E component  see L Component. [H76] 
eFolding Time  The time within which the amplitude of an oscillation increases or decreases by a factor e (e = 2.718...). [H76] 
E galaxy  In both Hubble's and Morgan's classifications, an elliptical galaxy. [H76] 
E layer  The part of Earth's ionosphere (about 150 km) where the temperature gradient reverses and starts to rise. It reflects "shortwave" radio waves. (also called KennellyHeaviside Layer) [H76] 
E+A Galaxy  One of the most enigmatic types of galaxy,
spectrally, are the "E+A"
galaxies. They appear to be a combination of an old elliptical galaxy
spectrum (E) with that of a significantly younger Astar population
(A), thought to be formed in a recent (t < 1 Gyr) episode of star
formation. "E+A" galaxies show strong Balmer absorption lines
(EW 
E Line  A Fraunhofer line at 5270 Å. It is a blend of Fe I and Ca I. [H76] 
eProcess  A hypothetical group of nuclear reactions by which the iron group is assumed to be synthesized. At temperatures > 5 × 10^{9} K and densities > 3 × 10^{6} g cm^{3} there are great numbers of collisions between highenergy photons and nuclei. These collisions break up the nuclei, the fragments of which promptly combine with other particles. Thus, there is in effect an equilibrium between formation and breakup. Since the iron group has the largest binding energies, the particles over the long run will tend to be trapped in these nuclei. The eprocess (the e stands for equilibrium) is presumed to occur in a supernova explosion. [H76] 
EarlyType Emission Stars  see Be Stars. [H76] 
EarlyType Spiral  In Hubble's classification, a spiral with a large nuclear bulge and closely coiled arms. [H76] 
EarlyType Stars  Hot stars of spectral types O, B, A, and early F. [H76] 
Earth  Third planet from the Sun. Mass 5.977 × 10^{27} g; polar radius 6356.9 km; equatorial radius 6378.17 km; mean density 5.517 g cm^{3}; effective temperature 287 K. Rotation period 23^{h}56^{m}4^{s}.1. Mean distance from Sun 149,598,500 km (8.3 ltmin): perihelion distance (early January) 147,100,000 km; aphelion distance 152,100,000 km; v_{orb} 29.78 km s^{1}; orbital period 365^{d}.2564; e = 0.0167, i = 0; obliquity (1973) 23°26'34". Albedo 0.39 (water and land about 0.2; snow and clouds about 0.8). Surface gravity 980 cm s^{2}; V_{esc} 11.19 km s^{1}. Precession 50'.256 per year; relativistic advance of perihelion 4'.6 per century. Atmosphere (by volume) 78% N_{2}; 20.9% O_{2}; 0.9% Ar, 0.03% C0_{2}. Particle density of Earth's atmosphere at sea level 10^{19} per cm^{3} = 1.3 × 10^{3} g cm^{3}. Atmospheric pressure at sea level 1.013 × 10^{6} dyn cm^{2}. Magnetic field at surface, about 0.5 gauss; in core, about 100 gauss. Core temperature about 6400 K; core density about 10 g cm^{3}. Age 4.6 ± 0.1 × 10^{9} years. First forms of life appeared about 3.2 to 3.5 × 10^{9} years ago (Homo sapiens appeared as a species about 10^{5} years ago). [H76] 
Earthlight  Light reflected from the Earth's atmosphere onto the dark part of the Moon. [H76] 
East Point  The point on the celestial horizon 90° clockwise from the north point. At the equinoxes the Sun rises in the east point. [H76] 
EBCCD  ElectronBombarded CCD An imaging device containing a thin target material which emits electrons by the photoelectric effect when illuminated and then magnetically focuses these electrons to impact onto a silicon CCD where they generate a large charge. [McL97] 
EBS  Electron Bombarded Silicon [LLM96] 
Eccentric  An orbit that has a high eccentricity that is, highly elliptical. [C95] 
Eccentric Anomaly  In undisturbed elliptic motion, the angle measured at the center of the ellipse from pericenter to the point on the circumscribing auxiliary circle from which a perpendicular to the major axis would intersect the orbiting body. (see Mean Anomaly; True Anomaly.) [S92] 
Eccentricity  (a) In astronomy, the extent to which an
elliptical orbit
departs from a circular one. It is usually expressed as a decimal
fraction, regarding a circle as having an eccentricity of 0. [A84]

Eccentrics  In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the Universe. [F88] 
Echelle  A type of diffraction grating with groove angles of 90°. With the grating at an angle of 45° the grooves resemble a staircase. [McL97] 
Echelon  A type of diffraction grating consisting of a number of equal thin glass sheets stacked on a slant. In use the light is reflected from the stepped side of the stack; d in the grating equation is very large, so that very high spectral orders are possible. [DC99] 
Eclipse  (a) Occultation of one celestial body by another
which passes
between it and the observer. The Solar eclipse is caused by the
passing of the Moon between the Sun and the Earth in this way; such an
eclipse may be complete (total) or incomplete (partial). Eclipsing
binary stars also accord with this pattern. Alternatively  and
exceptionally  a lunar eclipse is caused by the passage of the Earth
between the Sun and the Moon, so that the Earth's shadow falls across
the Moon, again either totally or partially, depending upon the
position of the observer. [A84]

Eclipse, Annular  A Solar eclipse (see Eclipse, Solar) in which the Solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun. [S92] 
Eclipse, Lunar  An eclipse in which the Moon passes through the shadow cast by the Earth. The eclipse may be total (the Moon passing completely through the Earth's umbra), partial (the Moon passing partially through the Earth's umbra at maximum eclipse), or penumbral (the Moon passing only through the Earth's penumbra). [S92] 
Eclipse, Solar  An eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's umbra), partial (observer in the Moon's penumbra), or annular. (see Eclipse, Annular.) [S92] 
Eclipse Year  The interval of time (346.62 days) between two successive passages of the Sun through the same node of the Moon's orbit. [H76] 
Eclipsing Binary  (a) A binary star of which, from the viewpoint of
Earth,
one of the two bodies regularly passes in front of the other. The
resulting variation is perceived luminosity of some eclipsing binaries
has led to their classification as variable stars. [A84]

Ecliptic  (a) Apparent linear path through the 12
constellations of the
zodiac that the Sun seems to take during one Earth year, also
representing therefore the "edge" of the plane of Earth's
orbit. Because the equator of the Earth is at an angle of more than
22° to the plane of its orbit, the ecliptic is at the identical
angle to the celestial equator, intersecting it at two points: the
vernal and autumnal equinoxes. [A84]

Eddington Approximation  An approximation used in the study of radiative transfer. It is the assumption that the ratio of the second moment of the radiation field to the mean intensity is everywhere equal to 1/3, the value of this ratio for an isotropic field. [H76] 
EddingtonLemaître Universe  A cosmological model in which the cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an expansion of the Universe. [Silk90] 
Eddington Limit  In essence, radiation pressure must not exceed gravity. It is the limit beyond which the radiation force on matter in the emitting region is greater than the gravitational forces that hold the star together. L_{E} = 4cGM/K_{s}, where K_{s} = Thomson and/or Compton scattering opacity. Eddington limit for a 1 M_{} star, 10^{38} ergs s^{1}. [H76] 
Eddington's Standard Model  A stellar model in which energy is transported by radiation throughout the whole star and the ratio of the radiation pressure to the gas pressure is assumed to be constant. [H76] 
Eddy Currents  Induced currents set up in a conductor by a
changing magnetic field. They occur in transformers and other
electrical devices. The currents produce a heating effect
corresponding to a loss of useful energy (eddycurrent
loss). Metal cores in electrical machines are usually laminated
(built of thin sheets) to reduce such losses; the surface layers
between the laminations have high electrical resistance.

Edge Effects  Absorption in the spectra of galaxies at the edges of some passbands by lines broadened by velocity dispersion. [H76] 
Effective Radius  The distance from the center of a galaxy within which half of the total luminosity is included (cf. Holmberg radius). [H76] 
Effective Temperature (T_{eff})  The temperature that a blackbody would have which emitted the same amount of energy per unit area as the star does: it is a temperature characteristic of the surface region. T_{eff} of the Sun is 5800 K. [H76] 
Effective Theory  Each part of the physical world can be described by a subtheory that applies over a certain distance scale or energy scale. Such subtheor ies are called effective theories. Explanations in a given effective theory can ignore much of the rest of the world, which has effects on the part of inter est through a few inputs or parameters. Every part of our description of the physical world is an effective theory, except the ultimate theory that is called the Primary Theory. [K2000] 
Eigenfunctions  The wave functions corresponding to the eigenvalues. Eigenfunctions represent the stationary states ("standing waves") of a system. [H76] 
Eigenstate, Eigenvalue  (a) The eigenvalue of a matrix M is a
number which satisfies the
equation

Eightfold Way  classification scheme for elementary particles established c. 1960. Forerunner of quark model. [D89] 
Eikonal Approximation  An approximation in which the oscillation of a wave front is replaced by the direction of the ray which is normal to the oscillation. [H76] 
Einstein Coefficient  An emission (or absorption) coefficient. A_{ji} is the coefficient of spontaneous emission; B_{ji}, is the coefficient of stimulated emission, where i is the lower level and j is the upper level. [H76] 
Einsteinde Sitter Cosmology  (a) A Friedmann model of the Universe in which the
spacetime continuum is not curved. [C97]

Einstein Effect  Displacement of spectral lines due to the gravitational redshift. [H76] 
Einstein Equations  The equations of Einstein's theory of gravity, called general relativity. The Einstein equations quantitatively specify the gravity produced by matter and energy. Since gravity is believed to be the principal force acting over very large distances, the Einstein equations are used in modern theories of cosmology. [LB90] 
Einstein Equivalence Principle  Foundation for curvcd spacetime, it states that bodies fall with the same acceleration and that physics in freely falling reference frames is independent of the velocity and location of the frames. [D89] 
Einstein's General Theory of Relativity  The theory of gravity in which the gravitational force is described mathematically by a curvature in space or spacetime. [D89] 
Einstein Static Universe  A cosmological model in which a static (neither expanding nor collapsing) Universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force. [Silk90] 
Einstein Universe  A world model of a static Universe with a positive cosmological constant, whose radius of curvature is constant and independent of time. [H76] 
Einsteinium  A radioactive transuranic element of the actinoid
series, not found naturally on Earth. It can be produced in milligram
quantities by bombarding ^{239}Pu with neutrons to give
^{253}Es (halflife 20.47 days). Several other shortlived
isotopes have been synthesized.

Ekman Layer  Upper boundary layer within which the amplitude changes exponentially. [H76] 
Elastic Collision  A collision between two particles which conserves the total kinetic energy and momentum of the system. For atomic collisions it is one involving energy less than the excitation potential of the atom. [H76] 
Elastic Scattering  Particle reactions in which the same particles emerge from the reaction as entered it (e.g. ^{} p > ^{} p). In inelastic scattering, where different and/or new particles emerge, energy is used to create new particles. [CD99] 
Electrodes  Also called gates or phases. Small electrically conducting plates connected to a voltage source (battery or power supply) and arranged in strip patterns to define the picture elements or pixels of a CCD. The plates create an electric field within the semiconductor which therefore forms a storage site for photogenerated charges. Also used as a generic term for any conductor with an applied voltage. [McL97] 
Electrodynamics  Study of the behavior of electromagnetic force in motion. [F88] 
Electromagnetic Field  Force field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in space. [G99] 
Electromagnetic Force  (a) Fundamental force of nature that acts on all
electrically charged
particles. Classical electromagnetics is based on Maxwell's and
Faraday's equations, quantum electromagnetics on the theory of quantum
electrodynamics (QED). [F88]

Electromagnetic Gauge Symmetry  Gauge symmetry underlying quantum electrodynamics. [G99] 
Electromagnetic Unit  EMU A system of electrical units based on the electromagnetic properties of an electric current. [H76] 
Electromagnetic Radiation  (a) "Waves" of electrical and magnetic
"disturbance", radiated as visible light, radio waves, or any other
manifestation of the electromagnetic spectrum. The distance between
successive crests of each wave is known as the wavelength, and varies
considerably between electromagnetic forms. The velocity of such
radiation in a vacuum is the speed of light. The units of
electromagnetic radiation are quanta or photons ("packets" of
energy). [A84]

Electromagnetic Spectrum  (a) Complete range of electromagnetic radiation,
from very shortwavelength (highfrequency) gammarays, through Xrays
and ultraviolet light to the small range of visible light, and further
to infrared radiation, microwave, and the comparatively
longwavelength lowfrequency radio waves. [A84]

Electromagnetic Wave  A pattern of electric and magnetic fields that moves through space. Depending on the wavelength, an electromagnetic wave can be a radio wave, a microwave, an infrared wave, a wave of visible light, an ultraviolet wave, a beam of X rays, or a beam of gamma rays. (see Photon) [G97] 
Electromagnetism  (a) One of the four fundamental forces of nature,
governing the electric and
magnetic interaction between particles. [C97]

Electron  (a) Negatively charged fundamental particle (also
called a beta
particle) found in the atoms of all elements, where it "orbits" (at
different energy levels and with different directions of spin) round
the central nucleus. The combined charge of the orbiting electrons is
balanced (in a neutral atom) by the charge of an equal number of
positively charged protons in the atomic nucleus. An electron is also
the fundamental unit of electricity. [A84]

Electron Affinity  (a) The work needed to remove an electron from
a negative ion and move it to infinity.

Electron Collider  Short for electronpositron collider. One import ant way to study particle interactions and search for new particles is to accelerate an electron and a positron to high energies and then collide them, using a detector to study what emerges. The energy to which they are accelerated is chosen to f it the question of interest. For example, to study CP violation in bqua rk decays, the energy is chosen to maximize the production of b's in an appropriate way, whereas to produce new heavy particles, the energy i s made as large as possible. All uses of electron colliders require ver y large luminosity (intensity). [K2000] 
Electron Conduction  A process in astrophysics occurring in highly ionized stellar interiors where the density is high, whereby the bulk of the energy is transported by "hot" electrons moving in one direction and cooler electrons in the other. In degenerate matter electron conduction, not radiation, is the main mechanism of energy transport. [H76] 
Electron Gas  a system of electrons whose mutual interactions are sufficiently weak that they can be regarded as moving independently, subject only to the effects of the exclusion principle. [D89] 
ElectronHole Pairs  When a photon is absorbed in silicon its energy causes an electron in the valence band to be ejected into the conduction band leaving a (positively charged) vacancy or hole in the valence band. [McL97] 
ElectronPhonon Scattering  Electron scattering by ions oscillating about equilibrium positions which form a perfect lattice. [H76] 
Electron Shells  Zones in which the electrons in atoms reside. Their radius is determined by the quantum principle, their population by the exclusion principle. [F88] 
Electron Temperature  The temperature that appears in the Maxwell distribution of velocities for electrons. [H76] 
Electronuclear Force  Single fundamental force thought to have functioned in the very early Universe and to have combined the attributes thereafter parceled out to the electromagnetic and the strong and weak nuclear forces. (see Grand Unified Theory) [F88] 
Electron Volt (eV)  (a) Unit of energy. Typically 110 eV is the amount
of energy per atom involved in chemical reactions. 1 eV is the energy
gained when an electron is accelerated by a potential of one volt. [D89]

Electrostatic Unit (esu)  A unit of charge defined as the charge which exerts a force of 1 dyne on a charge of equal magnitude at a distance of 1 cm. [H76] 
Electroweak Force  (a) The combination of the electromagnetic force and
the weak nuclear force which takes place at high energy. [C97]

Electroweak Interactions  The unified description of the weak interactions and electromagnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam. [G97] 
Electroweak Theory  (a) Theory
demonstrating links between the electromagnetic and the weak nuclear
forces. Indicates that in the high energies that characterized the
very early Universe, electromagnetism and the weak force functioned
as a single, electroweak force. Also known as the WeinbergSalam
theory. [F88] 
Element  Different elements are distinguished by the number of protons in their nuclei. All hydrogen atoms have one proton; all helium atoms have two protons; all oxygen atoms have eight protons. [C95] 
Elements, Besselian  Quantities tabulated for the calculation of accurate predictions of an eclipse or occultation for any point on or above the surface of the Earth. [S92] 
Elements, Orbital  (a)
Parameters that specify the position and motion of a body in
orbit. (see Osculating Elements; Mean Elements) [S92]

Elevation  The angle in degrees above the horizon toward the zenith or overhead point. Sometimes loosely called the "altitude" of a star, but not to be confused with height above sea level. Elevation angle is 90° minus the zenith distance (or zenith angle). [McL97] 
ElevenDimensional Supergravity  Promising higherdimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory. [G99] 
Ellipse  A plane curve in which the sum of the distances of each point along its periphery from two points  its "foci"  are equal. [F88] 
Elliptical Galaxy  (a) A galaxy that looks round or elliptical. One example is M87, in the constellation Virgo. [C95] 
 (b)
A galaxy without spiral arms and with an ellipsoidal
shape. Ellipticals have little interstellar matter and no blue giants
 the only giants are red, and they give ellipticals a slightly
redder color than spirals. The most massive galaxies known (about
10^{13} M_{}) as well as some of the least massive known, are
ellipticals. No giant elliptical is near enough for any individual
stars to be resolved. Ellipticals apparently produce only Type I
supernovae. [H76]

Ellipticity  A quantitative measure of the shape of a galaxy. A completely spherical galaxy has zero ellipticity. A galaxy shaped like a cigar has a very high ellipticity. [LB90] 
Elongation, Greatest  The instants when the geocentric angular distances of Mercury and Venus are at a maximum from the Sun. [S92] 
Elongation, Planetary  (a) The geocentric angle between a planet and the
Sun, measured in the plane of the planet, Earth and Sun. Planetary
elongations are measured from 0° to 180°, east or west of
the Sun. [S92] 
Elongation, Satellite  The geocentric angle between a satellite and its primary, measured in the plane of the satellite, planet and Earth. Satellite elongations are measured from 0° east or west of the planet. [S92] 
ELS  Eggen, LyndenBell and Sandage An influential paper published in 1962 by Olin Eggen, Donald LyndenBell, and Allan Sandage, who argued that the Galaxy formed from a single huge cloud of gas that rapidly collapsed. [C95] 
Emersion  The reappearance of a celestial body after eclipse or occultation. [H76] 
Emission  The process of transition of an electron from an outer orbit to an inner orbit around the nucleus results in a characteristic amount of energy being radiated (as line emission) that corresponds to the lost energy of the electron. [Silk90] 
Emission Coefficient  Radiant flux emitted per unit volume per unit solid angle. [H76] 
Emission Lines  Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula. Compare absorption lines. [F88] 
Emission Measure (EM)  The product of the square of the electron density times the linear size of the emitting region (in parsecs). [H76] 
Emission Nebula  An HII region whose spectrum consists of emission lines. [H76] 
Emission Spectrum  A spectrum consisting of emission lines produced in the laboratory by a glowing gas under low pressure. [H76] 
Emissivity  A measure of the efficiency of a source to radiate like a perfect black body; 0% is perfectly black and 0% is perfectly reflecting. [McL97] 
Empiricism  An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience. [F88] 
Enceladus  Third satellite of Saturn, about 500 km in diameter. Orbital period 1.37 days. Discovered by Herschel in 1789. [H76] 
Encke's Comet  The comet with the shortest known period (3.30 years) (a = 2.21 AU, e = 0.847, i = 12^{°}.4). It has been observed at every apparition since its discovery in 1819. Its period is gradually decreasing. Named after J. F. Encke, who computed its orbit. (It was discovered by Pons.) [H76] 
Encke's Division  (a) Gap within Saturn's Ring A. [A84]

Encounter  see Gravitational Encounter. [H76] 
Endoergic Process  A process in which some of the energy of the incoming particle is transferred to the nucleus. [H76] 
Endothermic Process  An adjective applied to a reaction in which a net input of energy is required for the reaction to occur. [H76] 
Energy  (1) The capacity to do work. (2) Manifestation of a particular variety of force. [F88] 
Energy Band  a continuous range of energies in a solid in which there are possible states for the electrons. Energy bands are separated from one another by energy gaps. [D89] 
Energy Curve  A plot of the intensity of the continuous spectrum versus the wavelength. [H76] 
Energy Density  The amount of energy in the form of radiation per unit volume, expressed in ergs cm^{3}. The energy density of blackbody radiation at temperature T is aT^{4}, where the radiation constant a = 7.56 × 10^{15} erg cm^{3} (K)^{4}. [Silk90] 
Energy Distribution  The amount of energy radiated at each range of wavelengths. [H76] 
Energy Gap  A range of energies in a solid for which there are no quantum states of the electrons. [D89] 
Energy Level  (a) Any of the several discrete states of energy
in which an
atom or ion can exist. For example, an orbital electron can exist
only in those energy levels that correspond to an integral number of
deBroglie wavelengths in a Bohr atom. [H76]

Energy Spectrum  (In cosmicray studies, a plot of number of particles versus energy. [H76] 
Ensemble Average  An average over an ensemble of all possible systems. [H76] 
Ensemble (of universes)  A hypothetical group of many universes of varying properties. Some physicists attempt to estimate how "probable" are the properties of our Universe by imagining it as a sample from an ensemble of universes. [LB90] 
Entanglement  the impossibility of expressing certain quantum mechanical states of a system with two or more parts as the conjunction of definite quantum states of the separate parts. [D89] 
Enthalphy (H)  The heat content of a body. H = U + pV, where U is the internal energy, p is the pressure, and V is the volume. [H76] 
Entrance Pupil  The real object or image which defines the limit of valid light paths through an optical system. [McL97] 
Entropy  (a) A thermodynamic property of a macroscopic body
which corresponds intuitively to the degree of disorder. [D89]

Eötvös Experiment  (a) An experiment performed in 1909 by the Hungarian
physicist
Eötvös to establish that the gravitational acceleration of a
body does not depend on its composition  i.e., that
inertial mass and gravitational mass are exactly equal. [H76]

Ep Galaxy  In Morgan's classification, an elliptical galaxy with dust absorption. [H76] 
Epact  The age of the Moon; the number of days since New Moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (see Gregorian Calendar; Lunar Phases) [S92] 
Ephemeris  (a) A list or tabulation of astronomical phenomena
that change with time. [McL97]

Ephemeris Hour Angle  An hour angle referred to the ephemeris meridian. [S92] 
Ephemeris Longitude  Longitude (see Longitude, Terrestrial) measured eastward from the ephemeris meridian. [S92] 
Ephemeris Meridian  A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by Terrestrial Dynamical Time (TDT). The ephemeris meridian is 1.002738T east of the Greenwich meridian, where T = TDTUT1. [S92] 
Ephemeris Second  The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 ephemeris time. [H76] 
Ephemeris Time  (a) Time based on the ephemeris second. Ephemeris
time is determined primarily from observations of the Moon
against the background of stars, whereas Universal Time is determined
from observations of the stars and depends on the
Earth's current rate of rotation. [H76]

Ephemeris Transit  The passage of a celestial body or point across the ephemeris meridian. [S92] 
Epicycle  (a) Circular orbit of a body round a point that is
itself in a
circular orbit round a parent body. Such a system was formulated to
explain some planetary orbits in the Solar System before they were
known to be elliptical. [A84]

Epicycle Theory  A means of accounting for the apparent motions of the planets in terms of circular motions in a geocentric cosmology. Each planet moves in a circle, the center of which moves in a circle of larger radius, and so on, the largest circles being centered on the earth. [Silk90] 
Epitaxial  A thin layer of differently doped semiconductor used in the construction of solidstate devices such as the CCD. [McL97] 
Epoch  (a) A point of time selected as a fixed
reference. [H76]

EPR  Abbreviation of A. Einstein, B. Podolsky and N. Rosen, who presented an argument in 1935 that the quantum mechanical description of certain composite physical systems cannot be complete. [D89] 
EPROM  Erasable Programmable Read Only Memory A small silicon chip containing thousands of individual locations which can be set to either a low or a high voltage level; a 0 or a 1. The settings can be erased by exposure to ultraviolet light. [McL97] 
Epsilon Eridani  A young orange dwarf star in the constellation Eridanus that is visible to the naked eye and lies just 10.7 lightyears away from the Sun. [C95] 
Epsilon Indi  An old orange dwarf star in the southern constellation Indus that lies 11.2 lightyears away from the Sun. [C95] 
Equation of Center  In elliptic motion the true anomaly minus the mean anomaly. It is the difference between the actual angular position in the elliptic orbit and the position the body would have if its angular motion were uniform. [S92] 
Equation Equinoxes  The Right Ascension of the mean equinox (see Mean Equator; Equinox) referred to the true equator and equinox; apparent sidereal time minus mean sidereal time. (see Apparent Place; Mean Place) [S92] 
Equation of State  (a) A relation between the pressure, temperature, and
density of a fluid. [H76]

Equation of Time  (a) The difference between Apparent and Mean Solar
Time. At Greenwich, apparent Solar noon varies between
11^{h}44^{m}05^{s} and
12^{h}14^{m}19^{s}. Maximum contribution from
Earth's orbital eccentricity, ~ 8 min; from Earth's obliquity, ~ 10 min.
Apparent and Mean Solar Time agree 4 times a year. [H76]

Equator  The great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (see Celestial Equator.) [S92] 
Equatorial Mount  The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located. only the polar axis need be driven by a motor to counteract the Earth's rotation. [McL97] 
Equilibrium  A condition of balance between the forces operating on or within a physical system, so that no accelerated motions exist among the parts of a system. For stable equilibrium, a small disturbance will eventually damp out. If a small disturbance continues to grow, the system is said to be in unstable equilibrium. [H76] 
Equilibrium Position  The position of an oscillating body at which no net force acts on it. [H76] 
Equinox  (a) One of two points in the sky that represent
where the Sun appears
to cross the plane of the Earth's equator. From the Earth's viewpoint
therefore, the Sun reaches one point at a quarter, the other at three
quarters of the way through the sidereal year: the vernal (spring)
equinox is thus on or around 21 March, the autumnal on or around 22
September. The actual points in the sky change slightly every year
through a process called precession. [A84]

Equipartition of Energy  (a) If all stars have the same kinetic energy,
equipartition of energy prevails. Because kinetic energy depends on
both a star's mass and its velocity, highmass stars must move more
slowly than lowmass stars, if equipartition of energy prevails. [C95]

Equivalence Principle  (a) The principle that it is impossible to
distinguish between
gravitational and inertial forces; gravitational
mass is precisely equal to inertial mass. [H76]

Equivalent Width  A measure of the total amount of energy subtracted from the continuous spectrum by an absorption line on a graph of relative intensity versus wavelength. Since the shapes of line profiles vary  e.g., one may be broad and shallow whereas another is narrow and deep  measurement is facilitated by transforming each profile into a rectangle whose base corresponds to zero intensity and whose area is the same as that of the true absorption line. [H76] 
Era  A system of chronological notation reckoned from a given date. [S92] 
Erbium
 
ERG  Extremely Red Galaxy 
erg  (a) The cgs unit of energy; the work
done by a force of 1 dyne acting over a distance of 1 cm. 1 erg
= 10^{7}joules = 1 g cm^{2} s^{2}. (sometimes
called dyne cm) [H76]

Ergodic Motion  Motion by one or more particles which fills phase space with uniform density after a sufficiently long time. [H76] 
Ergoregion  That part of space in which no physical object can remain at rest with respect to an observer at infinity; the dragging of inertial frames is so extreme that all timelike world lines rotate with the star. Technically, it is the region in which the asymptotically timelike Killing vector becomes spacelike. [H76] 
Ergosphere  The region surrounding the event horizon (but inside the stationary limit) of a rotating Kerr black hole (see Ergoregion). [H76] 
Eri  see Achernar. [H76] 
AS Eridani  An eclipsing binary whose secondary is close to its Roche limit. [H76] 
Eri  A fourthmagnitude K2 V star 3.30 pc distant. In 1973 van de Kamp announced that it has a planetlike object in orbit around it at a distance of about 8 AU and with a period of about 25 years. [H76] 
ERO  Extremely Red Object 
40 Eridani  A nearby triple system, 5 pc distant. Component A is K0 V; component B is a DA white dwarf; component C is M5e V. [H76] 
Eros  A small asteroid, No. 433 (axes 35 × 16 × 17 km) whose closest approach to Earth is less than 0.15 AU. Rotation period 5^{h}16^{m}12^{s}.913, orbital period 642 days, a = 1.48 AU, e = 0.223, i = 10^{°}.8; perihelion distance 1.084 AU. Discovered by G. Witt in 1898. [H76] 
Eruptive Galaxy  see Violent Galaxy. [H76] 
Eruptive Variable  see Cataclysmic Variable. [H76] 
ESA  European Space Agency [LLM96] 
Escape Velocity  (a) Speed an object must attain in order to free
itself
from returning to the parent body under the effects of gravity. [A84]

ESO  European Southern Observatory [LLM96] 
ESRF  European Synchrotron Research Facility [LLM96] 
ET  Ephemeris Time [LLM96] 
Eta Aquilae  A pulsating star in the constellation Aquila. It was the first Cepheid variable star discovered, in 1784. [C95] 
Etalon  Essentially an optical filter that operates by multiplebeam interference of light reflected and transmitted by a pair of parallel flat reflecting plates. [McL97] 
Ethernet  A system for linking computers with a single serial cable. [McL97] 
Euclidean Geometry  The geometry developed by the Greek Euclid about 300 BC. Euclidean geometry, like all geometries, deduces certain results from a set of starting assumptions. One of the critical assumptions of Euclidean geometry is that given any straight line and a point not on that line, there is exactly one line that can be drawn through that point parallel to the first line. One of the results of Euclidean geometry is that the interior angles of any triangle sum to 180 degrees. Euclidean geometry is the geometry we learn in high school. [LB90] 
Euler Number (Eu)  A number used in fluid dynamics defined by p / v^{2}, where p is pressure, density and v velocity. It is named after the German mathematician L. Euler (17071783). [JM92] 
Europa (J II)  One of the Galilean satellites of Jupiter 3600 km in diameter. Period 3.55 days, e = 0.00, i = 0^{°}.01, mean density 3.07 g cm^{3}. [H76] 
Europium
 
EUV  Extreme UltraViolet [LLM96] 
Evection  The small irregularity in the Moon's orbital motion due to Solar and planetary perturbations. [H76] 
EvenEven Nuclei  see 4N Nuclei. [H76] 
EvenOdd Nuclei  Nuclei that contain even numbers of protons but odd numbers of neutrons. [H76] 
Event  (a) A happenstance in the spacetime continuum
referenced by three spatial
coordinates and a complementary temporal ordinate. [C97]

Event Horizon  (a) The "edge" of a black hole; the interface between
fourdimensional space and a singularity. [A84]

Eventuality  a contingency concerning a system which is either true or false if it is definite, but which (in view of a fundamental conceptual innovation of quantum mechanics) may he indefinite. A nearsynonym for this term is `proposition'. [D89] 
Evershed Effect  The radial motion outward (from the central umbra) of the gases in the penumbral regions of sunspots. [H76] 
Evolution  (a) In Biology the theory that coniplex and
multifarious
living things developed from generally simpler and less various
organisms.

Exchange Correlation  The correlation of particles and spins which is embodied in a Slaterdeterminant wave function. [H76] 
Exchange Interaction  the spindependent part of the interaction between particles with spin. [D89] 
Excitation Potential  Amount of energy required to bring an electron from its ground state to a given excited state (measured in electron volts). [H76] 
Exclusion Principle  (a) Pauli's exclusion principle says that there
could not be more than one electron in each quantum state. [D89]

Existence Theorems  these are the theorems that assert the existence of mathematical objects satisfying a specific set of axioms. In the case of differential equations describing the time evolution of a physical system, they guarantee the existence of solutions to the equations that become unique if enough initial conditions are specified. [D89] 
Exoergic Process  A process in which energy is liberated. [H76] 
Expanding Arm  A spiral arm of neutral hydrogen lying between 2.5 and 4 kpc beyond the Galactic center and receding from it at about 135 km s^{1}. [H76] 
Expansion of Universe  Constant increase, with time, in the distance separating distant galaxies from one another. Expansion does not take place within individual galaxies or clusters of galaxies, which are bound together gravitationally, but evidences itself on the supercluster level. [F88] 
Exploding Galaxy  see Violent Galaxy. [H76] 
Explorer  A US series of satellites, many of which remain in orbit round the Earth fulfilling scientific functions. Explorer 1 was in fact the first US orbital satellite (launched on 31 January 1958) and was instrumental in discovering the inner Van Allen belt. [A84] 
Explosive Galaxy Formation  A theory of galaxy formation wherein the explosion of a large number of stars creates a giant shock wave that travels outward and compresses the surrounding gas. Galaxies form in the regions of highdensity gas. [LB90] 
Explosive Nucleosynthesis  (a) The nucleosynthetic processes which are
thought to occur in supernovae. These explosive processes are
thought to produce the nuclei from neon up to and including the
eprocess nuclei and possibly the rprocess
nuclei. Explosive carbon burning occurs for a temperature of about 2
× 10^{9} K and a density of 10^{4}10^{7} g
cm^{3} and produces nuclei
from neon to silicon. Explosive oxygen burning occurs for a
temperature of about 4 × 10^{9} K and produces nuclei from
silicon to calcium, and the eprocess occurs at a temperature
greater than 5 × 10^{9} K and produces the iron peak
nuclei. [H76]

Explosive Variables  see Cataclysmic Variables. [H76] 
Exponential Expansion  (a) Extremely rapid expansion. "Exponential" is a
mathematical term that precisely defines the rate of expansion. For
example, a balloon that doubles its size every second is expanding
exponentially. By contrast, a balloon whose radius is one inch after
one second, two inches after two seconds, three inches after three
seconds, and so on, is expanding linearly with time, rather than
exponentially. According to the inflationary universe model, the early
Universe went through a brief period of exponential expansion, during
which its size increased enormously. [LB90]

Extended Dimension  A space (and spacetime) dimension that is large and directly apparent; a dimension with which we are ordinarily familiar, as opposed to a curledup dimension. [G99] 
Extended Inflationary Universe  A version of the inflationary Universe theory proposed in 1989 by Paul Steinhardt and Daile La. Its key new feature was the suggestion that a new field interacts directly with the gravitational field, causing the strength of gravity to change with time. This causes the expansion of the Universe to slow down, allowing the bubbles forming at the end of inflation to catch up with the expansion and smoothly fill the Universe. [G97] 
Extended Source  In radio astronomy, formerly a source whose angular extent could be measured, as distinguished from a point source. Now, one which has a large angular extent and is strongest at longer wavelengths (distinguished from a compact source). Most extended sources tend to be polarized. [H76] 
Extinction  (a) Attenuation of starlight due to absorption and
scattering
by Earth's atmosphere, or by interstellar dust. The longer the
path through the dust, and the denser the dust, the more the
starlight is reddened. The normal relation is A_{V} =
0.8 mag per kpc. The total visual extinction toward the galactic
center is on the order of 25 magnitudes. [H76]

Extragalactic Astronomy  The field that deals with objects beyond the Milky Way, especially galaxies and quasars. [C95] 
Extragalactic Distance Scale  The set of distances to astronomical objects outside our galaxy. It is difficult to obtain distances to objects further than about 10 million light years with accuracies better than about 25%. [LB90] 
Extremal Black Holes  Black holes endowed with the maximal amount of force charge possible for a given total mass. [G99] 
Extrinsic  A semiconductor, such as silicon, which has been doped with impurity atoms to provide smaller energy band gaps for detection of lowerenergy photons. [McL97] 