A supermoon is a full moon or a new moon that nearly coincides with perigee—the closest that the Moon comes to the Earth in its elliptic orbit—resulting in a slightly larger-than-usual apparent size of the lunar disk as viewed from Earth. The technical name is a perigee syzygy (of the Earth–Moon–Sun system) or a full (or new) Moon around perigee.[a] The term supermoon is astrological in origin and has no precise astronomical definition.
Scientists have confirmed that the combined effect of the Sun and Moon on the Earth's oceans, the tide, is greatest when the Moon is either new or full, and that during lunar perigee, the tidal force is somewhat stronger, resulting in perigean spring tides. However, even at its most powerful, this force is still relatively weak, causing tidal differences of inches at most.