Since we don't make the defender decide which side of the planet his defences are then I don't see why we bother randomising sunward either. You can either assume that the sun is in the back of the Eldar (ie, it's day time) or you can take it from exactly where the ships entered low orbit to determine sunward. So if the player entered from the sunny side of the planet it's sun to the rear, if he entered from the nightside then there would be no sunward edge (ie, planet edge is sunward). If he entered at the edge then it's one of the short table edges.