Line of Sight is a game concept that determines how far a unit or building can see. Line of sight is shown by the brightest areas of the map. Areas that are visible but appear darker than the rest of the map show areas that have been explored but are not currently being observed (see Fog of War). Enemy units outside a nation's line of sight are hidden from view, while enemy buildings remain visible once they have been discovered. Science level 7 will extend a nation's line of sight to their entire territory. The Space Program wonder reveals all units, buildings, and terrain on the map, even those belonging to enemies.
With Science level 2, a player will be able to see their ally's line of sight. This is very useful as any reconnaissance gained by allies will also be visible to the player. This can be disabled if using custom rules.