From CBS News:
The largest full moon of 2013, a so-called "supermoon," will light up the night sky this weekend, but there's more to this lunar delight than meets the eye.
On Sunday, June 23, at 7 a.m. EDT, the moon will arrive at perigee -- the point in its orbit its orbit bringing it closest to Earth), a distance of 221,824 miles. Now the moon typically reaches perigee once each month (and on some occasions twice), with their respective distances to Earth varying by 3 percent.
But Sunday's lunar perigee will be the moon's closest to Earth of 2013. And 32 minutes later, the moon will officially turn full. The close timing of the moon's perigee and its full phase are what will bring about the biggest full moon of the year, a celestial event popularly defined by some as a "supermoon."
So, be sure to check out the moon Sunday night, particularly at Moonrise or Moonset, when it will look the biggest.
It's also a great time to set your Orrery. Be sure to order one from your favorite dealer today to record this annual event.