E

Earthshine

The sunlight reflected from the earth that dimly illuminates the dark position of the moon.

 

Eccentricity

The measure of the degree of flattening of an ellipse. It is the ratio of the distance between the foci to length of the major axis. A circle has zero eccentricity.

 

Eclipse

The cutting off the light, partially or totally, from one body when another body passes in front of it.

 

Eclipse limit

The maximum angular distance from either side of the node at which an eclipse can occur.

 

Eclipsing binary

A binary star whose orbital plane is seen nearly edgewise so that alternately one star appears to eclipse the other.

 

Ecliptic

The annual apparent path of the sun among the stars. It is the projection of the earth's orbit on the celestial sphere.

 

Electromagnetic radiation

Radiation consisting of wave phenomena associated with electrical and magnetic fields whose wavelengths include gamma rays, x-rays, ultraviolet, visible light, infrared, and radio.

 

Electromagnetic spectrum

The entire family of electromagnetic radiation.

 

Ellipse

A conic section. A closed curve formed by the intersection of a nonhorizontal plane and a circular cone.

 

Elliptical galaxy

An ellipsoidal galaxy that lacks spiral arms and any appreciable ainterstellar material.

 

Elongation

The apparent angle between the sun and a planet as seen from the earth. The difference between the longitude of the sun and a planet.

 

Emission line

A discrete bright line in a spectrum.

 

Emission nebula

A bright gaseuos nebula whose light is derived from the ultrviolet light of a star within or near the nebula.

 

Emission spectrum

A series of bright lines (emission lines) produce by a low-pressure incandescent gas.

 

Ephemeris

A table that gives the positions of celestial bodies at regular interval of time.

 

Epicycle

The small circle in the Ptolemaic system whose center moves along the circumference of the larger circle (defedent) while the planet moves along the circumference of the small circle.

 

Epoch

An arbitrary date selected as a point of reference to which astronomical observations are referred.

 

Equation of time

The difference between the apparent and mean solar time.

 

Equator

A great circle on the terrestrial sphere  whose points are 90^o from the north and south poles.

 

Equatorial mount

A telescope mount with one axis parallel to the earth's axis which rotates at the same rate as the earth by means of a clock drive.

 

Equinox

One of two intersection of the ecliptic and the celestial equator.

 

Erg

A unit of energy in the metric system. The amount of work accomplished by a force of one dyne moving a body a distance of one centimeter.

 

Escape velocity

The initial speed that a body must attain to overcome the gravitational pull of another body and escape into space.

 

Exosphere

The top layer of the earth's atmosphere where the molecules readily escape from the earth's gravitational pull.

 

Extragalactic

Outside or beyond the Milky Way.