There remains a not-always-friendly rivalry between the U.S. Army and the U.S. Navy – one that many militaries worldwide share. It probably comes down to the fact that one fights on land and the other on the sea. As such, some U.S. Army officers and Special Forces operators may not like to admit that they use certain “nautical terms,” notably those related to the time around “Nautical twilight.”
First, we need to remember that the day in “Military time” is still divided into 24 hours, beginning at midnight (00:00) and ending at 23:59; with each hour represented using four digits – instead of “AM” and “PM.” Thus 1 am is 01:00 or “Zero-One-Hundred Hours” while 1 pm is 13:00 or “Thirteen Hundred Hours.” This is meant to avoid confusion.
That’s simple enough for things that occur at a set time, but the sun doesn’t rise at the same time each day, and this is where the military must also know the time of day for both “Begin Mean Nautical Twilight” (BMNT) and “End Evening Nautical Twilight” (EENT).
In simple terms, BMNT is when the sky first starts to become light – that time of day when some college kids are just making it home after a long night, or when hunters will begin to gear up and head into the woods. By contrast, EENT is when the sky is fully dark after the sun goes down.
In other words, BMNT is literally the first moment of dawn, while EENT is the final moment of dusk.
So Why “Nautical Twilight?”
Even in the era before the electric light, towns and villages would have some forms of illumination that would make it hard to distinguish when dusk ended, and dark began – while most people woke as the sun rose. Without alarm clocks and precise timekeeping, it wasn’t exactly easy to wake precisely before dawn, especially as it changes daily and varies depending on one’s location.
However, it was far easier to see the location of the sun on open water.
Moreover, as TimeAndDate.com explains there are three types of twilight that include “Civil Twilight,” “Nautical Twilight” and “Astronomical Twilight” and all define the basis of how far the sun is below the horizon.
“Civil twilight occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon,” the website noted, adding, “Astronomical twilight occurs when the Sun is between 12 degrees and 18 degrees below the horizon.”
Nautical twilight is simply when the geometric center of the sun is 12 degrees below the horizon and it occurs every morning and evening. It dates back to when sailors used the stars to navigate the seas.
Military Importance of BMNT and EENT
The Army doesn’t generally fight at sea, but it still teaches BMNT and EENT to leaders at the Infantry Officer Basic training and Ranger School, among others.
During BMNT there is generally enough light for military personnel to conduct an operation without the need for artificial illumination, including night-vision goggles. The brief period provides a balance between darkness and visibility. It provides an opportune moment that can be used to launch or execute tactical operations and for warfighters to engage in stealth movements. More importantly, during BMNT, visibility gradually increases while an adversary may still be off-guard and less alert.
The June 6, 1944, D-Day landings 80 years ago are an example of the military employing BMNT as a start time for the invasion, as it allowed soldiers to reach the short and then navigate the beach under low light conditions. At the same time, the low light made it harder for the German defenders to spot and target them.
EENT also serves as a crucial time for navigation and military operations, but visibility will gradually decrease, and could then require the use of artificial illumination.
Being able to calculate the exact time of BMNT remains essential for military commanders to successfully synchronize and execute operations. Both BMNT and EENT differ in the geographic location as well as the time of the year, as it depends on the position of the sun relative to the Earth’s surface. To determine when it will occur requires knowing the date, Latitude, Longitude, and GMT (Greenwich Mean Time).
Other Related Terms
Because BMNT and EENT can be the times of day when an adversary might engage in an attack, military leaders are also taught to prepare for such a possibility. This can include maintaining a state of readiness that is known as “Stand To,” which is when troops are to have weapons ready and to be prepared for possible action.
While “Stand To” can be ordered at any time, standing ready for attack is often issued just before dawn or after dark. Warfighters are told to “Stand Down” when it is determined there is no threat.
There is also 0-Dark-Thiry (Zero Dark Thirty) which may be familiar from the 2012 movie about the effort to find and kill Al-Qaeda leader Usama bin Laden (UBL), and the film does show a clock with a location time of 00:30 as the mission begins. However, 0-dark-thirty may also mean any time it is dark between 00:01 and 05:00 hours local time. In the context of the movie, director Kathyrn Bigelow suggested, “It’s a military term for early morning before dawn, and it refers also to the darkness and secrecy that cloaked the entire decade-long mission.”
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