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KEPLER
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 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.
ABERDEEN: THE UNIVERSITY PRESS
