Bright nights: scientists explain rare phenomenon of 'nocturnal sun'

Researchers in Canada say 'zonal waves' in upper atmosphere may explain why people have reported oddly well-lit evenings since Roman times

The Romans referred to it as the "nocturnal sun". Later accounts describe it as an unexplained glow - bright enough to read a book by - that would sometimes light up the night sky.

Now researchers from York University in Canada have come up with a possible explanation for the rare phenomenon known as "bright nights". Using satellite data, two atmospheric scientists from the Toronto institution suggest that the bright nights are not due to the sun or meteors, but instead the result of converging "zonal waves" in Earth's upper atmosphere.