Twilight is the period of illumination that occurs both before sunrise and after sunset, representing a gradual transition between full daylight and complete darkness. This phenomenon is caused by the scattering of sunlight in the Earth’s upper atmosphere, even though the sun is positioned below the horizon. During this time, the sun’s rays are refracted and scattered by air molecules, providing a measurable level of light on the ground. For precise scientific measurement, twilight is formally divided into three distinct stages based on the sun’s angle below the horizon.
The Three Scientific Stages of Twilight
The brightest phase is Civil Twilight, which begins or ends when the sun’s center is exactly 6 degrees below the horizon. During this period, there is enough natural light available for people to conduct most ordinary outdoor activities without needing artificial illumination. The brightest stars and planets become visible, but the horizon remains clearly discernible.
Nautical Twilight occurs when the sun is positioned between 6 and 12 degrees beneath the horizon. The natural light level is significantly reduced, meaning most detailed outdoor work requires supplemental lighting. This phase is defined by conditions that allow mariners to distinguish the horizon line from the sky, a capability necessary for celestial navigation.
The deepest phase is Astronomical Twilight, defined by the sun’s center being between 12 and 18 degrees below the horizon. Once the sun drops past the 12-degree mark, the horizon is no longer visible to the unaided eye. This phase ends precisely when the sun reaches 18 degrees below the horizon, marking the beginning of true night as the last traces of scattered sunlight disappear.
Factors Affecting Twilight’s Duration and Timing
The length of time each twilight stage lasts is not constant globally, but is determined primarily by geographical and astronomical factors. The most significant variable is a location’s latitude, which dictates the angle at which the sun appears to set or rise relative to the horizon. Near the equator, the sun sets almost perpendicularly, causing it to pass through the 18 degrees of twilight in the shortest amount of time, sometimes as little as 24 minutes.
In contrast, at higher latitudes, the sun sets at a much shallower angle. This oblique path means the sun takes considerably longer to drop the full 18 degrees, extending the duration of twilight significantly. For example, at latitudes above 48 degrees near the summer solstice, the sun may never sink lower than 18 degrees, meaning true night does not occur at all.
The season also plays a role in the duration of twilight due to the Earth’s axial tilt. Twilight tends to be shortest around the equinoxes and longest around the summer solstice, especially at mid to high latitudes. While atmospheric conditions can affect the apparent brightness, they do not change the official, mathematically defined start and end times of the three twilight stages.
Why the Stages of Twilight Matter
The precise definitions of the three twilight stages have significant applications across multiple professional and regulatory fields. The boundaries of Civil Twilight, for example, are often incorporated into legal codes and regulations. These definitions dictate when vehicle headlights must be turned on, when certain hunting practices are allowed, and are a factor in aviation safety rules.
Nautical Twilight is important for its application in navigation. The ability to see the horizon while simultaneously viewing the first or last stars is fundamental for taking celestial measurements. This period ensures that sailors have a reliable window to use a sextant to determine their position.
The definition of Astronomical Twilight is most relevant to the field of astronomy. Sensitive astronomical observations require the complete absence of scattered sunlight that would otherwise interfere with telescope readings. Only once the sun is 18 degrees below the horizon can astronomers begin their most detailed work on faint celestial objects.