float distanceBetweenObjects = 50.0; // or however far apart you want them to be
float distanceFromCenter = (distanceBetweenObjects*0.5)/sin(a*0.5);
float accumulatedAngle = 0.0;
foreach( object )
{
    object.position.x = cos(accumulatedAngle) * distanceFromCenter;
    object.position.y = sin(accumulatedAngle) * distanceFromCenter;
    accumulatedAngle += a;
}