[Slick]

Here is a quote from the SE 4 gold manual under "combat":

Combat in a simultaneous game takes place when enemy forces are in the same location in a solar system. Different than in a Turn-Based game, combat does not begin automatically when enemy ships first enter the same location. Combat only takes place every 5 days in the 30 days of the turn month. If the enemy forces are at the same location on a 5th day, then combat will take place.