| aphelion | The point in the orbit of a solar system body where it is farthest from the Sun.
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| astronomical unit (AU) | The average distance between the Earthand the Sun.
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| 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.
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| ellipse | A closed, elongated curve describing the shape of the orbit that one body follows about another.
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| elliptical galaxy | A galaxy having an ellipsoidal shape and lackingspiral arms.
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| 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.
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| greatest elongation | The position of Mercury or Venus when it has the greatest angular distance from the Sun.
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| 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.
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| inferior planet | A planet whose orbit lies inside the Earth’s orbit.
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| Kepler’s laws of planetary motion | Three laws, discovered by Kepler,that describe the motions of the planets around the Sun.
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| 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.
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| perihelion | The point in the orbit of a body when it is closest to the Sun.
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| 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.
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| sidereal period | The time it takes for a planet or satellite to complete one full orbit about the Sun or its parent planet.
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| 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.
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| superior planet | A planet whose orbit lies outside the Earth’s orbit.
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| transverse velocity | The part of the orbital speed of a body perpendicular to the Sun between the body and the Sun.
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