Night Sky Glossary


So, you've listened to those planetarium wizards as they guided you through the night sky.   Or, you're planning to endure such an experience.    Well, this guide will help you -we hope- understand some of the terms they use during these presentations.    Perhaps you're just curious about the night sky and would like to know some of the vocabulary.  

We've penned the glossary below to help you understand the night sky.  If you want to convey either effusive praise or feedback that will facilitate our personal growth, please send to


ASTERISM: a recognizable star pattern within a constellation.    For instance, the Big Dipper is an asterism within the constellation Ursa Major.   Another example:  the teapot within Sagittarius the Archer.

ASTROLOGY:  the belief system that the celestial bodies and their configurations exert a direct influence on our lives.     In other words, it's like astronomy, but effectively marketed and therefore profitable.

ASTRONOMY:  The study of the Universe beyond Earth's atmosphere.  Of course, this definition is vague because outer space begins at 100 kilometers above Earth's surface.   Called the "Karman Line," this boundary is well within Earth's atmosphere.

AURORA:  the Aurora Borealis ("northern dawn") occurs when charged solar particles excite gases within the upper atmosphere, causing them to glow.  In an average year, we observe the aurora 5-8 times.     Aurora displays can be quite vibrant, producing distinctly red or green light; sometimes the displays are muted and appear white. 

BOLIDE:  a bright meteor that explodes during descent.   These bolides also are known to produce sound before and during the explosion. 

CELESTIAL EQUATOR:  the projection of Earth's equator onto the Celestial Sphere.  (see below.)    Envision the celestial equator as being similar to a luminescent ring encircling the planet above the equator. 

CELESTIAL SPHERE:   an imaginary construct that still appeals to humanity.   The notion that the entire sky is imprinted onto an Earth-centered sphere.   This model assumes the Universe is depthless: an absurd notion that, like many absurd notions, is valued for its convenience. 

CIRCUMPOLAR: a term referring to any celestial object that doesn't set at a given latitude.   The Big Dipper, for instance, is a circumpolar star pattern at mid-latitudes because it's always in the sky.   

COMET: (a "dirty snowball") a small object consisting primarily of ice and dust and originating from either the Oort Cloud or Kuiper Belt.    When a comet approaches the Sun, it develops two tails: a dust tail, created by the outflow of liberated dust particles; and an ion tail, a stream of charged particles repelled by the solar wind.     Examples: Halley's Comet; Comet Hale-Bopp.  (A comet is named after its discoverer.) 

CONSTELLATION: a star pattern often depicting an animal, object or mythological character.    The International Astronomical Union has recognized 88 distinct constellations along the entire sky (i.e.  both hemispheres.)  Famous constellations include Orion the Hunter; Virgo the Maiden; Aquila the Eagle and Pegasus the Winged Horse.

ECLIPTIC: the Sun's apparent annual path through the sky.   The Sun doesn't move through the sky, of course.  Earth moves around the Sun and makes it appear to move around us.  We once thought the Sun did revolve around Earth, until Copernicus and Galileo told us not to be so full of ourselves.    The ecliptic passes through thirteen constellations:   Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra Scorpius, OPHIUCHUS, Sagittarius, Capricornus, and Aquarius.   (The constellation in capital letters is the "thirteenth zodiac constellation," the one that astrologers don't recognize.)

INTERNATIONAL ASTRONOMICAL UNION:   the international organization responsible for naming celestial objects.     This was the band of sinister intelligences responsible for Pluto's demotion (August 2006.)

KARMAN LINE: the boundary separating Earth and outer space. By international agreement, the Karman Line's altitude is 100 kilometers (about 62 miles.)

KUIPER BELT: a disc of cometary nuclei from which short period comets (those with orbital periods of less than 125 years) originate.  The Kuiper Belt is aligned closely with the solar system disc.    

LUNAR ECLIPSE: the passage of the full moon into Earth's shadow.

MAGNITUDE SYSTEM:  the system astronomers use to measure the brightness of celestial objects.  The lower the number, the brighter the object. For instance, the faintest stars that the unaided human eye can observe have a magnitude of 6.    The brightest night sky star, Sirius, has a magnitude of       -1.42.   Venus at its brightest has a magnitude of -5.0.   

MESSIER OBJECT: a celestial object catalogued by French astronomer Charles Messier (1730-1817).  Messier's aim was to identify objects that other astronomers might confuse for comets.   Each Messier object is denoted with an M.  M1, for instance, is the Crab Nebula; M31 is the Andromeda Galaxy.

METEOR: the light we see in the sky when a meteoroid descends through the atmosphere.  

METEORITE: a fragment of a comet, asteroid or other solar system body discovered on Earth's surface.

METEOROID:  an outer space particle cast off by a comet or asteroid.  When Earth captures it, the meteoroid descends through the atmosphere.  The resultant excitation of the atoms around it create the light we see as a meteor.

MILKY WAY: two definitions.  1. the "light ribbon" extending across the sky.  This glow is the collective light of thousands of stars within the Milky Way Galaxy's disc.    2. the name given to our home galaxy. 

NEBULA: a gaseous interstellar "cloud" from which stars are born; or a cloud produced when a star explodes.  The Orion Nebula is an example of the former; the Crab Nebula is an example of the latter.   

NORTH STAR:  Polaris.   The moderately bright star which approximately marks the positon of the North Celestial Pole.    Polaris is not precisely on the North Celestial Pole, but is within a degree of it and therefore serves as a handy marker.      In the Northern Hemisphere, one can use its position to determine latitude.   As an example, the latitude of Portland (Maine, not Oregon, thank you very much) is about 43.5 degrees, so Polaris' angle above the horizon is also 43.5 degrees. 

OORT CLOUD: a spherical distribution of cometary nuclei enclosing the solar system.  Long period comets (those with orbital periods exceeding 125 years) originate from this cloud. 

PLANET: what defines a planet?   Well, in order for a body in our solar system to be a planet it has to:  (1) be in orbit around the Sun; (2) be spherical or nearly spherical and (3) to be large enough to have cleared the "debris" in its vicinity.     Our solar system has 8 recognized planets:  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.   (Pluto was demoted in August 2006.)

SATELLITE: a body in orbit around another body.    The Moon is a satellite of Earth.   This term, however, popularly refers to the spacecraft in orbit around Earth, such as the International Space Station and the Hubble Space Telescope. 

SCINTILLATION: the "twinkling" caused when star light pases through the atmosphere.  

SOLAR ECLIPSE: the occultation of the Sun by the new moon. 

STAR: this one is not as easy as one might think.  A star is a "plasma sphere," consisting of superheated gases and powered by thermonuclear fusion in the core.    Our Sun is the closest star to us. 

ZENITH: the point directly overhead. 

ZODIAC: Use this term in front of an astronomer and his/her kidney's will fail.  An astrological term referring to the constellations through which the Sun appears to move each year.    The astrological zodiac recognizes twelve such constellations; the astronomical zodiac consists of thirteen.