Astronomical Dictionary


Dictionary of Astronomical Definitions and Terms.
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Definition


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Culmination
A general term for the hightest and lowest points a planet or celestial body reaches in the sky relative to an observer. For northern observers, this occurs when the object is due South. For southern observers when it is due North..
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Culmination
A general term for the hightest and lowest points a planet or celestial body reaches in the sky relative to an observer. For northern observers, this occurs when the object is due South. For southern observers when it is due North..
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Declination
Distance along the hour circle between the celestial equator and a celestial body measured in degrees. Angles are positive if a point is North of the celestial equator, and negative if South. It is used, in conjunction with Right Ascension, to locate celestial objects.
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Descending Node
Also called the south node, the point in the orbit of an object, when it crosses the ecliptic whilst travelling north to south.
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Direct Motion
1 Rotation or orbital motion in an anticlockwise direction when viewed from the north pole of the Sun (i.e. in the same sense as the Earth); the opposite of retrograde.
2 The East-West motion of the planets, relative to the background of stars, as seen from the Earth.
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Dwarf Star
A star, which lies on the main sequence and is too small to be classified as a giant star or a supergiant star. For example, the Sun is a yellow dwarf star.
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Eccentricity
Literally, not circular. The eccentricity of an ellipse (orbit) is the ratio of the distance between its focii and the major axis. The greater the eccentricity, the more 'flattened' is the ellipse. Venus has the least eccentric and Pluto has the most eccentric orbit of all the planets.
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Eclipse
A chance alignment between the Sun, or any other celestial object, and two other celestial objects in which one body blocks the light of the Sun, or other body, from the other. In effect, the outer object moves through the shadow of the inner object.
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Ecliptic
The great circle formed by the intersection of the plane of the Ecliptic with the Celestial Sphere. The apparent path the Sun (and, approximately that of the planets) as seen against the stars. Since the plane of the Earth's equator is inclined at 23.5 degrees to that of its orbit, the ecliptic is inclined to the celestial equator by the same angle. The ecliptic intersects the celestial equator at the two equinoxes.
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Elongation
The angular distance of an interior planet from the Sun, as seen from the Earth. The term Greatest Elongation is applied to the inner planets, Mercury and Venus. It is the maximum elongation from the Sun. At Greatest Elongation, the planet will appear 50% phase.
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Equatorial Mount
A telescope mount so designed so that the two axes, which support it, are aligned, one to the polar axis and the other to the Earth's equator. Once an object is centred in the telescope's field of view, only the polar axis need be adjusted to keep the object in view. If the polar axis is driven at Sidereal rate, it will counteract the rotation of the Earth, keeping the object (except the Moon) stationary in the field of view.
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Equinox
Literally, the time of equal day and night. This is the time when the Sun crosses the celestial equator. There are two equinoxes; Vernal (Spring), around March 21st and Autumnal (Autumn) around September 23rd. On these dates, day and night are equal. Actual dates and times vary due to the Earth\'s precession. This term is frequently, although incorrectly, used to describe a point lying on the Ecliptic.
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Facula
Large, irregular, bright areas on the surface of the Sun, particularly near the limb. They precede the appearance of sunspots and can remain for some months afterwards.
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Filament
A strand of (relatively) cool gas suspended over the Sun (or star) by magnetic fields, which appears dark against the disc of the Sun. A filament on the limb of the Sun seen in emission against the dark sky is called a solar prominence.
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Galaxy
Vast star systems containing thousands of billions of stars, dust and gas, held together by gravity. Galaxies are the basic building blocks of the Universe. There are three main classes, Elliptical, Spiral and Barred, named after their appearance.
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Galilean Moons
Jupiter's four largest moons: Io, Europa, Ganymede and Callisto.
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Galileo
(1564-1642) Italian mathematician, astronomer, and physicist. First to use a telescope to observe the skies. Born at Pisa; died at Arcetri (near Florence). Galileo Galilei was born 3 days before Michelangelo died. His first name resulted from a Tuscan tradition which used a variation of the last name for first name of the eldest son. His father had planned a career in medicine for him, until Galileo heard a lecture on geometry, and was exposed to the works of Archimedes. Shortly thereafter he convinced his father to allow the switch to mathematics and science. His approach to science transcended mere observation. Instead, he would attempt to devise a critical experiment which would clearly demonstrate his theories. After quantifying observables during an experiment, he would set upon the task of deriving a mathematical relationship to describe a phenomenon with simplicity and generality. During services at the Cathedral of Pisa, he observed the movements of a chandelier swinging, caused by air currents. Some swings were short, while others were made in wide arcs. But, the time required to complete the swing seemed to be constant. When he later tested his theory with two pendulums of equal length, using arcs of different sizes, he found that the periods of the swing remained constant. Not until after Galileo's death was the principle of the pendulum used (by Huygens) to regulate a clock. During his lifetime, the lack of an accurate clock to measure small intervals of time would hamper many of the experiments of Galileo. He devised a thermoscope to measure temperature; but this invention was quite inaccurate. He also designed a hydrostatic balance, and published a description of same in 1586. His experimentation with falling bodies has become nearly legendary. He is supposed to have dropped two cannon balls of unequal weight (in the ratio 10:1) from the Tower of Pisa, showing that both stuck the ground at the same instant. It is doubtful that this experiment actually took place; but, it was performed earlier by Stevinus. By rolling bodies down an inclined plane, Galileo was able to slow down the actions, and clearly demonstrate that the rate of fall of a body was independent of its weight. This result showed that the physics of Aristotle were in error. The philosophic ramifications of this result were profound: According to Aristotle, it was necessary to apply a continuous force in order to keep a body in motion. Thus, the heavenly bodies were kept in motion by the perpetual efforts of angels. Buridan, however, claimed that no force was required to maintain motion, save the initial push given by God during the act of creation. Galileo further showed that the velocity of a falling body increased linearly, and the total distance increased as the square of the time. His analysis of multiple forces created a science of gunnery. Lacking the advances in mathematical analysis achieved by the application of algebra to geometry, which was later provided by Descartes and Newton, Galileo formulated all his proofs using the geometric methods of the Greeks. His book on mechanics also treated the square-cube law for strength of materials. Then, in 1609, he learned of the telescope (which was invented in Holland). With his own version of the telescope, he began to explore the heavens, and the age of telescopic astronomy was born. He found that there were mountains on the Moon, spots on the Sun, that the Sun rotated upon its axis each 27 days, and that Jupiter posessed four companions which revolved about that planet (the Galilean satelites: Io, Europa, Ganymede, and Callisto). Again, his findings were at odds with the writings of Aristotle. The satellites of Jupiter offered definite proof that not all astronomical bodies circled the earth, giving strong support for the system of Copernicus. In addition, the phases exhibited by Venus were required by the Copernican theory, but could not be accounted for with the Ptolemaic theory. He publicized his findings in a periodical called Sidereus Nuncius (Starry Messenger). Thinking that Pope Urban IV would be friendly to his views, he published his Dialogue on the Two Chief World Systems, in which two people (who advocated the Ptolemaic and Copernican viewpoints, respectively) argue their cases before a third individual. But, the Pope was convinced that the book's character, Simplicio, was an insulting charicature of himself, deliberately invented for that purpose. Thus, Galileo was charged with heresy, brought before the Inquisition, and made to renounce all but the Ptolemaic viewpoint. It was not until 1835, that Galileo's Dialogue was to be removed from the Catholic Index of Prohibited Books. And finally, in 1965, Pope Paul VI praised the work of Galileo, thus admitting the wrong committed by the Church.
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Geosynchronous Orbit
Sometimes known as a geostationary orbit, in which a satellite's orbital velocity is matched to the rotational velocity of the planet, and as such, a geostationary satellite would appear to be stationary relative to the Earth.
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Globular Cluster
A spherical cluster of older stars, often found in galaxies. These clusters form a sphere surrounding the center of the galaxy known as the halo. From the Earth, they are seen in the direction of the constellations Scorpio-Sagittarius. Globular clusters are known to contain large numbers of white dwarfs, which would indicate that they are ancient even by stellar standards.
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Fortuna
In Roman mythology, goddess of fortune, chance and luck.
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feldspar
A group of rock-forming minerals that make up about 60% of the Earth's crust.
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Fault
A fracture or zone of fractures in a planet's crust, accompanied by displacement of the opposing sides.
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Europa
In Greek mythology, a mistress of Zeus to whom he appeared as a gentle white heifer. Zeus persuaded her to take a ride on his back, and then he carried her away across the sea.
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Escarpment
A long, more or less continuous cliff or relatively steep slope facing in one general direction, produced by erosion or faulting.
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Encke
Johann Franz Encke (1791-1865) German astronomer at the Seeberg Observatory, Switzerland. Determined period of the comet discovered by Pons and showed it to be identical with comets of other years.
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Astronomical [1] [2] [3] [4] [5] [6] [7] [8] [9]

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