Only 100 years ago, the Milky Way was visible from the center of Paris. Here’s how we can get the night sky back

For over 100,000 years, humans have walked the Earth, and we have looked up at night and seen the stars and our celestial home, the Milky Way. Cultures around the world have stories and records that include this majestic, sublime sight.

But nearly three billion people can no longer see the Milky Way when they look up at the sky at night, and with it their connection to the cosmos – and to the sense of deep time it represents – has been lost.

The cause of this loss is light pollution, but it is a relatively new problem. Only about a century ago, the skies above some of the world’s largest cities were dark enough to see the gas clouds of the Milky Way and the countless flickering spots of light in the farthest reaches of the universe.

So what happened? And what can we do to make the darkness take over again?

The long legacy of lights

Light pollution is the scattering or shining of light upwards into the sky.

Lights help us see on the ground. But for a variety of reasons—from poor design to inefficient lighting to unnecessary lighting—light pollution can quickly increase in an area.

Light pollution also has various sources.

Much of it comes from street lights. They contribute 20 to 50% of light pollution in a city. But they are not the only source. Other sources include floodlights from ovals, billboards and lights on our homes – both inside and outside.

When we see a large building or an empty house at night with all the lights on inside but no blinds or coverings, we are dealing with light pollution.

A new problem

For thousands of years, people have been observing the Milky Way in detail – even capturing dark areas where dust obscures the starlight from behind.

For example, Aboriginal and Torres Strait Islander people in Australia have extensive records of the Great Celestial Emu, which is formed by the dark bands of dust in the Milky Way just near the Southern Cross constellation, an area called the Coalsack Nebula.

About a century ago, one could enjoy sublime sights like this in the night sky when standing in the middle of the so-called “City of Lights” – Paris.

In the late 1880s and 1910s, the French astronomer Guillaume Bigourdan observed numerous galaxies from the Paris Observatory.

In 1917, he claimed that the Milky Way could be seen from the Paris Observatory in summer when the sun was far enough below the horizon, around the time of nautical twilight – the time at night when you can no longer see the horizon at sea.

But it was around this time that light pollution began to become a problem in modern cities.

The Melbourne Observatory was founded in 1863 but ceased its astronomical activities in the early 20th century, partly because Melbourne’s light pollution prevented astronomers from accurately observing the night sky.

In 1924, the Mount Stromlo Observatory outside Canberra took over observations of the Milky Way. It was chosen because of its remote location and dark skies.

But even though the Australian capital was less than 10% of its current size in the 1950s and had less than 10% of the light pollution it has today, a new dark location had to be found as the Milky Way slowly disappeared from view. The site the scientists chose was eight hours away at the Siding Spring Observatory.

But even Siding Spring can now see the glow of Sydney – from 450 kilometers away.

What can be done?

If we made our modern lives more intelligent, the Milky Way could once again be visible from anywhere, even from the heart of Sydney, Paris or Los Angeles – just like it was 100 years ago.

An important aspect is the shielding of the lights. Instead of an open light, flat lights or shielded lights that prevent light from being scattered upwards are essential. They direct the light onto the ground and not up into the sky.

In Canberra, the Australian Capital Territory government and lighting operator Omexom have replaced streetlights to achieve precisely this goal: no upward stray light and controllable light.

In this way, Canberra has reduced its light pollution by about 30% in just a few years, as my colleagues and I report in a forthcoming article.

It is also important to turn off – or dim – unnecessary lights.

Canberra is doing this too. The city dims the street lights to 50% of their total brightness in the middle of the night. This way, the city saves energy – and reduces light pollution. For every 10% we dim the street lights, we reduce light pollution by 5%, as our upcoming article shows.

The color of the light is another part of the solution. Instead of using bright white LEDs and cold-colored light, we can use warm light, which is better for our eyes, our sleep patterns and local animals – and helps reduce light pollution.

With these simple measures, we can return to the not-so-distant times when we could see the Milky Way from anywhere on Earth.

We can reclaim the night sky.