from space.com: There is more to a "supermoon" than meets the eye.
Science governs the appearance of the largest full moon of the year, and this weekend you can check out the amazing lunar sight for yourself.
On Sunday (June 23), the moon will be at its closest point to Earth —
called perigee. This relatively close brush will happen as the moon
enters its fullest phase, creating the cosmic coincidence known as the
supermoon. At its fullest and closest, the moon will appear about 12
percent larger in the sky.
"It doesn't matter where you are, the full moon you're seeing will be
the biggest for 2013," Michelle Thaller, the assistant director of
science at NASA's Goddard Space Flight Center said. "… That 12 percent
size different can mean as much as a 30 percent change in the
brightness, so this will be a particularly bright supermoon."