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Astronomy Dictionary

Click on an underlined word for more information on that subject.



A caldera is a large depression at the top of a volcano, caused by collapse or explosion.


Callipus of Cyzicus (370 - 300 BC) was an ancient Greek who accurately measured the length of the seasons. Callipus improved the Greek calendar, reconciling the lunar month with the solar year, by introducing a unit of time called the Callippic cycle (it was an improvement on the Metonic cycle of 6939.6 days or 19 solar years or 235 lunar months; the Callippic cycle was 4 Metonic cycles). He also added to the (incorrect) theory of the motions of the plants, as spheres within spheres, adding 7 more spheres to Eudoxus' system.


Callisto is a large, icy, dark-colored, low-density outer moon of Jupiter that is scarred with impact craters and ejecta. It has a diameter of about 3,000 miles (4800 km), the second largest moon of Jupiter, roughly the size of Mercury. It orbits Jupiter at a mean distance of 1,170,000 miles (1,883,000 km). Its mass is 1.08x1023 kg. It takes Callisto 400.5 hours to orbit Jupiter. Callisto was discovered by Galileo and S. Marius (independently) in 1610.


The Caloris Basin is the largest surface feature on the planet Mercury. This circular depression was formed in a collision with an asteriod, and is about 1,300 kilometers (800 miles) across. The Caloris Basin is near Mercury's subsolar point, the point on Mercury that is closest to the Sun when Mercury is at perihelion (the point in its orbit when it is nearest to the Sun), so the Caloris Basin gets extremely hot; this is why it is called Caloris, which means "heat" in Latin.


Calypso is one of the 18 moons of Saturn. It was discovered by B. Smith, H. Reitsema, S. Larson, J. Fountain in 1980. It is irregularly-shaped and has a radius of 15.0x8.0x8.0 km. Calypso an orbit with Telesto (294,660 km from the center of Saturn); these 2 moons are also called the Tethys Trojans because they orbit Saturn in Tethys' orbit, Telesto is 60° ahead of Tethys, Calypso is 60° behind Tethys.


The Candarm is the robotic arm on the Space Shuttle. The robotic arm lets astronauts on the flight deck of the Space Shuttle manipulate objects (like satellites) in space. The arm was built by Spar Aerospace Ltd., Canada for the U.S. (NASA's) Space Shuttle Program. Canadarm was launched aboard the Space Shuttle Columbia STS-2 on the second shuttle flight on November 12, 1981. It was deployed for the first time on Friday Nov. 13, 1981, operated by astronaut Richard Truly.


[Abbreviation: Cnc] Cancer (the crab) is a very faint constellation of the zodiac that is shaped like a crab. Cancer is seen along the ecliptic and is located between Leo and Gemini. At the center of the crab's shell is a huge open cluster, the Beehive cluster, also known as Praesepe (M44). The open cluster M67 is nearby. The brightest star in Cancer is Acubens (meaning claw); the second-brightest is Al Tarf (meaning tip).


Canis major (The Great Dog) is a constellation near Orion. The brightest star in Canis Major (and the brightest star in the sky) is Sirius, also known as the dog star. Canis Major is one of the constellation Orion's hunting dogs (together with Canis Minor).


Canes Venatici (The Two Hunting Dogs [or greyhounds, called Astarion and Chara, who belong to the herdman Boötes]) is a small Northern Hemisphere constellation near Ursa Major and Boötes. The brightest star in Canis Venatici is Cor Caroli (alpha CVn), a blue-white star (magnitude 2.9) with a companion star (magnitude 5). Beta CVn is a star called Chara. The Whirlpool Nebula (M51), the Sab galaxy (M94), the Sunflower Galaxy (M63), and the globular cluster M3 (magnitude 6) can be found in Canis Venatici. The Canes Venaticids meteor shower occurs from January 13 - January 30. Canes Venatici was named by the Polish astronomer Johannes Hevelius in 1690.
Annie Jump Cannon (1863-1941) was an American astronomer who cataloged 225,300 stars in the HD (Henry Draper) catalog; every star is classified by its stellar spectrum. Cannon and Edward C. Pickering (director of the Harvard Observatory) published the original HD catalog (9 volumes) from 1918 to 1924. The catalog was later expanded by Cannon and Margaret W. Mayall in 1949.


Capella is a multiple star system containing at least 9 stars. This bright system is in the Northern Hemisphere, 45 degrees from Polaris (the northern pole star); it is in the constellation Auriga. The two brightest stars in Capella are a binary star system. They are both yellow (like our Sun) with masses 2.6 times and 2.7 times that of the Sun. One is 9 times as large as the Sun, the other is 12 times as large. Each gives off roughly 78 times the light as the Sun. These two stars are about 43 light years from Earth.


[Abbreviation: Cap] Capricornus (the goat) is a constellation of the zodiac. Capricorn is seen along the ecliptic between Sagittarius and Aquarius. The brightest stars in Capricornus are Prima Giedi (Alpha 1 Cap) and Secunda Giedi (Alpha 2 Cap). The globular cluster M30 is in Capricorn.


A carbonardo is a rare type of opaque black diamond; they are not used for jewels, but for items like drilling bits and abrasive wheels. They were once thought to have been formed as a result of a comet impact 2 billion years ago, but this is no longer thought to be true. The largest diamond ever found was a carbonardo that weighed over half a kilogram. Carbonadoes are found in Bahia, Brazil, South America. Unlike other diamonds, carbonadoes are not found in a crystallized form - they are found in irregular or rounded fragments. Carbonadoes have a hardness of 10 and a specific gravity (density) of 3.1-3.3. Diamonds have a very hard polycrystalline carbon structure.


Carme is Jupiter's fourteenth moon. Carme is 18.5 miles (30 km) in diameter and orbits 13,800,000 miles (22,600,000 km) from Jupiter. Carme has a mass of 9 x 1016kg. It orbits Jupiter in 692 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Carme. Carme was discovered by S. Nicholson in 1938.


The Cartwheel Galaxy has a ring-like structure that is the result of a head-on collision of two galaxies! It started out as a regular spiral galaxy that was hit by a smaller galaxy. The ring-like band of stars formed much like ripples form in water when a rock is tossed into it. The Cartwheel Galaxy is about 500 million light-years from Earth in the constellation Sculptor.


A Cassegrain telescope is a wide-angle reflecting telescope with a concave mirror that receives light and focuses an image. A second mirror reflects the light through a gap in the primary mirror, allowing the eyepiece or camera to be mounted at the back end of the tube. The Cassegrain relecting telescope (named for the French sculptor Sieur Guillaume Cassegrain) was developed in 1672. A correcting plate (a lens) was added in 1930 by the Estonian astronomer and lens-maker Bernard Schmidt (1879-1935), creating the Schmidt-Cassegrain telescope which minimized the spherical aberration of the Cassegrain telescope.


Giovanni Domenico Cassini (1625-1712) was an astronomer born in Italy who later became a naturalized French citizen. He discovered four of Saturn's moons (Tethys, Dione, Rhea, and Iapetus) and a dark division in Saturn's rings (called the Cassini Division).


The Cassini Division is the main, dark division between Saturn's largest rings (the A and B rings). This gap is 2,920 miles (4,700 km) wide and is 73,010 miles (117,500 km) from the center of Saturn. It was discovered by G.D. Cassini in 1675.


NASA's Cassini-Huygens spacecraft was launched from Earth on October 1997 to go on an 11-year mission to Saturn. The spacecraft was named after the main division in Saturn's rings, called the Cassini Division. Cassini sent back photos and scientific information from Venus (in 1998) and Jupiter (in 2000) - it used the planets' gravitational fields to boost the spacecraft towards Saturn. In 2004, Cassini will send a probe called Huygens (built by the European Space Agency) to the surface of Titan, (Saturn's biggest moon).


Cassiopeia is an easily-seen constellation that is in the far northern sky. It circles the polestar (Polaris) throughout the year and also straddles the Milky Way. The five major stars of Cassiopeia (also known as "The Lady of the Chair") are shaped like a "W" (or an "M," depending on your orientation). All of the stars in Cassiopeia are all less than second magnitude brightness. The brightest star in Cassiopeia is Schedar (alpha CAS), which is a multiple star that is pale rose in color and varies in magnitude from 2.2 to 2.8 magnitudes. The second-brightest, called Caph (beta CAS), is a white star of magnitude 2.4. Cassiopeia contains two open clusters, M52 (magnitude 7.3) and M103 (magnitude 7.4). The strongest radio source, Cassiopeia A, emanates from Cassiopeia; it is the remnant of a supernova which ocurred about A.D. 1660, and is 10,000 light years from us. The constellation Cassiopeia was named for Cassiopeia, the mother of Andromeda (and the wife of Cephus) in Greek mythology.


A catena is a chain of craters.


Henry Cavendish (1731-1810) was an English chemist and physicist. Cavendish discovered that hydrogen gas was a substance different from ordinary air (whose components he analyzed), he described the composition of water (hydrogen and oxygen) and made other important discoveries. Cavendish was the first person to determine Isaac Newton's gravitational constant and accurately measured the Earth's mass and density.


CCD stands for charge-coupled device; it is a type of digital camera containing an array of extremely light-sensitive capacitors. When a photon (a particle of light) hits a capacitor, the photon dispaces some electrons, generating a small current - charging the capacitor. A CCD can detect light coming in at rates as low as one photon per minute. In CCD astronomy, this light-sensitive integrated circuit is hooked up to a telescope, detecting even extremely faint images that were impossible to detect before. All optical observatories (including the orbiting Hubble Space Telescope) use this type of device. George Elwood Smith and Willard S. Boyle (both of Bell Labs) invented the charge-coupled device in 1969. In 1970, they used their CCD to built the first solid-state video camera.


Celestial coordinates are pairs of numbers (right ascension and declination) which are used to locate objects on the celestial sphere. They are similar to the coordinates of longitude and latitude on Earth.


The celestial equator is the projection of the Earth's equator onto the celestial sphere.


The celestial poles are the projection of the Earth's poles onto the celestial sphere.


The celestial sphere is an imaginary sphere whose center is the Earth. This sphere is used by astronomers to map celestial objects.


Anders Celsius (1701-1744) was a Swedish professor of astronomy who devised the Celsius thermometer. He also ventured to the far north of Sweden with an expedition in order to measure the length of a degree along a meridian, close to the pole, later comparing it with similar measurements made in the Southern Hemisphere. This confirmed that that the shape of the earth is an ellipsoid which is flattened at the poles. He also cataloged 300 stars. With his assistant Olof Hiorter, Celsius discovered the magnetic basis for auroras.


Centaurs are relatively small objects (half asteroid, half comet) that are in unstable orbits (on a time scale of millions of years) located between Jupiter and Neptune. About 9 centaurs have been found so far; the largest centaur is 2060 Chiron (which is about 170 km in diameter and has a detectable coma). Centaurs are objects that probably came from the Kuiper belt.


The center of mass is the location at which the entire mass of an object (or set of objects) may be considered for purposes of calculations. It is the point of the average weighted position in space of an object (or a collection of objects).
Centrifugal (meaning "center fleeing" in Latin) means directed outwards from the center.
Centrifugal force is a force that acts upon a body spinning on an axis. This force is directed away from the body. Centrifugal force is equal to, but opposite the centripeta force.
Centripetal (meaning "center seeking" in Latin) means directed towards the center.
Centripetal force is a force that acts upon a body moving in a curved path. This force is directed towards the center of the curvature of the path. It is equal to, but opposite the centrifugal force.


Cepheid variables are supergiant stars that regularly pulsate in size and change in brightness. As the star increases in size, its brightness decreases; then, the reverse occurs. The luminosity is proportional to the period. Cepheid Variables may not be permanently variable; the fluctuations may just be an unstable phase the star is going through. Polaris and Delta Cephei are examples of Cepheids.


Cepheus is a house-shaped constellation in the Northern Hemisphere. It is a circumpolar constellation; it never dips under the horizon in the Northern hemisphere. Errai (Gamma Cephei) is a third-magnitude orange subgiant star that will become the Earth's north polar star in 2,000 years. Cepheus is next to the constellations Andromeda and Perseus. Cepheus was named after King Cepheus in Greek mythology, and is abbreviated Cep.


Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by Italian astronomer Giuseppe Piazzi on January 1, 1801). Its mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 cataloged asteroids. It is about 578 miles (930 kilometers) in diameter. Ceres is now considered to be a dwarf planet.


Cetus is a constellation that straddles the celestial equator. Cetus means whale, and is abbreviated Cet. The first-known variable star Mira, is Omicron Ceti.


The Space Shuttle Challenger was the second NASA Space Shuttle to be used (Columbia was the first). Challenger's first launch was the sixth Space Shuttle launch, called STS-6 Challenger; it took off on April 4, 1983 (from Florida), and landed on April 16, 1983 (at Edwards Air Force Base, CA). The first crew consisted of Paul J. Weitz, Karol J. Bobko, Donald H. Peterson, and F. Story Musgrave. It orbited the Earth 81 times, flying for 5 days, 0 hours, 23 min, 42 seconds; the first Shuttle space walk occured on this mission (it lasted four hours, 17 minutes). After nine flights into space, STS-51L (the mission number) Challenger, exploded 73 seconds into its launch on January 28, 1986, killing the crew (Gregory Jarvis, S. Christa McAuliffe, Ronald McNair, Ellison Onizuka, Judy Resnik, Francis Scobee and Michael Smith).
Thomas Chrowder Chamberlain (Sept. 25, 1843 - Nov. 15, 1928) was an American geologist and teacher who proposed the planetesimal hypothesis of the formation of the Solar System. In this theory, a star is supposed to have passed near the Sun, pulling matter away from the Sun. Later, this matter is to have condensed into larger masses, forming the planets.


The orbiting Chandra X-ray Observatory is the world's most powerful X-ray telescope. This 45 ft long satellite observes X-rays from high-energy parts of the sky, like exploding stars (temperatures in the millions of degrees Celsius produce X-rays). Chandra was launched on the Space Shuttle Columbia (STS-93) on July 23, 1999. It orbits more than 1/3 of the way to the moon. Electrical power is generated by solar panels, which generate the 2000 Watts it uses (this is about the power that a hair dryer uses). The extraordinarily smooth, barrel-shaped mirrors are coated with a very thin film of iridium (a rare metal). Chandra was named to honor astrophysicist Subrahmanyan Chandrasekhar. Chandra also means "Moon" or "luminous" in Sanskrit.


Subrahmanyan Chandrasekhar (born Lahore, India, in 1910 -died Chicago, USA, in 1995) was an Indian-American astrophysicist who studied stellar physics, evolution, and black holes. He realized that the fate of dying stars depended upon their mass, and above a certain point (1.4 times the mass of the Sun, now known as the "Chandrasekhar limit"), a star will undergo extreme collapse and not simply becomes a white dwarf. He won the Nobel prize in physics in 1983. The orbiting X-ray Observatory Chandra was named to honor S. Chandrasekhar.


Carl Vilhelm Ludvig Charlier (1862-1932) was a Swedish astronomer who studied celestial mechanics, the calibration of photographic photometry, and the theory of lenses. Charlier also worked in statistics, including the theory of errors; he studied the distribution and motions of stars. Charlier showed that hotter stars and galactic clusters formed flattened systems. In Charlie's hierarchical model of the Universe, he argued that the Universe has infinite mass, and that the density of matter approaches zero as one goes farther into space (resolving Olber's paradox). A 100 km wide crater on Mars (at 68.6 south, 168.4 west, shown above) was named for Charlier in 1973.


Charon is the only moon orbiting the planet Pluto. It is small, about 1,172 km in diameter. It orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon's orbit takes exactly one Pluto day. Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades.


The Chicxulub crater at the tip of the Yucatán Peninsula is an impact crater that dates from 65 million years ago. It is 120 miles wide and 1 mile deep. It is probably the site of the K-T meteorite or comet impact that caused the extinction of the dinosaurs and other groups of organisms.


2060 Chiron (also known as Comet 95P/Chiron) is the largest-known centaur (half asteroid, half comet). It is in a very elliptical, unstable orbit (on a timescale of millions of years) located between Jupiter and Neptune. Chiron is about 170 km in diameter and has a detectable coma. It was discovered by Charles Kowal on November 1, 1977.


Chondritic meteors are stony meteors with chondrules, tiny glass spheres. These meteors are unchanged since their formation, shortly after the formation of the Sun. These meteors consist of elements also common in the Earth's core.


The chromosphere is the inner, reddish layer in the sun's atmosphere, between the photosphere and the corona. The chromosphere is visible during a total solar eclipse.


A circumpolar constellation is one whose apparent path seems to circle a celestial pole. A circumpolar constellation never sets; it is always above the observer's horizon.


A circumpolar star is one whose apparent path seems to circle a celestial pole. A circumpolar star never sets; it is always above the observer's horizon.


Cislunar refers to the space between the Earth and the orbit of the Moon. Compare to translunar.


A classical Cepheid is a young, Helium-burning, supergiant star (spectral class F-K) that regularly pulsates in size and changes in brightness (with a period from 1-50 days). As the star increases in size, its brightness decreases; then, the reverse occurs. The luminosity is proportional to the period, so the absolute magnitude of the star can be calculated from the observed period, which can then give the distance to the star.


A closed universe is a model of the universe in which the universe first expands (after the big bang), and then contracts, due to the gravitational forces of the matter in the universe. In this model, the universe is finite in space and time. Contrast with an open universe.


The closest star to us is the Sun! Other than that, the closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the Alpha Centauri system). Proxima Centauri is 4.3 light-years from the Sun. It has an absolute magnitude of 15.5.


A cluster is a group of stars or galaxies. Our own galactic cluster, the Local Group, is about 5 million light-years across and contains about 30 galaxies (the largest of which are the Andromeda galaxy, Triangulum, and our Milky Way). The next-closest cluster is the Virgo Cluster, which is about 60 million light-years from Earth.


CMB stands for Cosmic Background Radiation and is also abbreviated CMBR and CBR. CMB is the radiation (energy) which remains from the original Big Bang explosion which formed the universe. CMB has cooled as the universe has expanded and is now slightly less than 3 degrees above absolute zero.


COBE (Cosmic Background Explorer) is a NASA satellite that was launched in 1992. COBE detected extremely fluctuations (anisotropy), in the cosmic microwave background (the heat left over from the Big Bang). The differences in temperature were tiny, ranging from 2.7281 Kelvin (degrees above absolute zero) to 2.7280 Kelvin, but imply fluctuations in the density of matter in the early universe.


To collimate a telescope, one lines up the optical components, the lenses, mirrors, prisms, and eyepieces, into their proper positions. Collimating maximizes image quality.
Michael Collins (1930-2021) was an American astronaut and US Air Force pilot. Collins piloted NASA's 3-day Gemini 10 Mission, which was launched on July 18, 1966; this mission successfully rendezvoused and docked with a separate Agena target vehicle, and Collins walked in space twice during this mission. Collins piloted the Command Module pilot of Apollo 11 (the mission that landed Neil Armstrong and Edwin Aldrin Jr. on the moon, it flew from July 16 to July 24, 1969); Collins circled the moon while Armstrong and Aldrin landed on the moon, and later rendezvoused with them.


Columba (the dove, abbreviated Col) is a small constellation in the Southern Hemisphere. This constellation pictures a dove holding an olive branch in its mouth. Columba is a member of the "Heavenly Waters," a constellation family. Columba was named by the astronomer Bayer, but was populatized by the French astronomer Augustin Royer in 1679 (before that, Colmba's stars have been included in the constellation Canis Major). Alpha Col (the brightest star in Columba) is called Phaet; beta Col (the second-brightest star in Columba) is called Wezn. The globular cluster NGC 1851 and the blue magnitude 5 star (mu Col, a "runaway star") are in Columba.


The first Space Shuttle, STS-1 Columbia (OV-102), launched on April 12, 1981 (from Florida), and landed April 14, 1981 (at Edwards Air Force Base, CA). The first crew consisted of Commander John W. Young and Pilot Robert L. Crippen. It orbited the Earth 37 times, flying 1,074,567 miles in 2 days, 6 hours, 20 min, 53 seconds. After dozens of flights into space, STS-107 (the mission number) Columbia, disintegrated during atmospheric re-entry on February 1, 2003, killing the seven crew members (Rick D. Husband, Kalpana Chawla, William C. McCool, David M. Brown, Laurel B. Clark, Michael P. Anderson, Ilan Ramon).


A coma is the roughly spherical blob of gas that surrounds the nucleus of a comet. The coma is comprised of water vapor, carbon dioxide gas, and other neutral gases that have sublimed from the solid nucleus. The coma and the nucleus form the head of a comet.

A Few Comets Discovered Last Visit Next Visit Period
Halley 240 B.C. 1986 2061 76.00
Encke 1786 1997 2000 3.28
Biela 1772 1852 broken up 6.62
Faye 1843 1991 1999 7.34


A comet is a celestial body that orbits around the sun. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, CO2, and other gases) and a tail (dust and ionized gases). Its long tail of gas and dust always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible when they're near the sun in their highly eccentric orbits. NASA's Stardust Mission will visit Comet Wild 2 in 2004.


Compact stars are very dense stars with a radius much smaller than than the our Sun. Some compact stars include black holes, white dwarfs, and neutron stars.


A complex impact crater is a large crater with a single or many peaks in the middle of the crater. On the moon, these craters have diameters between about 12 and 110 miles (20 and 175 km), and the central uplift is usually one or a few peaks. Craters with diameters over 110 miles (175 km) can have more complex, ring-shaped uplifts within the crater.


The Compton Gamma Ray Observatory (CGRO) is an orbiting telescope that NASA launched in 1991. It observes the electromagnetic spectrum from 30 keV to 30 GeV. The CGRO led to the discovery of blazars. CGRO was named to honor Dr. Arthur Holly Compton, who studied the scattering of high-energy photons by electrons. It will fall into the remote Pacific on June 3, 2000. CGRO detected more than 400 gamma ray sources and recorded more than 2,500 gamma ray bursts.


Condensation is the process in which a vapor (gas) is cooled to the liquid phase. Clouds are formed by the condensation of atmospheric water vapor.


Conjunction is the apparent close approach of a planet to the Sun (or another planet), from the point of view of an observer on the Earth. A planet is in conjunction when the Sun is exactly between that planet and the Earth or, for Mercury and Venus (the two inferior planets), when that planet, the Sun, and the Earth are lined up. Mercury and Venus have two positions of conjunction: when either planet is directly between the earth and the Sun, it is in inferior conjunction; when either planet is on the far side of the Sun from the earth, it is in superior conjunction. During conjunction, a planet cannot be seen from Earth (unless it is in transit); it is either behind the Sun or is lost in the glare of the Sun.


Conservation of energy is a basic principle of physics stating that energy can never be created or destroyed, it just changes form. For example, the chemical energy in an atom's bond turns into heat during combustion.


A constellation is a group of stars that, when seen from Earth, form a pattern. There are 88 constellations, including the Andromeda (pictured above).


There are many families of constellations, constellations that are either close to one another or have some other relationship. Some constellation families include the Zodiac (the 12 constellations representing the signs of the Zodiac), the Ursa Major Family (10 constellations, including Ursa Major, Ursa Minor Canes Venatici, Bootes, Coma Berenice, Corona Borealis, etc.), the Perseus Family (9 constellations, including Cassiopeia, Cepheus, Andromeda, Perseus, etc.), the Hercules Family (19 constellations, including Hercules, Sagitta, Aquila, etc.), the Orion Family (5 constellations, including Orion, Canis Major, Canis Minor, Monoceros, and Lepus), the Heavenly Waters (9 constellations, including Delphinus, Columba, Equuleus, Vela, Puppis, etc.), the Bayer Group (11 constellations, including Hydrus, Dorado, Volans, Apus,etc.), the La Caille Family (13 constellations, including Norma, Fornax, Circinus, etc.).
Continental Drift
Forward Backward


Continental drift is the movement of the Earth's continents. The land masses are hunks of Earth's crust that float on the molten core. The ideas of continental drift and the existence of a supercontinent (Pangaea) were presented by Alfred Wegener in 1915.


The crust of the Earth is broken into plates. The plates are enormous chunks of rock that float atop the soft mantle. The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Continental plates are thicker, older, and less dense than oceanic plates. These plates are about 125 kilometers thick and are made of granite that is about 3 billion years old.


A continuous spectrum is a spectrum of emitted light that contains all wavelengths of the colors that compose white light (red, orange, yellow, green, blue, indigo, violet, from long to short wavelength). Continuous spectra are emitted by incandescent solids, liquids, or compressed gases. If some discrete lines are missing from a spectrum, it is an absorption spectrum (indicating the presence of elements that absorb particular wavelengths).


Convection is the transfer of heat by circulation through a gas or liquid.


The Copernican system is the model of the solar system in which the planets (including the Earth) orbit the sun. This controversial theory was developed by Copernicus.


Nicolaus Copernicus (1473-1543) was an amateur Polish astronomer who developed the revolutionary Copernican system, a model of the solar system in which all the planets orbit the Sun. His ideas overturned the old Ptolemaic System. His seminal work was De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orb"), published in 1543.


The core is the innermost layer of the Earth (or any planet, moon, or star). It is usually under great pressure and is very hot.


The Coriolis force is the force that results from the rotation of the Earth around its axis; it makes storms rotate counterclockwise in the Northern and clockwise in the Southern Hemispheres. The French engineer/mathematician Gustave-Gaspard Coriolis discovered this force in 1835. This force has no effect on the direction of water going down a drain.


The corona is the top layer of the sun's atmosphere; it extends for millions of miles beyond the Sun's surface. The corona has very high temperatures (over a million K) and a very low density. During a complete solar eclipse (pictured above), only the ghostly corona is visible.


Coronal holes are areas in the coronal where the Sun's magnetic field loops out into space instead of looping back into the Sun, areas of magnetic anomalies (they often occur at the poles). In X-ray photographs of the Sun, coronal holes are black areas. Coronal holes can last for months or years. The solar wind is emitted from coronal holes.


Coronal mass ejections (CME's) are huge, balloon-shaped plasma bursts that come from the Sun. As these bursts of solar wind rise above the Sun's corona, they move along the Sun's magnetic field lines and increase in temperature up to tens of millions of degrees. These bursts release up to 220 billion pounds (100 billion kg) of plasma. CME's can disrupt Earth's satellites.


Cosmic Background Radiation (abbreviated CMB, CMBR and CBR) is the radiation (energy) which remains from the original Big Bang explosion which formed the universe. This radiation has cooled as the universe has expanded and is now slightly less than 3 degrees above absolute zero.


Cosmic rays are very high energy particles that travel through space near the speed of light. The 10 most abundant elements in cosmic rays are hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, and sulfur. Cosmic rays many sources, including the Sun, supernovae, and distant galaxies. Some cosmic rays enter the Earth's atmosphere.
There is a new and very controversial theory that there are comets composed of frozen water that are constantly bombarding the Earth. These "cosmic snowballs" have (perhaps) been seen by the visible imaging system of the Polar Satellite. These frozen comets vaporize in the atmosphere, adding water vapor to the environment.


A cosmic year is the time it takes the Sun (and our entire solar system) to complete an orbit around the center of the Milky Way galaxy. A cosmic year is roughly 225 million years.


Cosmology is the study of the structure, origin, and evolution of the universe.


The cosmological red shift is a phenomenon in which light from distant sources is red-shifted because space is expanding (and objects in the universe are receding from other objects due to this expansion).


Cosmonaut is the Russian word for astronaut, a person who ventures into space.


The cosmos refers to a universe that is a complete, harmonious, and orderly system.


The Crab Nebula (M1) is a cloud of intergalactic gas and dust. It is the remnant of a supernova that was seen on Earth in 1054. The Crab Nebula can be found in the constellation Taurus.


Impact craters are the remains of collisions between an asteroid, or meteorite and a planet, planetoid, or moon.


Crater rays are lines of ejecta radiating from a crater.


The Crêpe ring (also called the C ring) is the inner ring of Saturn's three major rings; it is smaller and less visible than the A and B rings. It is visible using a small telescope.


Crepuscular rays are shafts of sun rays and shadows that energe from behind puffy cumulus clouds.


A crescent moon is part way between a half moon and a new moon, or between a new moon and a half moon.
A critical (or supercritical) fluid is a liquid/gas under extreme pressure. These critical fluids have unique characteristics, the density of a liquid and the mobility of a gas. Critical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth.


Cruithne is an asteroid about 3 miles (5 kilometres) in diameter that is co-orbital with the Earth, which means that it shares roughly the same orbit as the Earth. It is a Near-Earth asteroid (NEA 3753). From the Earth, it appears to have a horseshoe-shaped orbit; it gets close to the Earth, then it moves away again. It's orbit is highly inclined to Earth's orbit. At its closest approach (which happens ever 100,000 years, Cruithne comes to within 10 million miles (15 million km) of Earth (40 times the distance from the Earth to the Moon). Cruithne was named for the first Celtic tribal group that settled in the British Isles. Cruithne was discovered on October 10, 1986 by D. Waldron at Siding Spring Observatory, Coonabarabran, Australia.


The crust is the outermost layer of the lithosphere (the solid part of the Earth consisting of the crust (broken into plates) and the upper mantle).


Crux (or Crux Australis) is the scientific name of the Southern Cross constellation. This well-known, cross-shaped Southern Hemisphere constellation is on the Australian flag. The brightest star in Crux is Acrux (alpha Cru), a double-star system at the base of the cross. The second-brightest star is Becrux or Mimosa (Beta Cru); the third-brightest is Gacrux (Gamma Cru). The Jewel Box (also known as Kappa Crucis) is an open cluster of about 100 stars in the Southern Cross. Crux lies on the Milky Way and is surrounded by the constellation Centaurus on three sides.


A cryptovolcano is a volcano-like feature on a planet or moon. They may or may not be volcanoes. For examples, cryptovolcanoes have been seen on the surface of Ganymede, a moon of Jupiter (one is pictured above).


Crystals are solids whose atoms form a very regular pattern.


A Cubewano is a main Kuiper belt objects, ones that are not influenced by the resonances of the outer planets. Cubewanos are farther from the Sun than about 41 AU. Compare with Plutino.


Culmination is the highest altitude that a celestial object attains above the horizon (as seen from Earth); an object culminates as it crosses the observer's meridian.


The Mars rover Curiosity is a remote-controlled robotic vehicle sent to Mars by NASA. Rover Curiosity is a 899 kg (1980 lb), 10-foot long, six-wheeled vehicle (about the size of a small car). Curiosity contains many scientific instruments. It was launched on November 26, 2011, and landed in the Mars' Gale Crater on August 6, 2012. Curiosity was designed to study the geology and climate of Mars, to determine whether Mars ever could have sustained life, and to prepare for future Mars missions.


Curtate means shortened or abbreviated.
The curvature of space-time is a distortion of space-time that is caused by the gravitational field of matter. The degree of curvature depends on the strength of the gravitational field (which depends on the massiveness of the objects in that part of space). An object traveling in space moves along the curves in space-time.


Cygnus (the swan) is a constellation that is shaped like a large cross. It is also known as the Northern Cross. It is seen along the Milky Way in the northern hemisphere. At its tail is the very bright star Deneb (alpha Cygni), and at its head is the double star Albireo (beta Cygni).


Cygnus A (located in the constellation Cygnus) is the most powerful radio source visible from Earth. This double-lobed radio galaxy is 500,000 light-years wide and is located 600 million light-years from Earth. It was identified by Hey, Parsons and Phillips in 1946.


The Cygnus loop is a nebula, a supernova remnant in the constellation Cygnus. It covers a region in the sky six times the diameter of the full Moon. The supernova exploded about 15,000-20,000 years ago and is still expanding. The nebula is almost spherical with some bright filaments.
Astronomy Dictionary

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