ion round a circle whose centre travels round another circle, the
latter is called the deferent.

Ecliptic: The plane of the earth's orbital motion about the sun, which
cuts the heavens in a great circle. It is so called because
obviously eclipses can only occur when the moon is also
approximately in this plane, besides being in conjunction or
opposition with the sun.

Epicycle: A point moving on the circumference of a circle whose centre
describes another circle, traces an epicycle with reference to the
centre of the second circle.

Equant: In Ptolemy's excentric theory, when a planet is describing a
circle about a centre which is not the earth, in order to satisfy
the convention that the motion must be uniform, a point was found
about which the motion was apparently uniform,[4] and this point was
called the equant.

[Footnote 4: I.e. the _angular_ motion about the equant was uniform.]

Equinox: When the sun is in the plane of the earth's equator the lengths
of day and night are equal. This happens twice a year, and the times
when the sun passes the equator are called the vernal or spring
equinox and the autumnal equinox respectively.

Evection: The second inequality of the moon, which vanishes at new and
full moon and is a maximum at first and last quarter.

Excentric: As an alternative to epicycles, planets whose motion round
the earth was not uniform could be represented as moving round a
point some distance from the earth called the excentric.

Geocentric: Referred to the centre of the earth; e.g. Ptolemy's theory.

Heliocentric: Referred to the centre of the sun; e.g. the theory
commonly called Copernican.

Inequality: The difference between the actual position of a planet and
its theoretical position on the hypothesis of uniform circular
motion.

Node: The points where the orbit of the moon or a planet intersect the
plane of the ecliptic. The ascending node is the one when the planet
is moving northwards, and the line of intersection of the orbital
plane with the ecliptic is the line of nodes.

Occultation: Usually means when a planet or star is hidden by the moon,
but it also includes "occultation" of a star by a planet or of a
satellite by a planet or of one planet by another.

Opposition v. Conjunction.

Parallax: The error introduced by observing from some point other than
that required in