Astronomical Dictionary


Dictionary of Astronomical Definitions and Terms.
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Definition


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Magnitude
The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest star visible to the unaided eye, has magnitude 6. A decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Apparent magnitude of a star is the brightness as we see it from Earth, whilst absolute magnitude is a measure of its intrinsic luminosity. Lower numbers represent brighter objects.
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Meteor
Also known as a "shooting star" or "falling star", is a bright streak of light in the sky caused by a meteorite as it burns up in the Earth's atmosphere.
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Meteorite
A stony or metallic object from interplanetary space that impacts a planetary surface.
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Minor Planets
This term is preferred by astronomers, and is synonomous with the term asteroid.
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Moon
A naturally occurring satellite, or relatively large body, orbiting a planet.
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Magma
Molten rock material (liquids and gases).
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Mantle
The main bulk of a planet between the crust and the core; on Earth, the mantle ranges from about 40 to 2,900 kilometers (25 to 1800 miles) below the surface.
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Mare
A dark, low-lying lunar plain, filled to some depth with volcanic rocks.
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Mars
Planet fourth in order from the sun. In Roman mythology, god of war and discord (Greek name, Ares).
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Massif
A massive topographical feature, commonly formed of rocks more rigid than those of its surroundings.
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Mead
Margaret Mead (1901-1978) American anthropologist, author and lecturer on contemporary social issues.
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Mercury
The planet closest to the sun. In Roman mythology, the fleet-footed messenger god and escort of dead souls to the underworld (Greek name, Hermes).
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Mimas
In Greek mythology, a giant.
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Morphology
The study of structure or form.
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Mylitta
In ancient Phoenicia, a moon goddess who presided over fertility and childbirth.
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Nebula
A term used to describe celestial objects which have a fuzzy, or nebulous, appearance (from the Latin for cloud.), such as gas, or dust, clouds. Galaxies were once described thus..
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Nova
Literally, a new star. An existing star which suddenly increases its brightness by more than 10 magnitudes and then slowly fades. This happens when a star throws off a portion of its outer shell or envelope; after this happens, the star usually returns to normal. On the average, 25 such eruptions per year occur in a spiral galaxy such as our own (the Milky Way). But, only 2 or 3 new novae are usually discovered in our galaxy each year.
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Navka
Arab mother-goddess.
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Neptune
Planet eighth in order from the sun. In Roman mythology, god of the sea (Greek name, Poseidon).
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Nucleus
The frozen core of a comet which contains almost the entire cometary mass and is located in the comet's head.
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Occultation
This is when one celestial body, passes in front of, and obscures, another.
This refers to the same type of phenomena as an eclipse. [Distinguish from eclipse] From precise timings by different observers (almost exclusively amateur astronomers), a wide variety of information about celestial objects can be determined, including: planetary atmospheres, coordinates of distant radio sources, dimensions of objects with extended disks, radio and X-ray objects, and the heights of lunar mountains lying near the limb of the Moon.
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Open Cluster
A group of young stars, possibly bound together by gravity. Over 750 of them have been recorded. It is thought that the stars in these clusters are relatively young and have recently been formed from interstellar gas clouds. Prominent examples are the Pleiades and Hyades clusters in Taurus.
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Opposition
A planet is said to be "in opposition" when it appears opposite the Sun in the sky. For the outer planets, this is generally the closest they come to the Earth, hence when they are most easily visible.
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Optical Binary
Not a true binary system; two stars which appear in alignment from the perspective of the Earth which are actually an enormous distance apart. They are not physically associated with one another and lie at vastly different distances. Optical binaries are also known as visual binaries.
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Orbit
The path of one body around another due to the influence of gravity. For two-body systems, the possible orbits thus described are the five conic sections: straight line, circle, ellipse, parabola, and hyperbola.
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Astronomical [1] [2] [3] [4] [5] [6] [7] [8] [9]

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