The sky begins to lighten well before sunrise, a phenomenon known as twilight. Sunrise is officially defined as the precise moment when the sun’s upper edge appears to touch the visible horizon. This definition accounts for atmospheric refraction, which bends the sun’s rays and makes the solar disk appear slightly higher than its true position. This period of increasing light results from sunlight interacting with the Earth’s atmosphere.
The Mechanism of Light Before Sunrise
The light that illuminates the pre-dawn sky comes from the sun, even when the sun itself is still hidden below the horizon line. This is possible because the Earth is surrounded by its atmosphere, which extends many miles above the surface. Sunlight travels in a straight line until it enters this gaseous layer, where it encounters countless tiny molecules of nitrogen and oxygen.
The interaction between sunlight and these small molecules causes a process called Rayleigh scattering. This scattering is highly effective at deflecting the shorter, bluer wavelengths of light in all directions, diffusing the light across the upper atmosphere. Some of this scattered light is redirected back toward the ground, making the sky appear luminous even before the sun’s direct rays can reach a surface observer.
The amount of atmosphere the light travels through dictates the intensity and color of the light we see. Because the light path is long during twilight, blue light is scattered away, leaving longer-wavelength reds and oranges to dominate the light reaching the upper atmosphere. This scattered light then creates the soft, diffuse illumination of the dawn sky.
The Three Stages of Twilight
The pre-dawn period is divided into three distinct stages based on the sun’s angular position below the horizon. These stages provide a precise measure of available light, defined by how many degrees the sun’s geometric center is depressed below the horizon. This categorization moves from the darkest phase to the lightest phase, ending at sunrise.
Astronomical Twilight
Astronomical twilight begins when the sun is 18 degrees below the horizon. The sky remains dark enough for faint celestial objects, such as distant galaxies, to be observed without ambient light interference. For the average person, the illumination is practically imperceptible, and the sky appears completely dark. This phase marks the transition from night to day as scattered light enters the highest atmosphere.
Nautical Twilight
Nautical twilight starts when the sun reaches 12 degrees below the horizon. The illumination is sufficient for a mariner at sea to distinguish the horizon line. This visibility allowed sailors to take navigational measurements against the still-visible horizon. The horizon becomes discernible, but the outlines of terrestrial objects are difficult to make out without artificial lighting.
Civil Twilight
Civil twilight is the final and most practical stage, beginning when the sun is 6 degrees below the horizon. There is enough ambient light to conduct ordinary outdoor activities. Just before sunrise, most objects are clearly visible, and artificial lighting is no longer required for tasks like reading. The sky rapidly brightens as the sun nears the horizon.
Why Twilight Duration Changes
The time required to move through these three stages of twilight is not fixed and varies significantly based on latitude. This variability is due to the angle at which the sun’s path intersects the horizon line. The sun must travel 18 degrees vertically to complete the entire twilight process, from astronomical dawn to sunrise.
Near the equator, the sun’s daily path is nearly perpendicular to the horizon. This steep angle means the sun moves through the 18-degree twilight zone quickly, resulting in a total duration of only about 75 minutes. Twilight is brief, transitioning rapidly from dark to full daylight.
In contrast, at higher latitudes closer to the poles, the sun’s path is much shallower, or oblique, relative to the horizon. The sun must travel a longer distance along this shallow slope to achieve the 18-degree change in vertical depression. Consequently, the duration of twilight can stretch for several hours, leading to the prolonged summer evenings experienced in northern regions.