this opacity expression should do the opacity trick:
centerOpacity = 90;
edgeOpacity = 20;
center=thisComp.width*0.5; //center of comp on X axis
x=position[0];
if (x<= center){
linear(x,0,center,edgeOpacity,centerOpacity)
}else{
linear(x,center,thisComp.width,centerOpacity,edgeOpacity)
}
to make them move on a circle is going to take some trig which I'll leave the more math savvy contributors to tackle..
-Lloyd
https://aescripts.com