This particular Syzygy occur a little less often then once a year, the ideal alignment is like once every 20 though. The whole term 'Supermoon' was coined by some damn astrologer back in the late 70's and has a fair amount of cream-filling to it, so to speak. Since we basically get syzygy (alignment of three bodies in an approximate straight line) twice a month, new and full moon, a Supermoon happens to be when the full moon also happens to occur at perigee, that is, when the moon is closest to Earth on its own orbit. I suppose a perfect supermoon would be when a full moon occurs at the moons perigee simultaneous to the Earths own perihelion to the sun, since that would make for the largest and brightest full moon, which I'm much too lazy to calculate.