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 scatters in the atmosphere. 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. Twilight is divided into three parts based on how low the Sun is.

Other things can also make the night sky brighter, like very faint light from the upper atmosphere called airglow, and scattered light from the Sun and stars. Understanding sky brightness helps us appreciate how dark the night should be and how we can protect it from too much extra light.

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, causes gases like oxygen and sodium to 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 molecules and tiny particles called aerosols in our view, the more sunlight scatters. This makes the sky glow even after the Sun disappears.

Another source of light in the sky is the zodiacal light, which 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 just lit up by sunlight alone. The light from stars and the soft glow of our galaxy, the Milky Way, also gets scattered by the air around us. Stars that are not too faint, up to what we call V magnitude 16, add to this soft light in the sky. Distant galaxies and nebulae don’t add much to this effect.

A long time ago, in 1899, someone named Burns tried to measure how bright all the stars together are. He figured out 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. It is growing quickly, about 9.6% each year. In places with lots of lights and not enough rules to control brightness, the night sky can be 5 to 50 times brighter than it would be with no lights on. Because of this, many people, especially in developed countries, cannot see the Milky Way anymore.

Twilight

When the sun has just set, the sky becomes darker very quickly. This allows us to see a special glow in the sky 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 a mix of 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 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.

golden hour blue hour

Relative contributions

The table below shows how different natural lights add up to make the night sky glow at its darkest, 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.