The moment the sun disappears below the horizon, known as sunset, does not mark the beginning of full darkness. The period between sunset and true night is a gradual process that involves the scattering of sunlight in the upper atmosphere, creating a lingering glow. Accurately determining when the sky becomes “fully dark” requires understanding a specific astronomical metric. The answer is not a single, fixed time, but rather a variable moment defined by the sun’s precise angle beneath the horizon.
Defining True Darkness: The Three Stages of Twilight
The transition from day to night is formally divided into three distinct stages of twilight, each defined by how far the geometric center of the sun has dropped below the horizon, measured in degrees.
Civil Twilight
The first phase after sunset is Civil Twilight, lasting until the sun reaches six degrees below the horizon, a point known as civil dusk. During this time, there is generally enough natural light available for most outdoor activities without the need for artificial illumination, and the brightest stars and planets become visible.
Nautical Twilight
Once the sun dips past the six-degree mark, Nautical Twilight begins, continuing until the sun is twelve degrees below the horizon. In this stage, the horizon line remains discernible, which historically allowed mariners to take readings. However, the light level has dropped enough that artificial light is generally required for detailed outdoor tasks.
Astronomical Twilight
The final and darkest stage before true night is Astronomical Twilight, spanning from twelve degrees to eighteen degrees below the horizon. Although the sky is quite dark during this phase, a faint scattering of sunlight is still present in the upper atmosphere. The scientific definition of “full darkness” is the moment the sun’s center reaches eighteen degrees below the horizon, marking the end of astronomical twilight, or astronomical dusk. Beyond this point, the sky is as dark as it will get, provided there is no interference from the moon or artificial ground light pollution.
Why Darkness Time Changes: Seasonal and Geographic Factors
While the definition of full darkness—the sun being eighteen degrees below the horizon—is constant, the time it takes to reach that point varies widely based on geographic location and the time of year. This variability is primarily dictated by the angle at which the sun’s path intersects the horizon.
Geographic Latitude
Near the equator, the sun sets almost perpendicularly, meaning it drops below the eighteen-degree mark very quickly. This steep angle results in a short twilight period, often lasting only about twenty to twenty-eight minutes for each phase. By contrast, locations at higher latitudes experience a much shallower, more oblique angle of the setting sun. The sun essentially skims the horizon, which significantly prolongs the duration of all three twilight stages, sometimes stretching the transition to full darkness over several hours.
Seasonal Changes
Seasonal changes also play a significant role in the duration of twilight due to the Earth’s axial tilt relative to the sun. Near the summer solstice, particularly in high-latitude regions above approximately 48.5 degrees north or south, the sun never dips more than eighteen degrees below the horizon. In these areas, true astronomical night does not occur, and the sky remains in a state of perpetual astronomical twilight, a phenomenon often called “white nights.” Conversely, around the equinoxes, the sun’s angle relative to the horizon is such that the twilight periods are at their shortest for those mid-latitude locations. This dynamic interaction ensures that the time of full darkness is a continuously shifting target.
Tools and Methods for Calculating Local Darkness
Because the time of full darkness is specific to a precise location and date, it cannot be determined by a simple, universal formula. The most practical and accurate method for finding this time is to use specialized online calculators or mobile applications that incorporate complex astronomical equations. These digital tools use your geographical coordinates—latitude and longitude—and the current date to calculate the exact position of the sun.
When using these resources, the key is to look specifically for the time listed for “Astronomical Dusk” or “End of Astronomical Twilight.” Many general weather applications provide sunset times, but only specialized astronomical tools will calculate the definitive eighteen-degree mark. Government agencies, such as the U.S. Naval Observatory, also provide highly accurate tables and calculators for these precise astronomical events. To use these tools effectively, you must input your precise location, either by using your device’s GPS or by manually entering the latitude and longitude coordinates. This ensures the calculation correctly accounts for the local curvature of the Earth and the specific angle of the sun’s path at your exact spot.