These waves, called zonal waves, are influenced by severe weather on Earth’s surface and travel around the upper atmosphere. Dr. Shepherd and Dr. 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,” said Dr. Shepherd. “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, 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 Dr. 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, said Dr. Shepherd. “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, said Dr. Shepherd. It’s something to experience firsthand.
And to do that requires patience, luck and a very…