The full moon yesterday, June 23, 2013 at 11:32 UTC, is called a supermoon because it was not only the closest and largest full moon of the year, but its closest encounter with Earth for 2013. The term supermoon is not used within the astronomical community, which use the term perigee-syzygy or perigee moon According to NASA, a full moon at perigee is up to 14% larger and 30% brighter than one at its farthest point, or apogee. The full Moon occurring less than one hour away from perigee is a near-perfect coincidence that happens only every 18 years or so. Continue reading for more.
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. At lunar perigee the tidal force is somewhat stronger, resulting in perigean spring tides. But even at its most powerful this force is still relatively weak causing tidal differences of inches at most. As the tidal force follows an inverse-cube law, that force is 18% greater than average. However, because the actual amplitude of tides varies around the world, this may not translate into a direct effect.