aphelion The point in the orbit of a solar system body where it is farthest from the Sun. astronomical unit (AU) The average distance between the Earthand the Sun. eccentricity A measure of the extent to which an orbit departs from circularity. Eccentricity ranges from 0.0 for a circle to 1.0 for aparabola. ellipse A closed, elongated curve describing the shape of the orbit that one body follows about another. elliptical galaxy A galaxy having an ellipsoidal shape and lackingspiral arms. focus One of two points from which an ellipse is generated. For all points on the ellipse, the sum of the distances to the two foci is the same. greatest elongation The position of Mercury or Venus when it has the greatest angular distance from the Sun. impetus A theory of motion, developed in the fourteenth and fifteenth centuries, that motion could continue only so long as a force was at work. inferior planet A planet whose orbit lies inside the Earth’s orbit. Kepler’s laws of planetary motion Three laws, discovered by Kepler,that describe the motions of the planets around the Sun. major axis The axis of an ellipse that passes through both foci. The major axis is the longest straight line that can be drawn inside anellipse. perihelion The point in the orbit of a body when it is closest to the Sun. semimajor axis Half of the major axis of an ellipse. Also equal tothe average distance from the focus of a body moving on an elliptical orbit. sidereal period The time it takes for a planet or satellite to complete one full orbit about the Sun or its parent planet. stellar parallax The shift in the direction of a star caused by the change in the position of the Earth as it moves about the Sun. superior planet A planet whose orbit lies outside the Earth’s orbit. transverse velocity The part of the orbital speed of a body perpendicular to the Sun between the body and the Sun.