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Earth-approaching asteroid  An asteroid with an orbit that crosses the Earth's orbit or that will at some time cross the Earth's orbit as it evolves under the influence of the planets' gravity. See also near-Earth object.
eccentricity (of ellipse)  Ratio of the distance between the foci to the major axis. 
eclipse The cutting off of all or part of the light of one body by another; in planetary science, the passing of one body into the shadow of another. 
eclipsing binary star  A binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it. 
ecliptic  The apparent annual path of the Sun on the celestial sphere. 
effective temperature  See temperature (effective).
ejecta Material excavated from an impact crater, such as the blanket of material surrounding lunar craters and crater rays.
electromagnetic force  One of the four fundamental forces or interactions of nature; the force that acts between charges and binds atoms and molecules together. 
electromagnetic radiation  Radiation consisting of waves propagated through the building up and breaking down of electric and magnetic fields; these include radio, infrared, light, ultra-violet, x rays' and gamma rays. 
electromagnetic spectrum  The whole array or family of electromagnetic waves, from radio to gamma rays. 
electron A negatively charged subatomic particle that normally moves about the nucleus of an atom. 
element A substance that cannot be decomposed, by chemical means, into simpler substances. 
elementary particle  One of the basic particles of matter. The most familiar of the elementary particles are the proton, neutron, and electron. 
ellipse A curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same. 
elliptical galaxy  A galaxy whose appearance resembles a solid made of a series of ellipses and that contains no conspicuous interstellar material.
ellipticity The ratio (in an ellipse) of the major axis minus the minor axis to the major axis.
emission line  A discrete bright line in the spectrum. 
emission nebula  A gaseous nebula that derives its visible light from the fluorescence of ultraviolet light from a star in or near the nebula.
emission spectrum  A spectrum consisting of emission lines. 
energy level  A particular level, or amount, of energy possessed by an atom or ion above the energy it possesses in its least energetic state; also used to refer to the states of energy an electron can have in an atom. 
epicycle A circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent). 
equator A great circle on the Earth, 90' from (or equidistant from) each pole. 
equinox One of the intersections of the ecliptic and celestial equator; one of the two times during the year when the length of the day and night are the same. 
equivalence principle  Principle that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment. 
eucrite meteorite  One of a class of basaltic meteorites believed to have originated on the asteroid Vesta. 
event horizon The surface through which a collapsing star passes when its velocity of escape is equal to the speed of light; that is, when the star becomes a black hole. 
excitation The process of imparting to an atom or an ion an amount of energy greater than it has in its normal or least-energy state. 
exclusion principle  See Pauli exclusion principle
extinction Reduction of the light from a celestial body produced by the Earth's atmosphere, or by interstellar absorption. 
extragalactic Beyond our own Milky Way Galaxy. 
eyepiece A magnifying lens used to view the image produced by the objective of a telescope.
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