The third supermoon in 2014 will take place on Sept. 9. Earth's natural satellite will appear bigger and brighter than other regular full moons. Although not uncommon, a supermoon – scientifically called a perigee moon – can be quite a spectacle, especially when it's accompanied by what is called the "moon illusion."
"Generally speaking, full moons occur near perigee every 13 months and 18 days, so it's not all that unusual," Geoff Chester of the U.S. Naval Observatory told NASA Science News. "In fact, just last year there were three perigee moons in a row, but only one was widely reported."
Since the moon follows an elliptical orbit around Earth, it is natural that there are times when it passes closer to the planet. It's nearer to Earth by about 50,000 kilometers when it's on the perigee side of its orbit than when it's on the apogee side. Supermoons occur when a full moon nearly coincides with its perigee.
The moon at its perigee is more or less 30 percent brighter and 14 percent bigger than when it's on the apogee side of its orbit. Furthermore, when the moon is close to the horizon, it generally appears unnaturally large – a phenomenon called "moon illusion" that cannot yet be fully explained. Thus, a supermoon appears like a huge orb as it rises up the eastern horizon.
Although the occurrence of perigee moons is not unusual, there are rare incidents when it's nearly perfect. According to Chester in an earlier interview, near perfect supermoons like the one that occurred on March 19, 2011 (less than one hour from perigee) happen only once every 18 years or so.
