E

Eagle - name given to the Apollo 11 lunar lander than landed the first men on the Moon
Eagle nebula - emission nebula in Serpens. Thought to be a birthplace of stars.
Earth light - sunlight reflected from the Earth onto the Moon. Seen best at new Moon
eccentric - off centre, can be applied to a non circular orbit
echo - reflection of a wave from a surface (usually sound wave, could be radar wave)
echo sounder - an instrument using ultrasonics to find the depth of the sea
eclipse - see lunar eclipse and solar eclipse
eclipsing variable - a double star whose apparent brightness changes as one star eclipses the other
ecliptic - path along which the Sun and planets move in the sky
eddy current - current induced in a conductor due to a changing magnetic field across it
efficiency - of a machine. Useful energy output/useful energy input
eight fold way - hexagonal grouping of the first eight baryons
eject - throw out
elastic collision - a collision where all the kinetic energy is conserved
elasticity - property of a material that governs how its size and shape is affected by a force
elastic limit - point beyond which the extension is non-recoverable if the force is removed
electrode - conductor for carrying current into a gas or liquid
electrolyte - liquid containing oppositely charged ions
electromagnet - device made into a magnet when a current is passed though a coil of wire
electromagnetic - waves, due to a combination of electric and magnetic fields e.g. light
electron - a very small, fundamental charged particle. Orbits the nucleus of an atom
electron cloud - region in an atom where there is a high probability of finding an electron
electron gun - device that 'fires' a beam of electrons in a cathode ray tube
electron microscope - microscope using the wave properties of the electron to 'see' fine detail
electron volt - unit of energy 1.6x10^-19 J, written as eV.
electroplating - using electrolysis to plate one metal with another
electroscope - electrostatic device for measuring potential difference
electroweak force - unification of the electromagnetic and weak forces
ellipse - a regular and symmetrical closed curve rather like a squashed circle. Two foci.
elongation - the angle between the position of the Sun and a planet as seen from the Earth
e.m.f - electromotive force. Energy liberated by unit charge passing round the circuit
emit - give out
emission - giving out, as in an emission spectrum – one from light given out by a lamp
emissivity - the ability of a body to emit or absorb radiation compared with a black body
emitter - one region of a transistor
end correction - correction made to allow for the vibration of additional air just outside a tube
endoscope - an instrument using fibre optics to enable a surgeon to see inside the body
energy - quantity that measures the amount of work a body can do
energy level - allowed energy state of an electron within an atom
enrich - increase the amount of, as in 'enriched uranium' - uranium with more U235
entropy - measurement of the disorder of a system
epicycle - moving circle used to try to explain retrograde planetary motion
epoch - an instant in time
equation of time - difference between mean solar time (clock) and apparent solar time (sundial)
equatorial telescope - type of telescope mounting with one axis parallel to the axis of the Earth
equilibrium - no resultant translation (movement) or rotation. Forces balance.
equinox - point where the ecliptic cuts the celestial equator. Night and day of equal length
erg - obsolete unit of energy of the cgs system = 10^-7 J
error - mistake
error - in an experiment an uncertainty due to either the apparatus or the experimenter
escape speed - speed needed to escape from a planet or a star. (Escape velocity)
ether - imaginary light carrying substance filling all space. Existence disproved by Michelson-Morley.
Europa - one of the satellites of Jupiter
evaporation - molecules of liquid leaving a liquid surface at a temperature below its boiling point
event horizon - the "edge" of a black hole. Nothing can escape from within it.
excited state - energy level of an electron or atom above the ground state
expansion - getting bigger, as in thermal expansion - getting bigger when heated
exponential - decay, changes by equal fractions in equal periods of time (capacitor, radioactive)
extraterrestrial - outside the Earth
extrinsic - semiconductor where the conduction is mainly due to added impurities
eye lens - the lens nearest the users eye in an optical instrument