This is an astronomy story that comes up only once in a blue moon, like on Friday. Maybe.
When the full moon appears at 6:43 a.m. EDT Friday, it will become the second full moon of July. Geoff Chester of the U.S. Naval Observatory said the traditional definition of a blue moon is two full moons in a month.
It happens about every 2½ years and has nothing to do with color.
But there's a catch that may disqualify this as a blue moon.
Chester said an older definition from a Maine almanac calls a blue moon the fourth full moon in an astronomical season. This is only the second full moon of summer, so by that definition no blue moon.
The next blue moon will be in January 2018.
Explore further: Blues for the second full moon of July