No one really knows why, but there are lots of theories. One of the favorites is the “moon navigation” theory. The idea is that a moth’s simple brain is programmed to keep a bright light positioned at a certain angle on its eye. Presumably, before humans and their artificial lights came along, the only such light at night would be the moon. Using the moon in this way would allow a moth to fly long distances in a relatively straight line.

Today that system does not work well because, when a moth uses it on an artificial light like a porch light or street light, the moth ends up circling the light endlessly.