What is a supermoon?

The next full moon on May 24-25 is a supermoon, but the perigee full moon on June 23 will be the most “super” supermoon of five in 2013.


What’s a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. To the best of our knowledge, the term supermoon was coined by the astrologer Richard Nolle over 30 years ago. The term is only now coming into popular usage. Nolle has defined a supermoon 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. That’s a pretty generous definition and allows for many supermoons. The first “super” full moon, for 2013, is coming up on the night of May 24-25. By this definition, according to Nolle: There are 4-6 supermoons a year on average. And indeed the Farmers Almanac for 2013 has now adopted the term supermoon, saying there will be three supermoons in a row coming up over these next three months in 2013: May 24-25, June 23, July 22.

Photographs or other instruments can tell the difference between a supermoon and ordinary full moon. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image by Marco Langbroek of the Netherlands via Wikimedia Commons.


What did astronomers call these moons before we called them supermoons? We called them a full moon, or a new moon, at perigee. The moon is full, or opposite Earth from the sun, once each month. It’s new, or more or less between the Earth and sun, every month. And, every month, as the moon orbits Earth, it comes closest to Earth. That point is called perigee. The moon always swings farthest away once each month; that point is called apogee. No doubt about it. Supermoon is a catchier term than perigee full moon. We first became familiar with the supermoon label in the year 2011 when the media used supermoon 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 of 2011.



 Perigee full moon is a special kind of supermoon 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 come in concert 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.

 So what exactly is a supermoon? 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. How many supermoons in 2013? In 2013, according to the Farmer’s Almanac, there are only three supermoons, but we figure there are five and that the most super of the 2013 supermoons is the one 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 what supermoon means. 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 give us the most “super” supermoon of them all! Bottom line: The term supermoon doesn’t come from astronomy. It’s comes from astrology, and the definition is pretty generous so that there are 4 to 6 supermoons each year. This post explains what a supermoon is, how many will occur in 2013, and which moon is the most “super” of all the 2013 supermoons.

source and credit a earthsky