It was night, but people could see, almost as if it was day. There were no streetlamps, no flood lights, 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 this week 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.