Sky brightness

Sky brightness refers to how we perceive the light in our atmosphere. It describes how light scatters and diffuses through the air. While we often think of the night sky as black, it is rarely completely dark. If we removed the Moon and all light pollution, we would still see starlight and other natural glows. The brightness of the sky changes constantly throughout the day. The primary cause of this change depends on whether the Sun is above or below the horizon.

One major source of natural nighttime brightness is airglow. This is the collective name for processes in the upper atmosphere that release photons, which are particles of light. This process is driven primarily by ultraviolet (UV) radiation from the Sun. Airglow is about one-tenth as bright as the total glow of starlight. It requires very clear, dark skies to be seen in photographs. Different gases in the atmosphere create different colors of light through specific emission lines. For example, oxygen creates a green line at a wavelength of 557.76 nm. It also produces red lines at 630.0 nm and 636.4 nm. Sodium creates a yellow doublet at 589.0 and 589.6 nm.

These gases exist in specific layers of our atmosphere. The sodium emissions come from a sporadic layer about 10 km thick. This layer sits at an altitude between 90 and 100 km. Scientists believe this sodium comes from meteor ablation, which is when meteors break apart in the air. The red oxygen lines originate much higher, at about 250 km in the F-layer. The green oxygen emissions are more spread out. They peak in the upper mesosphere and the lower thermosphere. During the day, these emissions are roughly 1,000 times brighter than at night. This happens because the upper atmosphere is fully exposed to solar UV radiation. However, the glare from scattered sunlight is so strong that humans cannot notice this increase.

Another source of light is the indirect scattering of sunlight. This happens in two ways: from the atmosphere and from outer space. After the Sun sets, it still illuminates the upper atmosphere directly. The amount of light scattered depends on the number of scatterers, such as air molecules and aerosols. This relationship is described by the Beer–Lambert law. As the Sun drops lower, the intensity of scattered twilight decreases. When the Sun is 6° below the horizon, 99% of the atmosphere at the zenith is in Earth's shadow. However, at the horizon where the Sun set, 35% of the atmosphere remains directly illuminated. This illumination continues until the Sun reaches -12°. Between -12° and -18°, only the uppermost parts of the atmosphere at the horizon are still lit.

Twilight is the period between sunset and full darkness. Scientists divide twilight into three distinct stages based on the Sun's altitude. Civil twilight occurs when the Sun is between 1/4° and 6° below the horizon. Nautical twilight follows, occurring when the Sun is between -6° and -12°. Finally, astronomical twilight happens when the Sun is between -12° and -18°. Once the Sun is more than 18° below the horizon, the sky reaches its maximum darkness. During twilight, the sky can appear purplish. This is because yellow sodium emissions and red oxygen emissions are dominant. As astronomical darkness sets in, the green oxygen line becomes the main source of light.

History shows how our understanding of these lights has grown. In 1868, the physicist Anders Ångström studied the aurora borealis. He discovered a characteristic green line in the spectrum even when the aurora was absent. It took until the 1920s for scientists to fully identify these emission lines. They began to understand what caused the sky to glow. In 1899, a researcher named Burns measured the total brightness of all stars. He calculated that the light reaching Earth was equivalent to 2,000 first-magnitude stars. Today, we also account for zodiacal light, which is sunlight scattered by interplanetary dust in space. This light varies based on the position of the Earth and the dust itself.

Modern sky brightness is heavily affected by light pollution. This is the increase in sky brightness caused by human urbanization. As of 2023, light pollution is estimated to increase by 9.6% every year. In densely populated areas without strict controls, the night sky can be 5 to 50 times brighter than a natural dark sky. This effect is often much stronger than moonlight. Because of this, one-third of humanity and most people in developed countries cannot see the Milky Way. On a perfectly dark night at middle latitudes, the total brightness at the zenith is about 220 S10. This is a measurement used to describe how much light reaches a specific area of the sky.