This is handy expression for creating circle motion in AE.

I use this most often when creating circles that are rendered with Particlar. By setting the emiter to a light and adding this expression to the position property of that light, you get a perfect circle particle trail.

radius = 300;
angle = time * 200;
x = radius*Math.cos(degreesToRadians(angle));
y = radius*Math.sin(degreesToRadians(angle));
z = 0;
center = [400,300,0]; //or wherever you want it to be
add(center, [x,y,z]);