well this is a projectile physics game (Although some weapons are essentially heat seeking insta-kill weapons) to the idea is that such a device would be actually simulating the proper path for it's projectiles to travel and get to their targets. And yeah, Halley don't have a animation for turning planets (Vector thrusting anybody?) so I agree that's wierd. But frankly, it would looks just so wrong to have a cannons shells do a 180 degree turn just to hit a target, even if the shells are guided. Frankly, the current unit cannon isn't like the noah unit cannon of supcom 2, it isn't designed to turn. And even then, the concept for the unit cannon to from moon to planet, not planet to planet that could be on the other side of a solar system. So there is a lot to fix with the unit cannon it's self before we can even consider it to be a method of unit-logistics.