Orbital Angular Momentum (or angular momentum) (角動量): a vector equal to the cross product of the position vector and momentum of a particle.
Central Force (中心力): a force always along the particle’s position vector with the origin located at the force center.
Eccentricity (離心率): encodes the types of shape of conic section; for planetary motion, it is a measure of how elongated an elliptic orbit is.
Kepler's Laws (克卜勒定律): three laws describing the motion of planets around the Sun.
Plane Motion (平面運動): motion confined to a two-dimensional flat space.
Perihelion (近日點): the point in a planet's orbit where it is closest to the sun.
Aphelion (遠日點): the point in a planet's orbit where it is farthest from the sun.
Astronomical Unit (AU) (天文單位): a unit of distance equal to the average distance between the Earth and the Sun.