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Apogee: The point in the orbit of a celestial body when it is furthest from the earth.

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,[1] and this point was called the equant.

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.

Ejection: 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 theory, e.g. in geocentric places because the observations are made from the surface of the earth instead of the centre, or in heliocentric places because observations are made from the earth and not from the sun.

Perigee: The point in the orbit of a celestial body when it is nearest to the earth.

Precession: Owing to the slow motion of the earth's pole around the pole of the ecliptic, the equator cuts the ecliptic a little earlier every year, so that the equinox each year slightly precedes, with reference to the stars, that of the previous year.



  1. I.e. the angular motion about the equant was uniform.