The moon follows an oval or elliptical orbit, and a supermoon occurs when the moon is in the part of its orbit closest to Earth. At this point, known as "perigee," the moon is about 30,000 miles (50,000 kilometers) closer to the planet than at its farthest point, or "apogee." [In Photos: Glitzy Images of a Supermoon]

Tomorrow and Sept. 9, the moon will be full on the same day as perigee, and on Aug. 10, it will be full during the same hour as perigee, making it especially bright.