Earth axis;Axis about which the Earth rotates.
Earth's magnetic field;Dipole magnetic field of the Earth.
Earth's orbit;Elliptical orbit of the Earth about the Sun. The Earth's orbit is located in the plane of the ecliptic.
Earth's shadow;Shadow on the side of the Earth that is not facing the Sun.
Eccentricity;Ratio of the distance between the center and the focus of an ellipse to half the length of its major axis.
Eclipse;An eclipse occurs when one celestial body travels through the shadow of another.
Eclipses;Total or partial obscuring of a celestial body by the shadow of another body.
Ecliptic;Apparent great circle of the annual path of the Sun in the celestial sphere, as seen from the Earth.
Edge;Visible edge of a planet or celestial body when observed from a great distance.
Electromagnetism;The range of electromagnetic radiation: from high-energy gamma and X-ray radiation to infrared and radio waves.
Electron;Negatively charged light particle orbiting the atomic nucleus.
Electrons;Elementary particles in atoms, together with protons and neutrons.
Element;Chemical substance consisting of atoms of uniform structure.
Ellipse;Shape of the planetary orbits, described by Kepler's laws on the motion of planets.
Elliptical;Shape of the planetary orbits, described by Kepler's laws on the motion of planets.
Elongation;Angular distance between a planet and the Sun or a satellite and its planet.
Emission nebula;Nebula of gas and dust, which is heated by nearby stars, which results in luminance.
Energy;Capacity to perform work.
Equator;Great circle on a sphere whose plane is at right angles to the line connecting the poles.
Equatorial plane;Plane defined by the celestial equator, which is at right angles to the line connecting the poles.
Equinoxes;Intersections of the ecliptic and the plane of the celestial equator.
Erosion;Wearing down of rock and minerals on the surface of a planet or the Earth by exterior forces.
Eruptions;Expulsion of lava, or, on the Sun, of matter from the chromosphere.
Escape velocity;Minimum velocity of a body to escape the gravitational attraction of another body.
Event horizon;Boundary around a black hole, where the centrifugal velocity equals the speed of light.
Extinction;Apparent dimming of a celestial body as its light passes through the atmosphere.
Eyepiece;Optical system for the magnification of a generated image.
Fixed stars;Term used by astronomers in antiquity referring to stars that appear fixed to the celestial sphere.
Flare star;Red dwarf star that shows a sudden increase in brightness.
Flares;Sudden increases in solar activity.
FOC;(Faint object camera), camera in the HST for pictures of small sections of space.
Focal point;Point on the optical axis where parallel rays intersect when refracted.
Fraunhofer lines;Dark (adsorption) lines in the spectrum of the Sun.
Frequency;Number of oscillations per second (unit: Hertz)