The "supermoon" set to rise tonight will be the closest full moon since 1948 and until 2034.

Supermoons occur because the Earth is not always perfectly centered within the moon's orbit. The moon orbits the Earth in such a way that sometimes the moon is very close to the Earth, and sometimes it is farther away. The point at which the moon is farthest from the Earth is the apogee, and the moment at which it is closest is the perigee.