This function takes simple circle properties like the radius and degrees, and returns a point on a circle, around the original value. Using this as a position expressions, then calling the function like in the example below, will result in a circular motion that's easily customizable.

Move in a circle - After Effects Expression.jsx

function CircularMotion (theRadius, degrees, angleShift){
  degrees = (degrees === undefined) ? - 90 : -degrees + 90
  angleShift = (angleShift === undefined) ? 0 : -angleShift;
  theRadius = (theRadius === undefined) ? 100 : theRadius;
  xDir = Math.sin(degreesToRadians(degrees + angleShift));
  yDir = Math.cos(degreesToRadians(degrees + angleShift));
  return [xDir * theRadius, yDir * theRadius] + value;
}

CircularMotion (100, time * 90, 0)