The experience of a bright sky after the sun has set is common, stemming from both natural celestial mechanics and human intervention. When the sun drops below the horizon, the atmosphere continues to scatter light, illuminating the sky for hours depending on location and time of year. This natural phenomenon is often compounded by the artificial glow produced by human settlements, particularly in densely populated areas. Nighttime illumination is a combination of scattered sunlight, light pollution, and faint emissions from the upper atmosphere.
When the Sun Doesn’t Fully Set
The most common natural reason for a bright night sky is extended twilight, where the sun is below the horizon but still close enough for its light to be scattered by the upper atmosphere. This effect is pronounced in the summer months at higher latitudes, sometimes leading to the “white nights” seen near the Arctic and Antarctic Circles. The Earth’s axial tilt causes the sun to graze the horizon at these latitudes, resulting in prolonged illumination.
Astronomers divide this post-sunset period into three phases based on the sun’s angle below the horizon. Civil twilight occurs when the sun is between the horizon and six degrees below it, remaining bright enough that artificial lighting is often unnecessary and the horizon remains clearly visible. As the sun sinks further, the sky transitions into nautical twilight, which lasts until the sun is twelve degrees below the horizon. During this phase, the horizon is no longer discernible, but enough light remains for celestial navigation.
The sky is only considered truly dark once the sun drops eighteen degrees below the horizon, marking the end of astronomical twilight. Until this point is reached, the upper atmosphere is still high enough to be illuminated by the sun’s rays. This scattered light, primarily through Rayleigh scattering, diffuses across the sky, preventing the complete darkness required for observing the faintest celestial objects. The duration of these twilight periods can vary significantly, lasting about 24 minutes at the equator but extending for weeks near the poles.
Artificial Skyglow
A major contributor to nighttime brightness, particularly over urban centers, is artificial skyglow. This phenomenon is a form of light pollution caused by upward-directed light from ground sources, such as streetlights, buildings, and industrial facilities. The light travels up into the atmosphere, where it is redirected back toward the ground by atmospheric particles.
The scattering process involves both air molecules and larger particles like aerosols, dust, and water vapor. While Rayleigh scattering by tiny air molecules scatters shorter, blue wavelengths more effectively, larger pollutants and moisture cause Mie scattering. Mie scattering redirects all wavelengths of light more uniformly, contributing significantly to the dome of light visible over cities. The color and intensity of the glow depend heavily on the type of lighting used.
Newer lighting technologies, such as white light-emitting diodes (LEDs), often contain a higher proportion of blue light compared to older high-pressure sodium lamps. Since blue light scatters more easily, the increased use of these sources has resulted in brighter and more widespread skyglow. This dome of light fundamentally changes the visual experience of the night sky, reducing the contrast between celestial objects and the background.
Light from the Upper Atmosphere
Beyond scattered sunlight and light pollution, the Earth’s upper atmosphere generates its own faint, natural light through two distinct mechanisms. The first is Airglow, a continuous, diffuse emission of light that occurs globally. Airglow is a result of chemical reactions where atoms and molecules in the upper atmosphere, primarily oxygen and nitrogen, recombine after being energized by the sun’s radiation during the day.
This process, known as chemiluminescence, releases energy in the form of light photons, typically resulting in a faint green or red tint. Airglow is generally too dim to be seen by the unaided eye, but it is easily captured by sensitive cameras and is visible from orbit. It is distinct from the more dynamic phenomenon of the aurora, or the Northern and Southern Lights. Auroras are caused by charged particles from the solar wind colliding with atmospheric gases in the polar regions, producing vibrant displays.