IN THE millennia before street lights and smartphones, humans could, on rare occasions, walk around on a moonless night and see clearly. Looking up, they could see broad luminous patches of light stretching across the sky, which brightened the heavens in all directions as though it were daylight. People could read without candlelight, view small details in their surroundings, and make out landscapes in the distance. It was as if the world were illuminated by a hidden night-time sun.
The existence of bright nights is well accepted, but their cause remains a mystery. Frustratingly, sightings have almost entirely faded away in the past few decades, making it seem that any hope of solving the riddle was dimming. Now, though, one man says he has seen the solution.
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The earliest account of a bright night comes from Pliny the Elder, a Roman army commander who studied nature in his spare time. In his encyclopaedic Natural History of around AD 77, he wrote that the “phenomenon commonly called a nocturnal sun… a light emanating from the sky at night” has been seen many times. In 1988, a French atmospheric scientist named Michel Hersé produced the definitive collection of accounts of bright nights, which documented similar stories from the past millennium and all over the world. In French, they were nuit claires, and in German helle Nächte.
But sightings have become rarer. The most recent may be from 22 and 23 August 2001 at the Leoncito Astronomical Complex high in the Argentinian Andes. During that event, in Massachusetts and his colleagues reported a night sky that was .
There is an obvious reason why the frequency of reported bright nights might have fallen: it has to be dark for us to notice them, and these days, 99 per cent of people in Europe and North America sleep under an artificially lit sky.
Hersé’s book suggests that about one bright night used to be observed every year, but aside from that reveals no obvious temporal or geographical pattern. “I think you would have to have been in the right place at the right time, and in the right situation, to see one,” says , an astronomer at the , which works to combat light pollution.
Luminous smog
However, Barentine points to an interesting clue buried in 19th-century accounts. These frequently include a description of a “luminous smog” in the air. Astronomers and maritime observers said the effect was distinct from auroras or the faint nocturnal glow known as the zodiacal light, a pyramid shaped brightness produced when space dust reflects sunshine coming from below the horizon. This suggests there might be some sort of reflective haze hanging in the upper atmosphere.
Perhaps that could have been dust from volcanoes or meteors, says Barentine. Take the account of a diarist we know only as M. Toucher, writing near Paris on 30 June 1908. It is possibly no coincidence that this was the day of the Tunguska event, when a huge space rock exploded in the upper atmosphere over Siberia. People around the world reported a haze in the atmosphere for months afterwards, and light reflecting from the haze might explain why Toucher could write: “At 22.30… Very clear sky, full of stars which shine to the horizon. No moonlight. All the details of the landscape are visible.”
Despite Toucher’s observation during a night when there was no moonlight, some have wondered whether bright nights could simply be cloudless nights with a full moon and bright stars. However, in 1909 L. Yntema, a doctoral student at the University of Groningen in the Netherlands settled that question. After , he found a discrepancy in the light on bright nights. That seemed to point to some sort of atmospheric phenomenon as their cause. Yntema called it “Earthlight”.
Rolling on the waves
So, are we sure that this isn’t just a rare, mid-latitude aurora? That possibility was ruled out by Robert Strutt, son and heir to Lord Rayleigh, a physicist who, among other things, had discovered that the way gas molecules scatter light explains why the sky looks blue. The younger Rayleigh witnessed a bright night on 8 November 1929, and demonstrated that the light came from all directions. In an aurora, it typically comes in streaks.
Today bright nights may have all but vanished, but we do have certain advantages over Rayleigh – satellites, for example. In the late 1980s, , Canada, built a satellite instrument called , which could monitor waves of air as they rolled through the atmosphere. He soon found that these waves could pile up on top of one another to produce towers of pressurised air.
Along with the waves, Shepherd also studies how the chemical make-up of the atmosphere changes through the day. During daylight hours, ultraviolet radiation from the sun splits molecular oxygen into individual atoms. When the sun goes down, the atoms rejoin. This produces a small amount of light, called airglow.
Airglow is usually barely visible with the naked eye from Earth’s surface, but looking at WINDII readings, which spanned from 1991 to 2004, Shepherd noticed the airglow emissions varied wildly from night to night, and from place to place.
Earlier this year, it occurred to him that air waves and airglow could be connected. The waves might force the oxygen into a higher concentration, he thought, creating a more intense glow that could explain bright nights. “I don’t know why it came to me, but I said, ‘Ah, that’s the explanation’,” he says.
“Unlike an aurora coming in streaks, the light on a bright night comes from all directions”
To verify his suspicion, Shepherd first had to account for the sun’s activity, which can affect the brightness of airglow too. He and his colleague Young-Min Cho went back through WINDII data for 1992 and 1996, when the sun’s activity was at different levels. Cho wrote an algorithm that could search the data, discard any nights when there was an aurora and find times when waves might have piled up enough to produce a bright night. For both years, that analysis showed that the waves about 7 per cent of the time at any given spot on Earth.
That convinced them that the action of the waves was a greater influence on the airglow than increased solar activity. But it also indicated that you would get about 25 bright nights a year, which doesn’t tally with Hersé’s collected accounts. However, further analysis showed that stacked waves and a cloud-free night are not very likely to coincide at any given spot, reducing the expected frequency. “I think that’s pretty consistent with the historical record,” says Shepherd.
Shepherd, now 86 and retired, hasn’t been able to link any of the suitably stacked waves with eyewitness reports of bright nights, partly because modern reports are so rare. To do so would be a neat confirmation, though, so he is looking into crowdsourcing a bright night. “You could get together 500 to 1000 people via the web and cover different longitudes and potentially all nights of the year,” he says. Enough to brighten someone’s night, anyway.
This article appeared in print under the headline “Bright sky at night”
