Vocabulary Words

Absolute magnitude. Magnitude that a star would appear to have if it were at a distance of 10 pc from the Sun.

Absorption lines. Dark lines superimposed on a continuous spectrum.

Achondrite. A type of stony meteorite with no chondrules.

albedo Fraction of incident sunlight reflected by a planet or minor planet.

altitude a. distance above the surface of a planet, used in describing an atmosphere or a spacecraft orbit. b. Angle of elevation above the horizon, for the line of sight to a celestial object.

Annular eclipse. Eclipse of the sun in which a ring of the Sun remains visible around the Moon.

Aphelion. In a noncircular orbit around the sun, the aphelion is the position where a planet is farthest from the sun. For a noncircular orbit around the Earth, the apogee is the position where a satellite is farthest from the Earth.

Apparent magnitude. The brightness of an astronomical object, as observed on Earth and referred to the appearance of some objects chosen as standards. The scale of magnitudes is defined so that a difference of 5 magnitudes corresponds to a ratio of 100 in observed radiation intensity.

Astroid. Older name for minor planet. Object in orbit around the Sun, intermediate in size between meteoroids and planets.

Asteroid belt. The region of the solar system in which most asteroids have their orbits, between Mars and Jupiter.

Astrology. A system in which the positions of the Sun, Moon, and Planets are supposed to exert an influence on events on Earth. Originally a part of astronomy, astrology is today without scientific content. Astrology is strictly forbidden in the Bible.

Astronomical unit (AU). Semimajor axis of the Earth's orbit around the Sun; 149.6 million km. The distance from the Earth to the Sun.

Aurora Light emitted in the upper atmosphere in the far northern and southern latitudes. Produced by collisions of high-speed particles with atmospheric atoms and molecules, and strongly influenced by the Earth's magnetic field and solar activity.

Azimuth. Angle measured clockwise around the horizon, from 0* towards the north, through 90* to the east, 180* to the south, 270* to the west, and to 360* due north.

Barred spiral galaxy. Spiral galaxy with spiral arms connected to a spoke or bar that extends out from the galactic center. See spiral galaxy.

Big Bang Theory. Cosmological theory that assumes a universal expansion starting from an explosion in a very dense and compact stage. Some big bang theories predict continued expansion, others a slowing expansion to be followed by a reversal leading to a contraction.

Binary star. Double star with the two stars in orbit around one another.

Black dwarf. Final state of stellar evolution, when a star has used up all of its energy resources and can no longer radiate.

Black Hole. Body that is so massive and so compact that no light can leave its surface. Suggested by General Theory of Relativity.

Breccia. Type of rock composed of compacted fragments of other rocks; commonly produced by meteorite impacts on the lunar surface.

Cassegrain. Inventor of a type of reflecting telescope that now bears his name. In this design, light from the concave primary mirror is reflected a second time by a convex mirror, though a hole cut in the primary, to produce an image at the Cassegrain focus.

CCD. Charged-couple device. An electronic device for measuring light intensity, far more sensitive than photographic materials.

Celestial Equator. Great circle formed on the celestial sphere by the extension out of the Earth’s equatorial plane.

Celestial Sphere. Apparent spherical surface, centered on the Earth to which the stars seem to be fixed.

Center of mass. Mean position of the masses that comprise a system or larger body: for two bodies, the center of mass is a point on the line joining them. For a binary star system, the motion of each star can be computed about the center of mass.

Cepheid variable. A type of star with regular variation in its brightness, named for Delta Cephei, the first star recognized of this type. These stars play an important role in the determination of the scale of distances to galaxies.

Chondrite. A type of meteorite that has stony appearance and contains small spherical particles, chondrules.

Chondrule. Small round body (generally less than 1 mm) found in meteorites.

Chromosphere. A region of the solar atmosphere between the bright photosphere and the more extensive corona. Hard to observe because of it’s relative faintness.

Circumpolar stars. Stars that are close enough to the celestial pole that they do not rise and set each night but can be seen to move around the pole in a circular path. Latitude of observer determines which stars appear circumpolar.

Clouds of Magellan. Two irregularly shaped galaxies, on large and the other small, relatively near to our Milky Way galaxy. Visible by eye from the southern hemisphere. Usually denoted as LMC and SMC.

Cluster. A group of apparently near by objects; clusters of stars or galaxies. Star clusters are open or globular. Good examples of an open cluster is M45, The Pleiades in Taurus and the Hyades also in Taurus. A good example of a globular cluster is M44, Praesepe or “Beehive” cluster in Cancer.

Coma. Large gaseous region surrounding the nucleus of a comet, together making up the head.

Comet. Small body in the solar system, in orbit around the Sun. Some of its frozen material vaporizes during the closer parts of its approach to the Sun to produce the characteristic tail, behind the right head.

Conjunction. Closest apparent approach of two celestial objects. Planetary conjunctions were once considered important omens for events on Earth.

Constellation. A group of stars that seemed to suggest the shape of some god, person, animal or object. Now a term used to designate a region of the sky. There are 88 constellations.

Continuous spectrum. Radiation spectrum that displays a smooth variation of intensity as the wavelength changes, as opposed to a line spectrum that derives from sharply defined energies of atoms.

Core. Central region of a planet. The Earth’s core is liquid, possible with a small, solid inner core.

Corona. The outer (high-temperature) region of the solar atmosphere.

Coronagraph. Instrument that blocks out the bright light from the solar photosphere and so makes it possible to observe the chromosphere and corona.

Cosmic rays. Atomic nuclei and electrons that travel in space at very high speeds. Most cosmic rays that are detected on Earth come from distant parts of the galaxy, but some come from the Sun, especially during its active phases.

Cosmogony. A term sometimes used to describe the study of the origin of the universe, but more frequently now used in the restricted sense of the origin of the solar system.

Cosmology. The study of the origin and Large-scale features of the universe.

Crab nebula. Remnant of the supernova observed in 1054 AD, now observable as an expanding and tangled cloud of gas, with a pulsar at the center.

Declination. Angular distance of an object north or south of the celestial equator, measured in degrees. Thus the north celestial pole has a declination of +90 degrees.

Density. A measure of compactness: mass of an object divided by its volume.

Dwarf. Main-sequence star of low luminosity.

Eclipse. Blocking of light from one body by another that passes in front of it. Eclipse can be total or partial.

Eclipse path. Narrow path on the Earth’s surface traced by the Moon’s shadow during an eclipse.

Eclipsing binary star. Binary star whose mutual orbit is viewed almost edge-on so that light observed is regularly decreased each time one star eclipses the other.

Ecliptic. Path that the Sun appears to follow, against the stars on the celestial sphere, during the course of a year.

Ecliptic plane. Plane defined by the Earth’s orbit around the Sun.

Electromagnetic radiation. Radiation that carries energy through regular variations in its electric and magnetic force. Includes radio waves, infrared and ultraviolet radiation, visible light, X-rays and gamma rays.

Ellipse. Type of closed curve whose shape is specified in terms of its distance from one or two points. A circle is a special form of ellipse. In appearance, an ellipse is oval-shaped.

Elliptical galaxy. Type of galaxy that appears to have an elliptical shape and contains no visible interstellar material.

Elongation. Angle between the directions of a planet and the Sun, as viewed from the Earth.

Emission line. Bright line of a spectrum, with the wavelength defined by the energy levels of the atoms or molecule from which the radiation is emitted.

Emission nebula. A cloud of interstellar gas that glows because of ultraviolet radiation absorbed from a nearby hot star.

Emission spectrum. Spectrum that consists of emission lines.

Equinox. Two days each year when the Sun is above and below the horizon for equal lengths of time.

Escape velocity. Minimum speed that must be given to a body in order that it can travel away from its starting point and not be restrained by gravity to following a closed orbit. Often employed in relation to escape of spacecraft from Earth or molecules from a planetary atmosphere.

Extragalactic. Beyond the Milky Way galaxy.

Flare, solar. Rapid release of energy from a small region of the solar surface, observed as a sudden and localized brightening. Solar flares can emit cosmic rays as well as electromagnetic radiation.

Flare, star. Star that sometimes brightens very suddenly, with no regular repetition.

Fraunhofer lines. (Dark) absorption lines observed in solar or stellar spectrum.

Full moon. Observed once each month, when the Earth lies between the Sun and Moon, and the fully illuminated disk of the Moon can be seen.

Galactic cluster. Cluster of stars with an open (not too closely packed) appearance, as opposed to globular cluster.

Galactic equator. Central plane of the Milky Way galaxy.

Galactic latitude and longitude. System of coordinates useful in specifying the location of objects with respect to the galactic equator (for latitude) and the direction toward the galactic center (for longitude).

Galaxy. Large number of stars with their interstellar gas and dust, grouped into a region that is well separated from other galaxies. (Star clusters occur on scale much smaller than that of galaxies).

Galilean satellites. Four satellites of Jupiter discovered by Galileo who first named them Medicean satellites, after Cosimo de Medci, his patron.

Gamma rays Shortest wavelength electromagnetic radiation.

Gegenschein. Dim and hazy glow in the sky, seen in the direction opposite to the Sun, probably caused by sunlight reflected from many small dust particles.

Giant. Star with very large luminosity and radius, much more luminous than main-sequence stars with same surface temperature.

Gibbous Moon. Phase of the Moon during which more that half of the disk appears illuminated.

Globular cluster. Tight cluster of stars that is circular and symmetrical, as opposed to open or galactic clusters.

Granulation. Terrazzo-like appearance of solar photosphere when observed at high magnification.

Halo ( of galaxy). Region of a spiral galaxy that extends away from the flattened disk, and contains few stars, some globular clusters, making a roughly spherical overall shape for the galaxy.

Head (of comet). Bright part of a comet, containing the small nucleus and its surrounding coma.

Heliocentric. With the Sun at the center, as in the Copernican model for the solar system.

Hertzxprung-Russel diagram. Graph showing absolute magnitude or luminosity plotted against temperature or color index for individual stars.

Inferior planet. Mercury and Venus, those planets whose orbits lie between the Earth’s orbit and the Sun.

Interstellar dust. Small, solid particles or grains, probably mostly silicates and graphite.

Interstellar reddening. Relative reduction of the intensity of the shorter (blue) wavelengths of a spectrum, compared to the longer (red) wavelengths, caused by absorption and scattering of light by interstellar dust.

Ionosphere. Outer region of the Earth’s atmosphere where many of the atoms have been ionized by the absorption of solar ultraviolet radiation.

Ionization. The process of removing one or more electrons from an atom or molecule.

Irregular galaxy. Galaxy that has no symmetrical or spiral shape.

Jovian planet. Any of the large outer planets: Jupiter, Saturn Uranus, and Neptune.

Keplerian. An orbit that follows Kepler’s Law.

Latitude. Coordinate used to measure (in degrees) the angular distance of a point or celestial objects above or below an equator.

Light year. Distance that light travels in 1 year.

Limb. Edge of a bright object (Sun or planet) as viewed from Earth.

Local Group. Group of galaxies closest to the Milky Way galaxy; includes the Large and Small Magellanic Clouds, and Andromeda.

Longitude. Coordinate used to specify the position of a point or direction around (or parallel to) an equator.

:Luminosity. Rate at which radiant energy is emitted by a star or other object, usually expressed in erg/sec.

Luminosity class. Classification of stars on the H-R diagram within the same spectral class.

Magnetic field. Region surrounding a magnet or electric current, in which magnetic force can be detected in such a region, high-speed electrically charged particles will generally move along curved paths and radiate energy.

Magnetic field. Region surrounding a magnet or electric current, in which magnetic force can be detected in such a region, high-speed electrically charged particles will generally move along curved paths and radiate energy .

Magnetic pole. One of the two regions on Earth to which a compass needle will point. Poles also exist on magnets, and the magnetic fields of some electric currents can have an equivalent behavior.

Magnetoshpere. Region surrounding star or planet (including Earth) in which a magnetic field exists.

Magnitude. Scale for describing brightness of a celestial object. See apparent magnitude, absolute magnitude.

Main sequence. Band on the H-R diagram, populated by stars that derive their radiant energy from the fusion of hydrogen to helium in their cores. Along the man sequence, the more massive

Mare (plural: maria). (Latin) name applied to areas on the Moon’s surface that seemed sealike when first viewed from Earth through a telescope.

Meridian. Great circle, on the celestial sphere or the Earth, that passes through both north and south poles and an observer’s zenith or location.

Meteor. Glowing trail in the upper atmosphere, produced by meteoroid burning up as it moves at high speed.

Meteor shower. Numerous meteors seen in short time span as the Earth moves through a cloud of meteoroids, probably remnants of a comet and still following the comet’s orbit.

Meteorite. Remnant of meteoroid that has been partially eroded in passage through the Earth’s atmosphere before hitting the surface. Term now also applied to similar bodies that collide with the surfaces of the other planets and their satellites, producing craters.

Meteoriod. Large rock (but much smaller than minor planets) moving in an orbit in the solar system. Meteoroids that enter in the Earth’s atmosphere are termed meteors or meteorites, depending on their behavior.

Milky Way. Bright band that stretches across the sky, produced by large number of stars and other bright objects that lie near the equatorial plane of our galaxy.

Milky Way galaxy. Concentration of stars, gaseous nebulas, interstellar gas and dust in which the Sun and solar system are located.

Minor axis. Smallest diameter of an ellipse.

nadirThe point opposite the zenith on the celestialsphere.

Nebula. Object with nonstellar appearance. Objects originally labeled as nebulae are now known in include galaxies (Andromeda is one), clouds of gas and dust (Orion nebula), and supernova remnants (Crab nebula).

Neutron. Subatomic particle with mass closely similar to that of the proton but carrying no electric charge. A constituent of all atomic nuclei except hydrogen.

Neutron star. Star composed of neutrons except for a very thin surface layer of atoms. Neutron stars have masses similar to the Sun but dimensions not much larger than the Earth, and , as a result, have very high densities.

New General Catalog (NGC). Comprehensive listing of star clusters, nebulae and galaxies published in 1888 by J. E. L. Dreyer, director of Armagh Observatory, Ireland. Later supplemented by two Index Catalogs (IC).

New moon. Phase of the moon when its motion brings it between the Earth and Sun, and thus appears to us not to be illuminated.

Newtonian telescope. Type of reflecting telescope devised by Newton in which a small flat mirror deflects the light from the primary mirror deflects the light from the primary mirror to a focus outside the telescope tube. Thus Newtonian focus.

Nova. Abbreviation from nova stella. Latin for new star, literally meaning the sudden appearance of a star where none had previously been known. Term now applied to sudden large brightening of a star, followed by a less rapid decrease in brightness.

Occultation Eclipse f a planet or star behind the Moon or one of the planets.

Oort cloud. Suggested reservoir of comets, located in a spherical region around the Sun and about 50,000 AU in radius.

Open cluster. Galactic cluster of stars in which the individual stars can be seen, located within the spiral arm of disk of the galaxy.

Open universe. Cosmological model in which the universe expands forever.

Opposition. Planetary position when it is on the opposite side of the Earth from the Sun.

Opitcal binary. Pair of stars that appear to be related because they seem close together, but are actually situated at very different distances from the Earth.

Orbit. Path traced out by one object around another.

Penumbra. Part of a shadow from which some of the illumination object can still be seen.

Perigee. Point in an Earth satellite’s orbit where it is closest to the center of the Earth.

Perihelion. Place in an orbit around the Sun which is closest to the center of the Sun. A term most frequently encountered in describing cometary orbits.

Phases of the moon. Cycle of variations in the Moon’s appearance, produced by the changing Sun-Moon-Earth angle through each month. The result is a regular cycle of changes in the Moon’s brightness and apparent shape.

Photosphere. Bright apparent surface of a star from which most of the radiant energy is emitted. Not a solid surface but rather a region within the stellar (or solar) atmosphere.

Planetary nebula. Relatively thin shell of gas, blown off from and illuminated by a very hot star. Expands at high speed as it disperses into the interstellar medium.

Precession. a. of the Earth: slow, toplike motion the Earth’s rotation axis, caused by the gravitational effects of Sun and Moon on the Earth’s nonspherical shape. A result of this precession is a slow drift in the apparent position of the stars and other celestial bodies.

Pulsar. Object that emits pulses of radiation with extreme regularity. Pulse periods for different objects range between 1.5 thousandths of a sec and about 5 sec. Pulsars are thought to be rotating neutron stars, extremely compact objects of great density.

Quasar. Object that appears starlike but its actually extra-galactic, moving away from us at high speed. Distance, deduced from velocity-distance relation, is very large. Extremely luminous.

RR Lyrae variable. Class of variable stars with periods less than 1 day.

Red giant. Large star with relatively low temperature but high luminosity; a stage in stellar evolution after a star has left the main sequence.

Reddening. Alteration of a spectrum of light that has passed through a dusty region, produced by the preferential scattering and absorption of the shorter wavelength (blue) light, leaving the red light less affected.

Redshift. Shift of wavelengths to longer (redder) values, caused by either a relative velocity of separation of source and detector, or else by a gravitational field.

Reflecting telescope. Type of telescope in which the objective is a concave mirror.

Reflection nebula. Interstellar cloud of gas and dust that is seen by the light which it reflects from nearby stars.

Refracting telescope, refractor. Type of telescope in which the objective is a lens. Usually an achromat.

Refraction. Bending of light and other electromagnetic radiation in passing from one transparent medium to another.

Retrograde motion or rotation. Apparent backward (westward) motion of a planet, as seen against the stars (as opposed to the regular or prograde motion in an easterly direction). Also applied to the direction of rotation of a planet (such as Venus) about its own axis in the opposite direction to that of the other planets.

Right ascension (R.A.) Coordinate for measuring celestial longitude along the celestial equator; measured in hours and minutes.

Roche’s limit. Distance from one body within which gravitational forces would break up a second body.

Rotation. Movement (spin) of a body abut an axis that passes through that body. Distinct from revolution, which is motion in an orbit about some point or other body.

Satellite. Body that revolves in orbit around another body. Planets are satellites of the Sun, the Moon is a satellite of the Earth, and artificial satellites have been sent into orbit around the Earth, Moon, Mars and Venus.

Schmidt telescope. Type of reflecting telescope that uses a spherical primary mirror and a thin correcting lens across the full aperture.

Sidereal. Related to the stars. Thus sidereal day, month, period, year: lengths of time intervals specified by motion of some object relative to the stars, as opposed to apparent lengths of those time intervals that will depend on the Earth’s own movements.

Solar activity. Variable phenomena observed on the Sun. Some (such as the sunspot cycle) will be fairly regular, but individual spots, prominence will not display any regularity.

Solar nebula. Extended cloud of gas and dust from which the Sun, planets and other bodies of the solar system are thought to have formed.

Solar wind. Flow of high-speed electrically charged particles form the solar corona, outward through the solar system.

Solstice. Extreme positions reached by the Sun, north and south of the celestial equator. When the Sun is at these positions, the shadows it casts on the Earth’s surface are the longest or shortest (depending on whether you are making this observation in the northern or southern hemisphere).

Spectral class (type). Classification of stars by the spectral features, into groups designated by O, B, A, F, G, K, M. This grouping also corresponds to surface temperatures.

Spectrograph. Instrument for dispersing light into a spectrum and then photographing it.

Spectrophotometry. Measurements of the intensity of light in various parts of a spectrum.

Spectroscope. Measurement of the intensity of light in various parts of a spectrum.

Spectroscope. Instrument for viewing a spectrum. Usually contains a prism or grating that disperses the light.

Spectroscopic binary. Type of binary star whose components are not seen separately, but whose spectrum shows periodic wavelength shifts that can be interpreted as the results of orbital motion.

Spectrum. The spread or range of wavelengths in the radiation emitted by some body or region. The type of spectrum depends on the physical processes involved in the emission of the radiation.

Spiral galaxy. Type of galaxy that has a flattened shape, with arms of stars and gas and dust extending either from the nucleus or from short bars that are themselves connected to the nucleus.

Star cluster. Group of stars within a galaxy, either very closely packed (in globular clusters) or further apart ( in open clusters).

Stratosphere. One of the upper layers of the Earth’s atmosphere, above the troposphere that contains most of the weather, and below the ionosphere.

Summer solstice. extreme northern position of the Sun on the celestial sphere, north of the celestial equator (summer refers to the northern hemisphere).

Sunspot. Area that appears dark on the solar disc because the sunspot has a temperature somewhat lower than its surroundings.

Sunspot cycle. 11-year periodicity in the number of sunspots.

Supergiant. Most luminous class of stars.

Synodic period. Time interval between successive repetitions of the same apparent positions of the Sun, Earth, and a body. Thus synodic period for a planet, or synodic month for the Moon.

Terminator. Line between bright and dark areas on the Moon or planet, marking the boundary of the area illuminated by the Sun at that time.

Terrestrial planet. Mercury, Venus, Earth, or Mars.

Tidal force. Gravitational force that is stronger on one side of a body than on the other. On the Earth, the oceans can respond to tidal forces and move relatively easily. On a completely solid object, the tidal force can produce a deformation that might even break the body.

Total eclipse. Eclipse of the Sun in which the Moon completely hides the solar photosphere, or an eclipse of the Moon in which it passes completely into the umbra behind the Earth.

Transit. Passage of a celestial body across the meridian, or an instrument used to observe transits. Also passage of one body in front of another (without eclipsing it): for example, transits of Mercury across the face of the Sun.

Trojan asteroids. Asteroids that are in solar orbit, in the same orbit as Jupiter but precede or follow the planet by 60 degrees.

Troposphere. Layer of the Earth’s atmosphere, from sea level to about 10-15 km. Altitude, containing most of the weather.

Umbra. Central part of a shadow.

Van Allen belts. Regions around the Earth where the Earth’s magnetic field confines high-speed electrically charged particles, mostly protons and electrons.

Variable star. Star whose luminosity changes. This designation will include stars with explosive changes ( novae and supernovae) as well as cyclic changes (Cepheids and RR Lyrae).

Velocity. Speed in a designated direction. The rate at which a body changes its position is usually designated as speed, which the direction of motion is not considered. Velocity implies of definite direction.

Visual binary star. Binary star whose two components can be seen by telescope to be separate.

White dwarf. Star that is less massive than 1.4 times the solar mass, that has consumed almost all of its nuclear fuel and has contracted to a size not much larger than the Earth. Characterized by high surface temperature but small luminosity, as compared to the Sun. White dwarfs fall below the main sequence in the H-R diagram.

Winter solstice. Extreme southern position of the Sun on the celestial sphere, south of the celestial equator (winter refers to the northern hemisphere).

Zenith. Point on the sky directly overhead.

Zodiac. Band on the sky, centered on the ecliptic, and about 18 degrees wide, through which the Sun, Moon and planets appear to move through the course of each year.

Zodiacal light. Faint glow seen at night near the ecliptic, probably sunlight reflected by interplanetary dust.


sm. starBack to the other outline.


I still get really excited about things that I see in the night sky. That is why I started this mailing list. Those who join this list will get email letters about things like meteor showers, planets that are up and all kinds of other wonderful events. I hope to someday include short articles about constellations. I just want everyone else to have as much fun as my astronomy club and I are having. If you are interested, sign up.

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