The term “bomber's moon” refers to a bright full moon which illuminates the Earth almost like daylight. This idiom dates to the Second World War, referencing the idea that pilots used the light of a bomber's moon to zero in on their targets. People who lived through the Second World War, especially veterans who flew in the war, still use this term to refer to an especially bright moon, although it is unfamiliar to many people born in the years after the war.
In the Second World War, targeting equipment for bombs was not very precise. Unlike modern aircraft and weapons, which can be programmed with very accurate geographic coordinates, World War Two-era planes had to be aimed directly at targets. The brighter the lighting conditions, the easier it was for pilots and crews to see potential targets, so the night of a bomber's moon often involved a large number of bombing raids taking advantage of the great conditions.
In addition to lighting up the landscape like a searchlight, a bomber's moon also reflects from the surface of bodies of water, making navigation much easier. Pilots can follow the line of a river or look for a distinctive lake to guide themselves to their targets, which was useful in an era when visual navigation was the only way to get around. Some communities in the Second World War would string camouflage across bodies of water in the hopes of concealing them from pilots on the night of a bomber's moon.
Of course, the bomber's moon could also work against pilots. They could see the ground very well, but people on the ground could also see them, allowing them to target anti-aircraft guns at approaching aircraft. Fighter planes could also be sent up to attack bombers, which tended to be vulnerable since they were sluggish and less maneuverable than fighters. Pilots also appreciated the great visibility because it allowed them to confirm kills in the air, as opposed to murky conditions, which would obscure the fate of a plane after it was shot up in battle.
The German Luftwaffe and British Royal Air Force both took advantage of the bomber's moon in numerous raids over the holdings of the enemy. Citizens learned to fear the full moon, and many would take shelter in bright conditions before the air raid sirens went off, rightly assuming that bombing was probably going to occur at some point during the night.