astronomical unit: the average distance between Earth and the Sun
axis of symmetry: a line that divides a shape into two identical parts
calendar year: 365 days or 366 days (leap year)
centripetal: directed toward the centre of a circle
elliptical: having the shape of an ellipse or oval
epicycle: a circle that rolls along the circumference of another circle
focal point: one of two special points used in describing an ellipse
frequency: the number of cycles in a time period (f)
horizontal plane: a plane perpendicular to a radius of Earth; usually used to suggest that there is no vertical component to motion or forces
period: the time for one complete cycle (T)
periodic: recurring at regular intervals
sidereal year: the orbital period of Earth
tension: a stress that tends to stretch an object
trajectory: the path of a moving body through space
uniform circular motion: motion of an object with a constant speed along a circular path
vertical circular motion: motion in a circular path where one diameter of the circle is vertical
weightless: experiencing little apparent gravitational pull