Download .fla

var radius = 120;
var degrees = 0;

onEnterFrame = function (){
angle = degrees * (Math.PI/180);
degrees += 2;

xposition = radius * Math.cos(angle);
yposition = radius * Math.sin(angle);

moon_mc._x = xposition + earth_mc._x
moon_mc._y = yposition + earth_mc._y

updateAfterEvent();
}