Making some display object orbit around some other display object in ActionScript3.0 is easy.
Here is basic orbit function where all the magic happens:
function orbit(planet:MovieClip, sat:MovieClip, radius:Number, speed:Number):void
var currentDegrees:Number = 0;
currentDegrees += speed;
var radians:Number = getRadians(currentDegrees);
var posX:Number = planet.x + Math.sin(radians) * radius;
var posY:Number = planet.y + Math.cos(radians) * radius;
sat.x = posX;
sat.y = posY;
return degrees * Math.PI / 180;
planet - MovieClip around which another MovieClip is orbiting
sat (satellite) - MovieClip which orbits
radius - distance from satellite to planet
speed - orbit speed (can be positive or negative value)
Orbiting direction depends on speed value. Positive speed value will move satellite in counter-clockwise direction and negative speed value will move satellite in clockwise direction.
Advanced approach can use satellites as new planets, just as in star system you can have star in a center, some planets orbiting around star and than you can add moons around those planets orbiting with different speed and on different distances from planets.
Click on image to download example.