An Etymological Dictionary of Astronomy and Astrophysics

A family of fibrous mineral silicates that are incombustible, resistant to chemicals, and do not conduct electricity.
In the past asbestos has been widely used for a range of fireproof materials and in the building industry. Asbestos causes very serious health problems if the fibers are inhaled (bronchial cancer, laryngeal cancer, and mesothelioma).

Etymology (EN): M.E. albeston, abestus, from O.Fr. abeste, abestos, from L. asbestos “quicklime,” from Gk. asbestos, literally “inextinguishable,” from → a- “not” + sbestos, verbal adjective from sbennynai “to quench.”

Etymology (PE): Panbe-ye kuhi, literally “mountain cotton,” from panbé,
→ cotton, + kuhi “pertaining to mountains,” from kuh, → mountain.
Panbe-ye nasuz, literally “non-inflammable cotton,” from panbé

• nasuz “non-inflammable,” from na- “not,” → a- + suz-, suxtan, → burn.

A family of fibrous mineral silicates that are incombustible, resistant to chemicals, and do not conduct electricity.
In the past asbestos has been widely used for a range of fireproof materials and in the building industry. Asbestos causes very serious health problems if the fibers are inhaled (bronchial cancer, laryngeal cancer, and mesothelioma).

Etymology (EN): M.E. albeston, abestus, from O.Fr. abeste, abestos, from L. asbestos “quicklime,” from Gk. asbestos, literally “inextinguishable,” from → a- “not” + sbestos, verbal adjective from sbennynai “to quench.”

Etymology (PE): Panbe-ye kuhi, literally “mountain cotton,” from panbé,
→ cotton, + kuhi “pertaining to mountains,” from kuh, → mountain.
Panbe-ye nasuz, literally “non-inflammable cotton,” from panbé

• nasuz “non-inflammable,” from na- “not,” → a- + suz-, suxtan, → burn.

1. To move, climb, or go upward; mount; rise.
   

2. To rise to a higher point, rank, or degree; proceed from an inferior to a superior degree or level (Dictionary.com).

Etymology (EN): M.E. ascenden, from L. ascendere “to climb up, mount,” from → ad- “to” + scandere “to climb,” → scan.

Etymology (PE): Farâzidan, from farâz “above, over, aloft.”

1. To move, climb, or go upward; mount; rise.
   

2. To rise to a higher point, rank, or degree; proceed from an inferior to a superior degree or level (Dictionary.com).

Etymology (EN): M.E. ascenden, from L. ascendere “to climb up, mount,” from → ad- “to” + scandere “to climb,” → scan.

Etymology (PE): Farâzidan, from farâz “above, over, aloft.”

1a) A position or condition of dominance, superiority or control.

1b) An ancestor; forebear.

2a) Ascending; rising.

2b) superior; predominant.

See also: → ascend.

1a) A position or condition of dominance, superiority or control.

1b) An ancestor; forebear.

2a) Ascending; rising.

2b) superior; predominant.

See also: → ascend.

The point in an orbit where the orbiting body crosses a reference plane, such as the ecliptic or the celestial equator, going from south to north.
The celestial longitude of the ascending node is one of the elements of the orbit. → descending node.

See also: → ascend; → -ing; → node.

The point in an orbit where the orbiting body crosses a reference plane, such as the ecliptic or the celestial equator, going from south to north.
The celestial longitude of the ascending node is one of the elements of the orbit. → descending node.

See also: → ascend; → -ing; → node.

The act of ascending; ascent.

See also: Verbal noun of → ascend; → -tion.

The act of ascending; ascent.

See also: Verbal noun of → ascend; → -tion.

A standard code or protocol for displaying → characters as numbers. Each alphabetic, numeric, or special character is represented with a 7-bit binary number (a string of seven 0s or 1s). 128 possible characters are defined. For example, the ASCII code for uppercase C is 67 and for lowercase c is 99. Most computers use ASCII codes to represent text, which makes it possible to transfer data from one computer to another.

See also: Short for American Standard Code for Information Interchange.

A standard code or protocol for displaying → characters as numbers. Each alphabetic, numeric, or special character is represented with a 7-bit binary number (a string of seven 0s or 1s). 128 possible characters are defined. For example, the ASCII code for uppercase C is 67 and for lowercase c is 99. Most computers use ASCII codes to represent text, which makes it possible to transfer data from one computer to another.

See also: Short for American Standard Code for Information Interchange.

The powdery residue that remains after something is burnt. → ashen light.

Etymology (EN): M.E. a(i)sshe; O.E. asce, æsce; cf. Frisian esk, Dutch asch, O.N., O.H.G. aska, Ger. Asche; akin to Gk. azein “to dry up, parch,” L. arere “be dry,” → arid, Skt. asa- “ashes,” PIE root *as- “to burn, glow.”

Etymology (PE): Xâkestar, ultimately from Proto-Iranian *aika-âtar- literally “fire dust,” from *aika- (Pers. xâk “dust, earth,” → soil)

• *ātar- (Pers. âzar, âtaš, → fire); cf. Malâyeri xâkesar, Qasrâni xâksar.

The powdery residue that remains after something is burnt. → ashen light.

Etymology (EN): M.E. a(i)sshe; O.E. asce, æsce; cf. Frisian esk, Dutch asch, O.N., O.H.G. aska, Ger. Asche; akin to Gk. azein “to dry up, parch,” L. arere “be dry,” → arid, Skt. asa- “ashes,” PIE root *as- “to burn, glow.”

Etymology (PE): Xâkestar, ultimately from Proto-Iranian *aika-âtar- literally “fire dust,” from *aika- (Pers. xâk “dust, earth,” → soil)

• *ātar- (Pers. âzar, âtaš, → fire); cf. Malâyeri xâkesar, Qasrâni xâksar.

The faint glow occasionally observed on the unlit area of Venus in its crescent phase. Its cause is not known with certainty, but it might result from bombardment of atmospheric atoms and molecules by energetic particles and radiation, as with terrestrial airglow.

See also: → ash; → light.

The faint glow occasionally observed on the unlit area of Venus in its crescent phase. Its cause is not known with certainty, but it might result from bombardment of atmospheric atoms and molecules by energetic particles and radiation, as with terrestrial airglow.

See also: → ash; → light.

The largest of the world’s continents (about 41,440,000 sq. km.), bounded by Europe and the Arctic, Pacific, and Indian oceans.

See also: From L. Asia, from Gk. Asia, apparently first used by Herodotus (about 440 BC) in reference to Anatolia or to the Persian Empire, in contrast to Greece and Egypt. Of uncertain origin.

The largest of the world’s continents (about 41,440,000 sq. km.), bounded by Europe and the Arctic, Pacific, and Indian oceans.

See also: From L. Asia, from Gk. Asia, apparently first used by Herodotus (about 440 BC) in reference to Anatolia or to the Persian Empire, in contrast to Greece and Egypt. Of uncertain origin.

The apparent position of a body in the Solar System relative to the Sun, as seen from Earth. The main aspects are conjunction, greatest elongation, opposition, and quadrature.

Etymology (EN): Aspect from L. aspectus “looking, view, appearance,”
p.p. of aspicere “to look at,” from ad- “to” + specere “to look,” cognate with Gk. skeptesthai “to examine, consider,”
Av. spas- “to watch, look,” Skt. spaz “to see, behold, look at;” PIE *spek- “to observe, look”.

Etymology (PE): Nemud from nemudan “to show, demonstrate, exhibit, appear”

The apparent position of a body in the Solar System relative to the Sun, as seen from Earth. The main aspects are conjunction, greatest elongation, opposition, and quadrature.

Etymology (EN): Aspect from L. aspectus “looking, view, appearance,”
p.p. of aspicere “to look at,” from ad- “to” + specere “to look,” cognate with Gk. skeptesthai “to examine, consider,”
Av. spas- “to watch, look,” Skt. spaz “to see, behold, look at;” PIE *spek- “to observe, look”.

Etymology (PE): Nemud from nemudan “to show, demonstrate, exhibit, appear”

A series of experiments carried out in the early 1980s by Alain Aspect and his colleagues that showed the violation of
→ Bell’s inequality. Accordingly, quantum phenomena cannot be described by the → hidden variable theories, contrarily to the → EPR paradox interpretation.

See also: Alain Aspect (1947-); → experiment. Aspect et al., 1982, Physical Review Letters, Vol. 49, No. 25 and references therein.

A series of experiments carried out in the early 1980s by Alain Aspect and his colleagues that showed the violation of
→ Bell’s inequality. Accordingly, quantum phenomena cannot be described by the → hidden variable theories, contrarily to the → EPR paradox interpretation.

See also: Alain Aspect (1947-); → experiment. Aspect et al., 1982, Physical Review Letters, Vol. 49, No. 25 and references therein.

1. Roughness or unevenness of surface.
   

2. Physics: A microscopic projection on a metal surface resulting from normal surface finishing processes.
   

3. Geology: A site on a → fault surface where the two sides are held together by an area of higher strength than the areas surrounding it. When the stress on the fault exceeds the strength of the asperity, the asperity fails as an → earthquake.

Etymology (EN): M.E. asperite, from O.Fr. asperité “difficulty, painful situation,” from L. asperitas,
from asper “rough, harsh,” of unknown origin.

Etymology (PE): From zaft “thick, gross, rude,” (Steingass, Dehxodâ), + noun suffix -i.

1. Roughness or unevenness of surface.
   

2. Physics: A microscopic projection on a metal surface resulting from normal surface finishing processes.
   

3. Geology: A site on a → fault surface where the two sides are held together by an area of higher strength than the areas surrounding it. When the stress on the fault exceeds the strength of the asperity, the asperity fails as an → earthquake.

Etymology (EN): M.E. asperite, from O.Fr. asperité “difficulty, painful situation,” from L. asperitas,
from asper “rough, harsh,” of unknown origin.

Etymology (PE): From zaft “thick, gross, rude,” (Steingass, Dehxodâ), + noun suffix -i.

A company of persons gathered for a common reason, as for deliberation,
legislation, worship, or entertainment.
The act of assembling; the state of being assembled.

Etymology (EN): M.E. assemblee, from M.Fr., from O.Fr., from assembler “to gather together.”

Etymology (PE): Hamâyeš, from ham- “together,” → com- + âyeš “coming,” from ây- present stem of
âmadan “to come;” O.Pers. aitiy “goes;” Av. ay- “to go, to come,” aēiti “goes;” cf. Skt. e- “to come near,” eti “arrival;” Gk. eimi “I go,” L. ire “to go, walk,” eo “I go;” Tokharian AB i-; PIE *ei- “to go, to walk.”

A company of persons gathered for a common reason, as for deliberation,
legislation, worship, or entertainment.
The act of assembling; the state of being assembled.

Etymology (EN): M.E. assemblee, from M.Fr., from O.Fr., from assembler “to gather together.”

Etymology (PE): Hamâyeš, from ham- “together,” → com- + âyeš “coming,” from ây- present stem of
âmadan “to come;” O.Pers. aitiy “goes;” Av. ay- “to go, to come,” aēiti “goes;” cf. Skt. e- “to come near,” eti “arrival;” Gk. eimi “I go,” L. ire “to go, walk,” eo “I go;” Tokharian AB i-; PIE *ei- “to go, to walk.”

To state with assurance, confidence, or force; state strongly or positively; affirm.

Etymology (EN): From L. assertus, p.p. of asserere/adserere “to claim, lay claim to, declare,” from → ad- “to” + serere “to join together, range in a row,” from PIE root *ser- (2) “to line up.”

Etymology (PE): Sotvârdan from sotvâr, ostovâr “firm, reliable,” Mid.Pers. awestwâr, ôst “firm, reliable,” O.Pers./Av. sta- “to stand; set,” (Av. hištaiti, cf. Skt. sthâ- “to stand,” Gk. histemi “put, place, weigh,” stasis “a standing still,” L. stare “to stand”); PIE base *sta- “to stand.”

To state with assurance, confidence, or force; state strongly or positively; affirm.

Etymology (EN): From L. assertus, p.p. of asserere/adserere “to claim, lay claim to, declare,” from → ad- “to” + serere “to join together, range in a row,” from PIE root *ser- (2) “to line up.”

Etymology (PE): Sotvârdan from sotvâr, ostovâr “firm, reliable,” Mid.Pers. awestwâr, ôst “firm, reliable,” O.Pers./Av. sta- “to stand; set,” (Av. hištaiti, cf. Skt. sthâ- “to stand,” Gk. histemi “put, place, weigh,” stasis “a standing still,” L. stare “to stand”); PIE base *sta- “to stand.”

The act of asserting or something that is asserted.

See also: → assert; → -tion.

The act of asserting or something that is asserted.

See also: → assert; → -tion.

1. To give or allocate; allot.
   

2. To give out or announce as a task.
   

3. To designate; name; specify (Dictionary.com).

Etymology (EN): M.E. assignen, from O.Fr. assiginer “assign; appoint legally; allot,” from L. assignare “to mark out, to allot by sign,” from → ad- “to” + signare “to make a sign,” from signum “mark,” → sign

Etymology (PE): Nešârdan, from neš, → sign, + nuance suffix -âr.

1. To give or allocate; allot.
   

2. To give out or announce as a task.
   

3. To designate; name; specify (Dictionary.com).

Etymology (EN): M.E. assignen, from O.Fr. assiginer “assign; appoint legally; allot,” from L. assignare “to mark out, to allot by sign,” from → ad- “to” + signare “to make a sign,” from signum “mark,” → sign

Etymology (PE): Nešârdan, from neš, → sign, + nuance suffix -âr.

1. An appointment for a meeting.
   

2. The act of assigning; → assignment (Dictionary.com).

See also: Verbal noun of → assign.

1. An appointment for a meeting.
   

2. The act of assigning; → assignment (Dictionary.com).

See also: Verbal noun of → assign.

1. An act of assigning; appointment.
   

2. Something assigned, as a particular task or duty.
   

3. A position of responsibility, post of duty, or the like, to which one is appointed (Dictionary.com).

See also: Verbal noun of → assign.

1. An act of assigning; appointment.
   

2. Something assigned, as a particular task or duty.
   

3. A position of responsibility, post of duty, or the like, to which one is appointed (Dictionary.com).

See also: Verbal noun of → assign.

1. (v.tr.) To join or connect together; to bring together or into relationship in any of various intangible ways (as in memory or imagination). → dissociate.
   

2. (v.intr.) To come or be together as partners, friends, or join with other parts.
   

3. A person joined with another or others in business or at work; something that usually accompanies another thing.

Etymology (EN): M.E. associat “associated,” from L. associatus, p.p. of associare “to unite,” from ad- + sociare “to join,” from socius “ally, companion” (“follower”); PIE base *sek^w- “to follow.”

Etymology (PE): Âhazidan, from â- prefix + hazidan “to associate, accompany, follow,” from Av. hac-, hax- “to associate, follow, accompany” (hacenay- “getting together, association,” haxay-, hašy-, haš- “friend”), hacaiti “follows;”
hacā “from, out of;” O.Pers. hacā “from” (Mid.Pers. hâz- “to lead, guide,”
hac “from;” Mod.Pers. az “from”);
PIE base *sek^w- “to follow;”
cf. Skt. sac- “to be associated or united with,” sácate “accompanies, follows,” sácā “with;” Gk. hepesthai “to follow;” L. sequi “to follow.”
Âhazgâr, from âhaz- present stem of âhazidan + agent noun suffix -gâr, → -or, as in âmuzgâr, parhizgâr, sâzgâr, âmizgâr, sepuzgâr.

1. (v.tr.) To join or connect together; to bring together or into relationship in any of various intangible ways (as in memory or imagination). → dissociate.
   

2. (v.intr.) To come or be together as partners, friends, or join with other parts.
   

3. A person joined with another or others in business or at work; something that usually accompanies another thing.

Etymology (EN): M.E. associat “associated,” from L. associatus, p.p. of associare “to unite,” from ad- + sociare “to join,” from socius “ally, companion” (“follower”); PIE base *sek^w- “to follow.”

Etymology (PE): Âhazidan, from â- prefix + hazidan “to associate, accompany, follow,” from Av. hac-, hax- “to associate, follow, accompany” (hacenay- “getting together, association,” haxay-, hašy-, haš- “friend”), hacaiti “follows;”
hacā “from, out of;” O.Pers. hacā “from” (Mid.Pers. hâz- “to lead, guide,”
hac “from;” Mod.Pers. az “from”);
PIE base *sek^w- “to follow;”
cf. Skt. sac- “to be associated or united with,” sácate “accompanies, follows,” sácā “with;” Gk. hepesthai “to follow;” L. sequi “to follow.”
Âhazgâr, from âhaz- present stem of âhazidan + agent noun suffix -gâr, → -or, as in âmuzgâr, parhizgâr, sâzgâr, âmizgâr, sepuzgâr.

A → molecular cloud that is physically or apparently related to a star formation region.

See also: → associate; → molecular; → cloud.

A → molecular cloud that is physically or apparently related to a star formation region.

See also: → associate; → molecular; → cloud.

1. The act of associating, → associate; the state of being associated.
   

2. A group of → astronomical objects physically or apparently gathered in a particular area of sky, for example an → stellar association.
   

3. An organization of persons having a common interest. → dissociation.

Etymology (EN): Association, noun from → associate.

Etymology (PE): Âhazeš, verbal noun of → âhazidan→ associate. Anjoman, for the meaning 3, from Mid.Pers. anjaman, from Av. han-jamana, from han- “together” + jamana, from gam- “to come;” cf. Skt. samgamana “gathering together.”

1. The act of associating, → associate; the state of being associated.
   

2. A group of → astronomical objects physically or apparently gathered in a particular area of sky, for example an → stellar association.
   

3. An organization of persons having a common interest. → dissociation.

Etymology (EN): Association, noun from → associate.

Etymology (PE): Âhazeš, verbal noun of → âhazidan→ associate. Anjoman, for the meaning 3, from Mid.Pers. anjaman, from Av. han-jamana, from han- “together” + jamana, from gam- “to come;” cf. Skt. samgamana “gathering together.”

A celestial body making part of an astronomical association.

See also: → association; → member.

A celestial body making part of an astronomical association.

See also: → association; → member.

1. Of, characterized by, resulting from, or causing association.
   
2. In a mathematical operation, being independent of the sequence in which the elements are grouped. For ex., if a + (b + c) = (a + b) + c, the operation indicated by + (addition) is associative.

Etymology (EN): Adj. from associate.

1. Of, characterized by, resulting from, or causing association.
   
2. In a mathematical operation, being independent of the sequence in which the elements are grouped. For ex., if a + (b + c) = (a + b) + c, the operation indicated by + (addition) is associative.

Etymology (EN): Adj. from associate.

An algebra whose multiplication is associative.

See also: → associative; → algebra.

An algebra whose multiplication is associative.

See also: → associative; → algebra.

A basic rule in → group theory stating that if a, b and c are members of a group then (a * b) * c and a * (b * c) are members of the group.

See also: → associative; → axiom.

A basic rule in → group theory stating that if a, b and c are members of a group then (a * b) * c and a * (b * c) are members of the group.

See also: → associative; → axiom.

In mathematics, the rule that states that the result of two identical operations is independent of the sequence of these operations. For ex., in the addition operation, a + (b + c) = (a + b) + c = a + b + c. Multiplication of numbers is also associative.

See also: → associative; → law.

In mathematics, the rule that states that the result of two identical operations is independent of the sequence of these operations. For ex., in the addition operation, a + (b + c) = (a + b) + c = a + b + c. Multiplication of numbers is also associative.

See also: → associative; → law.

Of or relating to association; state of being associative.

See also: → associative + → -ity.

Of or relating to association; state of being associative.

See also: → associative + → -ity.

To take as granted or true; suppose.

Etymology (EN): M.E., from L. assumere “to take up,” from ad- “to, up” + sumere “to take,” from sub “under” + emere “to take.”

Etymology (PE): Âgarbidan, from âgarb, → assumption.

To take as granted or true; suppose.

Etymology (EN): M.E., from L. assumere “to take up,” from ad- “to, up” + sumere “to take,” from sub “under” + emere “to take.”

Etymology (PE): Âgarbidan, from âgarb, → assumption.

A fact or statement (as a proposition, axiom, postulate, or notion) taken for granted.

Etymology (EN): M.E., from L.L. assumption, assumptio “taking up,” from L. assumere, → assume.

Etymology (PE): Âgarb, from â-, nuance prefix, + garb, from Av./O.Pers. grab-, Av. gərəb- “to take, to seize;” cf. Mod.Pers. gereftan “to take; to assume;” Skt. grah-, grabh- “to seize, to take,” graha “seizing, holding, perceiving;” M.L.G. grabben “to grab;” E. grab “to take or grasp suddenly;” PIE *ghrebh- “to seize.”

Ar. farz “assumption, hypothesis.”

A fact or statement (as a proposition, axiom, postulate, or notion) taken for granted.

Etymology (EN): M.E., from L.L. assumption, assumptio “taking up,” from L. assumere, → assume.

Etymology (PE): Âgarb, from â-, nuance prefix, + garb, from Av./O.Pers. grab-, Av. gərəb- “to take, to seize;” cf. Mod.Pers. gereftan “to take; to assume;” Skt. grah-, grabh- “to seize, to take,” graha “seizing, holding, perceiving;” M.L.G. grabben “to grab;” E. grab “to take or grasp suddenly;” PIE *ghrebh- “to seize.”

Ar. farz “assumption, hypothesis.”

A positive declaration intended to give confidence; promise or pledge; guaranty; surety (Dictionary.com).

See also: → assure; → -ance.

A positive declaration intended to give confidence; promise or pledge; guaranty; surety (Dictionary.com).

See also: → assure; → -ance.

1. To declare earnestly to; inform or tell positively; state with confidence to.
   

2. To give confidence to; encourage.

Etymology (EN): M.E. as(e)uren, assuren, from O.Fr. aseurer, from L.L. assecurare, from as-, variant of → ad-, + secur-, → secure, + -a- thematic vowel, + -re infinitive suffix.

Etymology (PE): Âtenzidan, from â- intensive prefix, + tenz, → sure, + -idan infinitive suffix, → -ize.

1. To declare earnestly to; inform or tell positively; state with confidence to.
   

2. To give confidence to; encourage.

Etymology (EN): M.E. as(e)uren, assuren, from O.Fr. aseurer, from L.L. assecurare, from as-, variant of → ad-, + secur-, → secure, + -a- thematic vowel, + -re infinitive suffix.

Etymology (PE): Âtenzidan, from â- intensive prefix, + tenz, → sure, + -idan infinitive suffix, → -ize.

A small starlike symbol (*), used in printing or writing as a reference mark, as an indication of the omission of letters or words, to denote a hypothetical linguistic form, or for various arbitrary meanings.

Etymology (EN): M.E. astarisc, from L.L. asteriscus, from Gk. asteriskos “small star,” from aster-, → astro- + -ikos “diminutive suffix.”

Etymology (PE): Axtarak, from axtar “star” → astro- + -ak “diminutive suffix.”

A small starlike symbol (*), used in printing or writing as a reference mark, as an indication of the omission of letters or words, to denote a hypothetical linguistic form, or for various arbitrary meanings.

Etymology (EN): M.E. astarisc, from L.L. asteriscus, from Gk. asteriskos “small star,” from aster-, → astro- + -ikos “diminutive suffix.”

Etymology (PE): Axtarak, from axtar “star” → astro- + -ak “diminutive suffix.”

A group of stars in the sky which are traditionally imagined to present a pattern within a → constellation. Examples include the → Big Dipper, the → Northern Cross, the → Square of Pegasus, and → Orion’s Belt.

Etymology (EN): Gk. asterismos “a marking with stars, constellation,” from aster, → astro- + → -ism.

Etymology (PE): Axtargân, from axtar “star” → astro- + -gân suffix denoting collective nature.

A group of stars in the sky which are traditionally imagined to present a pattern within a → constellation. Examples include the → Big Dipper, the → Northern Cross, the → Square of Pegasus, and → Orion’s Belt.

Etymology (EN): Gk. asterismos “a marking with stars, constellation,” from aster, → astro- + → -ism.

Etymology (PE): Axtargân, from axtar “star” → astro- + -gân suffix denoting collective nature.

1. A small rocky object orbiting the Sun. There are millions of asteroids moving in orbits in the main → asteroid belt, between → Mars and → Jupiter and in the → Kuiper belt. The largest and the first discovered, → Ceres, about 1,000 km in size, is now classified as → dwarf planet (2006 IAU General Assembly). The largest asteroid in the solar system (Ceres apart), is → Pallas, with a size of 582 × 556 × 500 km. On the other hand, the smallest asteroid ever studied is the 2 meters space rock 2015 TC25, which was observed when it made a close flyby of Earth in October 2015. See also → near-Earth asteroid; → binary asteroid.
   

2. Math.: A → hypocycloid with four → cusps in which the → radius of the rolling → circle is a → quarter of the radius of the fixed circle. It has the → parametric equations x = a cos³θ, y = a sin³θ, where a is the radius of the fixed circle.

Etymology (EN): Gk. asteroeides “star-like,” from aster, → astro- + → -oid “like, resembling.”

Etymology (PE): Sayyârak “small planet,” from sayyâré, → planet, + -ak “diminutive suffix.”
Axtarvâr, from axtar, → astro-, + -vâr, → -oid.

1. A small rocky object orbiting the Sun. There are millions of asteroids moving in orbits in the main → asteroid belt, between → Mars and → Jupiter and in the → Kuiper belt. The largest and the first discovered, → Ceres, about 1,000 km in size, is now classified as → dwarf planet (2006 IAU General Assembly). The largest asteroid in the solar system (Ceres apart), is → Pallas, with a size of 582 × 556 × 500 km. On the other hand, the smallest asteroid ever studied is the 2 meters space rock 2015 TC25, which was observed when it made a close flyby of Earth in October 2015. See also → near-Earth asteroid; → binary asteroid.
   

2. Math.: A → hypocycloid with four → cusps in which the → radius of the rolling → circle is a → quarter of the radius of the fixed circle. It has the → parametric equations x = a cos³θ, y = a sin³θ, where a is the radius of the fixed circle.

Etymology (EN): Gk. asteroeides “star-like,” from aster, → astro- + → -oid “like, resembling.”

Etymology (PE): Sayyârak “small planet,” from sayyâré, → planet, + -ak “diminutive suffix.”
Axtarvâr, from axtar, → astro-, + -vâr, → -oid.

The region of the → solar system located between → Mars and → Jupiter
where over a million objects bigger than 1 km across orbit the Sun. Another region populated by minor bodies lies beyond the orbit of → Neptune, the → Kuiper belt.

See also: → asteroid; → belt.

The region of the → solar system located between → Mars and → Jupiter
where over a million objects bigger than 1 km across orbit the Sun. Another region populated by minor bodies lies beyond the orbit of → Neptune, the → Kuiper belt.

See also: → asteroid; → belt.

1. For an asteroid whose orbit is precisely known, a number and optionally a proper name, e.g. (7) Iris, (24101) Cassini, (99942) Apophis.
   

2. For an asteroid whose orbit is not known, a provisional designation composed of four elements: number.letter.letter.(optionally)number. The first number indicates the year of discovery. The first letter denotes the half-month of the discovery (A: first half of January, Y: second half of December; the letter “I” is excluded). The second letter and the following number indicate the order of discovery within the half-month. For example, the first asteroid discovered in the first half of May 1960 is: 1960 JA. Since more than 25 objects (without “I”) might be detected within a half-month, the number following the second letter indicates the number of 25 discoveries. Hence, 2001 SD3 was discovered in the second half of September 2001 and was the (D =) 4 + (25 x 3) or the 79th object found during that period.

See also: → asteroid; → designation.

1. For an asteroid whose orbit is precisely known, a number and optionally a proper name, e.g. (7) Iris, (24101) Cassini, (99942) Apophis.
   

2. For an asteroid whose orbit is not known, a provisional designation composed of four elements: number.letter.letter.(optionally)number. The first number indicates the year of discovery. The first letter denotes the half-month of the discovery (A: first half of January, Y: second half of December; the letter “I” is excluded). The second letter and the following number indicate the order of discovery within the half-month. For example, the first asteroid discovered in the first half of May 1960 is: 1960 JA. Since more than 25 objects (without “I”) might be detected within a half-month, the number following the second letter indicates the number of 25 discoveries. Hence, 2001 SD3 was discovered in the second half of September 2001 and was the (D =) 4 + (25 x 3) or the 79th object found during that period.

See also: → asteroid; → designation.

A group of asteroids that share the same or similar proper orbital elements (semi-major axis, eccentricity, inclination).

In 1918, the Japanese astronomer K. Hirayama first recognized some non random concentrations of asteroid elements. He noticed that certain “groups” of asteroids had similar orbital elements, and hence he first introduced the concept of “asteroid families,” and identified three of them: Koronos, Eos, and Themis. The names of these groups were chosen by the parent (brightest) asteroid that the smaller group asteroids follow.

Some of the more common asteroid families include the Trojans, which are actually not an asteroid family, but a group of asteroids caught in the Sun-Jupiter gravitational equilibrium points known as L3 and L4 → Lagrangian points.

See also: → asteroid; → family.

A group of asteroids that share the same or similar proper orbital elements (semi-major axis, eccentricity, inclination).

In 1918, the Japanese astronomer K. Hirayama first recognized some non random concentrations of asteroid elements. He noticed that certain “groups” of asteroids had similar orbital elements, and hence he first introduced the concept of “asteroid families,” and identified three of them: Koronos, Eos, and Themis. The names of these groups were chosen by the parent (brightest) asteroid that the smaller group asteroids follow.

Some of the more common asteroid families include the Trojans, which are actually not an asteroid family, but a group of asteroids caught in the Sun-Jupiter gravitational equilibrium points known as L3 and L4 → Lagrangian points.

See also: → asteroid; → family.

Systematic observation of the sky in particular searching for → asteroids that may have a close approach to the Earth. → near-Earth object.

Etymology (EN): → asteroid; → survey.

Etymology (PE): Bardid, → survey; sayyârakhâ plural of sayyârak, → asteroid.

Systematic observation of the sky in particular searching for → asteroids that may have a close approach to the Earth. → near-Earth object.

Etymology (EN): → asteroid; → survey.

Etymology (PE): Bardid, → survey; sayyârakhâ plural of sayyârak, → asteroid.

Of or relating to → asteroseismology.

See also: → spectropolarimetry; → -ic.

Of or relating to → asteroseismology.

See also: → spectropolarimetry; → -ic.

The study of the → internal structure of stars through the interpretation of their pulsation periods (→ stellar pulsation). The radial pulsations are the result of → sound waves resonating in the stars interior. Different → pulsation modes penetrate to different depths inside a star. If a large number of pulsation modes occurs, then the stellar interior, which is not directly observable, can be probed from oscillation studies because the modes penetrate to various depths inside the star. Using a complex
mathematical analysis, very detailed investigations of the structure of the star’s interior can be carried out. Applied to the Sun, it is called → helioseismology.

Etymology (EN): From → astero- “star,” from aster-,
→ astro-, + → seismology.

Etymology (PE): Axtarlarzešenâsi, from axtar “star,” → astro-, + larzešenâsi,
→ seismology.

The study of the → internal structure of stars through the interpretation of their pulsation periods (→ stellar pulsation). The radial pulsations are the result of → sound waves resonating in the stars interior. Different → pulsation modes penetrate to different depths inside a star. If a large number of pulsation modes occurs, then the stellar interior, which is not directly observable, can be probed from oscillation studies because the modes penetrate to various depths inside the star. Using a complex
mathematical analysis, very detailed investigations of the structure of the star’s interior can be carried out. Applied to the Sun, it is called → helioseismology.

Etymology (EN): From → astero- “star,” from aster-,
→ astro-, + → seismology.

Etymology (PE): Axtarlarzešenâsi, from axtar “star,” → astro-, + larzešenâsi,
→ seismology.

A layer of soft, partly molten, rock in the → Earth’s mantle, located at a depth of 100 to 250 km, over which the more rigid plates of the → lithosphere are in motion.

Etymology (EN): Asthenosphere, from Gk. asthenes “weak” + → sphere.

Etymology (PE): Sostsepehr, from sost “weak, tender” + sepehr, → sphere.

A layer of soft, partly molten, rock in the → Earth’s mantle, located at a depth of 100 to 250 km, over which the more rigid plates of the → lithosphere are in motion.

Etymology (EN): Asthenosphere, from Gk. asthenes “weak” + → sphere.

Etymology (PE): Sostsepehr, from sost “weak, tender” + sepehr, → sphere.

The optical system which is affected by → astigmatism.

See also: → astigmatism.

The optical system which is affected by → astigmatism.

See also: → astigmatism.

1. An imperfection in an optical system whereby light from a point source is formed into an image as a straight line, ellipse, or circle. The rays of light in two perpendicular planes appear as two lines at right angles.
   
2. A common eye defect in which the unequal curvature of one or more refractive surfaces of the eye, usually the cornea, prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.

Etymology (EN): From astigmatic, from Gk. → a- “without” + stigmatos, from stigma “a mark, spot, puncture.”

Etymology (PE): 1) Nâgerâvari, from nâ- “without, un” + gerâ, stem of gerâyidan “to converge,” + -var, agent forming suffix, + -i, noun forming suffix.

2) The same as above with -bini “seeing, discerning”.

1. An imperfection in an optical system whereby light from a point source is formed into an image as a straight line, ellipse, or circle. The rays of light in two perpendicular planes appear as two lines at right angles.
   
2. A common eye defect in which the unequal curvature of one or more refractive surfaces of the eye, usually the cornea, prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision.

Etymology (EN): From astigmatic, from Gk. → a- “without” + stigmatos, from stigma “a mark, spot, puncture.”

Etymology (PE): 1) Nâgerâvari, from nâ- “without, un” + gerâ, stem of gerâyidan “to converge,” + -var, agent forming suffix, + -i, noun forming suffix.

2) The same as above with -bini “seeing, discerning”.

Verbal form of → astration.

See also: → astration.

Verbal form of → astration.

See also: → astration.

The cyclic process in which interstellar matter is incorporated into newly formed stars, where it undergoes nuclear processing, is thus enriched with heavier elements, and then returns into the interstellar medium through supernova explosion or stellar winds to be used in the formation of a newer generation of stars.

Etymology (EN): Astration, from astrate, from astr-, → astro-,

• noun-forming suffix -ation.

Etymology (PE): Setâreš, from setâridan (from setâré “star”

• verb-making suffix -idan) + noun-builder -eš.

The cyclic process in which interstellar matter is incorporated into newly formed stars, where it undergoes nuclear processing, is thus enriched with heavier elements, and then returns into the interstellar medium through supernova explosion or stellar winds to be used in the formation of a newer generation of stars.

Etymology (EN): Astration, from astrate, from astr-, → astro-,

• noun-forming suffix -ation.

Etymology (PE): Setâreš, from setâridan (from setâré “star”

• verb-making suffix -idan) + noun-builder -eš.

A combining form with the meaning “pertaining to stars or celestial bodies” used in the formation of compound words. Variants aster-, and astr- before a vowel. → star.

Etymology (EN): Gk. astron “star,” akin to L. stella (Fr. étoile, from O.Fr. esteile, from V.L. *stela), Skt. str-, tara-, Av. star-, Mid.Pers. star, stârag, Mod.Pers. setâré, axtar, see below; cf. O.E. steorra, E. star,
Du. ster, O.H.G. sterro, Ger. Stern, PIE *ster- “star.”

Etymology (PE): Mod.Pers. axtar, → star, from Mid.Pers. axtar. The variants star-, estâr, estâré, setâré are obvious. Note also the following dialectal forms: (Lori, Laki) âsâra, (Tabari) essâra, (Laki) hasâra, (Shughni)
xiterj, xtarag. The form axtar is less straightforward, leading
some philologists to suggest different origins for setâré and axtar. According to W. Eilers (Iranica), axtar is a back-fomation from Mid.Pers. apâxtar “planet; north” produced by artificial dropping of the first component. Apart from phonological difficulties inherent in this suggestion, one must also explain how axtar meaning “planet” became a general designation for star, as for example in Mid.Pers. axtarmâr “astronomer,” despite the relatively infinitesimal number of planets known in ancient times.

W. Eilers’ suggestion is pure theoretical construction; no factual evidence support it. On the other hand, in Pahlavi texts, e.g. Bundahishn, axtar is extensively used for “star, planet, and the signs of zodiac.”

We suggest that both words star and axtar are etymologically related. This idea is based on
the fact that “s” and “x” phonemes interchange in Persian and other IE languages. For example, the PIE *swesor “sister” has evolved into Av. xvanhar-, Mid.Pers./Mod.Pers. xâhar (Skt. svasar-, L. soror, Fr. soeur, Gk. eor “daughter, cousin, relative,” Arm. k’oyr, O.H.G. swester, Ger. Schwester, Du. zuster, E. sister). Similarly, *saewel- “sun” has become Av. hvar- “sun,” Mid.Pers. xavr, Mod.Pers. xor, while
keeping its Av. h in Mod.Pers. hur “sun”
(cf. Skt. svar-, surya-,
Gk. helios, L. sol, Goth. sauil, Lith. saule). Finally, *su- “hog, pig, swine” also has changed its “s” into both “h” and “x” in Av. and Mod.Pers. hu- and xuk respectively (Skt. sukara- “boar, hog, pig,” Gk. hys, L. sus, Welsh hucc, Ger. Schwein, E. swine).

A combining form with the meaning “pertaining to stars or celestial bodies” used in the formation of compound words. Variants aster-, and astr- before a vowel. → star.

Etymology (EN): Gk. astron “star,” akin to L. stella (Fr. étoile, from O.Fr. esteile, from V.L. *stela), Skt. str-, tara-, Av. star-, Mid.Pers. star, stârag, Mod.Pers. setâré, axtar, see below; cf. O.E. steorra, E. star,
Du. ster, O.H.G. sterro, Ger. Stern, PIE *ster- “star.”

Etymology (PE): Mod.Pers. axtar, → star, from Mid.Pers. axtar. The variants star-, estâr, estâré, setâré are obvious. Note also the following dialectal forms: (Lori, Laki) âsâra, (Tabari) essâra, (Laki) hasâra, (Shughni)
xiterj, xtarag. The form axtar is less straightforward, leading
some philologists to suggest different origins for setâré and axtar. According to W. Eilers (Iranica), axtar is a back-fomation from Mid.Pers. apâxtar “planet; north” produced by artificial dropping of the first component. Apart from phonological difficulties inherent in this suggestion, one must also explain how axtar meaning “planet” became a general designation for star, as for example in Mid.Pers. axtarmâr “astronomer,” despite the relatively infinitesimal number of planets known in ancient times.

W. Eilers’ suggestion is pure theoretical construction; no factual evidence support it. On the other hand, in Pahlavi texts, e.g. Bundahishn, axtar is extensively used for “star, planet, and the signs of zodiac.”

We suggest that both words star and axtar are etymologically related. This idea is based on
the fact that “s” and “x” phonemes interchange in Persian and other IE languages. For example, the PIE *swesor “sister” has evolved into Av. xvanhar-, Mid.Pers./Mod.Pers. xâhar (Skt. svasar-, L. soror, Fr. soeur, Gk. eor “daughter, cousin, relative,” Arm. k’oyr, O.H.G. swester, Ger. Schwester, Du. zuster, E. sister). Similarly, *saewel- “sun” has become Av. hvar- “sun,” Mid.Pers. xavr, Mod.Pers. xor, while
keeping its Av. h in Mod.Pers. hur “sun”
(cf. Skt. svar-, surya-,
Gk. helios, L. sol, Goth. sauil, Lith. saule). Finally, *su- “hog, pig, swine” also has changed its “s” into both “h” and “x” in Av. and Mod.Pers. hu- and xuk respectively (Skt. sukara- “boar, hog, pig,” Gk. hys, L. sus, Welsh hucc, Ger. Schwein, E. swine).

Same as → archaeoastronomy, megalithic astronomy.

Same as → archaeoastronomy, megalithic astronomy.

The study of life throughout the Universe, also known as exobiology.

Etymology (EN): Astrobiology, from Gk. → astro- “star” + bio “life” + -logy “science, study.”

Etymology (PE): Axtarzistšenâsi, from axtar, → star, → astro- + zistšenâsi, → biology.

The study of life throughout the Universe, also known as exobiology.

Etymology (EN): Astrobiology, from Gk. → astro- “star” + bio “life” + -logy “science, study.”

Etymology (PE): Axtarzistšenâsi, from axtar, → star, → astro- + zistšenâsi, → biology.

A geological structure on the Earth’s surface from an ancient meteorite impact.

Etymology (EN): Astrobleme, from → astro- + Gk. blema “scar, wound, missile,” from ballein “to throw;” PIE *gwele- “to throw”.

Etymology (PE): Axtarxasts, from axtar “star,” → astro-

• xast “wounded; scratched,” from xastan “to wound, wound by scratching,” Mid.Pers. xst, xs “to injure,” Av. vixad- “to crush,” Proto-Iranian *xad- “to wound, hurt,” Skt. khad- “to hurt.”

A geological structure on the Earth’s surface from an ancient meteorite impact.

Etymology (EN): Astrobleme, from → astro- + Gk. blema “scar, wound, missile,” from ballein “to throw;” PIE *gwele- “to throw”.

Etymology (PE): Axtarxasts, from axtar “star,” → astro-

• xast “wounded; scratched,” from xastan “to wound, wound by scratching,” Mid.Pers. xst, xs “to injure,” Av. vixad- “to crush,” Proto-Iranian *xad- “to wound, hurt,” Skt. khad- “to hurt.”

The study of the chemical interactions between the gas and dust of the interstellar medium.

Etymology (EN): Astrochemistry, from → astro- “star” + → chemistry.

Etymology (PE): Axtaršimi, from axtar, → astro-, + šimi, → chemistry.

The study of the chemical interactions between the gas and dust of the interstellar medium.

Etymology (EN): Astrochemistry, from → astro- “star” + → chemistry.

Etymology (PE): Axtaršimi, from axtar, → astro-, + šimi, → chemistry.

The science dealing with the motion of satellites, rockets, and spacecrafts. It uses the principles of celestial mechanics.

Etymology (EN): Astrodynamics, from → astro- “star” + → dynamics.

Etymology (PE): Axtartavânik, from axtar, → astro-,

• tavânik, → dynamics.

The science dealing with the motion of satellites, rockets, and spacecrafts. It uses the principles of celestial mechanics.

Etymology (EN): Astrodynamics, from → astro- “star” + → dynamics.

Etymology (PE): Axtartavânik, from axtar, → astro-,

• tavânik, → dynamics.

A science concerned with the geology of solid bodies in the Solar system, such as planets, satellites, asteroids, and meteorites.

Etymology (EN): Astrogeology, from Gk. → astro- “star” + → geology.

Etymology (PE): Axtarzaminšenâsi, from axtar, → star, → + zaminšenâsi, → geology.

A science concerned with the geology of solid bodies in the Solar system, such as planets, satellites, asteroids, and meteorites.

Etymology (EN): Astrogeology, from Gk. → astro- “star” + → geology.

Etymology (PE): Axtarzaminšenâsi, from axtar, → star, → + zaminšenâsi, → geology.

A photographic instrument with great light gathering power which is used to photograph a large field in a single exposure.

Etymology (EN): Astrograph, from → astro- + → graph.

Etymology (PE): Axtarnegâr, from axtar, → astro-,

• -negâr, → -graph.

A photographic instrument with great light gathering power which is used to photograph a large field in a single exposure.

Etymology (EN): Astrograph, from → astro- + → graph.

Etymology (PE): Axtarnegâr, from axtar, → astro-,

• -negâr, → -graph.

An ancient instrument
for solving problems relating to time and the position of the Sun and stars in the sky. It had many uses, including telling time during the day or night, finding the time of sunrise and sunset and, thus, the length of the day, and locating celestial objects in the sky. It was widely used until replaced by the
→ sextant. The → planispheric astrolabe, which is the most common type of the instrument, is typically made up of a graduated disk hanging vertically, which is rotated so that it can be directed to the star chosen. The local time can thus be read from the face of the astrolabe, and different tables, at various latitudes, can be used. More specifically, a planispheric astrolabe is made up of the following main components: → mater, → tympanum, → rete, → alidade, → throne, → limb, → pin, → horse, → front, and → back.
The astrolabe was invented by Greeks, and some historians have attributed it to Hipparchus (c190-c120 BC). Modern, sophisticated versions (such as → prismatic astrolabe and
→ Danjon astrolabe), are used for high precision measurements of star positions. See also → particular astrolabe, → spherical astrolabe, and → universal astrolabe.

Etymology (EN): M.E., from O.Fr. astrelabe, from M.L. astrolabium, from Gk. astrolabos (organon) “star taking (instrument),” from astron “star,” → astro- + lambanein “to take.”

Etymology (PE): Ostorlâb, from Ar. usturlab, from Gk. astrolabos, as above.

An ancient instrument
for solving problems relating to time and the position of the Sun and stars in the sky. It had many uses, including telling time during the day or night, finding the time of sunrise and sunset and, thus, the length of the day, and locating celestial objects in the sky. It was widely used until replaced by the
→ sextant. The → planispheric astrolabe, which is the most common type of the instrument, is typically made up of a graduated disk hanging vertically, which is rotated so that it can be directed to the star chosen. The local time can thus be read from the face of the astrolabe, and different tables, at various latitudes, can be used. More specifically, a planispheric astrolabe is made up of the following main components: → mater, → tympanum, → rete, → alidade, → throne, → limb, → pin, → horse, → front, and → back.
The astrolabe was invented by Greeks, and some historians have attributed it to Hipparchus (c190-c120 BC). Modern, sophisticated versions (such as → prismatic astrolabe and
→ Danjon astrolabe), are used for high precision measurements of star positions. See also → particular astrolabe, → spherical astrolabe, and → universal astrolabe.

Etymology (EN): M.E., from O.Fr. astrelabe, from M.L. astrolabium, from Gk. astrolabos (organon) “star taking (instrument),” from astron “star,” → astro- + lambanein “to take.”

Etymology (PE): Ostorlâb, from Ar. usturlab, from Gk. astrolabos, as above.

One who practices → astrology; one who professes to foretell events by the aspects and situation of the stars.

Etymology (EN): Agent noun from → astrology.

Etymology (PE): Axtargu, a classical term used by e.g. Jalâleddin Rumi (Mowlavi),
13th century poet, → astrology.

One who practices → astrology; one who professes to foretell events by the aspects and situation of the stars.

Etymology (EN): Agent noun from → astrology.

Etymology (PE): Axtargu, a classical term used by e.g. Jalâleddin Rumi (Mowlavi),
13th century poet, → astrology.

A → pseudoscience based on the belief that the apparent positions and → aspects of a small number of celestial bodies influence the course of human life and terrestrial events.
Although the Sun and Moon have a gravitational influence on Earth, there is no known force that can cause celestial bodies to affect human affairs in the way claimed by → astrologers. Generally speaking, astrology is baseless and incoherent. In the astrological belief the influence of celestial bodies does not depend upon their distance from Earth, but on their positions and
apparent angular separations. Outer planets can have a similar degree of influence as the inner planets. As a consequence, the billions of planets in our Galaxy and in billions of other galaxies should also influence us, and logically the effect of those planets must overwhelm any influence of the planets we see. Nevertheless astrologers do not care, and this fact makes astrological deductions absurd even in their scheme. Historically, the planets → Uranus, → Neptune, and → Pluto were not used in astrological predictions. They were added from the 18th century onward, after their discovery. Now that Pluto is disqualified as a planet, will astrologers remove it from their theories? If the answer is negative, they must logically include the numerous other similar → dwarf planets (such as → Charon, → Quaoar, → Sedna) residing in the → Kuiper belt. In brief, astrology is a superstition chiefly based on ignorance and man’s need for mental contentment.

Etymology (EN): → astro-; → -logy.

Etymology (PE): Axtarguyi, literally “star-telling,” from axtar “star,” → astro- + guyi verbal noun from goftan “to tell, speak, talk;” Mid.Pers. guftan “to say, tell, utter;” O.Pers. gaub- “to say.”

A → pseudoscience based on the belief that the apparent positions and → aspects of a small number of celestial bodies influence the course of human life and terrestrial events.
Although the Sun and Moon have a gravitational influence on Earth, there is no known force that can cause celestial bodies to affect human affairs in the way claimed by → astrologers. Generally speaking, astrology is baseless and incoherent. In the astrological belief the influence of celestial bodies does not depend upon their distance from Earth, but on their positions and
apparent angular separations. Outer planets can have a similar degree of influence as the inner planets. As a consequence, the billions of planets in our Galaxy and in billions of other galaxies should also influence us, and logically the effect of those planets must overwhelm any influence of the planets we see. Nevertheless astrologers do not care, and this fact makes astrological deductions absurd even in their scheme. Historically, the planets → Uranus, → Neptune, and → Pluto were not used in astrological predictions. They were added from the 18th century onward, after their discovery. Now that Pluto is disqualified as a planet, will astrologers remove it from their theories? If the answer is negative, they must logically include the numerous other similar → dwarf planets (such as → Charon, → Quaoar, → Sedna) residing in the → Kuiper belt. In brief, astrology is a superstition chiefly based on ignorance and man’s need for mental contentment.

Etymology (EN): → astro-; → -logy.

Etymology (PE): Axtarguyi, literally “star-telling,” from axtar “star,” → astro- + guyi verbal noun from goftan “to tell, speak, talk;” Mid.Pers. guftan “to say, tell, utter;” O.Pers. gaub- “to say.”

A binary star in which the presence of an unresolved companion is revealed by small oscillations in the movement of the visible component caused by the gravitational influence of the unseen component.

See also: → astrometry; → binary.

A binary star in which the presence of an unresolved companion is revealed by small oscillations in the movement of the visible component caused by the gravitational influence of the unseen component.

See also: → astrometry; → binary.

The precise measurement of the positions and motions of → astronomical objects.

Etymology (EN): Astrometry, from → astro- “star” + → -metry “measuring.”

Etymology (PE): Axtarsanji, from axtar, → astro-,

• -sanji “measuring.”

The precise measurement of the positions and motions of → astronomical objects.

Etymology (EN): Astrometry, from → astro- “star” + → -metry “measuring.”

Etymology (PE): Axtarsanji, from axtar, → astro-,

• -sanji “measuring.”

A person trained to pilot, navigate, or otherwise participate as a crew member of a spacecraft.

Etymology (EN): Astronaut, from Gk. → astro- “star” + nautes “sailor,” from naus “ship” (cognate with Mod.Pers. nâv “ship;” Av./O.Pers. *nāv-, O.Pers. nāviyā- “fleet;” Skt. nau-, nava- “ship, boat;” Gk. naus, neus, L. navis; PIE *nāu- “ship”).

Etymology (PE): Fazânavard, from Ar. fazâ “space” + navard agent noun from navardidan “to travel, walk, pass by and over.”

Keyhânnavard, from keyhân “cosmos” + navard.

A person trained to pilot, navigate, or otherwise participate as a crew member of a spacecraft.

Etymology (EN): Astronaut, from Gk. → astro- “star” + nautes “sailor,” from naus “ship” (cognate with Mod.Pers. nâv “ship;” Av./O.Pers. *nāv-, O.Pers. nāviyā- “fleet;” Skt. nau-, nava- “ship, boat;” Gk. naus, neus, L. navis; PIE *nāu- “ship”).

Etymology (PE): Fazânavard, from Ar. fazâ “space” + navard agent noun from navardidan “to travel, walk, pass by and over.”

Keyhânnavard, from keyhân “cosmos” + navard.

The science and technology of space flight, including the building and operation of space vehicles.

See also: → astronaut.

The science and technology of space flight, including the building and operation of space vehicles.

See also: → astronaut.

One who specializes in astronomy. → professional astronomer, → amateur astronomer.

Etymology (EN): Astronomer, from → astronomy + → -er.

Etymology (PE): Axtaršenâs has a long history in Persian; it is abundantly used by Ferdowsi (A.D. 950-1020) in his great work Šâhnâmé (Shahnameh); from axtar “star” → astro- + šenâs contraction of šenâsandé “expert, knowlegeable, skilled,” from šenâxtan “to know, to recognize.” → astronomy.

One who specializes in astronomy. → professional astronomer, → amateur astronomer.

Etymology (EN): Astronomer, from → astronomy + → -er.

Etymology (PE): Axtaršenâs has a long history in Persian; it is abundantly used by Ferdowsi (A.D. 950-1020) in his great work Šâhnâmé (Shahnameh); from axtar “star” → astro- + šenâs contraction of šenâsandé “expert, knowlegeable, skilled,” from šenâxtan “to know, to recognize.” → astronomy.

Of or relating to → astronomy.
In non astronomical contexts, of enormous magnitude; immense.

Etymology (EN): Astronomical, from L. astronomic(us),
→ astronomy + → -ic,

• → -al adj. suffix.

Etymology (PE): Axtaršenâxti, from axtaršenâxt, variant of axtaršenâsi, → astronomy

• -i adj. suffix.
  Axtaršenâsik, from axtaršenâsi,
  → astronomy + re-introduced Mod.Pers. suffix -ik, → -ics.

Of or relating to → astronomy.
In non astronomical contexts, of enormous magnitude; immense.

Etymology (EN): Astronomical, from L. astronomic(us),
→ astronomy + → -ic,

• → -al adj. suffix.

Etymology (PE): Axtaršenâxti, from axtaršenâxt, variant of axtaršenâsi, → astronomy

• -i adj. suffix.
  Axtaršenâsik, from axtaršenâsi,
  → astronomy + re-introduced Mod.Pers. suffix -ik, → -ics.

A precise pendulum clock with separate dials for seconds, minutes, and hours. It was originally used by astronomers to calculate astronomical time.

See also: → astronomical; → clock.

A precise pendulum clock with separate dials for seconds, minutes, and hours. It was originally used by astronomers to calculate astronomical time.

See also: → astronomical; → clock.

A precisely measured fundamental quantity in astronomy, such as the → solar parallax, the
→ constant of aberration, and the → obliquity of the ecliptic.

See also: → astronomical; → constant.

A precisely measured fundamental quantity in astronomy, such as the → solar parallax, the
→ constant of aberration, and the → obliquity of the ecliptic.

See also: → astronomical; → constant.

Values in a reference system used to relate the position of a body on the celestial sphere.

See also: → astronomical; → coordinates.

Values in a reference system used to relate the position of a body on the celestial sphere.

See also: → astronomical; → coordinates.

The intersection of a plane perpendicular to the radius of the Earth through the observer’s eye with the celestial sphere. Same as → true horizon. Because the → celestial sphere has an infinite radius, two observers at different heights above sea level, but placed on the same vertical line, have the same astronomical horizon. Because of → dip of the horizon, the astronomical horizon always lies above the → sea horizon. But on land it is usually hidden by trees, hills, and buildings which determine the observer’s → apparent horizon.

See also: → astronomical; → horizon.

The intersection of a plane perpendicular to the radius of the Earth through the observer’s eye with the celestial sphere. Same as → true horizon. Because the → celestial sphere has an infinite radius, two observers at different heights above sea level, but placed on the same vertical line, have the same astronomical horizon. Because of → dip of the horizon, the astronomical horizon always lies above the → sea horizon. But on land it is usually hidden by trees, hills, and buildings which determine the observer’s → apparent horizon.

See also: → astronomical; → horizon.

A device used to observe and study → astronomical objects.

See also: → astronomical; → instrument.

A device used to observe and study → astronomical objects.

See also: → astronomical; → instrument.

The angle between the → equatorial plane and the true → vertical at a point on the surface.

See also: → astronomical; → latitude.

The angle between the → equatorial plane and the true → vertical at a point on the surface.

See also: → astronomical; → latitude.

A naturally occurring physical entity or association that lies beyond the Earth’s atmosphere and can be studied observationally. In other words, a gravitationally bound structure that is associated with a position in space,
but may consist of multiple independent astronomical objects. A list of astronomical objects includes → planets, → asteroids, → comets, → stars, → nebulae, galaxies (→ galaxy), → galaxy clusters, → pulsars, and → black holes. Note that → celestial body, → celestial object, and → heavenly body are less technical terms for these entities.

See also: → astronomical; → object.

A naturally occurring physical entity or association that lies beyond the Earth’s atmosphere and can be studied observationally. In other words, a gravitationally bound structure that is associated with a position in space,
but may consist of multiple independent astronomical objects. A list of astronomical objects includes → planets, → asteroids, → comets, → stars, → nebulae, galaxies (→ galaxy), → galaxy clusters, → pulsars, and → black holes. Note that → celestial body, → celestial object, and → heavenly body are less technical terms for these entities.

See also: → astronomical; → object.

A building, place, or institution designed and equipped for making → observations of astronomical phenomena.

See also: → astronomical; → observatory.

A building, place, or institution designed and equipped for making → observations of astronomical phenomena.

See also: → astronomical; → observatory.

The → angular → displacement of a point on the → celestial sphere due to the Earth’s → atmospheric refraction.

See also: → astronomical; → refraction.

The → angular → displacement of a point on the → celestial sphere due to the Earth’s → atmospheric refraction.

See also: → astronomical; → refraction.

A certain place whose characteristics, as to location, altitude, atmospheric conditions, etc., make it appropriate for astronomical observations.

See also: → astronomical; → site.

A certain place whose characteristics, as to location, altitude, atmospheric conditions, etc., make it appropriate for astronomical observations.

See also: → astronomical; → site.

One of a set of tables giving parameters used for calculations of positions of the Sun, the Moon, and the planets in particular in pre-telescopic astronomy. The oldest known astronomical tables are those of Ptolemy. In Modern astronomy it is usually replaced by the term → ephemeris. Same as → zij. See also → Toledan Tables,
→ Alfonsine Tables.

See also: → astronomical; → table.

One of a set of tables giving parameters used for calculations of positions of the Sun, the Moon, and the planets in particular in pre-telescopic astronomy. The oldest known astronomical tables are those of Ptolemy. In Modern astronomy it is usually replaced by the term → ephemeris. Same as → zij. See also → Toledan Tables,
→ Alfonsine Tables.

See also: → astronomical; → table.

One of the twilight phases when the Sun’s center lies between 12 and 18 degrees below the horizon. Astronomical twilight is followed or preceded by → nautical twilight. Most stars and other celestial objects can be seen during this phase. However, some of the fainter stars and galaxies may not be observable as long as the Sun is less than 18 degrees below the horizon. See also → civil twilight.

See also: → astronomical; → twilight.

One of the twilight phases when the Sun’s center lies between 12 and 18 degrees below the horizon. Astronomical twilight is followed or preceded by → nautical twilight. Most stars and other celestial objects can be seen during this phase. However, some of the fainter stars and galaxies may not be observable as long as the Sun is less than 18 degrees below the horizon. See also → civil twilight.

See also: → astronomical; → twilight.

1. A unit of length equal to 149 597 870 700 m exactly, with symbol “au” (re-definition at the International Astronomical Union’s 28th General Assembly in Beijing, China, August 20-31). The astronomical unit equals 1.5813 × 10^-5 → light-years and 4.8481 ×10^-6 → parsecs.
   

2. Previous definition: The radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2 π / k days, where k is the → Gaussian gravitational constant. This is slightly less than the semi-major axis of the Earth’s orbit.

See also: → astronomical; → unit.

1. A unit of length equal to 149 597 870 700 m exactly, with symbol “au” (re-definition at the International Astronomical Union’s 28th General Assembly in Beijing, China, August 20-31). The astronomical unit equals 1.5813 × 10^-5 → light-years and 4.8481 ×10^-6 → parsecs.
   

2. Previous definition: The radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2 π / k days, where k is the → Gaussian gravitational constant. This is slightly less than the semi-major axis of the Earth’s orbit.

See also: → astronomical; → unit.

The science of the celestial bodies and the Universe, dealing especially with the positions, dimensions, distribution, motion, chemical composition, energy, and evolution of celestial bodies and phenomena.

Etymology (EN): O.Fr. astronomie, from L. astronomia, from Gk. astronomia, from → astro- “star” + nomos “arranging, regulating,” related to nemein “to deal out.”

Etymology (PE): Axtaršenâsi, from axtar “star,” → astro-

• -šenâsi “knowledge” from šenâxtan “to know, to discern.”

The science of the celestial bodies and the Universe, dealing especially with the positions, dimensions, distribution, motion, chemical composition, energy, and evolution of celestial bodies and phenomena.

Etymology (EN): O.Fr. astronomie, from L. astronomia, from Gk. astronomia, from → astro- “star” + nomos “arranging, regulating,” related to nemein “to deal out.”

Etymology (PE): Axtaršenâsi, from axtar “star,” → astro-

• -šenâsi “knowledge” from šenâxtan “to know, to discern.”

The area of science which deals with → elementary particle and → high-energy phenomena in → astrophysics and → cosmology.

See also: → astro-; → particle; → physics.

The area of science which deals with → elementary particle and → high-energy phenomena in → astrophysics and → cosmology.

See also: → astro-; → particle; → physics.

The photography of stars, other celestial bodies, and stellar fields.

See also: → astro-, → photography.

The photography of stars, other celestial bodies, and stellar fields.

See also: → astro-, → photography.

The measurement of the intensity of light of celestial bodies.

Etymology (EN): Astrophotometry, from → astro- + → photometry.

Etymology (PE): Axtar-šidsanji, from axtar-, → astro-, + -šidsanji, → photometry.

The measurement of the intensity of light of celestial bodies.

Etymology (EN): Astrophotometry, from → astro- + → photometry.

Etymology (PE): Axtar-šidsanji, from axtar-, → astro-, + -šidsanji, → photometry.

Of or pertaining to → astrophysics.

See also: → astrophysics + → -al

Of or pertaining to → astrophysics.

See also: → astrophysics + → -al

A very fast moving, → collimated beam of → ionized gas at high temperatures associated with most classes of compact objects that spin and/or accrete matter from their surroundings, such as → protostars, → X-ray binary systems, and, at a larger scale, with → active galactic nuclei, → gamma-ray bursts, and → quasars. In general, jet sources host → accretion disks and are associated with → magnetic fields. Astrophysical jets, despite their different physical scales and power, are morphologically very similar, suggesting a common physical origin. For example, in one extreme, → active galactic nuclei jets have typical sizes ≥ 10⁶ pc, velocities near that of light c, and parent sources (→ massive black holes) with masses 10^6-9&nbsp Msun and luminosities ~ 10^43-48Lsun; while in the other extreme, → young stellar objects jets have typical sizes ≤ 1 pc, velocities ≤ 10^-3 c,
and emerge from low mass protostars with masses ~ 1 Msun and luminosities (0.1-2 × 10⁴) Lsun. Jets play an important → feedback role in the evolution of their host systems. See also: → jet launching.

See also: → astrophysical; → jet.

A very fast moving, → collimated beam of → ionized gas at high temperatures associated with most classes of compact objects that spin and/or accrete matter from their surroundings, such as → protostars, → X-ray binary systems, and, at a larger scale, with → active galactic nuclei, → gamma-ray bursts, and → quasars. In general, jet sources host → accretion disks and are associated with → magnetic fields. Astrophysical jets, despite their different physical scales and power, are morphologically very similar, suggesting a common physical origin. For example, in one extreme, → active galactic nuclei jets have typical sizes ≥ 10⁶ pc, velocities near that of light c, and parent sources (→ massive black holes) with masses 10^6-9&nbsp Msun and luminosities ~ 10^43-48Lsun; while in the other extreme, → young stellar objects jets have typical sizes ≤ 1 pc, velocities ≤ 10^-3 c,
and emerge from low mass protostars with masses ~ 1 Msun and luminosities (0.1-2 × 10⁴) Lsun. Jets play an important → feedback role in the evolution of their host systems. See also: → jet launching.

See also: → astrophysical; → jet.

An extraterrestrial → object whose physical properties and formation are studied in → astrophysics.

See also: → astrophysical; → object.

An extraterrestrial → object whose physical properties and formation are studied in → astrophysics.

See also: → astrophysical; → object.

A scientist who studies → astrophysics.

See also: → astro-; → physicist. The term astrophysicist was introduced by Greenwich astronomer Edwin Dunkin in 1869.

A scientist who studies → astrophysics.

See also: → astro-; → physicist. The term astrophysicist was introduced by Greenwich astronomer Edwin Dunkin in 1869.

The branch of → astronomy that deals with the → physics of → celestial objects and the → Universe in general. It relies on the assumption that the → laws of physics apply everywhere in the Universe and throughout all time. See also → observational astrophysics, → theoretical astrophysics.

See also: Astrophysics, from → astro- “star” + → physics.

The first use of the term astrophysics has been attributed to Johann Karl Friedrich Zöllner (1834-1882) in 1865. He defined it as a coalescence of physics and chemistry with astronomy (History of Astronomy: An Encyclopedia, ed. John Lankford, Routledge, 1997).

The branch of → astronomy that deals with the → physics of → celestial objects and the → Universe in general. It relies on the assumption that the → laws of physics apply everywhere in the Universe and throughout all time. See also → observational astrophysics, → theoretical astrophysics.

See also: Astrophysics, from → astro- “star” + → physics.

The first use of the term astrophysics has been attributed to Johann Karl Friedrich Zöllner (1834-1882) in 1865. He defined it as a coalescence of physics and chemistry with astronomy (History of Astronomy: An Encyclopedia, ed. John Lankford, Routledge, 1997).

Not having → symmetry.

See also: → asymmetry.

Not having → symmetry.

See also: → asymmetry.

Lack of symmetry; not symmetrical.

Etymology (EN): Gk. asymmetria “lack of proportion,” from asymmetros “ill-proportioned,” from → a- “not” + symmetros “commensurable, symmetrical.”

Etymology (PE): Nâhamâmuni, from nâ- “not” + hamâmuni “symmetry,” from ham- “together = syn” + -â-, euphonic affix,

• mun “measure” + -i, noun affix.

Lack of symmetry; not symmetrical.

Etymology (EN): Gk. asymmetria “lack of proportion,” from asymmetros “ill-proportioned,” from → a- “not” + symmetros “commensurable, symmetrical.”

Etymology (PE): Nâhamâmuni, from nâ- “not” + hamâmuni “symmetry,” from ham- “together = syn” + -â-, euphonic affix,

• mun “measure” + -i, noun affix.

A straight line which is approached, but never reached, by an infinite branch of a curve, and which can be regarded as a line tangent to the curve at infinity.

Etymology (EN): Gk. asymptotos “not falling together,” from → a- “not” + → syn “with” + ptotos “fallen,” verbal adj. from piptein “to fall”.

Etymology (PE): Nâhamsâv, literally “not touching each other,”
from nâ- “not” + ham “with” (akin to Gk. syn-) + sâv, agent noun of sâvidan “to touch.”

A straight line which is approached, but never reached, by an infinite branch of a curve, and which can be regarded as a line tangent to the curve at infinity.

Etymology (EN): Gk. asymptotos “not falling together,” from → a- “not” + → syn “with” + ptotos “fallen,” verbal adj. from piptein “to fall”.

Etymology (PE): Nâhamsâv, literally “not touching each other,”
from nâ- “not” + ham “with” (akin to Gk. syn-) + sâv, agent noun of sâvidan “to touch.”

Of or pertaining to an → asymptote.

See also: Adjective from → asymptote; → -ic.

Of or pertaining to an → asymptote.

See also: Adjective from → asymptote; → -ic.

The phenomenon wherein the → quarks within a → hadron get closer together, the force of containment gets weaker so that it asymptotically approaches zero for close confinement. According to → quantum chromodynamics, the quarks in close confinement are completely free to move about. On the contrary, the further we try to force the quarks apart, the greater the force of containment. The 2004 Nobel Prize in Physics was awarded to David Gross, Frank Wilczek, and David Politzer for their discovery of asymptotic freedom. This discovery established quantum chromodynamics as the correct theory of the → strong interaction.

See also: → asymptotic; → freedom.

The phenomenon wherein the → quarks within a → hadron get closer together, the force of containment gets weaker so that it asymptotically approaches zero for close confinement. According to → quantum chromodynamics, the quarks in close confinement are completely free to move about. On the contrary, the further we try to force the quarks apart, the greater the force of containment. The 2004 Nobel Prize in Physics was awarded to David Gross, Frank Wilczek, and David Politzer for their discovery of asymptotic freedom. This discovery established quantum chromodynamics as the correct theory of the → strong interaction.

See also: → asymptotic; → freedom.

A region of the → Hertzsprung-Russell diagram populated by evolving → low-mass to → intermediate-mass stars. These stars have an electron → degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 10⁴-10⁵ times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong → stellar wind, which forms a dusty → circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by → nucleosynthesis processes in the burning shells in the deep interior.

See also: → symptotic;
→ giant; → branch.

A region of the → Hertzsprung-Russell diagram populated by evolving → low-mass to → intermediate-mass stars. These stars have an electron → degenerate core of carbon and oxygen surrounded by two burning shells of helium and hydrogen. The H and He-burning shells are activated alternately in the deep layers of the star. An extended and tenuous convection envelope, having a radius of 10⁴-10⁵ times the size of the core, lies above these shells. The loosely bound envelope is gradually eroded by the strong → stellar wind, which forms a dusty → circumstellar envelope out to several hundreds of stellar radii. The convective envelope, stellar atmosphere, and circumstellar envelope have a rich and changing chemical composition provided by → nucleosynthesis processes in the burning shells in the deep interior.

See also: → symptotic;
→ giant; → branch.

For → stellar winds, same as → terminal velocity.

See also: → asymptotic; → velocity.

For → stellar winds, same as → terminal velocity.

See also: → asymptotic; → velocity.