** E = mc^{2}**: Einstein’s formula
(special theory of relativity) for the equivalence of mass and energy.

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** early universe**: The Universe during its first
minutes.

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** earthshine**: Sunlight illuminating the Moon
after having been reflected by the Earth.

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** eccentric**: Deviating from a circle.

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** eccentricity**: A measure of the flatness of an
ellipse, defined as the distance between the foci divided by the major axis.

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** eclipse**: The passage of all or part of one
astronomical body into the shadow of another.

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** eclipsing binary**: A binary star in which one
member periodically hides the other.

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** ecliptic**: The path followed by the Sun across
the celestial sphere in the course of a year.

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** ecliptic plane**: The plane of the Earth’s orbit
around the Sun.

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** ecosphere**: The region around a star in which
conditions are suitable for life, normally a spherical shell.

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** electric field**: A force set up by an electric
charge.

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** electromagnetic force**: One of the four
fundamental forces of nature, giving rise to electromagnetic radiation.

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** electromagnetic radiation**: Radiation resulting
from changing electric and magnetic fields.

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** electromagnetic spectrum**: Energy in the form
of electromagnetic waves, in order of wavelength.

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** electromagnetic waves**: Waves of changing
electric and magnetic fields, traveling through space at the speed of light.

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** electromagnetism**: The combined force of
electricity and magnetism, which follows the formulae unified by Maxwell.

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** electron**: A particle of one negative charge,
1/1830 the mass of a proton, that is not affected by the strong force. It is a lepton.

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** electron volt (eV)**: The energy necessary to
raise an electron through a potential of one volt.

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** electroweak force**: The unified electromagnetic
and weak forces, according to a recent theory.

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** element**: A kind of atom, characterized by a
certain number of protons in its nucleus.
All atoms of a given element have similar chemical properties.

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** elementary particle**: One of the constituents
of an atom.

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** ellipse**: A curve with the property that the
sum of the distances from any point on the curve to two given points, called
the foci, is constant.

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** elliptical galaxy**: A type of galaxy characterized
by elliptical appearance.

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** emission line**: Wavelengths (or frequencies) at
which the brightness of radiation is greater than it is at neighboring
wavelengths (or frequencies).

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** emission nebula**: A glowing cloud of
interstellar gas.

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** emulsion**: A coating whose sensitivity to light
allows photographic recording of incident radiation.

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** energy**: A fundamental quantity usually defined
in terms of the ability of a system to do something that is technically called
“work,” that is, the ability to move an object by application of force, where
the work is the force times the displacement..

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** energy, law of conservation of**: Energy is
neither created nor destroyed, but may be changed in form.

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** energy level**: A state corresponding to an
amount of energy that an atom is allowed to have by the laws of quantum
mechanics.

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** energy problem**: For quasars, how to produce so
much energy in such a small emitting volume.

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** ephemeris**: A listing of astronomical positions
and other data that change with time.
From the same root as ephemeral.

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** epicycle**: In the Ptolemaic theory, a small
circle, riding on a larger circle called the deferent, on which a planet
moves. The epicycle is used to account
for retrograde motion.

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** equal areas, law of**: Kepler’s second law.

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** equant**: In Ptolemaic theory, the point equally
distant from the center of the deferent as the Earth but on the opposite side,
around which the epicycle moves at a uniform angular rate.

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** equator**: (a) Of the Earth, a great circle on
the Earth, midway between the poles; (b) celestial, the projection of the
Earth’s equator onto the celestial spheres; (c) galactic, the plane of the disk
as projected onto a map.

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** equatorial mount**: A type of telescope mounting
in which one axis, called the polar axis, points toward the celestial pole and
the other axis is perpendicular. Motion around only the polar axis is
sufficient to completely counterbalance the effect of the Earth’s rotation.

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** equinox**: An intersection of an ecliptic and
the celestial equator. The center of
the Sun is geometrically above and below the horizon for equal lengths of time
on the tow days of the year when the Sun passes the equinoxes; if the Sun were
a point and atmospheric refraction were absent, then day and night would be of
equal length on those days.

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** erg**: A unit of energy in the metric system,
corresponding to the work done by a force of one dyne (the force that is
required to accelerate one gram by one cm/sec

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** ergosphere**: A region surrounding a rotating
black hole (or other system satisfying Kerr’s solution) from which work can be
extracted.

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** escape velocity**: The initial velocity that an
object must have to escape the gravitational pull of a mass.

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** event horizon**: The sphere around a black hole
from within which nothing can escape; the place at which the exit cones close.

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** evolutionary track**: The set of points on a
temperature-luminosity diagram showing the change of a star’s temperature and
luminosity with time.

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** excitation**: The raising of atoms to higher
energy states than the lowest possible.

** excited level**: An energy level of an atom
above the ground level.

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** exit cone**: The cone that, for each point
within the photon sphere of a black hole, defines the directions of rays of
radiation that escape.

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** exobiology**: The study of life located
elsewhere than Earth.

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** exponent**: The “power” representing the number
of times a number in multiplied by itself.

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** exponential notation**: The writing of numbers
as a power of 10 times a number with one digit before the decimal point.

** extended objects**: Objects with detectable
angular size.

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** extinction**: The dimming of starlight by
scattering and absorption as the light traverses interstellar space.

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** extragalactic**: Exterior to the Milky Way
Galaxy.

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** extragalactic radio sources**: Radio sources
outside our galaxy.

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** eyepiece**: The small combination of lenses at
the eye end of a telescope, used to examine the image formed by the objective.