Astronomical Dictionary
Letter: E


Dictionary of Astronomical Definitions and Terms.
Astronomical | E

 

Definition


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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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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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Enceladus
In Greek and Roman mythology, a giant, son of Titan and Gaea. Buried by an angry Zeus under Mount Etna. When the giant hisses and thrusts out his fiery tongue, Mount Etna erupts.
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Ejecta
Material thrown out of a volcano or impact crater.
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Eistla
In Norse mythology, a giantess.
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