Scientists attribute the bright nights of old to UV light’s effect

It was night, but people could see, almost as if it was day. There were no streetlamps, no floodlights, no candles, sun or moon. But they could read documents, make out pebbles on the ground and spot details of landscapes hundreds of yards away. Distant mountains were illuminated. Some called it the nocturnal sun.

Reports of observations like these, dating to ancient Rome, have long perplexed scientists and onlookers. Scientists in Canada may have an answer.

In a study published in Geophysical Research Letters, Gordon Shepherd and his colleague Young-Min Cho, atmospheric scientists at York University, explain how waves in Earth’s atmosphere may have made these ancient bright nights possible.

A bright night starts with a dull light called airglow, which is found more than 60 miles above the earth’s surface. Normally, Earth’s atmosphere is made up mainly of nitrogen and oxygen, in their molecular forms. That just means that rather than a single atom of oxygen, for example, two of them are stuck together. But up in those heights, ultraviolet light from the sun separates the atoms in these molecules.

At night, when the sun is gone, they come back together, releasing energy as they reunite. This energy is visible as light, and the presence of oxygen, which researchers focused on in the study, can make it appear green.

Scientific instruments are sensitive enough to detect it, but not human eyes — until a bright night, when an unexpected alignment of waves in our upper atmosphere amplifies that once invisible airglow, making it much brighter.

These waves, called zonal waves, are influenced by severe weather on Earth’s surface and travel around the upper atmosphere. Shepherd and Cho zeroed in on four kinds of zonal waves using images taken from a satellite deployed in the 1990s to measure airglow and other features of the atmosphere.

Usually, the waves peak in different places along their journeys around Earth. But “every once in awhile, the waves end up in the same spot,” Shepherd said. “Just imagine waves in the ocean piling up together. That makes a bigger wave.”

And when they superimpose like that, the intensity of the airglow increases so much that it’s possible for the naked eye to see it, and may explain those nocturnal suns of the past.

Once superimposed, the waves will stay that way for a while because they move so slowly, said Shepherd, so bright nights will last two to four nights. And, according to his analysis of the satellite images, one bright night can shine over areas as big as Europe.

In historical reports, people did not really mention what was going on in the sky, Shepherd said. “They were just aware that suddenly they could see things in their environment.”

With so much light polluting our nights now, it is nearly impossible to make out a bright night when it occurs in most places, let alone find a photograph of one. When many of the observations took place, cameras weren’t yet invented. And a photograph from Earth of the bright sky at night wouldn’t be that impressive, Shepherd said. It’s something to experience firsthand.

And to do that requires patience, luck and a very special place. The weather patterns that produce the waves leading to bright nights tend to favor middle latitudes. “If you pick one location, like New York City, it would only happen about once a year,” Shepherd said. And even then, you’d need a clear night and no light pollution.

“We have animal species that are disappearing. We have glaciers that are disappearing. And bright nights too are disappearing, because there are so many city lights everywhere,” Shepherd said. “There are going to be fewer and fewer places where people can see them, and if they did, they’d have to wait a long time.”