The Stars and Scopes Glossary: API Developer Reference Page

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2) Below is a list of all the glossary items currently in the database. Simple copy the anchor tags from the right column and insert it into your page's source.

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Glossary Reference
Aberration	In an optical system, any departure from a spherically converging final wave front.
Absolute Magnitude	The apparent magnitude of a star at 32.6 light-years (10 parsecs) away.
Achromatic	A refracting optical system without color correction.
Adrastea	A moon of Jupiter was discovered by David C. Jewitt and G. Edward Danielson on July 8, 1979. Adrastea is also designated as "Jupiter XV" and "S/1979 J1". In Greek mythology, Adrastea was a nymph, the daughter of Jupiter and Ananke.
Afocal Photography	A type of eyepiece projection photography in which the camera lens is placed very close to a telescope's eyepiece
Airy Disk	The central, brightest part of a normal Airy pattern.
Airy Pattern	The diffraction pattern of a telescope with central Airy Disk and surrounding diffraction rings. An ideal telescope objective produces a diffraction-limited Airy pattern from an infinitely small, luminous object point.
Aitne	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2001. Aitne is also designated as "Jupiter XXXI" and "S/2001 J11". In Greek mythology, Aitne was a nymph who was seduced by Zeus.
Albiorix	A moon of Saturn was discovered by Matthew Holm in 2000. Albiorix also is designated as "Saturn XXVI" and "S/2000 S11". In Celtic mytholoy, Albiorix, also known as Toutatis, is the god of tribal unity.
Alt-Azimuth Mount	A type of telescope mount designed to move up and down (altitude) and left and right (azimuth).
Amalthea	A moon of Jupiter was discovered by Edward Emerson Barnard on September 9, 1892. Amalthea was the first new moon of Jupiter to be discovered since Galileo Galilei discovered the Galilean moons in 1610. Amalthea is also designated as "Jupiter V". In Greek mythology, Amalthea was a nymph who nursed the infant Zeus (Jupiter) with goat's milk.
Ananke	A moon of Jupiter was discovered by Seth Nicholson at Mount Wilson Observatory on September 28, 1951. Ananke is also designated as "Jupiter XII". In Greek mythology, Ananke was the personification of destiny, unalterable necessity and fate.
Andromeda	The constellation "The Chained Maiden"
Angular Resolution	The angular size of the smallest detail of an astronomical object that can be distinguished with a telescope.
Anti-Light Pollution (ALP) Filter	A type of nebular filter that eliminates light frequencies from mercury and sodium vapor street lights.
Anti-Reflection Coatings	Interference coatings on a lens surface to increase light transmission.
Antlia	The constellation "The Air Pump"
Aoede	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Aoede is also designated as "Jupiter XLI" and "S/2003 J7". In Greek mythology, Aoede was the Muse of song, and was a daughter of Zeus (Jupiter).
Aperture	The measurement of the diameter of the main lens of an optical system.
Aphelion Aphelion of Earth	The Aphelion occurs when a planet is furthest to the Sun during its slightly elliptical orbit.
Aplanatic	To be free of coma and spherical aberration.
Apochromatic	An optical system without color aberration and free of secondary spectrum.
Apparent Magnitude	The relative brightness of a star as view from Earth.
Apus	The constellation "The Bird of Paradise"
Aquarius	Aquarius (The Water Bearer) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. There are 3 deep sky objects that are on the Messier catalog, the Globular Cluster M2, Globular Cluster M72, and the Open Cluster M73. Also in Aquarius, is the Saturn Nebula NGC-7009 and the Helix Nebula NGC-7293.
Aquila	The constellation "The Eagle"
Ara	The constellation "The Altar"
Arche	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2002. Arche is also designated as "Jupiter XLIII" and "S/2002 J1". In Greek mythology, Arche was one of the four original Muses, an addition to the earlier three (Aoede, Melete, and Mneme).
Arcminute	One-sixtieth of a degree of arc.
Arcsecond	One-sixtieth of a minute of arc. 1/3600 of a degree.
Ariel	A moon of Uranus was discovered by William Lassell on October 24, 1851. Ariel is also designated as "Uranus I". Ariel was the spirit who serves Prospero in Shakespeare's The Tempest.
Aries	Aries (The Ram) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. Aries has a few dim deep sky objects including the galaxies NGC-697, NGC-772, NGC-972 and NGC-1156.
Asterism	A recognizable grouping of stars which is a subset of a Constellation. For Example: 'The Big Dipper' is an asterism of the Constellation Ursa Major (The Big Bear) or 'Pleiades' is an asterism in the Constellation Taurus (The Bull).
Asteroid	Large piece of rock orbiting the sun, usually located in the asteroid belt between Mars and Jupiter.
Astigmatism	An optical aberration that causes the image of a point light source to appear as an ellipse.
Astronomical Season Rule	This is a rule to define a Blue Moon based on the "Maine Rule," but using the actual Equinoxes and Solstices instead of the circular orbit of a fictitious mean Earth to define the seasons.
Astronomical Unit	A measurement of distance equal to the distance from the Sun to the circular orbit of a Fictious Mean Earth or about 92,955,807 miles.
Atlas	A moon of Saturn was discovered by Richard Tirrile (Voyager 1) in 1980. Atlas also is designated as "Saturn XV" and "S/1980 S28". In Greek mythology, Atlas was a titan who held the sky up.
AU	Abbreviation for an "Astronomical Unit" which is a measurement of distance equal to the distance from the Sun to the circular orbit of a Fictitious Mean Earth or about 92,955,807 miles.
Auriga	The constellation "The Charioteer"
Aurora	Light emission in the sky around the north and south poles caused by charged particles from the solar wind hitting the Earth magnetic field.
Autonoe	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2001. Autonoe is also designated as "Jupiter XXVIII" and "S/2001 J1". In Greek mythology, Autonoe was a daughter of Cadmus, founder of Thebes, and the goddess Harmonia.
Autumnal	Having to do with the season of Autumn. The Autumnal Equinox is the beginning of Fall.
Averted Vision	An observing technique in which a faint object image may be made to appear brighter to the eye by not looking directly at it.
Back Focal Length	The distance from the final surface of an optical system to the focal plane.
Bandpass	The selectivity of a filter.
Barley Moon	The name of the Full Moon usually in September, if the September Full Moon is not the Harvest Moon. Specifically the first Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometime known as the "Fruit Moon" or "Barley Moon."
Barlow Lens	A small negative lens that amplifies and relocates a telescope's beam when placed just inside focus. It is used to obviate the use of short-focus eyepieces and in negative lens projection photography.
Beaver Moon	The name of the Full Moon usually in November, if the November Full Moon is not the Hunters Moon. Specifically the second Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometimes known as the "Frosty Moon."
Belinda	A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Belinda is also designated as "Uranus XIV" and "S/1986 U5". Belinda is the heroine of Alexander Pope's The Rape of the Lock.
Bianca	A moon of Uranus was discovered by Bradford A. Smith (Voyager 2) on January 23, 1986. Bianca is also designated as "Uranus VIII" and S/1986 U9". Bianca is the sister of Katherine (Kate) in Shakespeare's play The Taming of the Shrew.
Binary Star	Two stars revolving about each other; a double star.
Blue Moon	A "Blue Moon" was originally defined as the 3rd Full Moon in an season where 4 Full Moons occur. This is the "Maine Rule" as it was used by the Maine Farmers' Almanac. Due to a misinterpretation in a 1946 article in Sky & Telescope of on a 1943 Sky & Telescope article, a definition emerged of a "Blue Moon" being the second Full Moon in a calendar month. This simpler and popular rule is known as the "Sky & Telescope Rule." The Maine Rule uses the circular orbit of a fictitious mean Earth to define the seasons.
Bootes	The constellation "The Herdsman"
Boundary Layer, Thermal	The layer of relatively warm air on a telescope objective that is cooling down. The layer becomes harmful to definition if it becomes unstable, as by agitation in open-tube telescopes.
Buck Moon	The name of the Full Moon usually in July. Specifically the first Full Moon after the Summer Solstice (June 22 by "The Maine Rule"). Sometimes known as the "Thunder Moon" or the "Hay Moon."
Caelum	The constellation "The Chisel"
Caliban	A moon of Uranus was discovered by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars using the 200-inch Hale telescope on September 6, 1997. Caliban is also designated as "Uranus XVI" and "S/1997 U1". Caliban is the monster character from William Shakespeare's play The Tempest.
Callirrhoe	A moon of Jupiter was discovered by Spacewatch on October 6, 1999. At that time is was thought ot be an asteroid until it was discovered to be in orbit around Jupiter by Tim Spahr on July 18, 2000. Callirrhoe is also designated as "Jupiter XVII" and "S/1999 J1". In Greek mythology, Callirrhoe was a naiad. She was the daughter of Oceanus and Tethys.
Callisto	A moon of Jupiter was discovered by Galileo Galilei on January 7, 1610. Callisto is also designated as "Jupiter IV". In Greek mythology, Callisto was nymph and one of the many lovers of Zeus, only to be turned into a bear by Zeus' wife Hera.
Calypso	A moon of Saturn was discovered by Brad Smith, Harold Reitsema, Steven Larson and John Fountain on March 13, 1980. Calypso also is designated as "Saturn XIV" and "S/1980 S25". In Greek mythology, Calypso was a sea nymph who delayed Odysseus on her island of Ogygia for seven years.
Camelopardalis	The constellation "The Giraffe"
Camera Adapter	A telescope accessory that receives a T-Ring Adapter for attaching a Single Lens Reflex (SLR) camera to a telescope. Also called T-Adapter.
Camera Adapter Ring	See T-Ring Adapter.
Cancer	Cancer (The Crab) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. The Beehive Star Cluster (M44) can be found in Cancer.
Canes Venatici	The constellation "The Hunting Dog"
Canis Major	The constellation "The Big Dog"
Canis Minor	The constellation "The Little Dog"
Capricornus	Capricornus (The Sea Goat) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. The constellation is often depicted as a sea-goat, a goat with a fish's tail.
Carina	The constellation "The Keel"
Carme	A moon of Jupiter was discovered by Seth Nicholson at Mount Wilson Observatory in July 1938. Carme is also designated as "Jupiter XI". In Greek mythology, Carme was a Cretan spirit who assisted the grain harvest.
Carpo	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Carpo is also designated as "Jupiter XLVI" and "S/2003 J20". In Greek mythology, Carpo was a Horae, and a daughter of Zeus (Jupiter).
Cassegrain Telescope	In its classical form, a compound reflecting telescope employing a parabolic concave primary mirror and a small hyperbolic convex secondary mirror to form images. Variations of the Cassegrain design include the Maksutov-Cassegrain and the Schmidt-Cassegrain.
Cassiopeia	The constellation "The Queen"
Catadioptric Telescope	A physically-compact, long-focus telescope employing a combination of mirrors and lenses to fold the light path and form an image, as in a Maksutov-Cassegrain or Schmidt-Cassegrain.
Catadioptrics	An optical system that uses a combination of refractor and reflector lenses to produce a large aperture, compact optical system.
CCD	Stands for 'Charge Coupled Device'. CCD chips are the detectors used in digital cameras.
Celestial	Having to do with the heavens, the sky.
Celestial Equator	A line around the celestial sphere which divides its northern and southern hemispheres.
Celestial Poles	Two points (north and south) in the celestial sphere which the sky seems to rotate around.
Celestial Sphere	An observationally practical model of the sky as a sphere with fixed stars that rotates around the Earth.
Cell	A cell is a part of a telescope system which mounts a lens or mirror to the telescope.
Centaurus	The constellation "The Centaur"
Central Obstruction	In Newtonian and Catadioptric telescopes, the obstruction caused by the secondary mirror.
Cepheus	The constellation "The King"
Cetus	Cetus (The Sea Monster) while not a zodiacal Constellation is close enough to the Ecliptic than planets will pass through it. Inside of Cetus you can find the spiral galaxy M77. According to mythology, Cetus was sent by Neptune to attack the shores of the kingdom of Cassiopeia and Cepheus. They chained their daughter Andromeda was to a cliff as an offering to Cetus as appeasement but was rescued by Perseus, who used the Gordon's head to turn Cetus to stone.
Chaldene	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2000. Chaldene is also designated as "Jupiter XXI" and "S/2000 J10". In Greek mythology, Chaldene, the mother of Solymos by Zeus.
Chamaeleon	The constellation "The Chameleon"
Charge Coupled Device (CCD)	A type of solid-state silicon wafer designed to detect light.
Charon	A moon of the planet Pluto, also known as Pluto I. Charon is the largest of Pluto's Moons and was discovered in 1978 by James Christy. In Greek mythology Charon was the Ferryman who ferried souls to Pluto's underworld of the dead.
Chromatic Aberration	An optical problem cause by light going through a refractor lens and not all the light frequencies coming to focus at the same point. Usually it is apparent at high magnifications as rainbow edges on objects. An apochromatic lens system is design to solve chromatic aberration.
Chromosphere	The atmosphere of the Sun between the Photosphere and the Corona.
Circinus	The constellation "The Drawing Compass"
Classical Cassegrain	The original Cassegrain telescope design consisting of a parabolic concave primary mirror and a small hyperbolic convex secondary mirror.
Clock Drive	A motor that drives the polar axis of an equatorial telescope mounting, enabling long-exposure photography and continuous viewing at high magnifications.
Cold Moon	The name of the Full Moon usually in December Specifically the first Full Moon before Yule (December 22 by "The Maine Rule"). Sometimes called the "Long Night Moon" or the "Moon before Yule."
Collimation	The proper alignment of the optical elements of a telescope.
Color Aberration	See Chromatic Aberration.
Columba	The constellation "The Dove"
Coma	An asymmetrical off-axis aberration inherent in certain telescope designs.
Coma Berenices	The constellation "Berenice's Hair"
Comet	A small celestial object which orbits the sun in a long elliptical orbit. Comets are made up mostly of ice and dust. As comets approach the Sun in their orbit they develop a tail as gas and ice are blown off by the solar wind.
Comparator	A machine used for looking for parallax motion, proper motion, asteroids, or variable stars by quickly alternating between viewing two photographic plates from two different times.
Comparison Star	Star used to calibrate either the brightness or position of the unknown star.
Computerized	Self-pointing capability in a "GOTO" telescope mount.
Conjunction	An astronomical event where two celestial objects appear close to each other in the celestial sphere.
Constellation	A grouping or pattern of stars in the celestial sphere, usually representing a mythical person, animal or other object. Astronomers define constellations as areas, so that all celestial objects are located in one constellation.
Convection Currents	Warm air rising from a reflecting telescope's primary mirror, made turbulent by the open main tube.
Cordelia	The innermost moon of Uranus was discovered by Richard J. Terrile (Voyager 2) on January 20, 1986. Cordelia is also designated as "Uranus VI" and "S/1986 U7". Cordelia is the youngest daughter of Lear in William Shakespeare's King Lear.
Corn Moon	The name of the Full Moon usually in September, if the September Full Moon is not the Harvest Moon. Specifically the first Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometime known as the "Fruit Moon" or "Barley Moon."
Corn Planting Moon	The name of the Full Moon usually in May. Specifically the second Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Flower Moon" or the "Milk Moon."
Corona	The tenuous outmost part of the solar atmosphere.
Corona Australis	The constellation "The Southern Crown"
Corona Borealis	The constellation "The Northern Crown"
Corrector Plate	A donut-shaped refracting lens found is many catdioptric telescopes designs like a Schmidt-Cassegrain or a Maksutov-Cassegrain design.
Corvus	The constellation "The Raven"
Cosmology	The study of the formation, organization, and evolution of the universe.
Crater	The constellation "The Cup"
Crescent Moon Crescent Moon	The shape of the lit portion of the moon or a planetary disk where the lit portion of the disk is less than half the disk surface. The terminator appear concave.
Cressida	A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 9, 1986. Cressida is also designated as "Uranus IX" and "S/1986 U3". Cressida is the Trojan daughter of Calchas, a tragic heroine from William Shakespeare's play Troilus and Cressida.
Cross Hair Reticle	Cross hairs of a guiding eyepiece used in long-exposure astrophotography.
Crow Moon	The name of the Full Moon usually in March. Specifically the Full Moon before Ecclesiastical Vernal Equinox (March 21). Sometimes known as the "Worm Moon," "Sap Moon" or the "Lenten Moon."
Crux	The constellation "The Southern Cross
Cupid	A moon of Uranus was discovered by Mark R. Showalter and Jack J. Lissauer using the Hubble Space Telescope on August 25, 2003. Cupid is also designated as "Uranus XXVII" and "S/2003 U2". Cupid is a character in William Shakespeare's play Timon of Athens.
Cygnus	The constellation "The Swan"
Cyllene	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Cyllene is also designated as "Jupiter XLVIII" and "S/2003 J13". In Greek mythology, Cyllene was a nymph associated with Mount Kyllini, Greece. She was a daughter of Zeus (Jupiter).
Dark Matter	The undetected matter in the universe which is not luminous. We know of its existence because of how clusters of stars and galaxies rotate.
Dawes Limit	The limit of a telescope's angular resolution, expressed in arc seconds and equal to 4.56 divided by the telescope's aperture in inches.
Declination	The coordinate on the sky exactly analogous to latitude on Earth, measured north and south from the celestial equator.
Deep-Sky	A term used for dim, non-planetary, non-lunar astronomical targets, like nebulae and galaxies. The region of space beyond the solar system.
Definition	The ability of a telescope to reveal, in an extended object image, the contrast between two areas having nearly the same brightness.
Deformed Optics	Optics that have become stressed from being pinched in their cells.
Degree	A measurement of an angle where 360 degrees would be a circle.
Deimos	The smaller and outermost of the planet Mars' two moons. Deimos was discovered by Asaph Hall on August 11, 1877. In Greek mythology, Deimos was the son of Ares (Mars) and Aphrodite (Venus) and was the personification of "Panic".
Delphinus	The constellation "The Dolphin"
Desdemona	A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Desdemona is also designated as "Uranus X" and "S/1986 U6". Desdemona is the wife of Othello in William Shakespeare's play Othello.
Despina	A moon of Neptune was discovered by the Voyager Imaging Team on August 2, 1989. Despina is also designated as "Neptune V" and "S/1989 N3". In Greek mythology, Despina was a nymph who was a daughter of Poseidon.
Dewing	Nighttime condensation of water on exposed optical surfaces after the dew point has been reached. It may be delayed through the use of a dew cap.
Diagonal	An optic accessory with a mirror or prism used to place an eyepiece in a comfortable position for the observer using a telescope and to re-arrange the orientation of the image.
Diffraction	A modulation of light waves caused by interference in response to an obstacle in the path of propagation.
Diffraction Pattern	The image of a point light source formed by a telescope. Also called Airy Pattern.
Digital Photography	Photographic imaging in which images are digitized and stored in a computer instead of recorded on film.
Digital Setting Circles	A computerized setting circle system that simplifies locating celestial objects.
Dione	A moon of Saturn was discovered by Giovanni Cassini in 1684. Dione also is designated as "Saturn IV". In Greek mythology, Dione was a goddess and mother to Aphrodite.
Direct Objective Photography	A photographic technique that substitutes the telescope objective for the lens of the camera, focusing the image formed by the objective directly onto the film. Also called Prime Focus Photography.
Distortion	An optical aberration in which magnification varies across the field of view. In this aberration, the shape of the image is not a true copy of the object, even though it may be in sharp focus.
Dobsonian Mount	A type of alt-azimuth telescope mount designed to hold large Newtonian reflector telescopes. The mount consist of a rocker box which allows the telescope to move up and down which rests on a rotating platform which allows the telescope to be move left and right.
Doppler Equation	The relationship which describes how the change in frequency of light received from an object depends on the objects velocity from us.
Dorado	The constellation "The Goldfish"
Draco	The constellation "The Dragon"
Earth	Earth is the third planet from the Sun. Earth's diameter is 12,750 km and takes 365.25 days to orbit the Sun.
Easter Moon	The name of the Full Moon usually in April. Specifically the first Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Pink Moon," "Egg Moon," the "Pascal Moon" or the "Grass Moon."
Eclipse	An Eclipse occurs when one astronomical object moves into the shadow of another. The two types of eclipses of most interest to Earthlings, are Solar Eclipses when the Moon's shadow is cast across the surface of the Earth, and Lunar Eclipses where the Earth's shadow is cast onto the Moon. Lunar and Solar Eclipses usually occur two weeks from each other. Solar Eclipse always occur during a New Moon and Lunar Eclipses always occur during a Full Moon.
Ecliptic	The imaginary line across the sky representing the path of the Sun during the year. For practical purposes, it represents edge of the solar system. The Sun, Moon and all the planets, except Pluto, appear to travel along the ecliptic. The Ecliptic also intersects with the 13 zodiacal constellations.
Egg Moon	The name of the Full Moon usually in April. Specifically the first Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Pink Moon," "Pascal Moon," the "Easter Moon" or the "Grass Moon."
Elara	A moon of Jupiter was discovered by Charles Perrine at Lick Observatory in 1905 . Elara is also designated as "Jupiter VII". In Greek mythology, Elara was the mother of the giant Tityus, by Zeus.
Element	A substance which cannot be decomposed by chemical means into a simpler substance.
Elongation Elongation	Elongations occur when an inner planet's position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth's orbits their positions as viewed from the Earth are never very far from the position of the Sun. When a planet is at Elongation, it is furthest from the Sun as viewed from Earth, so it's view is best at that point. There are two kinds of Elongations: The Eastern Elongation occurs when the planet is in the evening sky and the Western Elongation Occurs when a planet is in the morning sky.
Emulsion	A light-sensitive coating on paper or film; consists of fine grains of silver bromide suspended in a gelatin.
Enceladus	A moon of Saturn was discovered by William Herschel in 1789. Enceladus also is designated as "Saturn II". In Greek mythology, Enceladus was a giant speared in a battle by the goddess Athena and thrown into the volcano Mount Etna. Earthquakes in the area were thought to be caused by Enceladus.
Encoder	A sensor or transducer for converting rotary motion or position to a series of electronic pulses.
Epimetheus	A moon of Saturn was discovered by Richard Waltker (Voyager 1) on December 18, 1966. Epimetheus shares the same orbit as the Saturnian moon Janus. Epimetheus also is designated as "Saturn XI" and "S/1980 S3". In Greek mythology, Epimetheus was a Titan, who with his brother Prometheus, acted as representatives of mankind.
Equatorial Mount	A type of telescope mount designed to be oriented to the movement of the sky around the North Star (Polaris).
Equinox	The Equinox is a point in the Earth's orbit where the tilt of the Earth lies on the same plane as the tangent of the Earth's orbit. There are two Equinoxes each year which marks the beginning of the Spring and Autumn seasons. Literally meaning 'Equal Night', Daylight and nighttime are of equal length on the Equinoxes. Like the Solstices, the Seasons between the Northern and Southern hemispheres are opposite of each other: The Vernal Equinox in the North occurs with the Autumnal Equinox in the South and visa-versa. The Equinoxes are counter pointed by the Summer and Winter Solstices.
Equuleus	The constellation "The Little Horse"
Erecting Prism	A telescope accessory to give a correctly oriented image in a refractor or a catadioptric telescope. Sometime referred to as a '45 degree diagonal.'
Erfle	A type of wide-angle eyepiece.
Eridanus	The constellation "The Celestial River"
Erinome	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Erinome is also designated as "Jupiter XXV" and "S/2000 J4". In Roman mythology, Erinome was a lover of Jupiter.
Erriapo	A moon of Saturn was discovered by John Kavelaars in 2000. Erriapo also is designated as "Saturn XXVIII" and "S/2000 S10". Erriapo was a giant from Gaulic mythology.
Euanthe	A moon of Jupiter was discovered by Scott Sheppard and team in 2001. Euanthe is also designated as "Jupiter XXXIII" and "S/2001 J7". In Greek mythology, Euanthe was the mother of the Graces.
Eukelade	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Eukelade is also designated as "Jupiter XLVII" and "S/2003 J1". In Greek mythology, Eukelade was one of the Muses, and a daughter of Zeus (Jupiter).
Euporie	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2001. Euporie is also designated as "Jupiter XXXIV" and "S/2001 J10". In Greek mythology, Euporie was a goddess of abundance and one of the Horae.
Europa	A moon of Jupiter was discovered by Galileo Galilei on January 7, 1610. Europa is the smallest of the Galilean moons and is also designated as "Jupiter II". In Greek mythology, Europa, for whom Europe is named, was one of Zeus' many lovers. She bore three sons: King Minos of Crete, King Rhadamanthus of the Cyclades, and Sarpedon.
Eurydome	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2001. Eurydome is also designated as "Jupiter XXXII" and "S/2001 J4". In Greek mythology, Eurydome was as the mother of the Graces by Zeus.
Evening Star	Either Mercury or Venus, whichever is most prominent, when it appears in the evening sky after sunset.
Eye Relief	The distance needed between the eye or the camera and the eyepiece to achieve the best field-of-view.
Eyepiece Projection Photography	Astrophotography where the telescope and eyepiece is used in place of the camera lens.
Facula	A calcium cloud in the Sun's chromosphere, seen in projection against the photosphere in white light and detectable most easily near the Solar limb.
Ferdinand	The outermost moon of Uranus was discovered by Matthew J. Holman, John J. Kavelaars, Dan Milisavljevic, and Brett J. Gladman on August 13, 2001. Ferdinand is also designated as "Uranus XX!V" and "S/2001 U2". Ferdinand is the son of the King of Naples in William Shakespeare's play The Tempest.
Field Curvature	Curvature of a telescope's focal surface inherent in certain telescope designs. In this aberration, the image does not fall on a flat plane. Thus, the focus changes from the center to the edge of the field of view. As the image is viewed, it appears sharp and crisp either at the center or at the edges of the field of view but not both.
Filaments	A solar prominence seen in projection against the Sun's photosphere.
Filar Micrometer	An eyepiece which has threads across the field of view for more accurate measurement.
Finder Telescope	A small auxiliary wide-field telescope used to locate objects for viewing through a larger telescope to which it is attached.
First Quarter Moon First Quarter Moon	For the week between the New Moon and the First Quarter Moon the sunlit side of the Moon grows as a waxing (growing) crescent until the disk of the Moon is half lit. This half lit point is known as the First Quarter Moon. After the First Quarter the sunlit part of the Lunar disk is called waxing gibbous as it grows to illuminate the full disk of the Moon.
Flares	A brief, sudden brightening in the sun's atmosphere that accompanies a burst of radiation from a sunspot.
Flower Moon	The name of the Full Moon usually in May. Specifically the second Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Milk Moon" or the "Corn Planting Moon."
Focal Length	The effective distance from a telescope objective to the focal plane. This distance is not apparent in compound telescopes.
Focal Plane	Plane at the focal length of a lens or concave mirror on which an object is focused.
Focal Ratio	The ratio of a telescope's effective focal length to its aperture.
Focus	Place at the focal length where light rays from an object are converged by a lens or concave mirror.
Focusing Mirror	A Primary Mirror which is moved to focus a catadioptric optical system.
Fornax	The constellation "The Furnace"
Francisco	A moon of Uranus was discovered by Matthew J. Holman, John J. Kavelaars, Dan Milisavljevic, and Brett J. Gladman on August 13, 2001. Francisco is also designated as "Uranus XXII" and "S/2001 U3". Francisco is a lord in William Shakespeare's play The Tempest.
Frequency	Number of waves that cross a given point per unit time, or the number of vibrations which occur per unit time.
Frosty Moon	The name of the Full Moon usually in November, if the November Full Moon is not the Hunters Moon. Specifically the second Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometimes known as the "Beaver Moon."
Fruit Moon	The name of the Full Moon usually in September, if the September Full Moon is not the Harvest Moon. Specifically the first Full Moon before the Autumnal Equinox (September 23 by "The Maine Rule"). Sometime known as the "Corn Moon" or "Barley Moon."
Full Moon Full Moon	A Full Moon appears when the side on the moon facing Earth is completely sunlit. At this point the Moon is at Opposition to the Sun: rising as the Sun sets and setting as the Sun rises. If the Earth actually passes between the Sun and the Moon so that it's shadow passes across the Lunar surface that event would be a Lunar Eclipse. The Full Moon is typically the worse time for deep sky observing because the "moon glow" affects the dark sky.
Galatea	A moon of Neptune was discovered by the Voyager Imaging Team in July 1989. Galatea is also designated as "Neptune VI" and "S/1989 N4". In Greek mythology, Galatea was one of the Nereids.
Galaxy	A structure consisting of billions of stars, gravitationally bound together. Our galaxy, the Milky Way Galaxy has about a 100 billion stars.
Ganymede	A largest moon of Jupiter and the largest moon in the solar system. Ganymede is larger than the planets Mercury and pluto. Ganymede was discovered by Galileo Galilei on January 11, 1610. Ganymede is also designated as "Jupiter III". In Greek mythology, Ganymede was the cup-bearer of the gods.
Gemini	Gemini (The Twins) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. The primary Stars are named after the twins Castor and Pollux, who are the sons of Zeus and Leda in Greek mythology.
Gibbous Moon Gibbous Moon	The shape of the lit portion of the moon or a planetary disk where the lit portion of the disk is more than half the disk surface. The terminator appear convex.
Globular Cluster	Large spherical group of stars bound together by gravity, mostly found in outlying regions of a galaxy.
GOTO	Self-pointing capability in a computerized telescope mount.
Grain Moon	The name of the Full Moon usually in August. Specifically the second Full Moon after the Summer Solstice (June 22 by "The Maine Rule"). Sometimes known as the "Sturgeon Moon" or the "Green Corn Moon."
Grass Moon	The name of the Full Moon usually in April. Specifically the first Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Pink Moon," "Egg Moon," the "Easter Moon" or the "Pascal Moon."
Gravity	The tendency for all matter to be attracted to all other matter.
Green Corn Moon	The name of the Full Moon usually in August. Specifically the second Full Moon after the Summer Solstice (June 22 by "The Maine Rule"). Sometimes known as the "Sturgeon Moon" or the "Grain Moon."
Gregorian Calendar	A revision of the Julian calendar ordered by Pope Gregory XIII on February 24, 1582 via the papal bull Inter gravissimas. to correct cumulating errors which the feast of Easter to drift eariier and earlier in the year. This is the current civil calendar used by most of the world.
Grus	The constellation "The Crane"
Harpalyke	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Harpalyke is also designated as "Jupiter XXII" and "S/2000 J5". In Greek mythology, Harpalyke was the incestuous daughter of Klymenos, King of Thrace.
Harvest Moon	The name of the Full Moon closest to the Autumnal Equinox (September 23 by "The Maine Rule").
Hay Moon	The name of the Full Moon usually in July. Specifically the first Full Moon after the Summer Solstice (June 22 by "The Maine Rule"). Sometimes known as the "Buck Moon" or the "Thunder Moon."
Hegemone	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Hegemone is also designated as "Jupiter XXXIX" and "S/2003 J8". In Greek mythology, Hegemone was one of the Graces, and a daughter of Zeus (Jupiter).
Helene	A moon of Saturn was discovered by the P. Laques and J. lecacheus on March 1, 1980. Helene also is designated as "Saturn XII" and "S/1980 S6". The moon was named after Helen of Troy.
Helike	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Helike is also designated as "Jupiter XLV" and "S/2003 J6". In Greek mythology, Helike was one of the nymphs that nurtured Zeus (Jupiter) in his infancy on Crete.
Heliometer	An instrument devised originally for measuring the diameter of the sun; now employed for delicate measurements of the distance and relative direction of two stars too far apart to be easily measured in the field of view of an ordinary telescope.
Hercules	The constellation "The Strong Man"
Hermippe	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2001. Hermippe is also designated as "Jupiter XXX" and "S/2001 J3". In greek mythology, Hermippe was a consort of Zeus and mother of Orchomenus.
Himalia	A moon of Jupiter was discovered by Charles Perrine at the Lick Observatory on December 3, 1904. Himalia is also designated as "Jupiter VI". In Greek mythology, Himalia was a nymph who bore three sons of Zeus.
Honey Moon	The name for the Full Moon closest to the Summer Solstice (June 22 by "The Maine Rule"). Usually in June.
Horizon	When observing the sky, the horizon is the line between earth and sky. The horizon can also be defined as the plane between the zenith and nadir.
Horologium	The constellation "The Clock"
Hour Angle	The Right Ascension setting circle of an equatorial mounting.
Hunger Moon	The name of the Full Moon usually in February. Specifically the second Full Moon after Yule (December 22 by "The Maine Rule"). Sometimes known as the "Snow Moon."
Hunters Moon	The name of the Full Moon after the Harvest Moon.
Hydra	A moon of the planet Pluto, also known as Pluto III. Hydra was discovered by the "Pluto Companion Search Team" in June 2005 in images taken by the Hubble Space telescope. In Greek mythology Hydra was a multi-headed dragon who guarded Pluto's underworld of the dead.
Hydra	The constellation "The Sea Serpent"
Hydrogen-Alpha Filters	A type of Solar Filter System consisting of an Energy Rejection Filter and a tunable bandpass filter. These filter system are used to view details of the Solar chromosphere like Solar Prominences.
Hydrogen-Beta (H-Beta) Filter	A type of nebular filter that isolates the hydrogen-beta line alone.
Hydrus	The constellation "The Water Snake"
Hyperion	A moon of Saturn was discovered by William & George Bond and William Lassell on September 16, 1848. Hyperion also is designated as "Saturn VII". In Greek mythology, Hyperion was a Titan and the 'god of observation'.
Iapetus	The third largest moon of Saturn was discovered by Giovanni Cassini on October 25, 1671. Iapetus also is designated as "Saturn VIII". In Greek mythology, Iapetus was a Titan.
Ijiraq	A moon of Saturn was discovered by the John Kavelaars in, 2000. Ijiraq also is designated as "Saturn XXII" and "S/2000 S6". In Inuit mythology, Ijiraq is a sort of boogeyman who captures children and hides them away forever.
Indus	The constellation "The Indian"
Inferior Planet	An Inferior Planet is a planet whose orbit lays inside the orbit of the Earth. These are: Mercury & Venus.
Interferometry	A method for increasing resolving power by combining light obtained by two or more telescopes.
Io	A moon of Jupiter was discovered by Galileo Galilei on January 7, 1610. Io is the innermost of the four Galilean moons and is also designated as "Jupiter I". In Greek mythology, Io was one of many of Zeus' lovers.
Iocaste	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Iocaste is also designated as "Jupiter XXIV" and "S/2000 J3". In Greek mythology, Iocaste was he mother/wife of Oedipus.
Iris	Thin overlapping plates which can be adjusted to change the diameter of a central opening.
Isonoe	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Isonoe is also designated as "Jupiter XXVI" and "S/2000 J6". In Greek mythology, Isonoe was one of the Danaides and a lover of Zeus (Jupiter).
Janus	A moon of Saturn was discovered by Audouin Dollfus on December 15, 1966. Janus shares the same orbit as the Saturnian moon Epimetheus Janus also is designated as "Saturn X" and "S/1966 S2". Janus is named after the two-faced Roman god of doors, gates, beginnings & endings.
Jovian	Having to do with the planet Jupiter.
Julian Calendar	A revision of the Roman religious calendar ordered by Julius Caesar in 46 BC to clarify the rules for a civil calendar. Not to be confused with a "Julian Day."
Julian Date	A Julian Date (JD) is a floating-point number rounded to the nearest Julian Day with the fraction of the day which has passed since 12 noon Greenwich Mean Time (UT or TT) of that day. This should not be confused with the "Julian Calendar" or the "Julian Date." Julian Dates are commonly used to simplify astronomical calculations.
Julian Day	A Julian Day (also known as a Julian day Nukmber - JDN) is an integer number counting the days since Monday, January 1, 4713 BC in the Proleptic Julian Calendar (that is extending the Julian Calendar to dates before is offical use in 45 BC). This should not be confused with the "Julian calendar or the "Julian Date." the Julian Day was proposed by Joseph Scaliger in 1583, during the Gregorian calendar reform. The "Julian" in "Julian day" refers to Scaliger's father, Julius Scaliger.
Juliet	A moon of Uranus was discovered by Stephen P. Synnott (Voyager 2) on January 13, 1986. Juliet is also designated as "Uranus XI" and "S/1986 U2". Juliet is the heroine of William Shakespeare's play Romeo and Juliet.
Jupiter	Jupiter is the fifth planet from the Sun. Jupiter's diameter is 143,000 km and takes 11.86 years to orbit the Sun. Jupiter is named after the Roman king of gods. In Eastern Asian cultures Jupiter is also known as the 'Wood Star.'
Kale	A moon of Jupiter was discovered in 2001. Kale is also designated as "Jupiter XXXVII" and "S/2001 J8". In Greek mythology, Kale was one of the Charities, goddesses of charm, beauty, nature, human creativity and fertility.
Kallichore	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Kallichore is also designated as "Jupiter XLIV" and "S/2003 J12". In Greek mythology, Kallichore was one of the Muses, and a daughter of Zeus (Jupiter).
Kalyke	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Kalyke is also designated as "Jupiter XXIII" and "S/2000 J2".
Kellner	An economy eyepiece employing a cemented doublet lens and singlet lens.
Kiviuq	A moon of Saturn was discovered by the Brett Gladman in, 2000. Kiviuq also is designated as "Saturn XXIV" and "S/2000 S5". The moon was named after a Hero/Giant from Inuit mythology.
Kuiper Belt	A region beyond the orbit of Neptune and exending about 1000 Astronomical Units from the Sun.
Kuiper Objects	Any of several small astronomical bodies in the Kuiper Belt. The largest Kuiper Belt Object is the Planet Pluto.
Lacerta	The constellation "The Lizard"
Larissa	A moon of Neptune was discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen on May 24, 1981. Larissa is also designated as "Neptune VII" and "S/1981 N1". In Greek mythology, Larissa was a nymph who was a lover of Poseidon (Neptune).
Last Quarter Moon Last Quarter Moon	The week between the Full Moon and the Last Quarter Moon the sunlit side of the Moon is called waning (shrinking) gibbous and is shrinking until the lunar disk is half in sunlight and half in shadow. This half lit point is the Last Quarter Moon. After the Last Quarter the sunlit side of the Moon continues to shrink as a waning crescent as the Moon approaches the Sun's position in the sky.
Leda	A moon of Jupiter was discovered by Charles T. Kowal at the Mount Palomar Observatory on September 14, 1974. Leda is also designated as "Jupiter XIII". In Greek mythology, Leda was queen of Sparta who was the mother of Castor, Polydeuces, Clytemnestra and Helen of Troy.
Lens	A piece of transparent material which is used in an optical instrument for forming an image by focusing rays of light.
Lenten Moon	The name of the Full Moon usually in March. Specifically the Full Moon before Ecclesiastical Vernal Equinox (March 21). Sometimes known as the "Worm Moon," "Sap Moon" or the "Crow Moon."
Leo	Leo (The Lion) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. Leo contains the 'Sickle' asterism which forms the lions head and four spiral galaxies from the Messier catalog M65, M66, M95 and M96.
Leo Minor	The constellation "The Lesser Lion"
Lepus	The constellation "The Hare"
Libra	Libra (The Balance) is a zodiacal constellation, that is, it lies across the ecliptic, an imaginary line across the sky that the Sun, Moon and all the planets except Pluto seem to move along. The constellation was once part of the constellation Scorpius (The Scorpion) until it was renamed by the Romans. The names of some of the stars in Libra still carry this ancient legacy of Scorpius (Zubenelgenubi ('southern claw'), Zubeneschamali ('northern claw'), and Zubenelakrab ('scorpion's claw')). After its association with Scoprpio the stars of Libra were associated with the constellation of Virgo as the 'Scales of Justice' of the goddess Astraea (Virgo), the goddess of justice.
Light Grasp	A measurement of the brightness a telescope can make a view. Light Grasp is the ratio between the area of a telescope's aperture and the area of an average human dark-adapted (scotopic) pupil.
Light Pollution Filter	A telescope filter that blocks the light of mercury and sodium vapor street lights, thus allowing faint nighttime celestial objects to be seen and photographed from cities.
Light-year	A measurement of distance base on how long it takes light to travel in a year in a vacuum (about 5,878,482,164,161 miles).
Long Night Moon	The name of the Full Moon usually in December Specifically the first Full Moon before Yule (December 22 by "The Maine Rule"). Sometimes called the "Cold Moon" or the "Moon before Yule."
Luminosity	The rate at which light is emitted from an object.
Luna	Earth's moon is Luna. Usually referred to as "The Moon."
Lunar	Having to do with the Moon.
Lunar Calendar	Any calendar system based on the phases of the Moon. Islamic, Hebrew and Chinese calendars are examples of lunar calendars.
Lupus	The constellation "The Wolf"
ly	Abbreviation for an "Light-year" which is a measurement of distance base on how long it takes light to travel in a year in a vacuum (about 5,878,482,164,161 miles).
Lynx	The constellation "The Lynx"
Lyra	The constellation "The Lyre"
Lysithea	A moon of Jupiter was discovered by Seth Nicholson at Mount Wilson Observatory in 1938. Lysithea is also designated as "Jupiter X". In Greek mythology, Lysithea was a daughter of Oceanus and one of Zeus' lovers.
Mab	A moon of Uranus was discovered by Mark R. Showalter and Jack J. Lissauer using the Hubble Space Telescope on August 25, 2003. Mab is also designated as "Uranus XXVI" and "S/2003 U1". Queen Mab, a fairy queen from English folklore who is mentioned in William Shakespeare's play Romeo and Juliet.
Magnitude	A measure of the apparent brightness of a celestial object.
Maine Rule	The rule for calculating a Blue Moon as the third Full Moon in a season with four Full Moons. It is named the "Maine Rule" from The Maine Farmers' Almanac. Seasons however are based on the circular orbit of a fictitious mean Earth defined from the Ecclesiastical Vernal Equinox (March 21) inferring Summer Solstice as June 22, Autumnal Equinox as September 23 and Winter Solstice as December 22 (Yule).
Maksutov-Cassegrain	A catadioptric telescope design characterized by a thick deeply curved Corrector Plate and usually an f/15 focal ratio.
Margaret	A moon of Uranus was discovered by Matthew Holman on August 13, 2001. Margaret is also designated as "Uranus XXIII" and "S/2003 U3". Margaret is the servant of Hero in William Shakespeare's play Much Ado About Nothing.
Mars	Mars is the fourth planet from the Sun. Mars' diameter is 6,790 km and takes 687 days to orbit the Sun. Mars is named after the Roman god of war. In Eastern Asian cultures Mars is also known as the 'Fire Star.'
Martian	Having to do with the planet Mars.
Mass	The total amount of material in an object.
Measuring Engine	A machine used to measure the position and brightness of objects on photographic plates.
Megaclite	A moon of Jupiter was discovered by Scott Sheppard and team in 2000. Megaclite is also designated as "Jupiter XIX" and "S/2000 J8".
Mensa	The constellation "The Table Mountain"
Mercurian	Having to do with the planet Mercury.
Mercury	Mercury is the closest planet to the Sun. Mercury's diameter is 4880 km and takes 88 days to orbit the Sun. Mercury is named after the Roman messenger god. In Eastern Asian cultures Mercury is also known as the 'Water Star.'
Meridian	A reference line in the sky that goes from one celestial pole to the other and passes through the zenith, dividing the eastern and western halves of the sky.
Messier Object	A non-stellar celestial object listed in the Messier Catalog.
Meteor	A fragment of material from space which falls into Earth's atmosphere and vaporizes.
Methone	A moon of Saturn was discovered by the Sebastien Charnoz on June 1, 2004. Methone also is designated as "Saturn XXXII" and "S/2004 S1". In Greek mythology, Methone was one of seven (beautiful) daughters of the giant Alkyoneus.
Metis	A innermost moon of Jupiter was discovered by Steven Synnott in March 4, 1979. Metis is also designated as "Jupiter XVI" and "S/1979 J3". In Greek mythology, Metis was the first wife of zeus and the mother of Athena.
Microphotometer	A machine used to measure the brightness of an object or a spectral line on a photographic plate.
Microscopium	The constellation "The Microscope"
Milk Moon	The name of the Full Moon usually in May. Specifically the second Full Moon after Ecclesiastical Vernal Equinox (March 21). Sometimes called the "Flower Moon" or "Corn Planting Moon."
Milky Way Galaxy	The galaxy in which our solar system resides.
Mimas	A moon of Saturn was discovered by William Herschel in 1789. Mimas also is designated as "Saturn I". In Greek mythology, Mimas was a giant slain by Heracules.
Miranda	A moon of Uranus was discovered by Gerard P. Kuiper on February 16, 1948. Miranda is also designated as "Uranus V". Miranda is Prospero's daughter in William Shakespeare's play The Tempest.
Mirror Coatings	Dielectric or aluminum coatings deposited on first-surface mirrors to greatly enhance reflectivity.
Mirror Lock	A feature in some catadioptric optical systems hat allows the focusing mirror to be loced into positions so that a "zero image shift" focuser can be used, usually for astro-imaging.
Mneme	A moon of Jupiter was discovered by Scott S. Sheppard and team in 2003. Mneme is also designated as "Jupiter XL" and "S/2003 J21". In Greek mythology, Mneme was one of the three original Muses.
Monoceros	The constellation "The Unicorn"
Moon After Yule	The name of the Full Moon usually in January. Specifically the first Full Moon after Yule (December 22 by "The Maine Rule"). Sometimes known as the "Wolf Moon" or "Old Moon."
Moon Before Yule	The name of the Full Moon usually in December Specifically the first Full Moon before Yule (December 22 by "The Maine Rule"). Sometimes called the "Cold Moon" of the "Long Night Moon."
Morning Star	Either Mercury or Venus, whichever is most prominent, when it appears in the morning sky before sunrise.
Mount	The mechanical portion of a telescope system which connect the telescope tube to a tripod or pier. There are two kinds of mounts: an Altazmuth mount whose motion is oriented left-right/up-down and an Equatorial mount which is angle at the latitude and whose motion is oriented around the motion of the celestial sphere.
Mundilfari	A moon of Saturn was discovered by Brett Gladman in 2000. Mundilfari also is designated as "Saturn XXV" and "S/2000 S9". In Norse mythology, Mundilfari was a giant and the father of the god Sol.
Musca	The constellation "The Fly"
Nadir	When observing the sky, the nadir is the point directly opposite of zenith and below of the observer (i.e. between the observer's feet).
Naiad	The innermost moon of Neptune was discovered by the Voyager Imaging Team in September, 1989. Naiad is also designated as "Neptune III" and "S/1989 N6". In Greek mythology, the Naiad were a type of nymph who presided over fountains, wells, springs, streams, and brooks, as river gods.
Narvi	A moon of Saturn was discovered by Scott Sheppard and team in 2003. Mundilfari also is designated as "Saturn XXXI" and "S/2003 S1". In Norse mythology, Narvi was the son of Loki who was killed to punish Loki for his crimes.
Nebula	Any one of dim, fizzy, non-stellar objects usually illuminated clouds of dust and gas in space.
Nebular Filters	These are high-end filters that target specific frequencies of light, used to enhance views of dim targets like nebulae and galaxies.
Negative Lens	A lens that cannot form an image when used alone, but can be used together with positive lenses to correct their aberrations, to amplify an optical system, or to relocate an image.
Negative Lens Projection Photography	The photographic technique of using a telenegative amplifying lens to enlarge the image formed by a telescope and to project the image directly into the camera.
Neptune	Neptune is usually the eight planet from the Sun although it is sometimes 9th when Pluto moves within it's orbit. Neptune's diameter is 49,500 km and takes 165 years to orbit the Sun. Neptune is named after the Roman god of the sea. Neptune was discovered on September 23, 1846 by French astronomer Urbain Le Verrier.
Neptunian	Having to do with the planet Neptune.
Nereid	The smallest moon of Neptune was discovered by Gerard Kuiper in May 1, 1949. Nereid is also designated as "Neptune II". In Greek mythology, the Nereid were sea-nymphs.
Neutral Density Filters	Gray Filters used to reduce light from bright targets like the moon without introducing false color.
New Moon New Moon	A New Moon occurs when the side on the moon facing Earth is completely in shadow. This means that the Moon and Sun are rising and setting in the same vicinity in the sky. If the Moon actually passes between the Sun and the Earth so that it's shadow passes across the Earth's surface that event would be a Solar Eclipse. New Moon is typically the very best time for deep sky observing.
Newtonian	A telescope employing a parabolic primary and a smaller, flat diagonal mirror to form images.
NGC	New General Catalog of Non-Stellar Astronomical Objects.
Nix	A moon of the planet Pluto, also known as Pluto II. Nix was discovered by Max Mutcher in June 2005 in images taken by the Hubble Space telescope. In Greek mythology Nix was the goddess of darkness and night and mother to Charon was the Ferryman who ferried souls to Pluto's underworld of the dead.
Norma	The constellation "The Level"
Oberon	A moon of Uranus was discovered by William Herschel on January 11, 1787. Oberon is also designated as "Uranus IV". Oberon is the king of the Fairies in William Shakespeare's play A Midsummer Night's Dream.
Objective	The principle optical element of a telescope that collects light and brings it to a focus.
Occultation	Occultation is when a body (usually the Moon) passes in front of (hides) a star or planet.
Octans	The constellation "The Octant"
Off-Axis Guider	An astrophotographic accessory featuring an off-axis prism that intercepts unused light and uses this light to transmit images of guide stars.
Off-Axis Masks	Masks that stop down the aperture of a reflecting or catadioptric telescope. Their apertures are located in an off-axis position to avoid obstruction by the telescope's secondary mirror.
Old Moon	The name of the Full Moon usually in January. Specifically the first Full Moon after Yule (December 22 by "The Maine Rule"). Sometimes known as the "Wolf Moon" or the "Moon After Yule."
Ophelia	A moon of Uranus was discovered by Richard J. Terrile (Voyager 2) on January 20, 1986. Ophelia is also designated as "Uranus VII". Ophelia is the daughter of Polonius in William Shakespeare's play Hamlet.
Ophiuchus	Ophiuchus (The Serpent Bearer) represents Aesculapius, the mythological healer of the sick. Ophiuchus might be considered the 13th Zodiacal Constellation. Over the centuries this constellation has move over the line known as the Ecliptic due to a wobble in Earth's rotation known as 'procession'. (The Ecliptic is the imaginary line that the Sun, Moon and all the planets except Pluto seem to move along. It represents the edge of the solar system.) Originally, the constellations Ophiuchus and Serpens were considered all part of the same 'Snake-Holder' constellation. Located inside Ophiuchus are the following interesting astronomical objects: Bernard's Star, several Messier Objects including (M9, M10, M12, M14, M19, M62, M107) and other open clusters like IC-4665, NGC-6633, also the nebula IC-4603-4604, and The Starfish galaxy (NGC-6240), a double galaxy.
Opposition Opposition	For planets outside the Earth's orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around Oppositions are the best time to view these. An Opposition occurs when the planet is opposite from the Sun, relative to the Earth. At Opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation. Also at Opposition the planet comes physically closest to the Earth in it's orbit so it appears as large as possible.
Optical Tube Assembly	The telescope part of a telescope system.
Orion	Orion (The Hunter) is one of the best known constellations in the sky, having an hourglass shape and the asterisms of Orion's Belt and Orion's Sword. Located in Orion's sword is one of the brightest nebula, Orion's Nebula (M42), which is a stellar nursery.
Orthoscopic	A eyepiece design invented by Ernst Abbe in 1880. This eyepiece, free of distortion, employs a triplet field lens and a singlet eye lens. It excels in planetary observing.
Orthosie	A moon of Jupiter was discovered by Scott Sheppard and team in 2001. Orthosie is also designated as "Jupiter XXXV" and "S/2001 J9". In Greek mythology, Orthosie was goddess of prosperity.
Oscilloscope	An instrument in which the variations in a fluctuating electrical signal appear as a visible wave on the fluorescent screen of a cathode ray tube.
OTA	Abbreviation for the "Optical Tube Assembly". It is the telescope part of a telescope system.
Oxygen III (O-III) Filter	A type of nebular filter that isolates just the two doubly ionized oxygen lines.
Paaliaq	A moon of Saturn was discovered by the Brett Gladman in, 2000. Paaliaq also is designated as "Saturn XX" and "S/2000 S2". T