# Animated circles

If we assume that a planet is traveling in a **perfect circle, how could we** **simulate this motion? **First, let's assume that our circle is centered at coordinates `(0, 0)`

and has a radius, r.

How can we simulate the motion of a planet orbiting around the perimeter of a circle?

Notice the position of the planet is based on the distance from the center (**radius r**) and the angle it has swept around the circle (**0 to 360 degrees**). These are known as **polar coordinates.**

However,** i**n order to draw this planet we need to define the planet's position using `x, y`

coordinates. These are known as **cartesian coordinates**.

The length of the triangle is `x`

, the height is `y`

, the hypotenuse is r and the angle at the origin is the planets current angle around its orbit. Now we just need to find the distance of `x`

and `y`

using basic trigonometry:

`\begin{align} x = t \times cos(\theta) \\ y = t \times sin(\theta) \\ \end{align}`

To animate the motion of a planet we can increment the angle θ by one degree at each frame and the planet will move around the origin in a circle. Your turn!

**Next we can try animating our own planet in the upcoming challenge.**