To the best of my knowledge, the term supermoon was coined by the astrologer Richard Nolle over thirty years ago. He defines it as "a new or full Moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit."
Photo credit: NASA/Bill Ingalls
Every month, as the Moon orbits Earth, it comes closest to Earth at perigee and farthest from Earth at apogee. I first became familiar with the supermoon label in the year 2011 when the media used the term to describe the Full Moon of March 19, 2011. On that date, the Full Moon aligned with proxigee - the closest perigee of the year - to stage the closest, largest Full Moon in all of 2011.
More often than not, the one day of the year that the Full Moon and perigee align also brings about the year's closest perigee (also called proxigee). Because the Moon has recurring cycles, we can count on the Full Moon and perigee to recur in periods of about one year, one month and 18 days.
Therefore, the Full Moon and perigee realign on March 19, 2011; May 6, 2012; June 23, 2013; August 10, 2014; September 28, 2015; November 14, 2016 and January 2, 2018. There won't be a perigee Full Moon in 2017 because the Full Moon and perigee won't realign again (after November 16, 2016) until January 2, 2018. By the way, all the Full Moons listed above are proxigee Full Moons.
Lunar Perigee and Apogee Calculator
To be called a supermoon, the full or new Moon doesn't actually have to align with perigee, in which case we would call it a perigee Full Moon or perigee New Moon. (There won't be a perigee New Moon in 2013, because the perigee New Moon happens in December 2012 and then in January 2014.)
By definition, a supermoon only has to be a new or full Moon "at or near (within 90% of) its closest approach to Earth." That means we have a number of supermoons in the span of one year. So how many supermoons are in store for 2013?
Most "super" supermoon of 2013 on June 23
In 2013, the moon comes closest to Earth on June 23 (356,991 kilometers) and swings farthest away on July 7 (406,490 kilometers). That’s a difference of 49,499 kilometers (406,490 – 356,991 = 49,499). Ninety percent of this 49,499-figure equals 44,549.1 kilometers (0.9 x 49,499 = 44,549.1). Presumably, any new or full moon coming closer than 361,940.9 kilometers (406,490 – 44,549.1 = 361,940.9) would be “within 90% of its closest approach to Earth.” In other words, that’s my understanding of the supermoon definition.
Perigee/New or Full Moon less than 5 hours apart: 1999-2020
If I figured everything correctly, that gives us a total of 5 supermoons in 2013: three Full Moons (May 25, June 23 and July 22) and two New Moons (January 11 and December 3). However, the perigee Full Moon on June 23 will feature the most "super" supermoon of them all!
Moon at Perigee and Apogee: 2001-2100