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, 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
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.