Why astronomy played a pivotal role in setting D-Day date

As the Allied forces planned to invade Normandy, France, during the Second World War, they needed to settle on some possible dates. While D-Day was originally scheduled for June 5, 1944, it was postponed by a day because of the weather. What often gets overlooked is why June 5 was a possibility in the first place.

The reasons come down to a mix of military strategy, as well a seemingly unlikely one: using astronomy to calculate the tides and moon phases to determine the best day for what would be the largest amphibious invasion in the history of warfare.

Traditional military thinking holds that the best time to launch an amphibious assault is during high tide, because it reduces the amount of beach the landing forces have to cross, thus making it harder to be picked off by the opposing forces.

Field Marshal Erwin Rommel, who was responsible for the German defence of France's Channel coast, was well aware of this.

"[The Allies] had more resources than the Germans. They could outlast the Germans any time, so Rommel's imperative was he had to keep the Allies from getting ashore and defeat them as soon as possible when they got ashore," said Roger Sarty, a history professor at Wilfrid Laurier University. "What Rommel appreciated was the vulnerability of landing craft."

The Germans erected defences that included huge sharpened logs in the tidal zone that were designed to tear the hulls of the landing craft. Many of these devices were mined, and wire was strung between them to create an almost impossible barrier. Rommel had these installed to ensure the Allies wouldn't be able to land at their preferred time, Sarty said.

For the Allies, this required them to adjust their thinking. They decided the landing timing would need to come shortly after low tide, so that specially-equipped troops would be able to destroy the barriers when the tide was low before the landing craft arrived. As the tide rose, this would help the ships get in and out easier.

This was only one piece of the puzzle though. Part of the Allied invasion required the use of paratroopers to go in several hours before the invasion to secure an 80-kilometre front in Normandy. Armed forces are weakest at their flanks, so the objective for the paratroopers was to secure these points.

Moon's role

"They wanted it to be a full moon so they would have the best possible visibility on all types of day and night," said William Bryant Logan, an American author who wrote about the factors behind the planning of the Allied invasion in Air: The Restless Shaper of the World.

Besides the need for a full moon and a low tide, the Allies also wanted to cross the English Channel with its convoys at night and needed "approximately 40 minutes of daylight preceding the ground assault to complete our bombing and preparatory bombardment," wrote the U.S. general in charge of the Allied forces, Dwight D. Eisenhower, in his memoirs.

The desired moon and tide conditions would occur during a full moon, which happens "when the Earth, sun and moon are nearly in alignment," according to the U.S. National Oceanic and Atmospheric Administration. Besides a bright moon, this also means "high tides are a little higher and low tides are a little lower than average," the NOAA said. This is known as a spring tide.

Why May D-Day didn't happen

Sarty said that while the Allies were originally planning on a May D-Day, they wouldn't be able to have the equipment they needed because of previous landings in the Mediterranean Sea, so June became the preferred date.

With this complex set of criteria in place, astronomy helped decide the dates — June 5-7, 1944 — because of the best tides and light conditions needed.

The next available option would have been June 19-21, and it turned out to be a blessing not to have waited until then because of "absolutely horrific spring storms," said Sarty.

"If they tried to go in two weeks later, it would have been fatal," he said. "It wouldn't have worked."

While June 5 was the original date selected, the invasion was postponed for a day because of a poor weather forecast that included strong winds and clouds that were too low over the English Channel and the French coast.

On June 6, roughly 150,000 Allied troops invaded Western Europe in Normandy, 14,000 of whom were Canadians. There were 1,074 Canadian casualties on D-Day.

Why Germans were surprised June 6

While the conditions weren't perfect on June 6, they were good enough, and the Germans were surprised for several reasons, said Logan. One was that the Allies made a better prediction about the June 6 weather, so the Germans thought an invasion was unlikely. Adding to this were the Beaufort Force 4 winds on June 6.

"People from Berlin said, 'Well, if it's Force 4, they're not invading,'" Logan said.

Interestingly, when the Germans were planning Operation Sea Lion, a 1940 amphibious assault into the United Kingdom, Logan said one of the reasons the Germans didn't go ahead with it was because the winds were Force 4.

"They said, 'Well, if we aren't going to do it with Force 4 winds, the Allies obviously won't do it with Force 4 winds.' But they were wrong about that," he said.

On D-Day, Germany's Rommel was in Berlin celebrating his wife's birthday, while half of the division commanders and a quarter of the regiment commanders were at a war-games planning exercise in the Brittany region, said Logan.

Less than a year after D-Day, the Germans surrendered to the Allies on May 7, 1945, bringing an end to the Second World War in Europe.

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