One potential method is:
1. Create a Mask to be the size of your orbit
2. Click on the Mask Path to select it, then copy it (edit->copy)
3. Now go to your smaller circle and select position and then paste.
What this does is paste a bunch of keyframes representing the circular motion path of the mask. You can then adjust the duration this plays out by spreading out the keyframes. You could have a completely different position animation prior to the orbiting keyframe, then ease into it. To have the other circles join is a matter of staggering the layers.
As this is the expressions forum, I guess its only right to have an expression, but from what you have said, I think the above method may suit better, possible not, but I'll leave that up to you.
To create a circular path with expressions, I have included a starter for 10. You would put this expression on the position property of the smaller circle. The First four lines are your variables which can be pickwhipped to sliders etc if needed or you can further derive their values. The radius is the radius of you orbit, the start angle is where on the orbit you want to start (from 0 -360), the speedmulitplier is how long (with that set to 1 it takes 36 seconds), and the centre is the centre of the orbit. I have set this to be the centre of the comp
radius=300;
startAngle=180;
speedMultiplier=100;
centre=[this_comp.width/2, this_comp.height/2];
angle = time * speedMultiplier + startAngle ;
x=radius * Math.cos(degreesToRadians(angle));
y=radius * Math.sin(degreesToRadians(angle));
add(centre,[x,y]);
Declan Smith
http://www.madpanic.tv
After Effects CS5.5/ FCS3 / Canon 7D / Canon XL2 / Reason / Cubase
"it's either binary or it's not"
