Sky brightness

Sky brightness is how bright or dark the sky looks to our eyes. It changes a lot between day and night.

During the day, the sky looks blue because sunlight spreads out in the air. This is called Rayleigh scattering.

At night, the sky is usually dark, but it’s never completely black. We can see stars, the Moon, and sometimes city lights that make the sky look brighter. This extra light from cities is called light pollution.

There are special times called twilight, which happen just after sunset or just before sunrise. During twilight, the Sun is just below the horizon, and the sky has a soft, glowing light.

Other things can also make the night sky brighter, like very faint light high up in the air called airglow. Understanding sky brightness helps us know how dark the night should be.

Airglow

Main article: Airglow

Airglow is the soft light that comes from the upper atmosphere at night. It happens when sunlight, especially invisible ultraviolet light, makes gases like oxygen and sodium glow. This light is much fainter than starlight, so you need very dark skies to see it clearly.

Scientists first noticed this glow in the 1800s when they studied the beautiful lights in the sky called the aurora. They found that even when those lights weren’t visible, a faint green glow remained. This green glow comes from oxygen high up in the sky, and there are also other colors like red and yellow from different gases. These gases glow because of the sun’s energy, creating a gentle light that makes the night sky never completely dark.

Indirect scattering of sunlight

Indirectly scattered sunlight comes from two places: the atmosphere around us and from outer space. After the Sun sets, it still lights up the upper atmosphere. The more air and tiny particles called aerosols we see, the more sunlight scatters. This makes the sky glow even after the Sun goes down.

Another source of light in the sky is the zodiacal light. This is sunlight reflecting off tiny bits of interplanetary dust between planets. The brightness of this light changes depending on where Earth is, where you stand on Earth, the time of year, and how the dust is spread out.

Scattered light from extraterrestrial sources

The sky isn’t lit up only by sunlight. Light from stars and the soft glow of our galaxy, the Milky Way, also scatter in the air around us. Some stars add to this soft light in the sky. Faraway galaxies and nebulae don’t add much to this effect.

Long ago, in 1899, a person named Burns tried to measure how bright all the stars together are. He found that all this starlight together is about as bright as 2,000 very bright stars. Other people have also done similar measurements since then.

Light pollution

Main article: Light pollution

Light pollution makes the night sky brighter, especially in cities. In places with many lights, the night sky can be much brighter than it would be with no lights. Because of this, many people can no longer see the Milky Way.

Twilight

When the sun has just set, the sky becomes darker quickly. This lets us see a special glow called airglow. This glow happens high up where the sun is still shining, even though we can't see it on the ground. During this time, we might see yellow and red lights that can make the sky look purple.

After the sun moves further below the horizon, these yellow and red lights fade away. The main light we see then comes from green glowing oxygen in the sky. When it gets completely dark, this green light is the strongest, and starlight scatters in the atmosphere. This change in light creates beautiful effects known as the golden hour and the blue hour.

Relative contributions

The table below shows how different natural lights add up to make the night sky glow when it is very dark, far from city lights.

The S₁₀ unit measures how bright a star looks when its light spreads out over a small patch of sky. On a very dark night, the whole sky looks about ~220 S₁₀ or 21.9 mag/arcsec² bright.

Two big natural lights change through the year and with the Sun’s activity: Airglow and Zodiacal light. Airglow changes with the Sun’s energy and the time of year. Zodiacal light depends on where you look in the sky compared to the Sun’s position. Sometimes, the sky can look almost twice as bright as usual under these natural lights.