# 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:

**x = r ***** cos(**θ**)**

**y = r ***** sin(**θ**)**

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.**

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