A blackout in time of war, or apprehended war, refers to the practice of collectively minimizing external light, including upward-directed (or reflected) light. This was done in the 20th century to keep the crews of enemy aircraft from being able to navigate to their targets simply by sight. In coastal regions a shore-side blackout of city lights would also help protect ships from being seen and attacked by enemy submarines farther out to sea. Lights can simply be turned off or light can sometimes be minimized by tarring the windows of large public structures. These benefits against air attack are now largely nullified in the face of a technologically sophisticated enemy. As early as World War II, aircraft were using radio-beam navigation (see battle of the beams) and targets were detected by air to ground radar, e.g. H2X. Today not only are night vision goggles readily available to air crews, but sophisticated satellite-based and inertial navigation systems enable a static target to be found easily by either an aircraft or a guided missile. During the Battle of the Atlantic in World War II the German U-boats were greatly aided in the "second happy time" with the sinking of unescorted ships in American coastal waters, because the ships were back lit by coastal lights. In any naval war this would still be an advantage which a blackout would help to nullify.
