Twilight is the period between full daylight and true night, defined by the sky remaining illuminated by scattered sunlight after the sun has set. It is formally divided into three distinct phases based on the sun’s solar depression angle below the horizon. Understanding these phases is necessary to accurately determine visibility, particularly during the middle stage, nautical twilight.
Defining the Twilight Stages
The brightest phase is Civil Twilight, which occurs when the sun is between 0 and 6 degrees below the horizon. During this period, there is enough light for most outdoor activities without artificial aid, and only the brightest stars and planets are visible in the sky.
Nautical Twilight follows, marked by the sun’s center being between 6 and 12 degrees below the horizon. This phase is significantly darker than civil twilight, but it retains enough illumination for practical purposes.
The final stage is Astronomical Twilight, which begins when the sun drops to 12 degrees and ends at 18 degrees below the horizon. Once the sun passes 18 degrees, the sky is considered fully dark, and the period of true night begins.
Visibility During Nautical Twilight
During Nautical Twilight, visibility balances atmospheric illumination and darkness. The key feature is the ability to distinguish the horizon line, which is why the stage is named for its utility to mariners. Even without moonlight, faint scattered light provides enough contrast to separate the sky from the sea surface.
The light intensity is low enough that the human eye’s cone cells, responsible for color and detailed vision, are largely ineffective. Vision is primarily handled by the rod cells, which allow for perception of general outlines and movement but not fine detail. This means while a person can discern the general outlines of ground objects, it becomes difficult to participate in detailed outdoor operations.
Artificial lighting is required for tasks such as reading maps or performing delicate work. Although the sky is dark, only the most prominent stars are bright enough to be clearly visible against the sky glow. This combination of a visible horizon and bright stars creates the optimal window for celestial navigation using a sextant.
Practical Applications and Duration
The specific visibility conditions of Nautical Twilight make it a designated period for practical applications in aviation and maritime operations. The U.S. Federal Aviation Regulations reference the start and end of this phase for determining when navigation lights must be operational. For military planners, the beginning of morning nautical twilight (BMNT) and the end of evening nautical twilight (EENT) are times of heightened security awareness.
The duration of this twilight stage is not fixed and varies significantly based on geographic latitude and the time of year. Near the equator, the sun’s path is nearly perpendicular to the horizon, causing a quick transition, with nautical twilight lasting approximately 23 to 26 minutes.
Moving to mid-latitudes, the duration extends to between 30 and 40 minutes per phase, while at high latitudes, this period can last for hours. In areas closer to the poles during the summer, the sun may never drop more than 12 degrees below the horizon, resulting in continuous Nautical Twilight throughout the entire night.