anomaly: the angular distance of a body from some reference point, apogee, in traditional astronomy. The mean anomaly is the distance a body would be from apogee after a given time if it were traveling with its average (mean) orbital speed.

apogee: the point in an eccentric or epicycle that is farthest from the earth, or the point in an epicycle that is farthest from the center of the deferent: marked A in Figure A.2.1.

concentric: a circle on which a point moves with uniform speed and therefore with uniform angular velocity, as seen from the center of the circle.

conjunction: the (essentially invisible) position in a planet's orbit at which its longitude coincides with the sun's.

coordinate system: the means by which the locations of celestial objects are specified. They rest on the common (Greek) conception of the heavens as a hollow shell of very large radius, centered on the earth and called the celestial sphere (see Figure A.2.2). Any position (S) on the sphere can be completely described by reference to its angular distance (ø) above or below a plane (LMN) bisecting the sphere, and the angular distance (θ) in that plane between the meridian through S and a given “zero” meridian. Note the resemblance to terrestrial coordinates. The Greeks developed not only the geographical system but also all systems of celestial coordinates. The latter are denominated by their respective fundamental planes, and their components are named as shown in the table A.2.1.