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This guide contains a listing of various terms that are used in astronomy, and in reference to astronomical observing.  Here, you will find key terms defined for your aid.  If you have further questions, please feel free to use the "Ask the Astronomers" form.

The Solar System

Solar System Planet  Moon
Ecliptic Plane Equatorial Plane Celestial Poles

Asteroids to Comets

Asteroid Meteor Meteorite Meteoroid Comet


Stars and Stellar Groupings 

Star Milky Way Galaxy Nova
Supernova Neutron Star Black Hole Quasar
Globular Cluster Open Cluster Constellation Double Star System
Star Classifications

Log Book Observation Terminology 

Seeing Transparency Magnitude Messier Object NGC Object

Solar System

This term refers to any stellar object (star) and attending satellites (planets and their moons). Our solar system begins with our own sun, Sol, a yellow star of medium intensity.  


Planets were originally given the name planetes by the Greeks.  The name means, "wanderers."  For thousands of years, people have noticed the bright bodies that move against the stable background of stars.  A planet is basically a body that revolves around a home star.  
The Astronomical League hosts a Planetary Observers Club for those who want to hone their skills of planetary observation.  HERE is information about this observing program.


Moons are satellites of other planetary bodies-- they revolve around a parent body--a planet.  The planet revolves around a sun.  Even if the moon is nearly as large as the planet, it is still technically a moon if it is orbiting another body.  
Earth's moon is named Luna.
The Astronomical League's Lunar Observers Club has a certificate program for observers who qualify.  HERE is information on this observing program.

Ecliptic Plane

The Earth's, and most other planets' orbit around the sun.  From Earth's perspective, the 12 zodiacal constellations are visible in procession unless occulted by the sun.

Equatorial Plane

The Earth's equator extended into space.  It is created by the Earth's rotation on its axis.  Orion's belt lies directly on the celestial equator.

Celestial Poles

The points that lie directly above and below Earth's geographical North and South Poles.   Polaris lies within 1 degree of the North Celestial Pole, and therefore does not appear to move during the course of the Earth's rotation over the period of an entire night.  There is no bright star near the South Celestial Pole, however.

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Asteroids are chunks of debris that maintain a relatively stable orbit between Mars and Jupiter.  This zone is called The Asteroid Belt.  Asteroids may be the result of a massive collision between a small planet that was in orbit beyond Mars, and one of Jupiter's outer moons.  
The Astronomical League's Asteroid Observing Club will help you learn to find and identify asteroids.  Click HERE for information on this observing club.


A meteor is a bit of extra-terrestrial debris (whether blown off of another world or moon due to some violent impact, or simply a bit of material that has been captured by Earth's gravity) that enters Earth's atmosphere.   Atmospheric friction causes the object, which may be as small as a grain of sand, or several tons, to glow.  The streaks that we see during meteor showers are bits of material burning out in our atmosphere.  On other solar system bodies than Earth, there may not be an atmosphere to blunt the object's impact.  When captured by the sun, no matter how large the chunk, it will be vaporized.  
The tails of comets, blown outward by the solar wind, leave particles behind in their wake.  Each time a periodic (returning) comet crosses the path of the Earth, a fine trail of particles is left in our path.  Common meteor showers are known for passage though the remainder of comet tails...Comets Swift-Tuttle, and Halley's among others.
The best time to watch a meteor shower is during the wee hours of the morning...after Midnight.  Although sporadic meteors may be seen at other times, those that appear before Midnight are basically having to race the Earth.  After Midnight, we can meet them head-on, and therefore get a much more spectacular showing.
Many meteorites show evidence of having been part of a larger, differentiated, parent body.  Differentiation refers to the fact that the world was composed of an inner core, outer core, mantel, and crust.
The Astronomical League has an observing program for those who are interested in observing meteors.  HERE is information on this observing program.


A meteorite is a meteor that actually makes it to land-fall.  Most are completely vaporized as they enter Earth's atmosphere; those that land are few, in comparison to the great amount of debris that leaves only a bright streak in the night sky.  Meteorites may be bits of other planets, asteroids, or other extra-terrestrial material.


A meteoroid is a small version of an asteroid, but is too small to be detected by Earth instruments until it enters Earth's atmosphere, and appears as a meteor.


Comets are chunks of ice and rock that may be the result of a collision between two moons, planetary bodies, or a small planet and a moon.  They are largely composed of various ices and rocky material.  As comets are drawn into the solar system, the closer they get to the sun, the more volatile they become.  Ices and gasses trapped in the comet body begin to "outgas."  The result is the hazy orb that surrounds some comets, or a spectacular tail as the outgassed material is blown back by the solar wind.    

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Stars are celestial bodies that undergo a process called thermonuclear fusion, which is the basic conversion of one element (hydrogen) into another (helium), and other heavier elements, through atomic forces.  The splitting of atoms, just as in hydrogen or atom bombs, produces enough heat energy to overcome the atoms mutual electrostatic repulsion.
The colors of stars indicate the temperature at which they burn.  Those that are white to blue-white are hottest.  On the other end of the spectrum, the red stars are cooler, and may be in danger of collapsing.

Milky Way

The proper name of the galaxy where our solar system is located.  Our galaxy is a spiral galaxy, with arms extending from a central nucleus.  The bright stars are chiefly located in this disc, as is the sun.  Our naked-eye view encompasses a band of light, brightest toward the center (galaxy nucleus) in the direction of Sagittarius, and is faintest in the direction of the galaxy arms toward Perseus.   


Galaxies are an agglomerate of millions to billions of stars.  Some are organized spirals, others are barred-spirals, spherical ellipticals, or disorganized irregulars. 
The two closest galaxies to the Milky Way are the Magellanic Clouds, visible only in the southern hemisphere.  The closest galaxy that northern hemisphere residents can view is Andromeda (M-31...Messier Object #31), visible in the autumn sky.


A nova occurs when a small, white-dwarf star is fed fresh hydrogen from a nearby companion star.  The hydrogen ignites explosively, making the star temporarily thousands of times brighter than normal.  This brightening may last for a few days to a few weeks for the observer.  Neither star is totally destroyed.  Some stars may go nova several times over their lifetimes.


A star is said to go "supernova" when its core collapses with a tremendous outpouring of energy.  In Type I, an overloaded white dwarf is completely disintegrated.  In Type II, some of the collapsed core stabilizes as either a neutron star or a black hole...depending on the star's mass.  Supernovae may become a billion times brighter than the sun for a period of weeks or months.

Neutron Star

Neutron stars are formed when the collapsed core left by a supernova causes its protons and electrons to combine into only neutrons.  They are about 15 miles in diameter.  They have strong magnetic fields, if they are still spinning rapidly.  These show up as pulsars and magnetars.  These are special classes of neutron stars that have strong radio emissions which can be picked up by radio telescopes.

Black Hole

A black hole is a stellar core of over three solar masses that has such a strong gravitational field that even light cannot escape it.  This region is only about 6 miles across for a three solar mass black hole.  Big black holes in the cores of galaxies may be larger than our solar system, and contain billions of solar masses.  The core of M-87 (a strong radio galaxy in Virgo) is one of the strongest radio sources.  The term M-87 is a Messier Object designation.


Quasars are well-fed black holes that are in the core of young galaxies.   The more in-falling material there is to feed it, the more energy is radiated outward from the accretion disc surrounding the black hole.  An accretion disc is simply material circling the black hole, gradually being drawn inward.  As the galaxy ages, this material is swept up, and the quasar turns into a normal galactic nucleus.

Globular Cluster

A ball of hundreds of thousands of stars, generally found above and below the plane of a galaxy.  Most are compact enough to resemble a slightly diffuse ball.  Many are very beautiful, when seen in large telescopes.  M-13 in Hercules is the best globular cluster for observation in northern hemisphere skies.  It is easily spotted with binoculars.

Open Cluster

An open cluster is simply a loose assemblage of hundreds of stars.  The Pleiades in Taurus is an easily spotted open cluster (resembles a tiny version of the Big Dipper), and one of the most spectacular.  In Greek mythology, the brightest stars in the cluster were known as the Seven Sisters.  These are stars in their infancy, and are still surrounded by the cloud of gas and dust that is the material from which they condensed.


Constellations are unrelated stars that appear to form a grouping in a recognizable pattern.  There are 88 official constellations.  All but about twenty of those are visible from Florida. 
For more information on constellations, click HERE and HERE.  Use your back button after visiting, if you want to return to this page. 

Double Star System

Often, thorough the telescope, two stars will appear to be positioned near each other, often side-by-side.  In "optical doubles," they are not related, and one will actually be far behind the other.  This accounts for almost a third of the double star images.  Two-thirds of the double stars that we know are actually visual binaries, and the two stars are orbiting around a common center of mass, with periods of decades to centuries.  The model for this orbit is a playground spinning teeter-totter.  The heavier of the two stars lies closer to the pivot point, and the less massive one lies farther out, but moves faster.  

Star Classifications

Stars are classified by temperature (from hottest to coolest)  in classes O, B, A, F, G, K, & M.  Sol, our sun, is a Class G star...yellow in color, and about 6000 degrees hot.  Antares in Scorpius is half the sun's temperature, and is reddish-orange in color, and a Type M.  Sirius is twice as hot as the sun, and is a Type A.  It appears blue-white in color.  The brightest stars of Orion's belt are Type O, and are among the hottest stars known in the galaxy--approaching 40,000 degrees hot.  The Pleiades are chiefly Type B stars.

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The stability of the atmosphere determines how much fine detail you can spot at high power with a telescope.  If the atmosphere is very calm, and detail is very crisp (the stars do not appear to twinkle, even at the horizon), the seeing is rated 10.  This is extremely rare.  If the atmosphere is extremely turbulent, and lunar and planetary detail are fuzzy even with your lowest magnifications, the seeing is considered to be Zero.  Typically, it ranges between 2 and 7.  This rating is rather subjective, based on your own personal observing experience.  As you get more practice, your ratings will be fine-tuned. 


Transparency is based on the magnitude of the faintest star naked-eye visible overhead.  It is rated from 1 (very hazy--with only a few 1st magnitude stars visible) to 6 (thousands of stars visible with the Milky Way quite obvious).  Light pollution plays a big role in the transparency around is rarely better than 2 or 3.  This is why dark sky observing sites are so important.


The magnitude scale for star brightness was established by Hipparchus in the second century B.C..  The first 20 stars to become visible in twilight were rated as 1st magnitude.  After about ten minutes, 2nd magnitude stars became visible.  These include Polaris and the seven brightest stars of the Big Dipper (Ursa Major).  About 15 minutes later, 3rd magnitude stars became visible, and so forth until about 90 minutes after sunset, the faintest 6th magnitude stars were visible.  We now rate each successive magnitude as 2.5 times fainter than the magnitude before.  For example, Polaris is 2nd magnitude, and about 2.5 times fainter than 1st magnitude Deneb.
You can find the magnitude scale in your sky map legends.  This includes printed maps from Sky & Telescope and Astronomy, Abrams Sky Calendar, all star atlases, EAAA sky maps, and computer maps (downloadable) such as Sky Globe and SkyMap.

Messier Object

In the late 1700's, Charles Messier (pronounced:  Sharl May'-see-ay) was trying to discover comets.  He found many fuzzy objects that immediately appeared to be a comet, but which did not move against the star field, as comets orbiting the sun.  Cometary movement becomes obvious in just a few hours.
Today, his list of 110 objects are some of the best galaxies, nebulae, and star clusters for northern hemisphere observers.  This list is ideal for beginning observers, for all of these objects are visible with telescopes with a 3" main objective, and many are visible with binoculars...some with the naked-eye.

You can earn the Astronomical League's Messier Club Certificate by completing the Messier observing program.  Click HERE to view information on this observing program.
If you only have binoculars, you may earn the Binocular Messier Club certificate.  HERE is information on this observing program.

NCG Object

NGC stands for New General Catalogue (which is actually over a century old) of deep sky objects.  Its over 7000 entries are numbered from 0 hours, right ascension eastward across the entire celestial sphere, both North and South.  It includes all of the Messier Objects, and many fainter objects which require larger telescopes.

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