Tides, moonlight, machines, and D-Day

Abstract

Bruce Parker’s article in the September 2011 issue of PHYSICS TODAY (page 35) presented a clear and thorough account of the important role of tide predictions for the Normandy invasion on 6 June 1944. The exceptionally rapid rise of the water level that morning meant that the engineers had only a short time to blow up beach obstacles while they were exposed just after low water. We can verify Parker’s statement that the water-level rise was “at least a meter per hour—perhaps even faster due to shallow-water effects.”Our calculation of the tide curve for the Normandy coast near Omaha Beach includes shallow-water constituents and gives a rate of rise of 1.5 meters per hour while the demolition teams were struggling to blow up the beach obstacles; the rate of rise exceeded 1.9 meters per hour shortly thereafter.11. D. W. Olson, R. L. Doescher,Sky Telesc. 87 (6), 84 (1994). Perhaps less clear is Parker’s comment about the time of moonrise. Parker states that the paratroopers needed to see their drop zones during the preceding night and therefore “there had to be a late-rising Moon.”According to our astronomical calculations for Omaha Beach (49°22’ N, 0°52’ W), the Moon had risen about 1.5 hours before sunset on the preceding day (5 June). It then arced across the sky during the night of 5–6 June and reached its highest point for that night just as the airborne assault began, about five hours after moonrise.11. D. W. Olson, R. L. Doescher,Sky Telesc. 87 (6), 84 (1994). We calculate that moonrise on 5 June occurred at 18:33 Greenwich Mean Time (GMT), that sunset at 20:04 GMT marked the beginning of civil twilight, and that the period of brightest moonlight fell near 23:22 GMT. Our calculations agree well with the approximate times determined by reading down the left edge of the official 1944 tidal and illumination diagram reproduced as Parker’s figure 4.General Dwight Eisenhower later explained that the Allied forces “wanted a moon for our airborne assaults,”22. D. D. Eisenhower, Crusade in Europe, Doubleday, Garden City, NY (1948), p. 239. and Winston Churchill agreed that “moonlight . . . would help . . . our airborne troops.”33. W. S. Churchill, Closing the Ring, Houghton Mifflin, Boston (1951), p. 591. Brigadier General James Gavin of the 82nd Airborne gives an eyewitness account. As his C-47 military transport aircraft approached a drop zone west of Sainte-Mère-Église, Gavin could clearly see that “the roads and the small clusters of houses in the Normandy villages stood out sharply in the moonlight.”44. J. M. Gavin, Airborne Warfare, Infantry Journal Press, Washington, DC (1947), p. 57. REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. D. W. Olson, R. L. Doescher,Sky Telesc. 87 (6), 84 (1994). Google Scholar2. D. D. Eisenhower, Crusade in Europe, Doubleday, Garden City, NY (1948), p. 239. Google Scholar3. W. S. Churchill, Closing the Ring, Houghton Mifflin, Boston (1951), p. 591. Google Scholar4. J. M. Gavin, Airborne Warfare, Infantry Journal Press, Washington, DC (1947), p. 57. Google Scholar© 2012 American Institute of Physics.