The period commonly referred to as “getting light in the morning” is scientifically known as dawn, the start of morning twilight. This gradual process begins long before the sun’s upper edge appears over the horizon, marking sunrise. Dawn represents a slow transition from night to the full illumination of day, governed by the sun’s position relative to the observer’s horizon. The intensity of this pre-sunrise light increases steadily as the sun rises closer to the horizon. This illumination is an effect of the earth’s atmosphere acting as a filter and reflector for sunlight.
How Light Appears Before Sunrise
The appearance of light when the sun is still below the horizon is a consequence of two distinct atmospheric phenomena. The first is atmospheric refraction, the bending of sunlight as it passes through layers of air with differing densities. When the sun is very low, its light rays enter the atmosphere at a shallow angle and are curved downward towards the earth’s surface. This bending causes the sun to appear slightly higher than its actual geometric position, allowing us to see its light minutes before sunrise.
The second, and more dominant, mechanism responsible for the overall illumination of the sky is Rayleigh scattering. Atmospheric molecules, primarily nitrogen and oxygen, scatter shorter wavelengths of light (blue and violet) far more effectively than longer wavelengths (red and orange). This scattered blue light illuminates the upper atmosphere and is diffused downward, creating the recognizable pre-dawn glow. Because sunlight travels a longer path through the atmosphere when the sun is below the horizon, the short-wavelength blue light is scattered widely, giving the sky its characteristic colors during this time.
The Three Stages of Dawn
Scientists divide morning twilight into three distinct phases, defined by the sun’s angular distance below the horizon.
Astronomical Twilight
The first stage is Astronomical Twilight, which begins when the sun is \(\text{18}^\circ\) below the horizon. The sky is still effectively dark to the casual observer. Faintest celestial objects, such as distant galaxies, are no longer visible due to the slight atmospheric illumination, marking the technical end of true night.
Nautical Twilight
The sky begins to lighten noticeably with the onset of Nautical Twilight, which occurs when the sun is \(\text{12}^\circ\) below the horizon. There is enough ambient light to distinguish the general outline of the horizon, though illumination is too faint for detailed outdoor activities. This stage historically allowed mariners to use a visible horizon line with major stars to determine their position at sea, giving the phase its name.
Civil Twilight
The period most people associate with “getting light” is Civil Twilight, which starts when the sun is \(\text{6}^\circ\) below the horizon. The illumination is sufficient for objects on the ground to be clearly discernible, and most outdoor activities can be conducted without artificial light. The horizon is clearly visible, and only the brightest stars and planets remain visible. Civil twilight ends precisely at sunrise, the moment the sun’s upper limb appears above the eastern horizon.
Factors That Shift the Timing
The clock time for these three stages of dawn is not constant and shifts significantly depending on geographic and seasonal variables. Latitude is the primary determinant of twilight’s duration because it affects the angle at which the sun appears to rise and set. Near the equator, the sun rises and sets at a nearly perpendicular angle to the horizon, causing the twilight phases to pass quickly, sometimes lasting as little as 24 minutes.
Moving toward the poles, the sun’s path becomes increasingly oblique, meaning it takes longer to pass through the \(\text{18}^\circ\) angular distance defining the twilight stages. At very high latitudes, twilight can last for hours. During summer months, the sun may not sink \(\text{18}^\circ\) below the horizon, resulting in continuous twilight instead of true night. The Earth’s axial tilt also means twilight duration is longer during summer months at mid-latitudes compared to winter months.
Local atmospheric conditions and human-made time designations also influence the perception and timing of dawn. Heavy cloud cover, dense fog, or air pollution can absorb and scatter light, dimming the sky and making the perceived start of twilight seem later. The use of time zones and Daylight Saving Time artificially shifts the clock time of dawn, even though the actual solar event remains consistent relative to the sun’s position.