It is a common observation that the sky remains illuminated for some time after the sun dips below the horizon. While many people use the terms “sunset” and “getting dark” interchangeably, there is a scientific distinction between these two phenomena. Sunset marks a precise astronomical event, whereas the progression to complete darkness is a gradual process that unfolds over several stages.
Defining Sunset
Sunset refers to the specific moment when the upper edge of the sun disappears below the western horizon as viewed from Earth. This event is a direct result of Earth’s continuous rotation. The atmosphere also plays a role, as light from the sun bends slightly due to atmospheric refraction, allowing us to see the sun for a few extra minutes even after it has fallen below the horizon.
The Science Behind Twilight
The reason it does not become instantly dark after sunset is due to the scattering of sunlight in Earth’s upper atmosphere. Even when the sun is no longer directly visible, its rays continue to illuminate particles and gases high above the horizon. These atmospheric components then scatter some of this sunlight back towards the ground. This scattered light keeps the sky bright.
How Darkness Progresses
The gradual progression from sunset to full darkness is categorized into three distinct phases of twilight, each defined by the sun’s angular position below the horizon. Civil twilight occurs when the sun is between 0 and 6 degrees below the horizon. During this phase, there is still enough natural light for most outdoor activities without artificial illumination, and the brightest stars become visible.
Following civil twilight is nautical twilight, which happens when the sun is between 6 and 12 degrees below the horizon. At this point, the horizon often remains discernible, which was historically important for mariners to navigate by the stars. More stars become visible as the sky darkens further.
The final phase, astronomical twilight, takes place when the sun is between 12 and 18 degrees below the horizon. During this period, the sky becomes truly dark, allowing for the observation of faint celestial objects such as nebulae and distant galaxies. Complete darkness is considered to have arrived once the sun drops more than 18 degrees below the horizon, as the atmospheric scattering of sunlight is minimal at this point.
What Affects the Transition to Darkness
Several factors influence the duration of twilight and how quickly darkness sets in. Latitude is a primary determinant; twilight lasts significantly longer at higher latitudes, closer to the Earth’s poles, because the sun’s path crosses the horizon at a more oblique angle. Conversely, near the equator, the sun’s path is more perpendicular to the horizon, resulting in a much shorter twilight period.
The time of year, or season, also affects twilight duration due to the Earth’s axial tilt and its orbit around the sun. This tilt changes the angle at which sunlight strikes the atmosphere. Atmospheric conditions, such as the presence of clouds, pollution, or a very clear sky, can alter the perceived brightness and overall duration of twilight.