# Circular orbits

# 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 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 coordinates. These are known as**cartesian coordinates**.The length of the triangle is , the height is , 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 and using basic trigonometry:

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.**