The moon becomes officially full tomorrow at 10:53 AM. Why a "supermoon?"

According to NASA the moon doesn’t orbit in a perfect circle. It travels in an ellipse that brings the moon closer to and farther from earth in its orbit. The farthest point in this ellipse is called the apogee and is about 405,500 kilometers from earth. Its closest point is the perigee which is an average distance of about 363,300 kilometers from Earth. During every 27-day orbit around earth, the moon reaches both its apogee and perigee. Full moons can occur at any point along the moon’s elliptical path, but when a full moon occurs at or near the perigee, it looks slightly larger and brighter than a typical full moon. That’s what the term “supermoon" refers to.

Source: nasa.gov