Apse:  An extremity of the major axis of the orbit of a body; a body is
    at its greatest and least distances from the body about which it
    revolves, when at one or other apse.

Conjunction:  When a plane containing the earth’s axis and passing
    through the centre of the sun also passes through that of the moon
    or a planet, at the same side of the earth, the moon or planet is in
    conjunction, or if on opposite sides of the earth, the moon or
    planet is in opposition.  Mercury and Venus cannot be in opposition,
    but are in inferior or superior conjunction according as they are
    nearer or further than the sun.

Deferent:  In the epicyclic theory, uneven motion is represented by
    motion 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.