An Etymological Dictionary of Astronomy and Astrophysics

An often temporary cessation of hostilities during wartime for a specific purpose. It may be unilateral or bilateral.

Etymology (EN): From cease from O.Fr. cesser “to come to an end, stop,” from L. cessare “to cease, go slow, be idle,” + → fire.

Etymology (PE): Âtašbas, from âtaš, → fire,

• bas, from bas kardan “to stop, end; leave.”

Of or relating to the sky or visible heavens.

Etymology (EN): M.E., from O.Fr., from M.L. celestialis, from L. cælestis “heavenly,” from cælum “heaven, sky.”

Etymology (PE): Âsmâni related to âsmân, → sky.

The Earth’s axis extended to the → celestial pole.

See also: → celestial; → axis.

→ astronomical object.

See also: → celestial; → body.

Any system of coordinates used to define a point on the celestial sphere (zenith distance, altitude, celestial latitude, celestial longitude, etc.).

See also: → celestial; → coordinates.

An imaginary great circle on the sky half-way between the → celestial poles. It is the projection of the → equator of the Earth on the sky.

See also: → celestial; → equator.

A small globe representing the celestial sphere, on which the apparent positions of the stars are indicated.

See also: → celestial; → globe.

A great circle on the → celestial sphere having a plane that passes through the center of the Earth at a right angle to the line formed by an observer’s → zenith and → nadir.

See also: → celestial; → horizon.

Angular distance → north or → south measured from the → ecliptic to a → celestial object.

See also: → celestial; → latitude.

Angular distance to an object measured eastward along the → ecliptic from the → vernal equinox.

Etymology (EN): → celestial; → longitude.

Etymology (PE): Derežnâ, → longitude; âsmâni→ celestial.

The branch of astronomy that deals with the calculation of motions of celestial bodies under the action of their mutual gravitational attractions.

Etymology (EN): → celestial; → mechanics.

Etymology (PE): Mekânik, → mechanics; âsmâni,
→ celestial.

The great circle on the → celestial sphere, passing through the two → celestial poles and the observer’s → zenith.

See also: → celestial; → meridian.

→ astronomical object.

See also: → celestial; → object.

The point of the sky, north or south, where the projection of the Earth’s axis of rotation intersects the → celestial sphere. They are at 90° relative to the → celestial equator. Because of → precession, the celestial poles describe a circle around the ecliptic’s poles every 25,800 years.

See also: → celestial; → pole.

An imaginary sphere, of large but indefinite dimension, used as a basis to define the position coordinates of celestial bodies. The center can
be the Earth, the observer, or any other point which plays the role of origin for a given system of coordinates. Seen from the Earth, the celestial sphere rotates around the → celestial axis every 23h 56m 04s (the → sidereal day), as a result of the Earth’s rotation. Two important circles on the celestial sphere are the → celestial equator and the → ecliptic. The angle between them, about 23.40 degrees, is known as the → obliquity of the ecliptic. The celestial equator and the ecliptic intersect at two points, → vernal equinox
and → autumnal equinox. The positions of the → celestial poles and therefore that of the → celestial equator move gradually on the celestial sphere, due to → precession.

See also: → celestial; → sphere.

1. General: A small compartment or bounded area forming part of a whole.
   

2. Fluid mechanics: A body of fluid (air, liquid, or → plasma) in which the process of → convection proceeds.
   

3. Electricity: A device for producing electric current by chemical action.

Etymology (EN): From L. cella “small room, hut,” related to L. celare “to hide, conceal,” from PIE base *kel- “conceal” (cf. Skt. cala “hut, house,” Gk. kalia “hut, nest,” kalyptein “to cover”).

Etymology (PE): Yâxté “small room, closet,” etymology unknown.

The official name of the centigrade temperature scale with the → ice point as 0° and the → boiling point of water as 100°. The Celsius scale uses a degree (the unit of temperature) which has the same magnitude as the degree on the → Kelvin scale:
T_C = T_K - 273.15. See also → Fahrenheit scale, → Rankine scale, → Reaumur scale.

See also: In honor of Anders Celsius (1701-1744), Swedish astronomer, originator of the first centigrade temperature scale. However, in his original scale Celsius had 100° for the ice point and 0° for the steam point; → scale.

An → asteroid whose orbit around the Sun lies typically between the orbits of → Jupiter and → Neptune Neptune (5 to 30 → astronomical units). The first Centaur, called → Chiron, was discovered in 1977, but since then more than 100 roughly similar objects have been found. Three centaurs, Chiron, 60558 Echeclus, and 166P/NEAT 2001 T4, have been found to display → cometary  → comas. Chiron and 60558 Echeclus are now classified as both asteroids and → comets. Most of the Centaur asteroids are probably dormant comets from the → Kuiper belt which have been pulled in by the gravity of → outer planets.

See also: → Centaurus; → asteroid.

The Centaur. A → constellation in the southern hemisphere covering an extensive area of about 1060 square degrees from R.A. 11 h to 15 h and
Dec. -30° to -64°. Abbreviation: Cen, genitive form: Centauri.

Centaurus is the ninth largest constellation in the sky, but it does not contain any → Messier objects.

The brightest star in constellation is → Alpha Centauri which is also the third brightest star in the sky. Beta Centauri, the second brightest star in Centaurus, also called → Hadar, is the eleventh brightest star in night sky. Among other bright stars of the constellation are: Menkent (θ Cen), γ Cen, ε Cen, and η Cen. There are three → meteor showers associated with the constellation: the Alpha Centaurids, the Omicron Centaurids, and the Theta Centaurids. The constellation contains several extragalactic objects, among which: Centaurus A (NGC 5128), Omega Centauri, and NGC 5139.

Etymology (EN): L. centaurus, from Gk. kentauros, cf. Av. gandarəwa- “a mythical monster killed by Kərəsâspa,” Skt. gandharva- “name of mythical beings related with Soma.”
In Gk. mythology, centaurs were half-man half-horse creatures living on Mount Pelion in Thessaly, northern Greece. They were followers of the wine god Dionysus and well known for drunkenness and carrying off helpless young maidens.

Etymology (PE): Kentâwros, from Gk. “Kentauros.” Arabicized Qenturis (قنطورس).

The closest (3.8 ± 0.1 Mpc) → radio galaxy with a physical age of about 560 Myr, associated with the massive → elliptical galaxy NGC 5128. The nucleus harbours a → supermassive black hole, with a mass (5.5 ± 3.0) × 10⁷ Msol derived from stellar → kinematics. A prominent → dust lane, with → starburst, crosses the central parts. Centaurus A shows a twin → jet in → radio and → X-ray bands, symmetrical on
parsec scales but with evident asymmetry on kpc scales. The main (i.e. northern) jet which is markedly brighter than the counterjet, is seen at a viewing angle of approximately 50°. From photoionization models for such a viewing angle, the → Lorentz factor of the jet is derived to be ≤ 5. A large number of radio and X-ray → knots is discernible in the jet on kpc scales with the radio knots of larger proper motions showing comparatively little X-ray emission (see S. Wykes et al. 2015, MNRAS 447, 1005, and references therein).

See also: Situated in the → Centaurus constellation.

The nearest large → supercluster. It is dominated by the → galaxy cluster A3526 (→ Abell catalog). The Centaurus supercluster is a long structure that stretches away from us. The most distant of the clusters, A3581, is about 300 million → light-years away.

See also: → Centaurus; → cluster.

1. The point that is equidistant from all the points on a circle or sphere.
   

2a) To place in or on a center.

2b) Telescope: To move a telescope in order to place the object of interest on the center of the field. → center of attraction, → center of gravity, → center of inertia, → center of mass, → center wavelength, → anticenter, → apocenter, → barycenter, → optical center, → pericenter.

Etymology (EN): M.E. centre, from O.Fr. centre, from L. centrum “center,” originally fixed point of the two points of a compass, from Gk. kentron “sharp point, goad,” from kentein “stitch,” from PIE
*kent- “to prick.”

Etymology (PE): Markaz “center,” from Ar.;
kayân, Mod.Pers. “the point made by the compasses, that is the center of a circle” [Dehxodâ]; markazidan, infinitive from markaz, → center,

• -idan infinitive suffix.

A point toward which a force on a body is always directed.

See also: → center; → attraction.

A fixed point in a body through which the resultant force of gravitational attraction acts. Same as → center of mass, → center of inertia, → centroid.

Etymology (EN): → center; → gravity.

Etymology (PE): Gerânigâh, from gerâni→ gravity

• -gâh “place.”

Same as → center of gravity,
→ center of mass, → centroid.

See also: → center; → inertia.

Same as → center of gravity,
→ center of inertia, → centroid, and → barycenter.

See also: → center; → mass.

Center of a filter passband measured at 50% of peak transmittance.

See also: → center; → wavelength.

Prefix denoting one-hundredth of, in metric units; e.g. centimeter, 0.01 of meter.

Etymology (EN): Fr., from L. centi,- “hundred,” from centum “hundred,” Gk. hekaton, Av. satem-, Mod.Pers. sad, Skt. satam-, P.Gmc. *hunda- “hundred,” Goth. hund, O.H.G. hunt, O.Ir. cet, Bret. kant,
PIE *kmtom “hundred.”

Etymology (PE): Sânti-, from Fr., → above paragraph.

A unit of length in the → metric system, equal to one-hundredth of a meter, which is the current unit of length in the → International System of Units (SI).

See also: → centi-; → meter.

One of the three obscured → Galactic center clusters, which contains the supermassive black hole → Sgr A*. The first stars observed in the Central cluster were evolved → massive stars showing strong He I emission lines (2.058 microns) in the near infrared K band. Subsequently more than 80 massive stars were detected including various types of → Wolf-Rayet stars, as well as → O-type and → B-type → supergiants and → dwarfs (see, e.g. Martins et al. 2007, A&A 468, 233).

See also: → central; → cluster.

An eclipse during which the axis of the lunar shadow cone intersects the Earth’s surface (in the case of solar eclipses) or the axis of the terrestrial shadow cone intersects the Moon’s surface (in the case of lunar eclipses). The total and annular solar eclipses are usually central. They can also be not central; then, they are visible only from places situated at high latitudes (M.S.: SDE).

A → force that is always directed toward a fixed point and whose → magnitude depends only on the distance from that point.

Mathematically, F is a central force if and only if: F = f(r)r₁ = f(r)r/r, where r₁ = r/r is a unit → vector in the direction of r. If f(r) < 0 the force is said to be → attractive toward the source. If f(r) > 0 the force is said to be → repulsive from the source. In other words, a central force is one whose → potential, V(r), depends only on the → distance from the source. → Gravitational force and → electrostatic force are central, with V(r)∝ 1/r.

See also: → central; → force.

A statement about the characteristics of the sampling distribution of means of → random samples from a given → statistical population. For any set of independent, identically distributed random variables, X₁, X₂,…, X_n,
with a → mean μ and → variance σ², the distribution of the means is equal to the mean of the population from which the samples were drawn. Moreover, if the original population has a → normal distribution,
the sampling distribution of means will also be normal. If the original population is not normally distributed, the sampling distribution of means will increasingly approximate a normal distribution as sample size increases.

See also: → central; → limit; → theorem.

A vast, turbulent region encircling the → Milky Way’s nucleus that contains a large fraction of the → Galaxy’s dense → molecular clouds and → star formation regions. Spanning -1 to +1.5 degrees of → Sgr A*, the CMZ is about 400 pc × 100 pc in size and contains at least 10⁷→ solar masses of → giant molecular clouds,
approximately 10% of the Galaxy’s molecular gas. The gas in the CMZ is at higher temperature than typical giant molecular clouds
and has high velocity dispersion reflecting the → turbulent nature of the gas in the area.

Despite these extensive molecular reserves, the → star formation rate within the CMZ is actually lower than expected based on the analysis of nearby → star-forming regions in the quiescent → Galactic disk. A common assumption is that this is a result of the extreme conditions within the CMZ, where the density, pressure, temperature, → velocity dispersion and → radiation field are all significantly greater than elsewhere in the Milky Way (Clark et al. 2018, The Messenger 173, 22 and references therein).

See also: → central; → molecular; → zone.

The uplift of the central parts of the → crater floor due to the impacting force of a large → meteorite.

The shock wave entering the Earth will first move in as a compressional wave (P-wave), but after passage of the compressional wave an expansion wave (rarefaction wave) will move back toward the surface. This will cause the floor of the crater to be uplifted and may also cause the rock around the rim of the crater to bent upward.

See also: → central; → peak.

The primary component of a → computer that processes instructions. It runs the → operating system and → applications, constantly receiving input from the user or active → software programs. The CPU has two typical components:

1. Control Unit, which extracts instructions from memory and decodes and executes them.
2. Arithmetic Logic Unit (ALU), which handles arithmetic and logical operations.

See also: → central; → processing; → unit.

Same as → central peak.

Etymology (EN): → central; → up; → lift.

Etymology (PE): → central; → uplift.

1. In an interference filter, the wavelength of peak transmission.
   
2. In a spectrograph, the wavelength corresponding to the middle of the range covered by the grating or grism.

See also: Central, adj. from → center; → wavelength.

Acting or moving in a direction away from the axis of rotation or the center of a circle along which a body is moving.

Etymology (EN): From Mod.L., coined 1687 by Sir Isaac Newton from L. centri-,
combining form of centrum, → center, + fugere “to run away, flee.”

Etymology (PE): Markaz-goriz, from markaz,
→ center, + goriz “running away,” from gorixtan, gorizidan “to run away,” Mid.Pers. virextan, proto-Iranian *vi-raik, from vi- “apart, asunder”

• *raik, Av. raek- “to leave, set free, let off,” Mid./Mod.Pers. reg/rig (in mordé-rig “inheritance,” Skt. ric- “to leave,” rinakti “gives up, evacuates,” Gk. leipein “to leave,” L. linquere “to leave,” from PIE *linkw-, from
  *leikw- “to leave behind” (cf. Goth. leihvan, O.E. lænan “to lend;” O.H.G. lihan “to borrow;” O.N. lan “loan”).

Of a point rotating in a circle round a central point, the outward acceleration away from the rotation axis. It corresponds to → centrifugal force. The centrifugal acceleration is given by ω x ω x r, or v²/r, where ω is → angular velocity, r the distance to the rotating axis, and v the → tangential velocity. The centrifugal and → centripetal accelerations are equal and opposite.

See also: → centrifugal; → acceleration.

A force in a rotating reference frame directed outward from the axis of rotation.

See also: → centrifugal; → force.

Acting or moving toward a → center or → axis.
→ centripetal acceleration, → centripetal force.

Etymology (EN): From Mod.L., coined 1687 by Sir Isaac Newton from L. centri- combining form of centrum “center” + petere “to fall, rush out;” cf. Av. pat- " to fly, fall, rush," Skt. patati “he flies, falls,” Mid.Pers. patet “falls,” opastan “to fall,” Mod.Pers. oftâdan “to fall;” Gk. piptein “to fall,” petomai “I fly;” PIE base *pet- “to fly, to rush.”

Etymology (PE): Markaz-gerâ, from markaz, → center,

• gerâ “inclining,” from gerâyidan “to incline toward;” Mid.Pers. grâyitan, → diverge.

The rate of change of the → tangential velocity of a body moving along a circular path. The direction of centripetal acceleration is always inward along the → radius vector of the → circular motion. The magnitude of the centripetal acceleration is related to the → tangential velocity (v) and → angular velocity (ω) as follows: a_c = v²/r = rω². According to → Newton’s second law, an object undergoing centripetal acceleration is experiencing a → centripetal force.

See also: → centripetal; → acceleration.

The force exerted on an object in → circular motion which is directed toward the center and keeps the body in motion.
Centripetal force produces → centripetal acceleration, according to → Newton’s second law: F_c = ma_c. Its direction is always inward along the → radius vector, and its magnitude is given by: F_c= ma_c = mv_t²/r = mω²r.

See also: → centripetal; → force.

1. Same as → center of gravity, → center of inertia, → center of mass, and
   → barycenter.
   

2. In a triangle, the point where the three medians converge.

See also: → center + → -oid.

A class of luminous, → yellow supergiants that are pulsating
variables and whose period of variation is a function of their → luminosity.
These stars expand and contract at extremely regular periods, in the range 1-50 days.
Their highest brightness and surface temperature occur when their expansion velocity
is greatest. Similarly, their minima in brightness and temperature occur when they are in the contraction phase. The longer the period, the more luminous the star. The process that drives the pulsation of → Cepheid variables is the → kappa mechanism. In fact, Cepheids provide one of the most powerful tools for measuring distances to other galaxies (→ period-luminosity relation).
However, this method is limited to the distance of the → Virgo cluster of galaxies (15-20 → Mpc) even with the → HST or the largest ground-based telescopes. One particularly special Cepheid is the North Star, → Polaris. See also → RR Lyrae star.

Etymology (EN): Named after the prototype → Delta Cephei discovered by John Goodricke in 1784. → Cepheus.

A → variable star belonging to the class of → Cepheids.

See also: → Cepheid; → variable.

A → constellation in the Northern Hemisphere lying next to → Cassiopeia. It contains several pulsating variable stars, including the prototype → Cepheid variable Delta Cephei.
Abbreviation: Cep, genitive: Cephei.

Etymology (EN): In Gk. mythology, Cepheus, king of Ethiopia, who was married to the beautiful → Cassiopeia, and was also father of princess → Andromeda.

Etymology (PE): Kefeus, from Gk. Cepheus. Arabicizd form qifâvus (قیفاووس).

Once qualified as the largest known → asteroid, Ceres is now classified as a → dwarf planet (2006 IAU General Assembly). It is approximately 950 km across, and resides with tens of thousands of asteroids in the main → asteroid belt; it is the largest body of the belt. Its mass is 9.4 × 10²⁰ kg,
its → rotation period 9.074 hours, its → orbital period 4.60 years, and its → semi-major axis 2.767 AU.
NASA’s → Dawn spacecraft, which was placed in orbit around Ceres in 2015, has mapped its surface in great detail from a distance. Dawn caught sight of bright spots that soon resolved into more than 130 bright patches, most of them tied to craters. The most prominent of these spots lie inside the crater → Occator. The patches turned out to be carbonate salts, which only form in the presence of water. Since water skips to gas almost immediately on the dwarf planet’s surface, the discovery of carbonates suggested that there was liquid beneath the dwarf planet’s crust. Aside from craters, the only outstanding feature On Ceres is a single mountain, Ahuna Mons. It formed about 250 million years ago when plumes of saltwater and mud rose and erupted from within Ceres.

Etymology (EN): Ceres in Roman mythology was the goddess of growing plants and of motherly love. She was equivalent to Demeter in Gk. mythology.

European Organization for Nuclear Research, founded in 1954, and located on the Franco-Swiss border near Geneva, Switzerland. CERN is one of the world’s largest centres for scientific research. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter, i.e. the → elementary particles. The instruments used at CERN are particle → accelerators and → detectors. Currently it has 20 Member States.

See also: CERN, acronym of the organization’s original name Centre Européen pour la Recherche Nucléaire.

A complex of astronomical telescopes and instruments located approximately 80 km to the East of La Serena, Chile, at an altitude of 2,200 m. CTIO headquarters are located in La Serena, Chile, about 480 km north of Santiago. The principal telescopes on site are the 4-m Victor M. Blanco Telescope and the 4.1-m Southern Astrophysical Research (SOAR) telescope. One of the two 8-m telescopes comprising the Gemini Observatory is co-located with CTIO on the Association of Universities for Research in Astronomy (AURA) property in Chile, together with more than 10 other telescopes and astronomical projects.

See also: Cerro “mountain” in Spanish; Tololo a proper name; → inter-; American, from America, → North America Nebula; → Observatory.

Determined, fixed; established beyond doubt or question; indisputable. → determinism.

Etymology (EN): From O.Fr. certain, from V.L. *certanus, from L. certus “sure, fixed,” originally a variant p.p. of cernere “to distinguish, decide.”

Etymology (PE): Tâštig, from Mid.Pers. tâštig “certain,” tâšitan “to cut, cleave, create,” Mod.Pers. tarâšidan, Gilaki tâštan “to shave, scrape, cut,” Av. taš- “to cut, fashion, shape, form,” taša- “ax, hatchet,” tašan- “creator, maker,” cf. Skt. taks- “to cut, chop, form by cutting, make, create,” taksan “carpenter,” Gk. tekhne “art, skill, craft, method,” L. textere “to weave;” PIE base *tek- “to shape, make.”

The fact, quality, or state of being certain, especially on the basis of evidence. Something that is certain. → uncertainty; → uncertainty principle.

See also: Noun from → certain.

A soft ductile chemical element of the → alkali metal group;
symbol Cs. It is found in several → silicate minerals, including pollucite. The metal oxidizes in air and reacts violently with water. → Atomic number 55; → atomic weight 132.9054; → melting point 28.4°C; → boiling point 669.3°C; → specific gravity 1.873 at 20°C; and → valence +1. Cesium has several radioactive isotopes, among which ¹³⁴Cs
with a half-life of 2.07 years and ¹³⁷Cs with a half-life of 30.3 years. Cesium was discovered spectroscopically in 1860 by W. Bunsen and G. Kirchhoff in mineral water from Durkheim.

See also: From L. caesius “bluish gray,” which was the color of the cesium line in the spectroscope, + → -ium.

→ atomic fountain clock.

See also: → cesium; → clock.

The Whale, or Sea Monster. A large, rather inconspicuous → constellation in the equatorial region of the sky at R.A. 1h 30m, Dec. -10°. Its brightest star (Diphda) is a 2nd magnitude and contains → Mira Ceti, the first-known variable star, and the → Seyfert galaxy M77. Abbreviation: Cet; genitive form: Ceti.

Etymology (EN): Named after the sea monster in Gk. mythology sent by Poseidon to punish the Queen → Cassiopeia for bragging that she or her daughter → Andromeda was more beautiful than the Nereides. But → Perseus rescued Andromeda.

Etymology (PE): Ketus, from Gk., Arabicized form Qeytas (قیطس).