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A - B - C

Select the letter that (probably) leads off the word in question. Please suggest new words or corrections.



  • A.U.: Abbr. See astronomical unit.

  • absolute magnitude: A quantity giving an astronomical object's intrinsic brightness defined by the apparent magnitude (see below) of an object placed at a distance of 10 Parsecs.

  • accretion disk: The theoretical nebular cloud from which the planets and the Sun coalesced into the solar system.

  • adaptive optics: System by which a telescope utlizes computer technology to correct for atmospheric effects on its image.

  • Airy disk: A disk of light seen through a telescope with about 80% of all the light from the star. It is surrounded by concentric rings, each fainter and fainter. Often used as a benchmark for determining the quality of a telescope's image. The better and more defined the disk is, the better the optical quality.

  • altitude-azimuth system: A system for celestial navigation. Rather than having a telescope who's center of rotation is centered on the celestial north pole, the telescope simply relies on an up-down, right-left bearing system, allowing it to move freely in any direction.

  • annular eclipse: This is a partial solar eclipse in which the Moon is not close enough to the Earth to completely block out the Sun's light. When the Moon is centered on the Sun there is still an uncovered ring of sunlight.

  • aperture: The diameter of a telescope's light collecting surface (either a lens or mirror).

  • aphelion: The apoapsis of an object orbiting around the Sun.

  • apoapsis: The point in an orbit farthest from the body being orbited.

  • apogee: The apoapsis of an object, commonly used for objects orbiting Earth.

  • apparent magnitude: The apparent brightness of an object as viewed from Earth, regardless of its intrinsic magnitude value.

  • arc minutes/seconds: Units of measurement in the sky based on a full circle of 360 degrees. Each degree is divided into 60 arc minutes (symbol is ' ) and each arc minute (symbol is " ) is divided into 60 arc seconds. For reference, the Full Moon is about 30 arc minutes (or 30' ) in diameter when viewed from Earth.

  • asterism: A named collection of stars in the sky that is not part an official constellation. Ex. the Big Dipper.

  • asteroid: A small, rocky, celestial body that revolves around the Sun with characteristic diameters between a few and several hundred kilometers. Also called minor planet, planetoid.

  • asteroid belt: A collection of asteroids in orbit about the Sun lying between Mars and Jupiter. It is believed that this belt formed as the result of a planet breaking apart or failing to coalesce at all.

  • astronomical unit: A unit of measurement based on the average distance Earth is from the Sun in its orbit. One astronomical unit is equal to about 93 million miles.

  • astronomy: The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

  • atmospheric "seeing": This term describes the observing conditions through which astronomers view the heavens. Bad "seeing" indicates that the atmosphere is not good for observing because of high winds, fluctuating temperatures, turbulence, haze, etc.



  • Bailey's Beads: phenomenon that occurs during a solar eclipse by which sunlight shines through the gaps produced by Moon craters, mountains, etc. The sunlight produces bright areas that appear like "beads".

  • barred galaxy: A type of galaxy in which there appears to be a "bar". The purpose and origin of the bar is not well understood, but they are often very active.

  • binary stars: A star system in which two or more stars orbit each other around a center of mass. They can share material between themselves.

  • black hole: An object in space who's gravity is so strong that nothing inside can escape, not even a ray of light. The center of the Milky Way (and many other galaxies) is thought to be a large black hole. A theory of their origin is that they are collapsed stars.

  • Blue Moon: term for the second full Moon in a single month. This usually happens a couple times a year.

  • brown dwarf: A star which does not have enough mass to ignite thermonuclear fusion in the core.



  • Cassini Division: A major gap between the A and B rings around Saturn. It was first discovered by Giovanni Domenico Cassini in the 1600's.

  • Callisto: Jupiter's second largest moon. It is composed of about 40% ice and 60% rock. It was discovered by Galileo in 1610.

  • CCD: see "charged-coupled device"

  • celestial: Of or relating to the sky or heavens.

  • Celestial Equator: The equator in the sky that corresponds to the equator on Earth.

  • Cepheid variable: Any of a class of intrinsically variable stars with exceptionally regular periods of light pulsation. Commonly used to determine celestial distances.

  • cerium oxide: A polishing agent commonly used for polishing and figuring optical surfaces.

  • charged-coupled device: A highly sensitive digital imaging device used to collect light data through a telescope.

  • Charon: The only known moon of the planet Pluto.

  • chromosphere: An incandescent, transparent layer of gas, primarily hydrogen, several thousand miles in depth, lying above and surrounding the photosphere of a star, such as the sun, but distinctly separate from the corona.

  • comet: solar system object in orbit around the Sun that is composed of ice, rock, and other chemicals. They are often described as a "snowball" in space.

  • conjunction: The position of two celestial bodies (very often one of them being the Sun) when they have the same celestial longitude (right ascension) when viewed from Earth.

  • constellation: A group of stars that are used as a system of mapping the night sky. The entire sky is broken up into multiple constellations, such as Andromeda, Perseus, and Draco to name a few.

  • corona: The luminous, irregular envelope of highly ionized gas outside the chromosphere of the Sun.

  • coronagraph: A telescope or an attachment for a telescope equipped with a disk that blacks out most of the Sun, used to photograph the Sun's corona.

  • cosmic dust: Interstellar dust that makes up much of the matter in galaxies.

  • cosmic wind: A force that pushes debris and cosmic dust along through galaxies.

  • crater: A depression on a body's (planet, moon, asteroid, etc.) surface originating from, among other things, the collision of another celestial body.


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