Forget the 'Supermoon': What's Actually Happening at Lunar Perigee

A full moon is when the sun, Earth and moon line up, with the Earth in the middle. As seen from the surface of the Earth, the moon is fully illuminated. Because the Earth is constantly revolving around the sun and the moon is constantly revolving around the Earth, a full moon is an instantaneous event, occurring when the moon is exactly opposite the sun.