Full moon at Normandy: Celestial Sleuth corrects D-Day historical record
Jayme Blaschke | June 4, 2019
June 6 marks the 75th anniversary of the D-Day invasion, arguably the biggest turning point of World War II. While the invasion of Normandy involved a never-before seen mobilization of people and resources, the role astronomy played in the operation's planning is often overlooked.
Texas State University astronomer, physics professor and Texas State University System Regents' Professor Donald Olson has applied his distinctive brand of celestial sleuthing to the events of June 1944 to correct the historical record regarding the airborne phase of the assault, involving paratroopers and glider-borne soldiers, and also to highlight astronomy's influence on the beach landing.
Olson's findings are published on the website of Sky & Telescope, available now at www.skyandtelescope.com/astronomy-news/astronomy-d-day-sun-moon-tides/.
Capturing two bridges close to the coast was crucial to the success of the invasion. Allied control meant two things: the prevention of a devastating counterattack from nearby German armored divisions, and the facilitation of the invasion's ability to break out into a large swath of occupied France. The element of surprise was so important to the success of the operation that the bridge would be assaulted by British soldiers arriving in silent-flying gliders. Within about 10 minutes of the gliders' landing, the British forces had captured both bridges intact, now known as Pegasus Bridge and Horsa Bridge.
Dozens upon dozens of historical accounts insist upon the need of a "late-rising moon" to ensure the success of the airborne attacks, arguing the initial stages needed complete darkness to ensure secrecy for the approach, but moonlight was necessary for the final landings and assault. Olson quickly realized something was amiss. A "late-rising moon" contradicted other historical statements from allied leaders such as General Dwight D. Eisenhower, Admiral Chester Nimitz and Prime Minister Winston Churchill, who stressed the importance of moonlight illuminating every stage during the night of June 5-6.
Furthermore, a late-rising moon did not match the known tidal conditions during the invasion. Checking specially-designed astronomical software, Olson confirmed the D-Day moon was not late-rising at all. In fact, it was just the opposite.
"It's not a late-rising moon. The moon actually rose before sunset on June 5 and remained in the sky all night long," Olson said. "It didn't set until after sunrise. It was reaching the highest point in the sky at 1:19 a.m., near the time of the British Pegasus Bridge assault and just as the American airborne operations began.
"But why do so many authors use the same phrase, 'a late-rising moon?' They're all following Cornelius Ryan, who wrote that in his book, The Longest Day," Olson said. "It's pretty clear that he's the one who spread it, but he's not the one who said it first. I traced it back to Walter Bedell Smith."
General Smith served as Eisenhower's chief of staff from 1942 through 1945. In 1946, Smith wrote a series of six magazine articles for the Saturday Evening Post to explain the reasoning behind the Allied actions for major events in the European Theater, and it is here the first erroneous reference appears:
"For the airborne landings … we needed a late-rising full moon, so the pilots could approach their objectives in darkness, but have moonlight to pick out the drop zones," Smith wrote.
In the decades following World War II, the invasion planners were clear that, in addition to the need for plentiful moonlight throughout the night, the time of sunrise and the effects of the lunar phase on the tides played a crucial role in the date selected.
The position of the sun and moon, relative to Earth, determines both the strength of the tides and the times of high and low waters. The Allies required a low tide near sunrise, and, on this part of the Normandy coast, such a tide occurs only near the times of either new moon or full moon. Those criteria left the Allies with three potential dates: June 5, 6 and 7, none of which featured a "late-rising moon."
"A spring invasion in May or June was ideal, because that would leave the entire summer for the Allied forces to drive back the German forces before bad weather set in with the coming of fall and winter," Olson said. "Invasion preparations weren't complete by May, so General Dwight D. Eisenhower postponed the assault until June.
"The Allies wanted low water so they could blow up the German's beach obstacles, but they also wanted rising water so that they could beach a craft and not get stranded," Olson explained. "If they landed on a falling tide, the landing craft would be stuck there for as much as the 12 hours. That's an important part of the D-Day plan—rising water, just after low tide."
That narrow window of opportunity also worked against the Allies. The difference between low and high tide on Normandy's beach was an incredible 19 feet. Near the time of low tide on D-Day, the Germans' underwater defenses were exposed for the Allies' demolition teams to destroy. The problem was that the demolition teams had only 30 minutes to accomplish their task—under enemy fire—before the rising tide became too deep. By 7 a.m., the water level was rising one foot every 10 minutes, and accelerating. This massive tidal surge resulted in demolition teams successfully clearing only five of the planned 16 gaps through the underwater defenses on Omaha Beach. The resulting loss of life related to the remaining underwater defenses contributed to the beach's nickname of "Bloody Omaha."
"The commemorations and media coverage will rightly focus on the heroism of the Allied soldiers, sailors and airmen who began the liberation of France," Olson said. "But we can also take this opportunity to appreciate the role astronomy played in the planning and execution of that world-changing event."