let's say your mid is 250, 250, then rotating your objects around it would look like this:
2 instance variables for your sprites: ccc and speed. Speed = the angle difference each step. ccc decides the direction.
code:
system every 0.2 seconds: sprite set ccc to choose(-1,1)
sprite set angle to sprite.angle+sprite.ccc*sprite.speed
sprite set position to x = 250+cos(sprite.angle)*distance(sprite.x,sprite.y,250,250) and y = 250+sin(sprite.angle)*distance(sprite.x,sprite.y,250,250)