I made this illusion based on the Tusi motion method, which takes his name from Nair al-Din Tusi, a Persian astronomer from the 13th century. The principle established by Tusi states that:
If a circle with diameter A rotates on the inner boundary of a circle with diameter 2A, then a point on the circumference of the inner circle traces a straight line.

As you can see, any ball in the inner circumference is following a perfect linear movement. This is created with 8 Containers where the balls move linear in a loop.

The delay between balls must be always the same as the speed of the animation. An ease-in-out cubic-bezier(.4, 0, .6, 1) animation is required to get the circular shape.