction pattern, such as the faint rings around the Airy disk of an optical image. This allows the telescope to resolve finer details.
   
2. Over a telescope aperture, the use of a screen that progressively cuts down, from the center to the edge of the aperture, the amount of light transmitted, in order to reduce diffraction.
   
3. A mathematical treatment carried out on data received from an interferometer before the Fourier transformation is calculated to obtain the spectrum.

Etymology (EN): Apodization from v. apodize, → a- “negation prefix” + pod from Gk. podos “foot” (compare with Pers. pâ, see below)

• -tion “noun forming suffix;” literally “removing feet,” i.e. suppressing the secondary maxima at the side of the Airy spot.

Etymology (PE): Pâzodâyi “removing feet,” from pâ “foot,” Mid.Pers. pâd, pây , Av. pad-, Skt. pat, Gk. pos, gen. podos, L. pes, gen. pedis, PIE *pod-/*ped-. Zodâyi, n. from zodudan “to polish, clean,” Mid.Pers. uzdâtan,
Av. uzdâ-, from uz-, → ex-, + dâ- “make, create.”

The point on an elliptic orbit at the greatest distance from the principal focus. Also knwon as → apocenter.

Etymology (EN): From → apo- “away from, off,” + → focus.

Etymology (PE): From apâ-, → apo-, + kânun→ focus.

The point on an elliptic orbit at the greatest distance from the principal focus. Also knwon as → apocenter.

Etymology (EN): From → apo- “away from, off,” + → focus.

Etymology (PE): From apâ-, → apo-, + kânun→ focus.

Of or pertaining to an → apogalacticon.

See also: → apo-; → galactic.

Of or pertaining to an → apogalacticon.

See also: → apo-; → galactic.

The point at which a celestial body is farthest from the center of a galaxy; opposite of → perigalacticon.

Etymology (EN): From → apo- “away from, off” + galacticon,
→ galaxy.

Etymology (PE): From apâ, → apo-, + kahkašân, → galaxy.

The point at which a celestial body is farthest from the center of a galaxy; opposite of → perigalacticon.

Etymology (EN): From → apo- “away from, off” + galacticon,
→ galaxy.

Etymology (PE): From apâ, → apo-, + kahkašân, → galaxy.

The point in the orbit of the moon or an artificial satellite that is farthest from the terrestrial center and at which the body’s velocity is at a minimum.

Etymology (EN): From Fr. apogée, from L. apogæum, from Gk. apogaion “away from the earth,”
from → apo- “off, away” + gaia/ge “earth.” According to Dehxodâ, the term owj used in Persian is neither Ar. nor Skt. (contrary to the opinions of Khwarazi and Biruni respectively), but the corrupt form of the above Gk. term.

Etymology (PE): Apâzam, from apâ-, → apo-, + Av. zam- “the earth,” Mid.Pers. zamig, Mod.Pers. zami, zamin “the earth;” cf. Skt. ksam, Gk. khthôn, khamai “on the ground,” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth”.

The point in the orbit of the moon or an artificial satellite that is farthest from the terrestrial center and at which the body’s velocity is at a minimum.

Etymology (EN): From Fr. apogée, from L. apogæum, from Gk. apogaion “away from the earth,”
from → apo- “off, away” + gaia/ge “earth.” According to Dehxodâ, the term owj used in Persian is neither Ar. nor Skt. (contrary to the opinions of Khwarazi and Biruni respectively), but the corrupt form of the above Gk. term.

Etymology (PE): Apâzam, from apâ-, → apo-, + Av. zam- “the earth,” Mid.Pers. zamig, Mod.Pers. zami, zamin “the earth;” cf. Skt. ksam, Gk. khthôn, khamai “on the ground,” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth”.

An eclipse (of the Sun or Moon) which takes place when the Moon is at the → apogee of its orbit. The solar apogee eclipses, when they are not partial, are always → annular. The maximum duration of an apogee solar eclipse is 6h 15m (between the → first contact and the → fourth contact). The maximum duration of a lunar apogee eclipse, between the two exterior contacts of the Moon with the → penumbra, is 6h 18m (the maximum totality being 1h 44m) (M.S.: SDE).

See also: → apogee; → eclipse.

An eclipse (of the Sun or Moon) which takes place when the Moon is at the → apogee of its orbit. The solar apogee eclipses, when they are not partial, are always → annular. The maximum duration of an apogee solar eclipse is 6h 15m (between the → first contact and the → fourth contact). The maximum duration of a lunar apogee eclipse, between the two exterior contacts of the Moon with the → penumbra, is 6h 18m (the maximum totality being 1h 44m) (M.S.: SDE).

See also: → apogee; → eclipse.

The → full Moon when our natural satellite is at its farthest position from the Earth. The difference in apparent size with respect to the → perigee full Moon represents a difference in distance of just under 50,000 km between → apogee and → perigee, given the Moon’s average distance of about 385,000 km. Also called → full micro Moon.

See also: → apogee; → full; → moon.

The → full Moon when our natural satellite is at its farthest position from the Earth. The difference in apparent size with respect to the → perigee full Moon represents a difference in distance of just under 50,000 km between → apogee and → perigee, given the Moon’s average distance of about 385,000 km. Also called → full micro Moon.

See also: → apogee; → full; → moon.

The → point in the → orbit of a → satellite of → Jupiter that is farthest from Jupiter’s center. → perijove.

See also: → apo- + jove, from L. Jovius “Jupiter,” → Jovian.

The → point in the → orbit of a → satellite of → Jupiter that is farthest from Jupiter’s center. → perijove.

See also: → apo- + jove, from L. Jovius “Jupiter,” → Jovian.

A member of a class of → near-Earth asteroids (NEAs) that have orbital → semi-major axes greater than that of the Earth (> 1 → astronomical unit) but → perihelion distances less than the Earth’s → aphelion distance (less than 1.017 AU); thus, they cross the Earth’s orbit when near the perihelia of their orbits. They are named for the prototype Apollo.

See also: → Apollo; → asteroid.

A member of a class of → near-Earth asteroids (NEAs) that have orbital → semi-major axes greater than that of the Earth (> 1 → astronomical unit) but → perihelion distances less than the Earth’s → aphelion distance (less than 1.017 AU); thus, they cross the Earth’s orbit when near the perihelia of their orbits. They are named for the prototype Apollo.

See also: → Apollo; → asteroid.

The point in a lunar orbit that is farthest from the center of the Lune. Also → apocynthion.

Etymology (EN): From → apo- “away from, off” + lune “moon,” from L. luna; PIE *louksnâ- “moon,” literally
“luminous, bright;”
compare with O.Pers. raucah-, Av. raocah- “light, luminous; daylight,” Skt. roka “brightness, light,” cognate with Gk. leukos “white, clear,” L. lux “light” (also lumen, luna), PIE *leuk- “light, brightness”. The Mod.Pers. words rowšan, raxšân “bright, clear,” ruz “day,” foruq “light,” and afruxtan “to light, kindle” also belong to this family, as well as the E. light, Ger. Licht, and Fr. lumière.

Etymology (PE): Apâmah, apâmâh, → apocynthion.

The point in a lunar orbit that is farthest from the center of the Lune. Also → apocynthion.

Etymology (EN): From → apo- “away from, off” + lune “moon,” from L. luna; PIE *louksnâ- “moon,” literally
“luminous, bright;”
compare with O.Pers. raucah-, Av. raocah- “light, luminous; daylight,” Skt. roka “brightness, light,” cognate with Gk. leukos “white, clear,” L. lux “light” (also lumen, luna), PIE *leuk- “light, brightness”. The Mod.Pers. words rowšan, raxšân “bright, clear,” ruz “day,” foruq “light,” and afruxtan “to light, kindle” also belong to this family, as well as the E. light, Ger. Licht, and Fr. lumière.

Etymology (PE): Apâmah, apâmâh, → apocynthion.

An appliance or device for a particular purpose: an X-ray apparatus. An integrated group of materials or devices used for a particular purpose.

Etymology (EN): From L. apparatus “equipment, preparation,” from p.p. of apparare “to prepare,” from ad- “to” + parare “make ready”.

Etymology (PE): Dastgâh “any manufacturing instrument, a loom; ustensils”.

An appliance or device for a particular purpose: an X-ray apparatus. An integrated group of materials or devices used for a particular purpose.

Etymology (EN): From L. apparatus “equipment, preparation,” from p.p. of apparare “to prepare,” from ad- “to” + parare “make ready”.

Etymology (PE): Dastgâh “any manufacturing instrument, a loom; ustensils”.

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

Etymology (EN): O.Fr. aparant, from L. apparentem, pr.p. of apparere “to appear,” from ad- “to” + perere “to come forth, be visible”.

Etymology (PE): 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”.

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

Etymology (EN): O.Fr. aparant, from L. apparentem, pr.p. of apparere “to appear,” from ad- “to” + perere “to come forth, be visible”.

Etymology (PE): 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.

See also: → apparent; → diameter.

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

See also: → 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.

See also: → apparent; → distance.

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

See also: → apparent; → distance.

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

See also: → apparent; → field.

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

See also: → 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.

See also: → apparent; → horizon.

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.

See also: → 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).

See also: → apparent; → magnitude.

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).

See also: → apparent; → magnitude.

The moment when the center of the Sun crosses the meridian. Same as true noon.

See also: → apparent; → noon.

The moment when the center of the Sun crosses the meridian. Same as true noon.

See also: → apparent; → noon.

Same as → apparent position.

See also: → apparent; → place.

Same as → apparent position.

See also: → apparent; → place.

1. The position on the celestial sphere at which a heavenly body would be seen from the Earth at a particular time.
   

2. A position given by the coordinates calculated for a star, if it were seen from the Earth’s centre, relative to the → real equator and the → real equinox, at a certain date. It includes the displacements from one heliocentric direction, given in a stellar catalogue, due to
   → precession, → nutation, → aberration, → proper motions, → annual parallax, and light gravitational deviation (M.S.: SDE).

See also: → apparent; → position.

1. The position on the celestial sphere at which a heavenly body would be seen from the Earth at a particular time.
   

2. A position given by the coordinates calculated for a star, if it were seen from the Earth’s centre, relative to the → real equator and the → real equinox, at a certain date. It includes the displacements from one heliocentric direction, given in a stellar catalogue, due to
   → precession, → nutation, → aberration, → proper motions, → annual parallax, and light gravitational deviation (M.S.: SDE).

See also: → apparent; → position.

The instant of time when the object is in the East and the geometric → zenith distance is equal to 90° plus the → horizontal refraction plus the semidiameter minus the → parallax.

See also: → apparent; → rising.

The instant of time when the object is in the East and the geometric → zenith distance is equal to 90° plus the → horizontal refraction plus the semidiameter minus the → parallax.

See also: → apparent; → rising.

The instant of time when the object is in the West and the geometric → zenith distance is equal to 90° plus the → horizontal refraction plus the semidiameter minus the → parallax.

See also: → apparent; → setting.

The instant of time when the object is in the West and the geometric → zenith distance is equal to 90° plus the → horizontal refraction plus the semidiameter minus the → parallax.

See also: → apparent; → setting.

The time interval between two successive → upper transits of the → true equinox of date.

See also: → apparent; → sidereal; → day.

The time interval between two successive → upper transits of the → true equinox of date.

See also: → apparent; → sidereal; → day.

The → mean sidereal time corrected for the → nutation and shift in the obliquity of the ecliptic that occurs as a result of the Moon’s gravitational effect. Apparent sidereal time differs from mean sidereal time in that the → true vernal equinox point is used.

See also: → apparent; → sidereal; → time.

The → mean sidereal time corrected for the → nutation and shift in the obliquity of the ecliptic that occurs as a result of the Moon’s gravitational effect. Apparent sidereal time differs from mean sidereal time in that the → true vernal equinox point is used.

See also: → apparent; → sidereal; → time.

The duration of one rotation of the Earth on its axis (→ Earth’s rotation), with respect to the → apparent Sun. It is measured by successive transits of the apparent Sun over the lower branch of a → meridian.

See also: → apparent; → solar day.

The duration of one rotation of the Earth on its axis (→ Earth’s rotation), with respect to the → apparent Sun. It is measured by successive transits of the apparent Sun over the lower branch of a → meridian.

See also: → apparent; → solar day.

The time based on the motion of the → apparent Sun and kept by dividing the day into 24 equal hours.

See also: → apparent; → solar; → time.

The time based on the motion of the → apparent Sun and kept by dividing the day into 24 equal hours.

See also: → apparent; → solar; → time.

The → true Sun as seen by an observer on Earth. The term “apparent Sun” is used in contrast to → mean Sun, which refers to an average of the Sun’s position). See also: → apparent solar time and → mean solar time.

See also: → apparent; → Sun.

The → true Sun as seen by an observer on Earth. The term “apparent Sun” is used in contrast to → mean Sun, which refers to an average of the Sun’s position). See also: → apparent solar time and → mean solar time.

See also: → apparent; → Sun.

→ Apparent magnitude in the visual wavelengths, around 5600 Å. → visual magnitude.

See also: → apparent; → visual; → magnitude.

→ Apparent magnitude in the visual wavelengths, around 5600 Å. → visual magnitude.

See also: → apparent; → visual; → magnitude.

Judging from what is apparent. As far as one knows or can see.

See also: → apparent; → -ly.

Judging from what is apparent. As far as one knows or can see.

See also: → apparent; → -ly.

A period during which a → planet, → asteroid, or → comet is observable, generally between two successive → conjunctions of the body with the Sun.

Etymology (EN): M.E. apparicioun, from O.Fr. apparition, from L.L. appritionem “an appearance,” from L. apparitus, p.p. of apparere “to appear.”

Etymology (PE): Padidâri, n. from → padidâr + -i; padidâreš, verbal n. from *padidâridan.

A period during which a → planet, → asteroid, or → comet is observable, generally between two successive → conjunctions of the body with the Sun.

Etymology (EN): M.E. apparicioun, from O.Fr. apparition, from L.L. appritionem “an appearance,” from L. apparitus, p.p. of apparere “to appear.”

Etymology (PE): Padidâri, n. from → padidâr + -i; padidâreš, verbal n. from *padidâridan.

1. Come into sight; become visible or noticeable.
   

   2. To have the appearance of being; seem.

Etymology (EN): From M.E. apperen, aperen, from O.Fr. aparoir, aperer, from L. apparere, from → ad- + prarere “to come forth, become visible.”

Etymology (PE): Padidâr šodan, literally “become apparent,” from padidâr, → apparent, + šodan, → become.

1. Come into sight; become visible or noticeable.
   

   2. To have the appearance of being; seem.

Etymology (EN): From M.E. apperen, aperen, from O.Fr. aparoir, aperer, from L. apparere, from → ad- + prarere “to come forth, become visible.”

Etymology (PE): Padidâr šodan, literally “become apparent,” from padidâr, → apparent, + šodan, → become.

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

Etymology (EN): From Fr. aperception, from N.L. apperceptionem, from ap- variant of → ad- before p + → perception.

Etymology (PE): 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.

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

Etymology (EN): From Fr. aperception, from N.L. apperceptionem, from ap- variant of → ad- before p + → perception.

Etymology (PE): 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.

1. General: The act of applying to a particular purpose or use.
   

2. Computers: A program that performs a specific function for the user, as contrasted with an environment or operating system. Some examples are Microsoft Windows, Microsoft Word, and Adobe Photoshop. The word “application” is used because each program has a specific application for the user.
   

See also: Verbal noun of → apply.

1. General: The act of applying to a particular purpose or use.
   

2. Computers: A program that performs a specific function for the user, as contrasted with an environment or operating system. Some examples are Microsoft Windows, Microsoft Word, and Adobe Photoshop. The word “application” is used because each program has a specific application for the user.
   

See also: Verbal noun of → apply.

A software with a specific function, such as a word processor or game. Contrast with operating system software.

See also: → application; → software.

A software with a specific function, such as a word processor or game. Contrast with operating system software.

See also: → application; → software.

A set of topics in physics intended for a particular or practical use. Applied physics programs are usually interfaces between pure physics and technology.

See also: Past participle of → apply; → physics.

A set of topics in physics intended for a particular or practical use. Applied physics programs are usually interfaces between pure physics and technology.

See also: Past participle of → apply; → physics.

To make use of; to put to use especially for some practical purpose.

To put into operation or effect.

To lay or spread on; to be placed or remain on.

To make a request especially in the form of a written → application.

Etymology (EN): From M.E. ap(p)lien, from O.Fr. aploiier “apply, use, attach,” from L. applicare “to attach to, connect;” figuratively, “devote (oneself) to, give attention,” from → ad- “to” + plicare “to fold,” → explain.

Etymology (PE): Âtâhidan, from â- intensive/nuance prefix, + tâh “fold, plait, ply” (on the model of L., as above, → explain);
Mid.Pers. tâg “piece, part” + -idan infinitive suffix.
Bé kâr bordan, from bé “to; for; in; on; with; by; according to,” → ad hoc; kâr, → work, bordan “to carry, bear, conduct, take,” → range.

To make use of; to put to use especially for some practical purpose.

To put into operation or effect.

To lay or spread on; to be placed or remain on.

To make a request especially in the form of a written → application.

Etymology (EN): From M.E. ap(p)lien, from O.Fr. aploiier “apply, use, attach,” from L. applicare “to attach to, connect;” figuratively, “devote (oneself) to, give attention,” from → ad- “to” + plicare “to fold,” → explain.

Etymology (PE): Âtâhidan, from â- intensive/nuance prefix, + tâh “fold, plait, ply” (on the model of L., as above, → explain);
Mid.Pers. tâg “piece, part” + -idan infinitive suffix.
Bé kâr bordan, from bé “to; for; in; on; with; by; according to,” → ad hoc; kâr, → work, bordan “to carry, bear, conduct, take,” → range.

1. Arrest (someone) for a crime.
   

   2. → Understand or → perceive. Often to hold in opinion but without positive certainty.

See also: From L. apprehendere “to take hold of, grasp,” from → ad- “to” + prehendere “to seize,” → prehend.

1. Arrest (someone) for a crime.
   

   2. → Understand or → perceive. Often to hold in opinion but without positive certainty.

See also: From L. apprehendere “to take hold of, grasp,” from → ad- “to” + prehendere “to seize,” → prehend.

1. Anxiety or fear that something bad or unpleasant will happen.
   

   2. Understanding; grasp.
      

   3. The action of arresting someone (Oxford Dictionary, lexico.com).

See also: Verbal noun of → apprehend.

1. Anxiety or fear that something bad or unpleasant will happen.
   

   2. Understanding; grasp.
      

   3. The action of arresting someone (Oxford Dictionary, lexico.com).

See also: Verbal noun of → apprehend.

1. (v.intr.) To come near or nearer, as in space or time.
   (v.tr.) To come or go near or nearer to in quality, character, time, or condition.
   

2. (n.) The act of drawing near; nearness or close approximation; the method used or steps taken in dealing with or accomplishing.

Etymology (EN): M.E. approchen, from O.Fr. aprochier, from L. appropiare, from ad- “to” + propius “nearer,” comparative of prope “near”.

Etymology (PE): 1) Nazdidan from nazd “near,” Mid.Pers. nazd, Av. nas- “to come near, approach, reach,” nazdišta- “nearest, next,” nazdyo “nearer to,” compare with Skt. nas- “to approach, to reach”

• -idan “infinitive forming suffix”.
  

2. Nazdeš, from nazd, nazdidan + -eš “noun forming suffix.”

1. (v.intr.) To come near or nearer, as in space or time.
   (v.tr.) To come or go near or nearer to in quality, character, time, or condition.
   

2. (n.) The act of drawing near; nearness or close approximation; the method used or steps taken in dealing with or accomplishing.

Etymology (EN): M.E. approchen, from O.Fr. aprochier, from L. appropiare, from ad- “to” + propius “nearer,” comparative of prope “near”.

Etymology (PE): 1) Nazdidan from nazd “near,” Mid.Pers. nazd, Av. nas- “to come near, approach, reach,” nazdišta- “nearest, next,” nazdyo “nearer to,” compare with Skt. nas- “to approach, to reach”

• -idan “infinitive forming suffix”.
  

2. Nazdeš, from nazd, nazdidan + -eš “noun forming suffix.”

The act of approving; approbation; formal permission or sanction.

See also: → approve; → -al.

The act of approving; approbation; formal permission or sanction.

See also: → approve; → -al.

To confirm or sanction formally; ratify.

Etymology (EN): M.E., from O.Fr. aprover “approve, agree to,” from L. approbare “to assent to as good, regard as good,” from → ad- “to” + probare “to try, test something (to find if it is good),” from probus “honest, genuine,” → prove.

Etymology (PE): Âsandidan, from â- + sandidan, as in pasandidan (with pati-) “to cherish, approve;” ultimately from Proto-Ir. *sand- “to appear, seem good;” cf. Av. sənd- “to appear, seem (good);” Manichean Mid.Pers. shyn “apparent, prominent, splendid;” Mid.Pers. passand “to approve;” Khotanese sad- “to appear, seem, show;” Skt. chand- “to appear, please;” L. censere “to appraise, value, judg” (Cheung 2007).

To confirm or sanction formally; ratify.

Etymology (EN): M.E., from O.Fr. aprover “approve, agree to,” from L. approbare “to assent to as good, regard as good,” from → ad- “to” + probare “to try, test something (to find if it is good),” from probus “honest, genuine,” → prove.

Etymology (PE): Âsandidan, from â- + sandidan, as in pasandidan (with pati-) “to cherish, approve;” ultimately from Proto-Ir. *sand- “to appear, seem good;” cf. Av. sənd- “to appear, seem (good);” Manichean Mid.Pers. shyn “apparent, prominent, splendid;” Mid.Pers. passand “to approve;” Khotanese sad- “to appear, seem, show;” Skt. chand- “to appear, please;” L. censere “to appraise, value, judg” (Cheung 2007).

1. Nearly exact or correct; near or approaching a certain state, condition, or goal.
   

2. (v.tr.) To come close to; be nearly the same as.
   (v.intr.) To come near or close, as in degree, nature, or quality.

Etymology (EN): From L. approximatus, p.p. of approximare “to draw near to,” from ad- “to” + proximare “come near,” from proximus “nearest,” superlative of prope “near”.

Etymology (PE): 1) Nazdin from nazd “near” + -in, adjective making suffix, in particular superlative, as in bišin, kamin, kehin, mehin, etc.
2) Nazdinidan from nazdin, as above, + -idan infinitive forming suffix.

1. Nearly exact or correct; near or approaching a certain state, condition, or goal.
   

2. (v.tr.) To come close to; be nearly the same as.
   (v.intr.) To come near or close, as in degree, nature, or quality.

Etymology (EN): From L. approximatus, p.p. of approximare “to draw near to,” from ad- “to” + proximare “come near,” from proximus “nearest,” superlative of prope “near”.

Etymology (PE): 1) Nazdin from nazd “near” + -in, adjective making suffix, in particular superlative, as in bišin, kamin, kehin, mehin, etc.
2) Nazdinidan from nazdin, as above, + -idan infinitive forming suffix.

Not exactly, but nearly or roughly.

See also: Adverb of → approximate.

Not exactly, but nearly or roughly.

See also: Adverb of → approximate.

1. General: A guess or estimate; nearness in space, position, degree.
   

2. Math., Physics: Result that is not quite exact, but is within the limits of accuracy required for a given purpose.

Etymology (EN): Approximation, verbal noun of → approximate.

Etymology (PE): Nazdineš, verbal noun of nazdinidan,
→ approximate.

1. General: A guess or estimate; nearness in space, position, degree.
   

2. Math., Physics: Result that is not quite exact, but is within the limits of accuracy required for a given purpose.

Etymology (EN): Approximation, verbal noun of → approximate.

Etymology (PE): Nazdineš, verbal noun of nazdinidan,
→ approximate.

A situation in which two heavenly bodies apparently approach
each other. A close → conjunction in which no → occultation actually occurs.

Etymology (EN): L. appulsus, from appellere, appulsum “to drive to,” from
ad, → ad- + pellere “to drive”.

Etymology (PE): Hamtâxt from ham- “against; together” + tâxt, from tâxtan “to rush upon, run, assault”.

A situation in which two heavenly bodies apparently approach
each other. A close → conjunction in which no → occultation actually occurs.

Etymology (EN): L. appulsus, from appellere, appulsum “to drive to,” from
ad, → ad- + pellere “to drive”.

Etymology (PE): Hamtâxt from ham- “against; together” + tâxt, from tâxtan “to rush upon, run, assault”.

Relating to an → apsis.

See also: → apsis; → -al.

Relating to an → apsis.

See also: → apsis; → -al.

→ Rotation of the → line of apsides in the plane of the orbit in the same direction as the → revolution of the → secondary body. The major axis of the Earth’s orbit rotates by 11.6 arcseconds per year.

See also: → apsidal; → motion.

→ Rotation of the → line of apsides in the plane of the orbit in the same direction as the → revolution of the → secondary body. The major axis of the Earth’s orbit rotates by 11.6 arcseconds per year.

See also: → apsidal; → motion.

→ advance of perihelion, → relativistic precession.

See also: → apsidal; → precession.

→ advance of perihelion, → relativistic precession.

See also: → apsidal; → precession.

The line connecting the two apsides, i.e. the major axis of an elliptical orbit. → apsis.

See also: Apsides, pl. of → apside; → line.

The line connecting the two apsides, i.e. the major axis of an elliptical orbit. → apsis.

See also: Apsides, pl. of → apside; → line.

The point of greatest or least distance of the orbit of a celestial body from a center of attraction. The closest point is the → periapsis, the further point the → apoapsis.

Etymology (EN): L. apsis “arch, vault,” from Gk. hapsis “loop, arch,” from haptein “fasten together”.

Etymology (PE): Mod.Pers. habâk “top of the head; the summit of a mountain”.

The point of greatest or least distance of the orbit of a celestial body from a center of attraction. The closest point is the → periapsis, the further point the → apoapsis.

Etymology (EN): L. apsis “arch, vault,” from Gk. hapsis “loop, arch,” from haptein “fasten together”.

Etymology (PE): Mod.Pers. habâk “top of the head; the summit of a mountain”.

Exactly suitable; appropriate; suited to a purpose.

Etymology (EN): M.E., from L. aptus “fitted, suitable, fastened,” from p.p. of *apere “to fasten;” akin to Hittite hap- “to attach;” PIE *ap- “to grasp, take, reach”.

Etymology (PE): Niyâw “apt, suitable, appropriate,” from Mid.Pers.

Exactly suitable; appropriate; suited to a purpose.

Etymology (EN): M.E., from L. aptus “fitted, suitable, fastened,” from p.p. of *apere “to fasten;” akin to Hittite hap- “to attach;” PIE *ap- “to grasp, take, reach”.

Etymology (PE): Niyâw “apt, suitable, appropriate,” from Mid.Pers.

The Bird of Paradise. A constellation in the southern hemisphere, at R.A. = 16 h, Dec. = -75°. Abbreviation: Aps; genitive form: Apodis.

Etymology (EN): L. apus “a kind of swallow,” from Gk. apous “without feet, sand martin,” from → a- “without”

• pous “foot”.

Etymology (PE): Morq-e behešti “bird of paradise,” from morq “bird” + behešt “paradise”.

The Bird of Paradise. A constellation in the southern hemisphere, at R.A. = 16 h, Dec. = -75°. Abbreviation: Aps; genitive form: Apodis.

Etymology (EN): L. apus “a kind of swallow,” from Gk. apous “without feet, sand martin,” from → a- “without”

• pous “foot”.

Etymology (PE): Morq-e behešti “bird of paradise,” from morq “bird” + behešt “paradise”.