TERMINOLOGY / GLOSSARY
 

Term Description:

Altitude Angular distance, usually measured in degrees, above the horizon
Aphelion The point farthest from the sun in an object's orbit
Apogee The point in a satellites orbit when it is farthest from the Earth
Apparent Magnitude A measure of the brightness of a celestial object as seen by an observer; brighter objects have smaller numerical magnitudes.
Asterism A notable pattern of stars, such as the Big Dipper, forming part of a constellation; not a constellation itself.
Asteroids Small, rocky objects that orbit the Sun; mainly between Mars and Jupiter
Astronomical Unit Unit of measurement, abbreviated AU, that is the distance between the Earth and the Sun; 93 million miles
Averted Vision An observing trick of looking at an object in the eyepiece is to look a little to one side of the target. This "averted vision" places the target on a more light-sensitive area of the eye's retina where it will pop into view.
Azimuth A celestial object's distance from true north measured eastward in degrees along the horizon
Binary Stars A double star system; two (or more) stars revolving about a common center of gravity
Black Hole A collapsed massive star; so dense that not even light can escape the pull of its gravity
Brown Dwarf A star with insufficient mass to initiate a fusion reaction
Cassegrain Telescope A reflecting telescope in which a secondary mirror reflects light back through a hole in the center of the primary mirror.
Catalogues Lists of stars and deep sky objects, i.e., M11 (Messier Object No. 11); NGC (New General Catalog); IC (Index Catalog); SAO (Smithsonian Astrophysical Observatory); GCVS (General Catalog of Variable Stars)
Celestial Equator The projection onto the sky of the Earth's equator.
Celestrial Pole The projection of Earth's north and south pole axes onto the celestrial sphere.
Celestrial Sphere An imaginary hollow sphere surrounding the Earth on which the stars appear to be fixed.
Circumpolar The stars or constellations that are closer to the celestial pole than the horizon and therefore never set
Comets A body, probably resembling a "dirty snowball" between 0.1 and 100 miles across that travels through the solar system in an elliptical orbit of random inclination. A comet grows a tail if it comes close enough to the sun.
Conjunction The alignment or close alignment of two or more astronomical bodies.
Constellation One of 88 recognized star figures into which the sky is divided; also refers to the historical, mythological or other figures that represented earlier divisions of the sky.
Coordinate System The use of Right Ascension and Declination to locate an object in the sky.
Corona The outermost layer of the sun and many other stars; a faint halo of extremely hot (million-degree) gas
Crater The impression left in a surface of solid planets or moons by the impact of meteorite, asteroid or comet
Dark Skies Prime sky conditions for viewing without light pollution from moon or man
Dark Matter A form of matter that exerts gravitation force but does not emit any detectable light or radiation; dark matter comprises most of the mass of the universe but its exact nature remains unknown
Dark Nebula A dust cloud blocking the light from behind.
Dec Abbreviation of Declination, which along with Right Ascension determines a position on the sky. Declination is measured north (+) and south (-) from the celestial equator and specified in degrees and minutes.
Declination The angular distance of a celestial object above or below the celestial equator; the celestial sphere equivalent of latitude
Diffuse Nebula An Emission or Reflection nebula
Dust Micron-size solid particles of somewhat uncertain composition, probably carbon, iron or silicates.
Dwarf Star The remnant core of a sun-like star, which is extremely small, hot and dense
Ecliptic The plane of Earth's orbit around the sun; all the planets except for Mercury and Pluto have orbits in nearly virtually the same plane
Elongation Apparent angular distance of a member of the solar system from the sun as seen from the earth.
Emission Nebula A gas cloud radiating by stimulated emission.
Galactic Cluster A loose cluster of small number of stars; aka Open Cluster.
Galaxy A large system of millions to hundreds of billions of stars; sometimes containing large amounts of dust and gas. Our Galaxy is called the Milky Way.
Globular Cluster An ancient, spherical, densely populated cluster in orbit around the galaxies halo
Great Rift From Cygnus to Aquilla, the Milky Way appears to be divided lengthwise by enormous dust clouds.
Great Red Spot A high altitude storm cloud on Jupiter that was first observed in the 1600s
Greatest Elongation When a planet attains its greatest apparent angular distance from the sun in the sky.
Inferior Inferior conjunction is when an "inferior planet" (Mercury or Venus) passes between the earth and the sun.
Inferior Conjunction The place in which a planet whose orbit is inside that of the Earth, passes between the Earth and the Sun
Light Pollution Light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical object
Light Year The distance light travels in a year - about 6 trillion miles.
Lunar Eclipse The passage of the moon into the earth's shadow
LY Abbreviation for Light-Year
Magnitude Astronomical measurement of the brightness of an object; the lower the number, the bright the object
Main Sequence Star A star in the prime of its life; where hydrogen inside is undergoing nuclear fusion
Meridian A great circle on the celestial sphere passing through the north and south celestial poles and the zenith (overhead point) of a given place
Messier Charles Messier was a French Comet Hunter. His catalog (first published in 1781) of nebulous objects was originally intended as a list of "junk" to avoid when searching for comets.
Meteor A track of light in the sky resulting from the vaporization of solid matter entering the earth's atmosphere; commonly called "shooting stars" or "falling stars"
Meteor Shower The appearance of many meteors during a short period of time; as when the earth passes through a comet's orbit
Meteorite The part of a meteor that survives its passage through the earth's atmosphere
Milky Way The name of our Galaxy in which our Solar System is located; the band of light that encircles the entire sky and stretches from horizon to horizon that results from the combined light of billions of stars in the Galaxy's disk
Moon A smaller body orbiting a larger body; often refers to Earth's moon
Nebula Any of various types of extended objects in the sky; see individual names.
Nebulae Plural form of "nebula"
Neutron Star A collapsed, extremely dense star (i.e., a billion tons per cubic centimeter) consisting almost entirely of neutrons; the final state of a star about twice as massive as the sun
NGC New General Catalogue, compiled by JLE Dreyer in 1888; later amended by the two Index Catalogues (IC)
Northern Lights The emission of light when charged particles from the solar wind, caused by magnetic storms on the sun, slam into and excite atoms and molecules in the Earth's upper atmosphere
Nova A star that suddenly flares in brightness by a factor of hundreds of thousands
Occulation The eclipse of one celestial object by another; as when the moon or a planet passes in front of a star.
Old An old star is very old indeed, about 10- billion years or more.
Oort Cloud A vast spherical halo of icy objects around the solar system; the repository of long period comets
Open Cluster A irregular, loose cluster of stars of common origin; found in the spiral arms of the Galaxy
Opposition The moment when a planet farther from the sun than the Earth appears exactly on the opposite side of the sun in the sky; it is the best time to observe a planet
Optical Doubles Two stars at different distances that lie along nearly the same line of sight and thus appears close together
Perigee The point in a satellite orbit when it is closest to the Earth
Perihelion The point at which a body in an elliptical orbit around the sun is at its closest distance to the earth.
Planet A large rocky or gaseous body that orbits a star
Planetary Nebula A bright nebula thrown off by a dying star.
Precession A 26,000 year wobbling cycle of the Earth's rotation axis that causes a gradual shift of the celestial poles and celestial coordinates
Pulsar A celestial object that emits radio energy (sometimes light) in short, regular bursts; probably a rapidly rotating neutron star
Quasars The extremely energetic early stage in the formation of an active galaxy, characterized by an output of radiation
RA Abbreviation of Right Ascension which along with Declination defines a position in the sky. RA is measured eastward along the celestial equator with the zero point at the vernal equinox; it is specified in hours, minutes and seconds.
Reflection Nebula A dust cloud reflecting starlight.
Reflector Telescope A telescope that uses a curved mirror to gather light
Refractor Telescope A telescope that uses a glass lens to gather light
Retrograde The apparent reversal of the normal west to east motion of outer planets of outer planets as the Earth passes them
Right Ascension The celestial equivalent to lines of longitude, which runs through the celestial poles and are perpendicular to the celestial equator; they are scaled in hours, increasing eastward from 0h and going full circle to 24h
Rotation The spinning of a planet or other objects on its axis.
Sidereal Time A time scale based on the true period of Earth's rotation; the time from star rise to star rise; 4 minutes shorter than the 24 hour day
Solar Eclipse The passage of the moon's shadow across the earth
Solar System The system containing the sun and all the smaller bodies in orbit around it
Spectrum The array of colors or wavelengths that appears when light from an object is dispersed
Star A self luminous sphere of hot gas held together by gravity; ordinary stars generate energy by nuclear fusion in their cores
Stellar Classification Stars given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are O, B, A, F, G, K, and M. O Stars are the hottest; M the coolest. The numbers are simply subdivision of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright. M stars are numerous but dim. Our sun is designated G2.
Sunspot A relatively cool dark area where magnetic fields piece the solar surface
Supergiant Star Only the most massive stars become supergiants, after passing through the giant stage
Superior Conjunction The place in which a planet whose orbit is inside that of the Earth, passes on the far side of the Sun with respect to the Earth
Supernova An unusually violent explosion of a star which results in an increase in brightness of hundreds of millions of times.
Supernova Remnant Leftover gas from a supernova explosion.
Variable Star A star whose apparent brightness changes over time
White Dwarf A small dense hot white star; the final stage in the evolution of stars with masses similar to the sun's or smaller
Young Young for a star must be interpreted in the context of the average stellar lifetime of billions of years. A young star is one that is no more than a few million years old.
Zenith The point in the sky directly over an observer's head
Sun Alluding to our own Sun; sun and star are interchangeable
Parsec Equal to 3-1/4 Light Years
Horizon Imaginary horizontal line lying at eye level in all directions from you; 90 degrees from the zenith
Energy Parts of the spectrum detectable only by instruments; i.e., X-rays, infrared, microwave, etc.
Airglow The sky's natural fluorescence. It's caused by the sun's invisible but powerful ultraviolet radiation, which excites our atmosphere's gases to glow like the hand of a luminous watch
Event Horizon The boundary of a black hole where space and time curve back unto themselves.
Singularity The black hole's center where an entire star weighing 2 million planet Earths has been crushed down to less than the size of an atom
White Hole Suggested by Einstein and mathematician Nathan Rosen that a tunnel, a wormhole, could provide a conduit for the black hole and its contents to reemerge in another place or time
Hubble Flow One of the names for the expansion of the universe
Earthshine The crescent moons dark unlit portion glows; traditionally called "the old moon in the new moon's arms". A portrait of our own planet's light reflected back to our eyes
Ashen Light An enveloping halo that spills around the planet Venus to produce a complete ring when the crescent is very thin
Planesphere A wheel like device displaying the orientation of the stars on any chosen date and time
Association Family members of stars forming a large and unconfined cluster
Junkosphere Space garbage producing a girdle of scrap metal surround our world
Meteoroids Tine, nonglowing chunks of stone and metal out in space
Crepuscular Rays During a sunset, bright pinkish streams will appear if the air si dusty or humid with distant cumulus clouds
Antisolar Rays In some parts of the world, crepuscular rays often cross the sky to meet at the point opposite the sun
Time Dilation According to Einstein's special relativity theory, time warps and slows on its own
Moon Illusion Perhaps the most powerful mirage of all, the moon seems enormous when down near the horizon, a visual effect caused by its proximity to foreground terrestrial objects
Maunder Minimum The period of 1640 to 1710 where the total loss of the sunspot cycle on the sun created a period of extraordinary cold