aborigine: From Latin, “people who have been present from the beginning”; When capitalized refers to indigenous Australians.
absolute magnitude: The brightness of a star as seen from a standard distance, defined as 10 parsecs.
accretion: A gradual growth or increase in size due to a slow buildup of material.
active galactic nucleus: The core of an active galaxy; usually abbreviated AGN.
active galaxy: A type of galaxy not defined by its shape or structure; these galaxies have a core, or nucleus (the active galactic nucleus), that is thought to generate huge amounts of energy.
afocal photography: A method of astrophotography that places the camera near the telescope eyepiece with both lenses focused at infinity.
albedo: The fraction of sunlight that is reflected off the surface of an object in the solar system; objects with high albedo (near 1) are bright, while objects with low albedo (near 0) are dark.
alt-az mount: Altitude-azimuth mount is a type of telescope mount involving axes that will move both horizontally and vertically.
altitude: The vertical height or elevation of an object, measured from the horizon or an arbitrary plane.
aperture: An opening designed to let light through; the lens diameter in a telescope.
aphelion: The point in the orbit of a planet (or other celestial body) where it is farthest from the Sun
apparent magnitude: The brightness of a star as seen from Earth.
archeoastronomy: A combination of archeology and astronomy that involves the study of religion, folklore, celestial myths, and all ancient astronomical rituals and ideas.
armillary: A spherical astronomical instrument used to measure the longitude and latitude of planets and stars.
asterism: A small group of stars that is not a constellation.
asteroid: A rocky or metallic, atmosphere-free body that orbits around the Sun, usually less than 1,000 kilometers in diameter.
astrolabe: An instrument using spherical projections to locate the position of celestial bodies.
astrology: The study of how the movements of the stars and planets affect us and our lives.
astronomical compendium: A small measuring device used during the Renaissance; could contain map, sundial, compass, astrolabe, sextant, and other astronomical tools.
astronomy: The study of stars, planets, and other bodies outside Earth's atmosphere.
astrophotography: Photography used in astronomy; can involve photography through telescopes.
atmosphere: The gaseous material surrounding planets; the air surrounding Earth.
azimuth: The direction of an object (planet, star, or other object in the cosmos) measured around the horizon clockwise.
barred spiral galaxy: A type of galaxy with arms of stars that spiral out from a linear central bar.
Big Bang theory: The scientific explanation for the origin of the universe, championing the notion that the universe spontaneously originated 15 billion years ago in a colossal explosion
binary star: A two-star system, where the individual stars orbit their common center of mass, often appearing to revolve around each other.
black hole: A location in space with very high mass and density; gravity prevents anything, including light, from escaping such a region.
blazar: A very energetic quasar that emits electromagnetic radiation.
cairn: a pile of stones used as a landmark, usually covering an underground chamber.
cataclysmic variable star: A type of variable star whose brightness is altered suddenly and violently by occasional explosions from within the star.
catadioptric telescope: A telescope that uses both mirrors and lenses to reflect and gather light; a compound telescope.
CBAT: Central Bureau for Astronomical Telegrams.
celestial sphere: An imaginary hollow sphere surrounding Earth, with all the stars painted on the inside.
charged coupled device: A computer chip with a grid of embedded light-sensitive detectors; (CCD).
closed universe: The theory that if the density of the universe is greater than the critical density, expansion will eventually cease, and the universe will someday collapse into to a single point.
codex: A manuscript book.
collimation: Creating parallel axes, or alignment of binocular or telescope optics to their mechanical axes.
comet: An ice-rock body composed of ice and nonvolatile dust, constituted mainly of frozen water or gas. Comets typically have five parts: a nucleus, a hydrogen cloud, a coma, a dust tail, and an ion tail.
constellation: A configuration of stars.
convection: The transfer of heat through the circulation of heated parts of liquid or gas.
convective zone: The top 15 percent of the Sun, where convection takes place.
coordinate system: The indexing of two or more terms (or coordinates) so that the intersection of their points is a meaningful way to locate objects.
cosmology: The study of the origins of the universe.
crater: A hole in the ground, usually hemispherical, resulting from the impact of a heavy, falling object.
critical density: The density of the universe at which the fate of the universe changes from one that expands forever (open universe) to one that will eventually stop expanding and begin contracting (closed universe).
dark matter: Material that does not give off any radiation, and is suspected of making up a sizeable fraction of the universe.
dark nebula: A cloud of gas and dust that is large and dense enough to block out the light given off by something behind it.
declination: Similar to latitude it is the angular distance to a celestial object, measured from the celestial equator; it helps identify the positions of stars and other celestial bodies.
Dobsonian mount: A type of telescope mount that uses a simplified altitude-azimuth design; the telescope maintains position by the friction created between the telescope bearings and the mount. Promoted by John Dobson of the Sidewalk Astronomers.
doppler effect: The observation that the pitch of a sound changes when it moves toward or away from an observer.
dualism: The sixteenth-century philosophical notion that mind and matter are completely separate substances, but mutually affected by God.
eccentric: Elliptical, deviating from circular.
eclipse: The passing of one celestial body in front of another.
eclipsing binary star: A type of binary star system whose orbital plane lies close enough to our line of sight that it appears to eclipse itself, appearing as a variable star.
ejecta: Material that has been thrown out, or ejected, from something, usually material from an impact crater.
elliptical galaxy: A type of galaxy that is shaped like an ellipse.
emission nebula: A very hot gas cloud; light from nearby stars causes these nebulae to emit radiation as transitions take place from high-energy to low-energy states.
epicycle: A circle in which a planet moves, while at the same time rotating around its own center. Required in geocentric views of the solar system.
equinox: One of two times each year when the Sun crosses the equator, so day and night are equally long—generally, March 21 and September 23. Also, the place on the celestial sphere where the ecliptic crosses the celestial equator.
erosion: The state of being diminished or reduced over time, usually through repeated application of natural elements such as water, wind, or ice.
escape velocity: The speed at which something needs to travel in order to escape the gravitational pull of a star or planet.
event horizon: The distance from the center of a black hole inside which everything approaching the black hole falls inward toward the singularity.
exit pupil: The ratio of the diameter of the objective lens to the binoculars’ magnifying power.
extrasolar: A descriptive term for planets that are not in our solar system.
extraterrestrial: Occurring outside the atmosphere and boundaries of Earth.
eye relief: The distance the viewer's eyes need to be from the binocular eyepiece in order to take in the entire field of view.
finder scope: A small telescope that attaches to a larger one; allows you to align your view on certain stars or planets.
fireballs: Meteors that appear brighter than anything in the sky except for the Sun and Moon.
focal length: In optics, refers to the distance from the center of a lens to the point where it is in focus. In a telescope, this is usually the length of the tube.
galactic cluster: A grouping of stars that is less dense than a globular cluster; an irregular mass, usually with no central core, generally having no more than a few thousand stars.
galaxy: A large group of stars, gasses, and dust held together by gravitation and separated from similar systems by vast regions of space.
gas giant: A large planet, such as Jupiter, that is composed mostly of gaseous elements instead of rocky material.
geocentric: A view that Earth is at the center of the solar system.
German equatorial mount: A telescope mount designed to follow the movements of planets or stars across the sky.
globular cluster: A very dense grouping of stars existing inside or next to a galaxy, containing anywhere from 100,000 to 1 million stars.
gravitational lensing: A phenomenon that occurs when an object has enough mass that it can bend light passing by it from an object behind it, causing observers on Earth to receive multiple images of the same object.
greenhouse effect: The gradual raising of the temperature of Earth's surface and atmosphere, due to a buildup of carbon dioxide and water vapor which traps the radiation from the Sun.
habitable zone: The area surrounding a star at a distance from the star where the temperature is between the freezing and boiling points of water.
heliocentric: A view that the Sun is at the center of the solar system.
hemisphere: Half of a sphere.
hydrothermal vent: A fissure or crack in the bottom of an ocean through which hot liquids or other materials pass up from the planet's interior
intelligent life: Generally considered to be life forms that can communicate with humans.
irregular galaxy: A type of galaxy that is not arranged in any orderly pattern and has no observable symmetry, such as a spiral or ellipse.
kiva: Partially underground chambers used for spiritual rituals by Native Americans.
latitude: The location of a point on Earth's surface north or south of the equator, expressed in degrees.
lens: A piece of glass or plastic having either curved or straight surfaces; used in optical instruments to focus light rays.
longitude: the number of degrees a body is located east or west of an arbitrarily defined starting point (usually the prime meridian) on Earth's surface.
magnetic field: A condition generated by electrons moving through space.
magnification: The state of apparent enlargement, especially when something is viewed through an optical tool such as a telescope.
magnitude: In the context of astronomy, the brightness of a star as seen from Earth; the brighter a star, the smaller its magnitude.
maria: Latin for ocean, refers to dark patches on the Moon, many of which are ancient lava flows.
medicine wheel: A rock arrangement closely resembling a spoked wheel, used by Native Americans.
megalith: A large, rough stone used for buildings or monuments in prehistoric times.
megaparsec: A unit of measurement for distance in space; equals 1 million parsecs.
meridien: an imaginary line on the Earth or the celestial sphere that goes from pole to pole through the observer's location (or a point directly above it).
Messier's catalogue: A description of 110 galaxies and other nonstar objects as observed and documented by Charles Messier in 1774.
meteor: Seen as a bright streak in the sky (sometimes called a shooting star or falling star) it is a tiny grain of dust or pebble that burns up when it hits Earth's atmosphere. Meteors come from comets and other space debris.
meteor shower: A regular occurrence where multiple meteors are visible.
meteorite: A meteor that survives passage through the atmosphere and crashes into Earth.
meteoroid: Small piece of rock or dust floating freely in space, before they enter the Earth's atmosphere and burn up as meteorites.
moon: A natural satellite of a planet.
mutation: A permanent change, either physical or chemical.
NASA: National Aeronautics and Space Administration. Founded in 1958, the American space agency has been responsible for a number of remarkable breakthroughs in aeronautics and space exploration.
near-Earth asteroids: Asteroids that come within approximately 120 million miles of the Sun.
nebula: A large cloud consisting of dust and gas that exists in interstellar space.
neutron star: A very dense core of material that forms after a star explodes.
nova: A star that has a sudden bright period, then a sharp decline in brightness.
nuclear fusion: The process by which the nuclei of two atoms fuse together to form a heavier element; large amounts of energy are produced by this fusion.
nucleosynthesis: The process by which hydrogen nuclei combine in pairs to form helium nuclei, and so on to form the elements in the early universe and inside stars.
observatory: A building or location equipped to watch astronomical phenomena.
occultation: In astronomy, the occurrence of one large celestial body moving in front of another, smaller one.
orbit: The path one object takes as it revolves around another, usually due to external forces such as gravity.
orbital resonance: The condition resulting from a two-planet or satellite system where one orbital period is twice that of the other. Resonances are possible with more than two planets; three of the Galilean satellites of Jupiter are in 1:2:4 resonances with each other.
organism: A collection of interdependent parts that survives and thrives by acting as a single unit.
origin: The point at which things begin; zero.
parapegma: A stone tablet that allowed Greek astronomers to make connections between dates and planetary movements, a very early predecessor to the planisphere.
parsec: A unit of measurement for distances in space that equals 3.26 light-years. It's defined as the distance at which the radius of Earth's orbit around the Sun would measure 1 arc-second.
perihelion: The point in the orbit of a planet (or other solar system object) where it is nearest the Sun.
photosphere: The surface of the Sun, the deepest layer that can be visibly observed.
planet: A large body that revolves around a star, such as the Sun in our solar system, and is not sufficiently massive to ignite and undergo nuclear fusion.
planetarium: A room, usually in a science museum or observatory, where celestial objects and events are projected onto the walls and ceiling.
planetary nebula: Gas ejected from a star that is approaching the end of its life; resembled a planet to early telescopic observers.
planisphere: The projection of a sphere onto a flat surface, or plane. In astronomy, a projection of the celestial skies into a plane; also called a star wheel.
plume: A long, rising column of gas or smoke.
prime meridian: The zero line for longitude at the British Royal Greenwich Observatory, in Greenwich England.
prism: A transparent object that disperses light and separates it into the individual colors of the spectrum.
prograde motion: The regular movement of planets in the sky, from west to east; opposite of retrograde, which is east to west.
protostar: A clump in a cloud of gas which heats and collapses, but is not yet a true star because it does not perform nuclear fusion.
pulsar: A neutron star that spins extremely rapidly.
pulsating variable star: A type of variable star in which the actual layers of the star expand and contract, giving off varying amounts of light.
quadrant: An instrument consisting of a graduated arc with a plumb line, used to measure the altitude of celestial bodies.
quasar: Means quasi-stellar object; a class of celestial objects that look like stars when photographed, but have high redshifts and give off strong radio emissions.
radiation: The emission of energy as electromagnetic waves.
radiative zone: The region outside the core of the Sun where energy travels from the interior to outer layers.
radio galaxy: A galaxy that gives off radio waves.
red giant: A star near the end of its lifetime with a relatively low surface temperature and large diameter when compared to our Sun; the star is cool because its energy is spread out over such a large diameter.
redshift: The shifting of an object's spectrum toward longer wavelengths due to its motion away from an observer.
reflecting telescope: A telescope that uses a concave mirror to gather light.
reflection nebula: A dust and gas cloud that reflects the light from nearby stars.
refracting telescope: A telescope that gathers light through a lens, sending the light to the eyepiece.
Renaissance: The historical period in Europe between the fourteenth and seventeenth centuries that was characterized by a rebirth of interest in art, music, astronomy, humanism, and other cultural and sociological events.
retrograde motion: When internal and external planetary orbits coincide in such a manner that it appears the planet has reversed directions in the sky; movement from east to west; opposite of prograde motion.
right ascension: Similar to longitude; a measurement from 0 to 24 hours that helps identify the positions of stars and other celestial bodies.
satellite: A celestial body that orbits another, larger, celestial body that is not a star; also called a natural satellite, as distinct from a robotic artificial satellite.
SETI: Search for Extraterrestrial Intelligence; also the SETI Institute, a research institution located in Mountain View, California.
sextant: An instrument used to measure the angular distance between stars or planets.
Seyfert galaxy: A type of active (spiral) galaxy that emits low-energy gamma rays.
silicate: A common rocky material containing silicon, oxygen, and one or more metals; the primary component of some types of asteroids as well as many other solar system bodies.
singularity: In the Big Bang theory, the point where time becomes zero and the density goes to infinity. Also can refer to the point at the center of a black hole.
solstice: One of two points on the ecliptic where the distance from the equator is at its maximum; summer solstice occurs around June 22, winter solstice around December 22. These are the longest and shortest days in the Northern Hemisphere.
spacecraft: A man-made vehicle designed to pass through Earth's atmosphere into orbit or beyond.
spectroscopy: The process of passing light through a prism to study different wavelengths.
spectrum: The color band created when white light is scattered.
spiral galaxy: A type of galaxy with arms of stars that appear to spiral out from a central core.
star: A self-luminating ball of gas that produces energy from internal nuclear reactions.
star party: A gathering held in a dark-sky location, where amateur astronomers bring telescopes and get together to observe celestial objects and events.
steady state theory: The notion that the universe has been around forever and is slowly expanding.
subatomic: Pertains to the particles contained within an atom, such as electrons, neutrons, or protons; smaller than an atom
subsurface: Means beneath the surface layer.
supernova: An explosion at the end of the life of a large star; it can be up to a billion times brighter than the Sun.
terrestrial: Relating to Earth.
theodolite: An instrument used by surveyors and astronomers for measuring both altitude and azimuth.
tidal flexing: A condition that occurs when the gravitational pull of a planet tugs on one of its satellites, causing the satellite's crust to flex up and down.
tidal heating: The warming of a planet due to tidal flexing.
transit: In astronomy, the occurrence of something small in our solar system moving directly in front of something larger.
universe: Everything that exists; the cosmos.
variable stars: Stars that change brightness periodically.
wavelength: The distance between corresponding peaks in different phases of a wave.
white dwarf: A small, very dense star object that cannot collapse into itself any further; one possible result for a low-mass star at the end of its life.
white hole: The theoretical “other end” of a black hole.
Worm hole: The theoretical connection between a black hole and a white hole.
Zodiac: Greek for “circle of animals,” a group of twelve symbols and mythological stories that correspond to twelve constellations.