Stories of a nocturnal sun have stumped scientists for hundreds, even thousands, of years. Dating back to ancient Rome, there have been tales of day-like evenings even without moonlight and there are no plausible explanations. Now, researchers may have cracked the case.

According to a report from the American Geophysical Union, scientists analyzed satellite data and suggested that waves in the upper atmosphere gathering over specific locations can boost the naturally occuring phenomenon of airglow, resulting in bright nights.

Airglow is a dull light that comes from the different chemical activities in the upper edges of the atmosphere. The green tint occurs when the molecular oxygen breaks into individual atoms. Upon their recombination, it produces extra energy that casts a greenish tint to the sky.

York University researchers Gordon Shepherd and Youngmin Cho set out to figure out what is causing airglow to increase and become visible at specific locations.

The team scanned data from the Wind Imaging Interferometer (WINDII) instrument, isolating airglow events that are bright enough to be seen by the human eye. Then, they matched these events to the activity of zonal waves. It turned out that when the peaks of certain waves aligned, bright nights events that are four to 10 times brighter than regular airglow are produced.

"This [study] is a very clear, new approach to the old enigma of what makes some night skies so remarkably bright, and the answer is atmospheric dynamics," said astronomer Jürgen Scheer from the Instituto de Astronomía y Física del Espacio in Buenos Aires, who was not connected to the study. "We now have a good idea which dynamical phenomena are behind [airglow] events of extreme brightness."

Unlike in ancient times, bright nights can be quite difficult to detect in modern times because of the presence of so much light pollution in most areas in the world. Even researchers who are dedicated to studying phenomenon likely haven't seen a true bright night with their naked eyes.

The study was published in the journal Geophysical Research Letters.