Montana Earth Science Picture of the Week

D-Day Moon

Photo courtesy of U. S. Naval Observatory

In honor of Veterans' Day (Nov. 11) let's look at how the Moon played a role in the planning of one of the most important invasions ever; the invasion of the northern coast of France (Normandy) on June 6, 1944 (D-Day). The invasion is re-enacted in the movie, "Saving Private Ryan."

Background . . .
Hitler's forces occupied France so the allies (Americans, British, Canadians) were faced with the task of crossing the English Channel (see map below) in order to invade and begin working their way toward Germany.

Moonlight and tides . . .
In planning the invasion, the phases of the Moon and the tides caused by the Moon were a major consideration. The air force wanted some daylight before the first men stormed the beaches in order to maximize the effectiveness of their bombing runs. However, the army insisted that the ships transport the men cross the channel at night in order to achieve a surprise attack at dawn and then have a full day to get established. The Germans believed that the attack would come at high tide because that would give the allies the shortest stretch of open beach to cross. . . During the highest of high tides, only 10 m of beach is exposed, whereas with the very lowest of low tides, anywhere from 200-400 m of beach is exposed.

A decision is made . . .
The allies decided on a landing that would reach the beach soon after a very low tide, which happens around the time of a full moon, and again when there is a new moon. On these days the Sun, Earth, and Moon are aligned so that the Sun is working with the Moon, rather than against it to cause especially high and low tides for a period of 2-3 days. In this situation (called "spring tide") the high tides would occur around mid-day and midnight. Low tides would happen near 6 a.m. and 6 p.m. The allies actually deicded that the first men should reach the beach soon after a low tide. Landing at low tide would allow the boats that dropped the men to avoid some of the obstacles that would be submerged during a higher tide. Also, a landing at low tide would make it more difficult for the German guns to pick off the invaders as the stepped out of the boats. In order for the boats to free themselves after dropping the men, the allies decided that a rising tide would be best. (At Omaha Beach the tide rose 8 feet between 6:30 and 8:00 a.m.). The allies also wanted at least half-moon the night of the crossing to provide some light for the ship and for the paratroopers who were dropped into France five hours before the beach was attacked. This moonlight was also crucial to the pilots of gliders who needed to land their quiet planes that carried vehicles and soldiers.

The ideal tide and moonlight situation . . .
The best scenario was for the men to land on the beach as the tide was rising (soon after low tide), soon after first light, following a night with suitable moonlight. These conditions were present on June 5, 6, and 7 of 1944. H-Hour (the time when the first forces would reach the beach) was set for 6:30 a.m., an hour after first light, and an hour after the low tide mark.

The best laid plans . . .
Because of stormy weather, the invasion was postponed until June 6. A full moon provided light for the crossing and a low tide at 5:25 a.m. The first men reached the beach shortly before 7 a.m. and by 8:30a.m. all landings had ceased. Strong winds and the smoke of battle hindered execution of the invasion. Although thousands of allied soldiers died on the beaches, the invasion was a success, marking the beginning of the end for Hitler . If you are interested in learning more, consider reading the book called D-Day by the late Stephen Ambrose.

More about the beaches . . .
The Allies gave various parts of the coast code names (see map below). Americans were responsible for the areas called Utah and Omaha, the British took Sword and Gold, and the Canadians invaded the segment called Juno. Although the moon is the main influence for tides, they are also affected by he depth and shape of the sea floor, shape and orientation of the coastline, and proximity to larger parts of the ocean. In the English Channel low tide comes later in the east. As a result, the timing of the landings (H-Hour) was staggered so that the Americans would land at 6:30 a.m., the British at 7:25 (Gold) and the Canadians at 7:45.

Map courtesy of Microsoft

Term: spring tide

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D-Day by Stephen Ambrose
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By Rod Benson
Earth Science Teacher at Helena High School

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