Quoted: While there is no strict definition of a supermoon, explained Jim Lattis, director of UW Space Place, it is a special astrophysical case that occurs within 24 hours of when a full moon is at perigee. In other words, the moon within a day of being at its fullest, and at a point in its path that is closest to Earth.
“The closest approach is going to happen roughly every 27 days. And a full moon happens every 29½ days,” said Jennifer Stafford, an astronomy graduate student at the University of Wisconsin-Madison. “So it’s not every month that we’re going to have a supermoon.”