Astronomy - Jargon


Guide to Astronomy:

Glossary of Terms Used in Astronomy

A , B , C , D , E , F , G , H , I , J , K , L , M , N , O , P , Q , R , S , T , U , V , W , X , Y , Z

A

Absolute magnitude     The brightness of a celestial object from a distance of approximately 33 light years. Brighter objects have smaller numerical magnitudes. For example, the sun has an absolute magnitude of +5.

Absorption nebula    A nebula which is seen as a silhouette as it absorbs light from farther away. Also called a dark nebula.

Apparent magnitude    The brightness of a celestial object as seen from earth. Brighter objects have smaller numerical magnitudes. For example, the sun has an apparent magnitude of approximately -26.

Altitude     The distance of an object above the horizon measured in degrees with the horizon being 0 degrees and straight overhead being 90 degrees. Also the elevation of an object above a given point, such as mean sea level.

Analemma     Figure 8 pattern showing the sun's declination for each day of the year.

Asterism     A distinct grouping of stars that is not one of the 88 officially recognized constellations.

Asteroid     Also called a minor planet. A very small "planet" in our solar system. Most are in an asteroid belt with orbits between Mars and Jupiter, but some, the Apollo asteroids, have orbits that cross the earth's.

Autumnal equinox     The point at which the sun crosses the celestial equator as it passes southward along the ecliptic.

Azimuth     The distance from north on the horizon to another point on the horizon directly below a celestial object. Measured clockwise from the north in degrees, with north being 0 degrees, east 90 degrees, south 180 degrees, west 270 degrees and north again completing a circle of 360 degrees.

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B

Baily's Beads   During a solar eclipse, the effect created by the solar disk shining between the mountains on the moon's edge just prior to and following totality.

Binary star     A double star system. May contain two or more stars that revolve around each other.

Black hole     A region of space where the mass is so dense that theoretically, not even light can escape its gravity.

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C

Celestial equator     An imaginary circle on the celestial sphere directly above the equator of the earth.

Celestial poles     The two points on the celestial sphere directly above the poles of the earth. Also, the intersection of the earth's axis with the celestial sphere. The imaginary sphere of the sky on which all celestial objects appear.

Cepheid variable     A class of variable star whose variations in brightness follow a regular pattern. Named after Delta Cephei, a star in the constellation Cephius, it was the first of its type to be discovered. These variables are important because they allow astronomers to measure distances in the universe.

Circumpolar stars Stars which, because of their closeness to the celestial pole, appear never to set.

Cluster     A group of stars of common origin in relative proximity to one another.

Conjunction     The alignment of two or more celestial objects along the same line of right ascension.

Constellation     One of 88 recognized groups of stars named for some object, animal, or mythical figure.

Contact     During eclipses, occultations and transits the points at which the edges of the disks of the bodies involved appear to touch. For example, during a solar eclipse, first contact is when the leading edge of the moon first touches the sun; second contact is when the trailing edge of the moon touches the same side of the sun; third contact is when the leading edge of the moon touches the far side of the sun; and fourth contact is when the trailing edge of the moon touches the far side of the sun.

Corona     The outermost layer of the sun and other stars, faintly visible as a halo of hot gas. Often prominent during a solar eclipse.

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D

Declination     The distance of a celestial object north or south of the celestial equator. Measured in degrees, with the celestial equator being 0 degrees, celestial north pole being 90 degrees, and celestial south pole being -90 degrees. Similar to latitude.

Diamond ring effect     During a total solar eclipse, the effect created by the faint "ring" of the corona and the last point of light from the solar disk as the eclipse becomes total. It occurs again on the opposite side as the solar disk slips out of totality.

Double star     Two or more stars that appear very close to each other on the celestial sphere. In a true double, they are actually close to each other. In an optical double they are simply in the same line of sight.

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E

Earthshine     Sunlight reflected off the earth as seen from the moon. Hence our ability to see faintly, at times, that part of the moon which is not illuminated by the sun.

Eclipse     The passage of one celestial body between the sun and another, dimming or eliminating the light from the sun.

Eclipse, annular     A solar eclipse where the distance from the moon to the earth is greater than the length of the umbra of the moon's shadow. Only the penumbra falls on the earth. Along what would be the path of totality, a ring of the solar disk called an annulus can still be seen around the moon.

Eclipse, lunar     The passage of the moon through the earth's shadow.

Eclipse, partial     An eclipse where the solar disk is only partially obscured.

Eclipse, solar     The passage of the moon between the sun and the earth causing the moon's shadow to fall on the earth.

Eclipse, total     An eclipse where the solar disk is completely obscured.

Ecliptic     An imaginary circle on the celestial sphere along which the sun appears to travel. The sun's path along this circle.

Elevation     Height above sea level. Also the angular distance of an object above the horizon. See (altitude).

Elongation     The distance from the sun to a celestial object, measured in degrees along the right ascension east to west. For example, the moon's elongation when it is full is 180 degrees; when it is new, it is 0 degrees.

Emission nebula     A gaseous cloud which radiates energy received from a hot star. Many have a characteristic reddish color.

Epoch     A date selected as a point of reference for the data under consideration. Used to define the equatorial coordinate system or the orbital parameters of a celestial object.

Equatorial coordinates     The coordinates (given in RA and Dec, right ascension and declination) that describe the position of an object on the celestial sphere. Measured relative to the celestial equator and the vernal equinox.

Equinox     The intersection of the sun's ecliptic and the celestial equator. The times of the year when the sun is at those points. Equinox comes from the Latin meaning equal night. It is at these times that day and night are of equal length.

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G

Galaxy     A great system of stars, dust and gas. There are three types: spiral, elliptical and irregular.

Giant     A star which is brighter and larger than other stars of its color and temperature.

Globular cluster     A spherical group of hundreds of thousands of stars of common origin.

Graticule     A scale in the focal plane of telescopes or other optical instruments for measuring and locating objects.

Greek alphabet     The brightest stars in each constellation are assigned Greek letters to indicate their relative brightness.

ALP alpha (brightest) NU nu
BET beta XI xi
GAM gamma OMI omicron
DEL delta PI pi
EPS epsilon RHO rho
ZET zeta SIG sigma
ETA eta TAU tau
THE theta UPS upsilon
IOT iota PHI phi
KAP kappa CHI chi
LAM lambda PSI psi
MU mu OME omega

     

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H

H-R Diagram     A diagram which charts the absolute brightness (luminosity) of stars as a function of their temperatures (derived from their color or spectral type). It is named for astronomers Ejnar Hertzsprung and Henry Norris Russell who developed it in the early 20th century.

Herschel object     One of many non-stellar objects discovered by William Herschel in the late eighteenth century. These are identified on star charts as a number preceded by an H. Herschel is often called the father of modern astronomy for his observational work. He also discovered Uranus, two of its moons, and the 6th and 7th moons of Saturn.

Horizon coordinates    The coordinates (given as altitude and azimuth) of a celestial object relative to the observer's horizon.

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I

IC object   A deep-sky object listed in the Index Catalogues, published in 1895 or 1908 as supplements to the New General Catalogue of 1888. They are identified on star charts as a number preceded by IC or I.

Inferior conjunction     A conjunction during which a planet whose orbit is inside that of the earth's passes between the earth and the sun.

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L

Light year     The distance light travels in a year. Approximately six trillion miles.

Luminosity     The energy a celestial object radiates. Its absolute brightness. Measured in magnitudes.

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M

Magnitude     The brightness of a celestial object. A change of 5 magnitudes indicates an increase or decrease by a factor of 100. Brighter objects have smaller numeric values. Expressed as positive and negative numbers.

Main sequence     A class of stars in the process of stable hydrogen burning. Most stars fall into this category. They form a distinct band across the H-R Diagram, hence the name main sequence.

Meridian     An imaginary circle on the celestial sphere which passes through the poles and the zenith of the observer.

Messier object    One of the 110 objects in a catalogue of fixed non-stellar nebulous objects published by Charles Messier in 1787. On a star chart, Messier objects are identified by a number preceded by M.

Meteorite     A small chunk of rock or metal that was not entirely vaporized as it fell through the earth's atmosphere.

Meteor     A small chunk of rock or metal that has entered the earth's atmosphere and emits a streak of light as it is vaporized. Commonly known as a 'shooting star'.

Meteor shower     A display of meteors, all of which are moving in parallel paths through space.

Meteoroid   A small chunk of rock or metal in the solar system, often debris in a comet's orbit, sometimes a chip off an asteroid.

Milky Way     The spiral galaxy which contains our solar system. Many of its stars appear as a band of light (which we also call the Milky Way) circling the ecliptic. Still, the vast majority of the stars in the center and far side of our galaxy are hidden from sight by clouds of dark dust. Our solar system is approximately one third of the way in from the edge to the center. The Milky Way Galaxy contains about 100,000,000,000 stars.

Moon   Earth's natural satellite.

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N

Nadir     The point on the celestial sphere directly below the observer. The opposite of zenith.

Nebula     A more or less clearly defined region of gas or dust, often luminous, that can be observed optically.

Nebulosity     The luminous or absorptive effect which is observed in a nebula.

Neutron star     A collapsed, extremely dense star composed almost entirely of neutrons.

NGC object     One of a numerical listing of non-stellar objects in the New General Catalogue published in 1888 by J.L.E. Dreyer. On star charts NGC objects appear as numbers without the NGC prefix.

Non-stellar     A celestial object which is either not a star or, under the observing conditions of its discovery, could not be resolved as separate stars. Examples include galaxies and star clusters.

North celestial pole     The point about which northern stars seem to move due to the earth's daily rotation on its axis.

Nova    A star that suddenly flares in brightness by hundreds or thousands of times.

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O

Occultation     The eclipse of one celestial body by another, such as the moon or a planet passing in front of a star.

Open cluster     An irregular group of stars of common origin.

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P

Parallax     The apparent change in position of an object when viewed from two different locations. Used to measure the distance to nearby stars.

Penumbra     During an eclipse, the area of partial shadow surrounding the umbra. A shadow where the illuminating body is not completely hidden.

Polaris     A star, able to be seen with the naked-eye, currently located very near the north celestial pole. Its position makes it convenient for use in navigation on the surface of the earth.

Planetary nebula     A shell of gas given off by a dying star which glows from energy radiated from that star.  

Precession     The slow wobble in the earth's axis and its effect on the location of celestial objects relative to the earth.

Proper motion     Apparent motion of an object across the sky measured by its relative position to background stars.

Pulsar     A rotating neutron star that emits radio energy in short regular "pulses."

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Q

Quasar     Called a "quasi-stellar" object, this is a source of radio energy and light with an extremely large redshift and, therefore, probably very far away.

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R

Radiant     The portion of the sky from which meteors in a shower seem to radiate.

Red giant     A very bright, cool star.

Red shift     The lengthening of visible light waves from an object as it travels away from us resulting in all colors being shifted toward the red end of the spectrum. A method whereby distances to the most remote objects in the universe can be measured. The more distant the object, the greater the redshift.

Reflection nebula     A dust cloud which reflects a star's light to us.

Retrograde motion     The apparent backward drift of a planet as the earth catches up to, and passes it in its orbit.

Right ascension     The angle of an object from the vernal equinox. Measured eastward along the celestial equator in hours, minutes and seconds. Similar to longitude.

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S

Sidereal time    Star time. Due to the earth's motion in orbit around the sun, the stars make one more revolution overhead per year than the sun. Thus, there are 366.25 sidereal days per year, instead of 365.25 solar days.

South celestial pole     The point about which southern stars seem to move due to the earth's daily rotation on its axis.

Spectral analysis     The graphing and analysis of an object's radiation which has been broken down into its component colors, wave lengths or frequencies.

Spectral type     A classification for stars based on their temperatures, O B A F G K M. The temperatures are derived from spectral analysis.

Summer solstice     The time and date when the sun reaches its northern most declination on the celestial sphere.

Superior conjunction     A conjunction in which a planet whose orbit is inside that of the earth passes the far side of the sun.

Supernova    The explosion and devastation of a massive star resulting in a brilliant flash.

Supernova remnant     Gas left over after a supernova that can be detected by sight, or by radio or x-ray emissions. The Crab Nebula can be detected all three ways.

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T

Terminator     The edge of the lighted area of the moon or a planet. The line between night and day.

Totality     The portion of an eclipse where the solar disc is completely obscured. (Between second and third contact.)

Train   The path of light seen behind a meteor.

Transit     The passage of a celestial object across the disk of another. Also, the passage of a celestial object across the meridian.

Tropic of Cancer     23 degrees 27 minutes north latitude on earth. When the sun is at its northern most declination, it is at the zenith at the Tropic of Cancer.

Tropic of Capricorn     23 degrees 27 minutes south latitude on earth. When the sun is at its southern most declination, it is at the zenith at the Tropic of Capricorn.

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U

Umbra     During an eclipse, that part of the earth or moon's shadow where the solar disk is completely hidden. Also, the darkest part of any shadow.

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V

Variable star     A star whose apparent brightness varies regularly.

Vernal equinox     The point at which the sun crosses the celestial equator as it moves northward along the ecliptic.

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W

Winter solstice     The time and date when the sun reaches its southern most declination on the celestial sphere.

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Z

Zenith     A point on the celestial sphere directly above the observer.

Zodiac     The 12 constellations that circle the ecliptic.

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 Please email me if you find any discrepancy in these definitions.

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