The sky everyone could see

Here is a thing that was true for every human being who ever lived, until about three generations ago, and is no longer true for most of us. On a clear night, away from the moon, you looked up and saw the galaxy. Not a scatter of stars. The whole pale river of the Milky Way arching from horizon to horizon, vivid enough that cultures the world over read pictures and roads and rivers into it. It was the most widely shared sight in human history. Then, in a single century, we turned on the lights and lost it.

A sky we can measure ourselves out of

This is not nostalgia. It is measurable, and it has been measured. In 2016 a team led by Fabio Falchi published a world atlas of artificial night-sky brightness, and the headline finding was stark: about a third of humanity, and roughly eighty percent of people in North America, can no longer see the Milky Way from where they live. The light from our own cities, scattered back down by the atmosphere, has drawn a permanent grey veil over the dark. For most city-born people now, a truly dark sky is not a memory being lost. It is an experience they have simply never had.

The scale of the loss is easy to underrate because it crept up gas lamp by gas lamp, then street light by street light, each one reasonable on its own. No one decided to switch off the galaxy. We did it by accident, one useful light at a time.

What the bright sky was for

For our ancestors the night sky was not decoration. It was instrument, calendar, and map, the only one available, and they read it with a precision we have mostly forgotten because we no longer need to.

The rising of particular stars told farmers when to plant and herders when to move. The ancient Egyptians timed the flooding of the Nile, and so their new year, to the heliacal rising of Sirius, the morning it first reappeared in the dawn after months below the horizon. Across the world, from Greece to the Andes to Aboriginal Australia, the first appearance of the Pleiades cluster set the agricultural year. Sailors steered by the stars long before they had anything else to steer by, and the Polynesian navigators who found and settled the scattered islands of the Pacific did it by holding a memorised map of rising and setting stars in their heads across thousands of kilometres of open ocean.

To do any of that you need the sky we have erased. A faint smear of the brightest dozen stars through urban haze will not navigate a canoe to a speck of land a month away. The sky was a working tool, and we decommissioned it without noticing it had ever been one.

The galaxy as a thing everyone owned

Because the bright sky was universal, it became one of the deep common features of human culture, the way fire and water and death are common. Nearly every people that left a record left a reading of the Milky Way. The Greeks saw spilled milk, which is where the word comes from. Many cultures across the Americas and northern Asia saw a road or river along which the dead travelled. In East Asia it was a celestial river separating two lovers among the stars, met one night a year. These are not random coincidences. They are independent answers to the same overwhelming nightly fact, a fact that pressed on everyone equally because there was no escaping it and nothing competing with it after dark.

That is the part hardest to feel from inside a lit city. The night sky was not a special occasion. It was the ceiling of the world, present every clear night of every life, more reliably than almost anything else a person would ever see.

How recently it went

The break is shockingly recent. Widespread gas street lighting arrives in the early nineteenth century; electric light spreads through the cities of the industrial world from the 1880s onward, and the great brightening of the suburbs and highways is a phenomenon of the last seventy or eighty years. There are people alive who remember the Milky Way over towns that have not shown it in decades.

The story most often told to capture this is the Northridge earthquake of 1994, which knocked out power across Los Angeles before dawn. In the version that has passed into legend, anxious residents called observatories and emergency lines to report a strange giant silvery cloud overhead. It was the Milky Way, suddenly visible over a darkened city, unrecognised by people who had never seen it. Whether or not every detail of the calls is exact, the truth underneath it is not in doubt: an entire metropolis had grown up unable to recognise the most ordinary sight in the history of the species.

To imagine the past honestly you have to put that sky back. Set any scene before about 1850 outdoors at night and the galaxy is overhead, casting its faint light on whatever you are picturing, exactly as it did for the people in the scene. They lived under a ceiling we have painted over. Getting the past right means remembering to look up and see what they could not help seeing.