Thursday, June 6, 2019

Requirement for World War II D-Day: Full Moon !

graphic showing orbit of moon around Earth, illustrating spring and neap tides
Spring Tides, the greatest tides of a lunar month, occur when the gravity of the Moon and the Sun are in direct alignment with the Earth, which occurs during Full Moon and New Moon phases of the Earth's Moon. Neap Tides, when the Sun's gravity partially cancels-out the gravitational pull of the Moon resulting in lower tides, occur during the Moon's First and Last Quarter phases. Spring Tides were a prerequisite for a successful D-Day invasion on 1944 June 6.
(Graphic Source: National Ocean Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce)

By Glenn A. Walsh
Reporting for SpaceWatchtower

On Tuesday Morning, 1944 June 6, 75 years-ago today on a date known as “D-Day”, Allied Forces landed on the coast of Normandy in France, to begin the liberation of German-occupied France in Operation Overlord during World War II. The date of D-Day was no accident. The phase of the Moon was critical if D-Day was going to be a success!

Operation Overlord was actually the code-name for the entire Battle of Normandy. The Normandy Landings, commonly known as D-Day, was officially known as Operation Neptune, as it was to be the largest sea-borne invasion in history. In Roman mythology, Neptune was the god of the sea. Of course, Neptune also became the name of the eighth planet from the Sun, first observed by Johann Galle on 1846 September 23, from calculations developed by Urbain Le Verrier.

The D-Day invasion force consisted of 5,000 ships and landing craft carrying 130,000 soldiers across the English Channel from England to the Normandy beaches. More than a thousand air transports carried another 24,000 paratroopers and glider men, who landed in zones behind enemy lines. Just before sunrise, the Allied airplanes and ships started a constant bombardment on German strongholds along the coast. The nations participating in the D-Day invasion were the United States, Canada, Great Britain, Australia, Czechoslovakia, France, Norway, and Poland. 

Military planners set certain conditions for the Normandy invasion. First, it had to be a complete surprise to the Germans defending the French coast. Even though the Germans were expecting some type of invasion some day, they did not when or where it would occur.

In fact, U.S. General George S. Patton, who was well-respected by the German High Command, prepared a phantom army in eastern England for a supposed alternate invasion of France at Pas de Calais in Operation Fortitude. Through false intelligence leaks to German intelligence and deceptive military radio traffic (the 1944 version of “fake news”), as well as fictitious props and decoys to be observed by German air reconnaissance, German military planners were successfully convinced that the Allied invasion would occur at the narrowest section of the English Channel.

Also necessary for a successful invasion would be early morning time-of-day, reasonable weather and sea conditions, the lowest and then highest tides possible, and a Moon phase of Full Moon.

There are two reasons why the phase of the Moon was so important.

  1. Spring Tides - Although D-Day occurred during the season of Spring, Spring Tides have nothing to do with the Spring season. Each day, the gravity of the Moon causes high and low tides along ocean coast-lines. Twice a month, at the Full Moon and New Moon phases of Earth's Moon, the Earth, Moon, and Sun are in a straight alignment known as a Syzygy. At this time, the gravity of the Sun reinforces the gravity of the Moon to provide the highest and lowest tides of the month; these are known as Spring Tides.
    Special Note: Summer in June of 1944 would not begin until Wednesday, 1944 June 21 at 13:02:45 Greenwich Mean Time (GMT - equivalent to today's Coordinated Universal Time) / 9:02:45 a.m. U.S. Eastern War Time (equivalent to today's Eastern Daylight Saving Time) / 3:02:45 p.m. British Double-Summer Time. During World War II, Great Britain advanced clocks two hours ahead of normal time (Greenwich Mean Time) during the warm-weather months, while America advanced clocks one hour ahead of Standard Time, called “War Time,” for the duration of the war. Allied Invasion Forces utilized British Double-Summer Time.
  2. Full Moon - A Full Moon can provide illumination at night (after dusk) and early morning (before dawn), for areas where there is no artificial illumination, for aircraft pilots, glider pilots, and paratroopers.

The best tides were extremely important for the landing of troops on the Normandy beaches to be successful. Military planners needed the lowest tides possible at the beginning of the invasion near sunrise, while the highest tides possible during the actual ship landings later in the morning. During only a three-day period twice a month, when Spring Tides were available (during Full Moon and New Moon phases), could the needs of these military plans be satisfied.

Originally, U.S. General Dwight D. Eisenhower, Supreme Commander of the Expeditionary Forces in Europe, had planned to implement D-Day in May, but the invasion force was not ready. He decided that the invasion force would be ready by June 5, when Spring Tides and the Full Moon would be advantageous (even though Full Moon did not actually occur until June 6). However on June 4, British military weather forecasters advised him that high winds and heavy seas made a June 5 landing impossible. So, General Eisenhower reluctantly agreed to a 24-hour delay.

Had bad weather also prevented a June 6 landing in Normandy, the D-Day landings may have been postponed until the June 18 to 20 period of time (probably June 19) during the New Moon phase when Spring Tides would also be available. The New Moon phase occurred on Tuesday, 1944 June 20 at 17:00 Greenwich Mean Time (GMT) / 1:00 p.m. U.S. Eastern War Time / 7:00 p.m. British Double-Summer Time.

However, an invasion during the New Moon phase did have the major disadvantage of no illumination from the Moon during the early morning, pre-sunrise paratrooper and glider landings behind enemy lines. So, General Eisenhower really did want to execute D-Day on June 6, which would also maximize the number of fighting days on the European Continent during the warm-weather months.

A very low tide at the very beginning of the invasion would expose mined obstacles, such as stakes, ramps, tetrahedrons, hedgehogs, and Belgian Gates, all connected with barbed wire, which the Nazis had placed in the ocean near shore specifically to prevent invasion ships from landing. A very low tide would allow engineers in demolition parties to destroy these obstacles before the landing ships arrived.

Military planners also planned on having these demolitions executed during a rising tide. This was necessary since they needed the landing craft to land on the beach as soon as the obstacles were gone. And, the landing craft needed a high tide to land on the beach.

However, the engineers only had about a half-hour, between initial landings of the demolition crews (on June 6 at 4:30 Greenwich Mean Time / 12:30 a.m. U.S. Eastern War Time / 6:30 a.m. British Double-Summer Time) and the rising water level of the rising tide, to perform their demolitions. In the case of Omaha Beach, code name for one of two beaches where American troops landed, it was not enough time. The military planners had planned on the demolition crews providing 16 gaps in the Nazi's wall of obstacles; the engineers only had time to create 5 such gaps. This is why Omaha Beach is considered the bloodiest landing of the five beaches where troops landed.

American troops also landed at Utah Beach, while Canadian troops landed at Juno Beach. British troops landed at Gold and Sword Beaches.

A key part of the plan was to land troops behind enemy lines long before sunrise on D-Day, to capture or destroy rail and road bridges, secure control of causeways, and secure control of key roads and intersections. This was accomplished by U.S. and British Airborne Divisions through landings by paratroopers and glider men. Paratroopers and silent gliders started landing in Normandy on June 5 around 23:30 Greenwich Mean Time / 7:30 p.m. U.S. Eastern War Time / June 6, 1:30 a.m. British Double-Summer Time, following pathfinder paratroopers who had jumped about an hour earlier.

So, military planners considered it very important to have a bright Full Moon available to guide the Airborne Divisions to the drop-sites, and to guide the paratroopers and glider men to their landing sites. The time of the phase of Full Moon on D-Day (June 6) occurred at 18:58 Greenwich Mean Time / 2:58 p.m. U.S. Eastern War Time / 8:58 p.m. British Double-Summer Time.

Until recently, there was a mistaken belief that, to ensure darkness and complete surprise to Nazi troops as aircraft approached Normandy, military planners had planned on a late-rising Moon. In other words, the Moon would not rise, and create the needed night-time illumination, until after the aircraft had reached Normandy. This mistaken belief was promoted by a magazine article published in the Saturday Evening Post on 1946 June 8 by General Eisenhower's chief-of-staff during the war, General Walter Bedell Smith. In the article, General Smith wrote, “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.”

Early this month, in a Sky and Telescope Magazine on-line article, the record is corrected by Texas State University Professor Donald Olson. Citing astronomical calculations, Professor Olson found that the Moon actually rose about an hour and a-half before sunset on June 5 and remained in the sky the rest of the evening and the early morning of June 6 until around sunrise. Over Normandy, Moon-rise occurred on June 5 at 18:30 Greenwich Mean Time / 8:30 p.m. British Double-Summer Time, when the Moon was 99 per-cent illuminated by the Sun; sunset was June 5 at 20:01 Greenwich Mean Time / 10:01 p.m. British Double-Summer Time.

Local lunar transit, when the Moon reaches the highest point in the sky on a particular day, occurred over Normandy on June 5 at 23:19 Greenwich Mean Time / June 6 at 1:19 a.m. British Double-Summer Time. This was equivalent to June 5, 7:19 p.m. U.S. Eastern War Time, even though the Moon would not rise in America until later in the evening (Moon-rise in Pittsburgh was June 5, 7:40 p.m. U.S. Eastern War Time).

Actually, this is typical behavior of a Full Moon. Every month, on or near the day of Full Moon, the Moon rises around the time of sunset, stays visible in the sky providing the brightest Moon-light of the month (weather-permitting) all-night-long, and sets around the time of sunrise.

The success of D-Day was truly a team effort, and science and technology played a critical role in the Allied victory in Normandy. Astronomers, meteorologists, geologists, geographers, engineers, military planners, radio operators, aircraft pilots and crews, glider pilots, paratroopers, and combat field medics were an important part of the invasion, in addition to the thousands of soldiers, sailors, and Marines who landed on the beaches that historic day.

Internet Links to Additional Information ---

More detailed information on Astronomy, Tides, and the D-Day Landings from
Sky and Telescope Magazine:
Link >>>

Link 1 >>>
Link 2 >>>

Full Moon: Link >>>

Spring Tides: Link >>>

Syzgy: Link >>>

Related Blog Posts ---

"Astronomy & World War II." Sun., 2014 Sept. 7.

Link >>>


"WWII Medals Return to Earth in Time for Veterans' Day." Tue., 2014 Nov. 11.

Link >>>


Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
              Thursday, 2019 June 6.

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Glenn A. Walsh, Informal Science Educator & Communicator: >
Electronic Mail: < >
Project Director, Friends of the Zeiss: < >
SpaceWatchtower Editor / Author: < >
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
  < >
* Adler Planetarium, Chicago:
  < >
* Astronomer, Educator, Optician John A. Brashear:
  < >
* Andrew Carnegie & Carnegie Libraries:
  < >


  1. Fascinating! Good work, Glenn

    Best wishes,


  2. Thank you Glenn for sharing, and may we never forget these true Heroes...!

  3. A terrific article! So much heroism by so many. Truly, the Greatest Generation. Thanks for this post.

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