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eccentric
deviating from a circle (used to describe the shape of an orbit)
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eccentricity
the extent to which a body’s elliptical orbit deviates from a circle
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eclipse
an event in which one body passes in front of another, blocking it partially or completely from view; a specific type of occultation
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eclipsing binary
a binary star with an orbital plane oriented so that one star passes in front of the other, thus completely or partially blocking the light from the other star during each orbital period
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ecliptic
the plane of Earth’s orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane
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ejecta
material thrown about by an impact or volcano
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electromagnetic radiation
the various forms of light; includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays
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electromagnetic spectrum
the spectrum encompassing the entire range of electromagnetic radiation (light)
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electron
a subatomic particle with a negative electric charge; electrons surround the atomic nucleus and are much less massive than protons or neutrons
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electron volt
a unit of energy equal to the energy gained by an electron that falls through a potential difference of one volt; 1.60 x 10^-19 joule
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element
a fundamental unit of matter; consists of a fixed number of protons, although the number of neutrons and electrons can vary
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elliptical galaxy
a gravitationally bound system of stars in a spherical or elliptical shape with no spiral structure
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elongation
the apparent angular separation of an object from the sun
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emission
the discharge of electromagnetic radiation from an object
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emission nebula
a cloud of very hot gas that is being illuminated from within by the radiation of energetic, young stars
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ephemeris
a table that gives the positions of astronomical objects at certain intervals of time
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equatorial mount
a telescope mount in which one axis lies parallel to Earth’s rotational axis; the motion of the telescope about this axis can compensate for Earth’s rotation
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equinox
the two times of year when the sun crosses the celestial equator, giving day and night an equal 12-hour length everywhere on Earth
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escape velocity
the velocity an object or rocket needs to escape the gravitational clutch of a more massive object
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evening star
the planet Venus when it appears in the evening sky
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event horizon
the boundary of a black hole from inside which light cannot escape
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exit pupil
the image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece
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exobiologists
a person who studies the origin, development, and distribution of 'living' systems that may exist outside of Earth
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extragalactic
beyond the Milky Way Galaxy
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extrasolar
beyond the sun
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extraterrestrial
beyond Earth
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eye relief
the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view
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eyepiece
a magnifying lens used to view the image produced by a telescope’s primary lens or mirror.
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