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Glossary

aberration:slight displacement of stars resulting from movement of the earth through space
absolute magnitude (M):the magnitude that an object would appear if it were at a distance of ten parsecs
absolute visual magnitude (Mv):the absolute magnitude of an object when viewed through a special yellow filter which approximates the visual range of the human eye
absorption nebula:a dark nebula, absorbs the light of stars beyond it and blocks them from our view
altitude:angular distance of an object above the horizon
angstrom:a unit measuring one-ten thousandth of a micrometer, used in wavelength
anullar eclipse:a solar eclipse in which a ring, or annulus, of the solar protoshere remains visible
aphelion:an objects farthest position from the sun during its orbit
apparent magnitude (m):magnitude as seen from earth
apparent solar time:time determined by the position of the sun in the sky
asterism:a noticable pattern of stars which make up part of a constellation
asteroid:a minor planet which does not orbit any of the major planets
astronomical unit (AU):the average distance of the earth to the sun, 149,598,770 kilometers
autumnal equinox:the intersection of the ecliptic and the celestial equator that the sun passes each year on its way to southern, or negative, declinations
bailey's beads:a chain of several bright beads of white light, visible just before or after the totality of a solar eclipse. This is an effect of bits of photosphere shining through the valleys on the moons edge
bayer designation:Greek letters assigned to each star of a constellation, usually according to brightness, by Johann Bayer in 1603
belts:dark bands in the clouds of gas giants such as Jupiter
binary star:a double star, a system containing two or more stars
black hole:a region of space in which matter is packed so densely that nothing, not even light, can escape its gravitational field, according to Einstein's general theory of relativity
cassini's division:the major division of saturn's rings which seperates the A-ring from the B-ring
celestial equator:the imaginary great circle which lies along the earth's equator on the celestial sphere
celestial longitude:longitude measured in degrees along the ecliptic to the east of the vernal equinox
celestial poles:the points in the sky where the earth's axis, extended into space, intersect the celestial sphere
celestial sphere:the imaginary sphere surrounding the earth, with the stars and other astronomical objects attached to it
cepheid variable:a star that varies in the manner of delta cephei. The absolute magnitude of these stars can be calculated from their periods of variation, and by comparing the absolute magnitude with the apparent magnitude, their distance can be found
chromoshpere:a layer in the sun and many other stars just above the photosphere. during an eclipse, the solar chromosphere glows reddish from hydrogen emmission
circumpolar:refers to a star, asterism, or constellation that is close enough to the celestial pole that, from the latitude from which you are observing, it never appears to set
comet:a body between 0.1 and 100 miles across that travels through the solar system in an elliptical orbit of random inclination to the ecliptic. A comte may have a tail if it travels close to the sun
conjunction:the alignment of two celestial bodies that occurs when they reach the same celestial longitude. This causes the bodies to appear very close in the sky
constellation:one of 88 parts into which the sky is divided; also refers to the historical, mythological, or other figures that represented earlier divisions of the sky
contact:the stage of an eclipse, occulation, or transit when the edges of the apparent disksof astronomical bodies seemto touch. In a solar eclipse, first contactis when the advancing edge of the sun first touches the moon, second contact is when the advancing edge of the sun touches the opposite side of the moon, beginning totality, third contactis when the trailing edge of the sun touches the trailing edge of the moon, and at fourth contact the eclipse is over
corona:the outermost layer of the sun and of many stars, a faint halo of extremely hot, million degree gases
Crepe ring:saturn's innermost ring, also known as the c-ring, which extends inward to the planet from the brightest ring, the b-ring
crescent:one of the phases of the moon or the inner planets, as seen from earth, caused by the relative angles of light and the observers viewpoint.
declination:the celestial coordinate analagous to latitude, measured in degrees north (+) or south (-) of the celestial equator
diamond-ring effect:an effect created when the total phase of a solar eclipse is about to begin, when the last bailey's bead - a remianing bit of the photosphere - glows so intensely in contrast with the sun's faint corona that it appears as a jewel on a ring
double star:a system containing two or more stars. in a true double, the stars are physically close to eachother, in an optical double, they appear in the same direction away from earth but are actually far apart
earthshine:sunlight reflected off the earth which dimly lights the side of the moon which does not receive direct sunlight
eclipse, lunar:the passage of the moon into the earth's shadow
eclipse, solar:the passage of the moon's shadow onto the earth
ecliptic:the apparent path the sun follows across the sky during the year, the same path is also followed approximately by the planets and the moon
ejecta blanket:chunks of rock, usually extending from one side of a crater, that were ejected during the crater's formation
elongation:angular distance in celestial longitudefrom the sun in the sky
emission lines:extra radiation at certain specific wavelengths in a spectrum, compared with other wavelengths
emission nebula:a gas cloud that receives energy from a hot star, allowing it to give off radiation in emission linessuch as those of hydrogen. The characteristic reddish radiation of many emission nebula is mostly from the hydrogen-alpha line
Encke's division:a thin division in the a-ring of saturn
equinox:one of the two intersections of the ecliptic and the celestial equator
filament:a dark region snaking across the sun, a prominence seen in projection against the solar disk
fireball:an extremely bright meteor, usually with an apparent magnitude between -5 and -20
Flamsteed number:a number assigned to a star in a given constellation, in order of right ascension, in the 1725 catalog of John Flamsteed
galactic:pertaining to our galaxy, the milky way
galactic equator & poles:the equator and poles in a coordinate system in which the equator is placed along the plane of our galaxy
galaxy:a giant collection of stars, gas, and dust
giant:a star brighter and larger than most stars of its color and temperature; stars become giants when they use up all the hydrogen in their cores and leave the main sequence phaseof their life cycle
gibbous:a phase of a moon or planet in which more than half of the side we see is illuminated
globular cluster:a spherical grouping of stars of a common origin, usually containing very old stars
graben:on the surface of a planet or satellite, a long and narrow region between two faults that have subsided
half moon:the first and third quarter phases of the moon in which one half of the side we see is illuminated
hour angle:the siderial time elapsed since an object was on the meridian, or, if the hour angle is negative, before the object reaches the meridian (the hour angle equals the difference between the objects right ascensoion and your meridian)
hour circle:a line along which right ascension is constant, lying on a great circle which passes through the celestial poles and the object
Hubble's law:the relationship between the velocity and distance of galaxies and other distant objects, which shows that the universe is expanding
hydrogen-alpha line:the strongest spectral line of hydrogen in the visible part of the spectrum; it falls in red, so the emission hydrogen-alpha line is a certain wavelength of red, and theabsorption hydrogen-alpha line is the absence of that certain wavelength of red
IC:the prefix used before numbers assigned to nonstellar objects in the Index Cataloguespublished as supplements to the New General Catalogue of J.L.E. Dryer
inferior conjunction:the conjunction in which a planet whose orbit is inside that of earth passes between the earth and the sun
intrinsic brightness:the amount of energy an object gives off, its true brightness independent of the effects of distance or dimming by intervening material
ionized hydrogen:hydrogen that has lost its electron, creating ionized hydrogen gas which is found in stars and nebula, consisting of free electrons and protons
the number of days since noon on january 1, 4713 BC; this is used by astronomers to calculate intervals between events by subtracting julian days, eliminatingthe need of tracking leap years and other calender details
libration:the turning of the visible surface of the moon, allowing us to see different details around the limb
light year:the distance light travels in a year, which equals 9,460,000,000,000 km or 63,240 AU
limb:the edge of the apparent disk of an astronomical body; such as the sun, moon, or a planet
magnitude:a logarithmic scale of brightnessin which each change of five magnitudes is equivilant to a change by a factor of 100; adding one magnitude corresponds to a decrease in brightness by a factor of 2.512
main sequence star:the longest period of a stars life when hydrogen is being fused into helium at its core; these stars form a band through the middle of a Russel-Hertzprung diagram
maxima:the time when a variable star reaches its maximum brightness
meridian:the great circle passing through the celestial poles and your zenith
messier catalog:a list of 103 nonstellar, deep sky objects compiled by Charles Messier in 1770, subsequently expanded to 110 objects
meteor:a meteoroid while passing through the atmosphere
meteorite:the part of a meteoroid that survives its passage through the atmosphere
meteor showerthe appearance of many meteors during a short period of time, usually as the earth travels through a comets orbit
minima:the time when a variable star reaches its minimum brightness
mira variable:a long period variable star, such as omicron ceti
nebula:a region of gas and dust in a galaxythat can be observed optically
NGC:the prefix used before numbers assigned to nonstellar objects in the New General Catalogue published in 1888 by J.L.E. Dryer
neutron star:a small, densestar resulting from the collapse a dying star, in which protons and neutrons are compressed together, forming a star of all neutrons which prevent it from further collapse
nova:an explosion within a star, which causes it to suddenly increase in brightness
nutation:a slight wobble in the direction of the earth's orbit which is superimposed on that of precession with a period of 19 years
oblate:a nonspherical shape created by rotating an ellipse around its shorter axis; the equitorial diameter of an oblate body is greater than its polar diameter
occulation:the hiding of one celestial body behind another
open cluster:an irregular grouping of stars of a common and possibly recent origin, also called a galactic cluster