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
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).
According to Illingworth: The Macmillan Dictionary of Astronomy, London,
1979, p.108: The amount that must be added to the mean solar time
to obtain the
apparent solar time, i.e. it is the difference in time as measured by a
sundial and by a clock. The equation of time varies through the year; it
has two maxima and two minima and is zero on four dates: April 15/16,
June 14/15, Sept. 1/2, Dec. 25/26. A positive value indicates that
apparent time is ahead of mean time; the greatest positive value is 16,4
minutes, the greatest negative value being 14.2 minutes. The curve is
the sum of two components, each reflecting a nonunifority in the
apparent motion of the sun; one component arises from the ellipticity of
the earth's orbit, the other from der inclination of the ecliptic to the
celestial equator (see mean sun).
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.