MAA Glossary

E


Eccentricity
Degree to which a an ellipse is flattened.

Eclipse
Obscuring of one celestial body by another, either by direct superposition or by the casting of a shadow. Solar eclipses are of three kinds, total (when the moon completely eclipses the sun), annular (when a complete ring of sunlight escapes around the edge of the sun), and partial.
Lunar eclipses are of two types, total and partial. See also Occultation

Ecliptic
Plane of earth's orbit projected upon the celestial sphere (the apparent path of the sun).

Ellipse
Oval path of a planet around the sub, with the sun at one of the two foci.

Elongation
When, as viewed from the earth, an inferior planet attains its greatest angular distance from the sun, it is said to be at either western or eastern elongation.

Equation of Time (ET)
This is the correction, in minutes and seconds, to be applied to local time apparent time (sundial time) for deriving Local Mean Time (LMT), or Local Solar Time (LST).

Equator, celestial
A great circle on the celestial sphere midway between the two apparent poles of rotation. It corresponds to the plane of the earth's equator extended until it cuts the sphere.

Equator, galactic
A great circle drawn along the Milky Way.

Equinox
The intersection of the ecliptic (apparent path of the sun) with the celestial equator. Vernal equinox is the intersection that the sun crosses about March 21; autumnal equinox denotes the crossing point about Sept. 21.

Eyepiece
The lens combination magnifying the image formed by the objective.