The night sky and light pollution

For all of human history people looked up and saw the galaxy we live in. Today one person in three on Earth can no longer see the Milky Way from home. This lesson teaches you to read the sky, and to understand what we are losing.

The sky is the oldest book there is. Light pollution is closing it.

Look

Look at the sky from a city and you might see a few dozen stars. From a truly dark place, far from any town, you can see thousands, and the Milky Way stretches across the sky like a cloud.

The difference is light pollution: artificial light that escapes upwards and makes the sky glow. That glow washes out the faint stars.

Drag the slider in the tool from 1 (a truly dark site) to 9 (inner city). The stars are real: they come from a catalogue of the 9,000 or so brightest stars in the sky. Watch how many disappear.

This is not only about stargazing. Night-time light also affects animals that navigate or hunt in the dark, and it wastes energy by lighting the sky instead of the ground. This connects to our Red Initiative and its Light Initiative for night-sky preservation.

Understand

Astronomers measure brightness with magnitudes. It is a backwards scale: bright stars have small numbers.

• Sirius, the brightest star at night, is about −1.5.
• The faintest stars most people can see from a dark site are about magnitude 6 to 7.
• In a city, the limit may be magnitude 4 or brighter.

The Bortle scale, published by the amateur astronomer John Bortle in 2001, sorts skies into nine classes. Each class comes with a typical faintest visible magnitude:

Bortle class	Description	Faintest stars
1	Excellent dark site	7.6 to 8.0
3	Rural sky	6.6 to 7.0
5	Suburban sky	5.6 to 6.0
7	Suburban/urban transition	4.6 to 5.0
9	Inner-city sky	4.0 at best

Because there are many more faint stars than bright ones, each magnitude you lose hides a lot of them. In the Yale catalogue, about 5,000 stars are brighter than magnitude 6, but only about 500 are brighter than magnitude 4.

A 2016 world atlas found that more than 80% of people, and more than 99% of people in the United States and Europe, live under light-polluted skies.

Master

Magnitude is logarithmic. For two stars with fluxes F₁ and F₂:

m₁ − m₂ = −2.5 · log₁₀(F₁ / F₂)

A difference of 5 magnitudes is exactly a factor of 100 in flux, so one magnitude is a factor of 100^(1/5) ≈ 2.512.

Why do star counts rise so steeply? If stars were spread uniformly through space, all with the same luminosity, the number brighter than magnitude m would grow as 10^(0.6 m): about four times more stars for each fainter magnitude. The real Galaxy is not uniform, so the observed growth is smaller, around three times per magnitude near the naked-eye limit. You can check it with the catalogue counts in the tool’s data table.

Sky glow adds a background of its own brightness. A star is visible only if it stands out enough against that background, so a brighter sky raises the limiting magnitude. The Bortle limits are rough observer-based ranges, not a physical law, and your own result depends on your eyes, dark adaptation and the weather.

What works against light pollution, according to DarkSky International’s lighting principles: light only what needs light, only when it needs it, no brighter than necessary, pointing downwards with full shielding, and in warm colours (less blue light, which scatters more in the atmosphere).