Sky brightness

The sky is not always black.

During the day, the sun makes the sky very bright. After the sun sets, the sky stays light for a while. This is called twilight.

At night, the sky can still have a soft glow. This glow comes from parts of the air high up. Tiny bits of dust in space also reflect light. Many stars also help make the sky bright. Even city lights can make the night sky glow. The sky is full of light.

The sky is not always dark at night. Many things make it glow.

One source is called airglow. This is a soft light from the upper atmosphere. It happens because of UV radiation from the sun. This light comes from parts of the air like oxygen and sodium. Oxygen can make green or red light. Sodium can make yellow light.

Another source is zodiacal light. This is sunlight that hits dust in space. The dust reflects the light back to us. Starlight also helps. The air scatters light from many stars. This makes the sky look soft instead of black.

Twilight is the time after the sun sets. The sky stays bright because the sun still hits the upper air. As the sun goes deeper below the horizon, the sky gets darker. Eventually, it reaches astronomical darkness.

In cities, light pollution makes the sky much brighter. This happens when man-made lights shine upward. In some places, the sky is 50 times brighter than it should be. This can hide the Milky Way from our view.

The sky is not always pitch black at night. We call the way we see light in the sky sky brightness. This refers to how light scatters and spreads out through the air. During the day, the Sun is the main reason the sky is bright. At night, the sky still has many sources of light. These include things like starlight and the glow of our own atmosphere.

One way the sky glows is through a process called airglow. This happens in the upper atmosphere because of UV radiation from the Sun. This energy causes different gases to release light. For example, oxygen can create green or red light. Sodium can also create a yellow glow. This sodium comes from a layer about 10 km thick. Scientists believe it comes from meteors breaking apart in the air.

Scientists have studied these lights for a long time. In 1868, a physicist named Anders Ångström looked at the aurora borealis. He found a green line in the light even when there was no aurora. Later, in the 1920s, researchers began to understand these emission lines. They learned that these gases release photons, which are tiny particles of light. This helped them identify what was actually making the sky glow.

There are many specific numbers that describe the night sky. Airglow is about one tenth as bright as the total glow of starlight. The sky also has zodiacal light, which comes from sunlight hitting dust in space. On a very dark night, the total brightness at the zenith is about 220 S10. This is a special unit used to measure how much light reaches us. Even starlight adds to this, as the air scatters light from many stars.

We can also see how brightness changes during twilight. Twilight is the time after sunset or before sunrise. It is divided into three parts based on the Sun's position. Civil twilight happens when the Sun is between 1/4° and 6° below the horizon. Nautical twilight occurs between 6° and 12° below. Finally, astronomical twilight happens between 12° and 18° below. Once the Sun is deeper than 18°, the sky reaches its maximum darkness.

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.