The blackout lasted throughout the war, plunging Britain into darkness every night. Lights could not be shown outside at night for any reason at all. Cars drove without headlights, streetlights were switched off and even torches were banned. It made life not only inconvenient, but also dangerous. Several people were run over by cars or bicycles, while others fell down holes or over the edge of bridges that they could not see.
The reason for the blackout was the fear of German bomber aircraft. It was known and expected that the bombers would come by night as well as by day. During daylight British anti-aircraft guns and fighters could see the German aircraft to try to shoot them down, but at night the German bombers would be almost impossible to see. They could roam at will over Britain's skies to rain down death and destruction. And the German bombing would be much more accurate and deadly if the German airmen could see what they were aiming at. If the ground was in darkness, aiming would be difficult so the Germans would be able to do less damage.
Two men watch the sky for German bombers from a suburban doorstep during the blackout. They are illuminated by a flare, dropped by German bombers to help them aim their bombs at night.
It was not only bombaiming that the blackout was designed to frustrate. Navigating an aircraft in 1939 was usually a business of following landmarks on the ground: rivers of a particular shape, road junctions, villages with unusual shapes, large buildings standing alone among fields. At night very few of these landmarks would be visible, unless they were lit up. If every house in a village had its lights on, the pilot of an aircraft high above could make out the shape of the village and would know where he was. But if the lights were all out, the pilot would have to rely on less reliable features. Rivers and lakes reflect moonlight, so water features could be used as navigational aids, but these are few and far between. Even worse one river bend can look much like another.