American soldiers wade ashore on Omaha Beach at the beginning of the Allied invasion of France on June 6, 1944. The timing of the D-Day invasion was influenced by the phase of the Moon. Planners wanted moonlight to help paratroopers and glider pilots landing behind the beaches after midnight. They also wanted the tides to be at the right level to make it least difficult for troops to move across the beaches. Scientists used mechanical computers to calculate the best time to meet those conditions. [U.S. Coast Guard]