Sky brightness

Sky brightness describes how the sky appears to the eye and how it scatters and diffuses light. It is obvious that the night sky is not totally black. Without the Moon and artificial light pollution, only direct starlight would be visible.

The sky’s brightness changes dramatically throughout the day, and the main reasons for this change also shift. During the day, with the Sun above the horizon, direct scattering of sunlight is by far the biggest source of light. At twilight—the period after sunset or before sunrise—the situation is more complex and needs further breakdown.

Twilight is split into three segments, each covering six degrees of the Sun’s position below the horizon. In civil twilight, the Sun’s disk center lies between a quarter degree and six degrees below the horizon. In nautical twilight, the Sun’s altitude is between six and twelve degrees below the horizon. In astronomical twilight, the Sun is between twelve and eighteen degrees down. Once the Sun is more than eighteen degrees below the horizon, the sky generally reaches its darkest state.

The natural glow of the night sky comes from several sources: airglow, indirect scattering of sunlight, scattering of starlight, and light pollution.

**Airglow**

In 1868, physicist Anders Ångström studied the aurora borealis’s spectrum and found that even on nights without aurora, a characteristic green line remained. It was not until the 1920s that scientists began to identify and understand the emission lines in aurorae and the sky itself, and what caused them. The green line Ångström saw is an emission line at 557.76 nm, produced by the recombination of oxygen in the upper atmosphere.

Airglow is the overall term for various processes in the upper atmosphere that release photons, mainly driven by the Sun’s ultraviolet radiation. It is about one-tenth as bright as the combined glow of starlight, so clear, dark skies are needed to photograph it. Several emission lines dominate: a green oxygen line at 557.7 nm, a yellow doublet from sodium at 589.0 and 589.6 nm, red oxygen lines at 630.0 and 636.4 nm, and various hydroxyl bands.

The sodium emissions come from a thin, sporadic sodium layer about 10 km thick at an altitude of 90–100 km, above the mesopause in the ionosphere’s D-layer. How sodium reaches these mesospheric heights is not well understood, but it is thought to come mainly from meteor ablation. The red oxygen lines originate at about 250 km altitude, in the F-layer. The green oxygen emissions are more spread out, with peaks in the upper mesosphere and lower thermosphere.

During the day, sodium and red oxygen emissions are dominant and roughly 1,000 times brighter than at night, because the upper atmosphere is fully exposed to solar UV. However, this effect is not noticeable to the human eye, as the glare of directly scattered sunlight outshines and hides it.

**Indirect scattering of sunlight**

Indirectly scattered sunlight comes from two places: the atmosphere itself and outer space. In the first case, after the Sun has just set, it still directly lights up the upper atmosphere. The amount of scattered sunlight is proportional to the number of scatterers—air molecules and aerosols—in the line of sight, which increases as the Sun nears the horizon. This attenuation follows the Beer–Lambert law.

The intensity of scattered twilight decreases as the Sun drops further below the horizon and illuminates less of the atmosphere. When the Sun’s altitude is below six degrees, 99% of the atmosphere in the zenith direction is in Earth’s shadow, and second-order scattering takes over. But at the horizon in the direction of sunset, 35% of the atmosphere along the line of sight is still directly lit, continuing until the Sun reaches twelve degrees below the horizon. From twelve to eighteen degrees, only the uppermost parts of the atmosphere along the horizon, directly above where the Sun set, remain lit. After that, all direct illumination stops and astronomical darkness begins.

A second source of sunlight is the zodiacal light, caused by reflection and scattering of sunlight by interplanetary dust. Its intensity varies with Earth’s position, the observer’s location, the time of year, and the composition and distribution of the dust.

**Scattered light from extraterrestrial sources**

Sunlight is not the only light scattered by air molecules. Starlight and the diffuse light of the Milky Way are also scattered, and stars up to visual magnitude 16 contribute to this diffuse scattered starlight. Other sources, like galaxies and nebulae, do not add significantly.

The total brightness of all stars was first measured by Burns in 1899, who calculated that the total brightness reaching Earth was equal to that of 2,000 first-magnitude stars. Later measurements by others confirmed this.

**Light pollution**

Light pollution is a growing source of sky brightness in urban areas. As of 2023, it is estimated to be increasing at a rate of 9.6% per year. In densely populated regions without strict light pollution controls, the entire night sky is regularly 5 to 50 times brighter than it would be with all lights off. Often, light pollution’s effect far exceeds natural sources, including moonlight. Because of urbanization and light pollution, one-third of humanity—and most people in developed countries—cannot see the Milky Way.

**Twilight**

When the Sun has just set, the sky’s brightness drops rapidly, allowing the airglow from high altitudes to become visible.