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 2014, comes on New Year’s Day, with the January 1 new moon. By this definition, according to Nolle:
There are 4-6 supermoons a year on average.
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 new moon or 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 in periods of about one year and 48 days:
March 19, 2011
May 6, 2012
June 23, 2013
August 10, 2014
September 28, 2015
November 14, 2016
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 14, 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 wasn’t a perigee new moon in 2013, because the perigee new moon happened in December 2012 and will happen next on January 1, 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 2014? By this definition, the new moon or full moon has to come within 361,863 kilometers (224,851 miles) of our planet, as measured from the centers of the moon and Earth. Therefore, the year 2014 gives us a total of five supermoons: two January new moons, and the full moons of July, August and September.
The full moon on August 10, 2014, will showcase the closest supermoon of the year (356,896 kilometers or 221,765 miles). However, the new moons on January 1 and January 30 will not be all that far behind, featuring the year’s second-closest and third-closest supermoons, respectively. On January 1, the moon turns new less than 10 hours before reaching lunar perigee – the moon’s closest point to Earth in its orbit. On January 30, the moon turns new nearly 12 hours after reaching lunar perigee.
In 2014, the moon comes closest to Earth on August 10 (356,896 kilometers), and swings farthest away some two weeks before, on July 28 (406,567 kilometers). That’s a difference of 49,671 kilometers (406,567 – 356,896 = 49,671). Ninety percent of this 49,671-figure equals 44,703.9 kilometers (0.9 x 49,671 = 44,703.9). Presumably, any new or full moon coming closer than 361,863.1 kilometers (406,567 – 44,703.9 = 361,863.1) would be “at or near (within 90% of) its closest approach to Earth.”
If I figured everything correctly, that gives us a total of 5 supermoons in 2014: two new moon supermoons (January 1 and 30) and three full moon supermoons (July 12, August 10 and September 9).
However, the perigee full moon on August 10 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 2014, and which moon is the most “super” of all the 2014 supermoons.