Circular Motion vs Harmonic Motion
Up/down keys to rotate the circle.
Space to switch 3D lighting on/off (visible when the circle is almost flat).
The red circle is vertical at startup. You can make it horizontal/vertical by down/up keys.
The green dot moves with a uniform circular motion. When the circle is horizontal switch lighting off. The green dot moves now with an harmonic motion with respect to the red line segment, that is more slowly when it is close to the end points and more quickly when it is closer to the middle of the line segment.
Thanks to Blue-Monkey for the 3D circle drawing script.