A). Kepler's first law of planetary motion ... "Each planet moves
in an elliptical orbit, with the Sun at one focus of the ellipse."
B). Heliocentric motion ... motion in an orbit around the Sun.
C). Eccentricity ... what makes an ellipse different from a circle.
The eccentricity of a circle is zero.
The eccentricity of the Earth's orbit is very small.
The eccentricity of a comet's orbit is big.
D). Retrograde motion ... the apparent, temporary, backward
motion of a planet on its path through the stars.
