What’s a supermoon? It’s a new or full moon closely coinciding with perigee – the moon’s closest point to Earth in its monthly orbit. An astrologer, Richard Nolle, coined the term supermoon over 30 years ago, but now many in astronomy use it as well. Are supermoons hype? In our opinion … gosh, no, just modern folklore. They’ve entered the popular culture (check out Sophie Hunger’s music video in this post, for example). And they can cause real physical effects, such as larger-than-usual tides. According to the definition of supermoon coined by Nolle, the year 2015 has a total of six supermoons. The new supermoons of January, February and March have passed. The full supermoon of August has also passed, and now we’re coming up on the year’s closest supermoon, on the night of September 27-28, which will also be called a Blood Moon and which will also undergo a total lunar eclipse. Finally, there will be a third full supermoon in October. Follow the links below to learn about the supermoons of 2015.
What is a supermoon? We confess: before a few years ago, we in astronomy had never heard that term. To the best of our knowledge, astrologer Richard Nolle coined the term supermoon over 30 years ago. The term has only recently come 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, which is why there are so many supermoons. By this definition, according to Nolle:
There are 4-6 supermoons a year on average.
Some astronomers have complained about the name … but we like it! And it’s entered the popular culture. for example, Supermoon is the title track of Sophie Hunger’s new album, due for release in May, 2015. It’s a nice song! Check it out in the video below.
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, once each 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 it 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.
When are the supermoons of 2015? By Nolle’s definition, the new moon or full moon has to come within 361,836 kilometers (224,834 miles) of our planet, as measured from the centers of the moon and Earth, in order to be considered a supermoon.
By that definition, the year 2015 has a total of six supermoons. The first supermoon, for 2015, came with the January 20 new moon. The new moons on February 18 and March 20 were also considered supermoons, according to Nolle’s definition, and that same definition dictates that the full moons of August, September and October will be supermoons, too. Thus, the full moon supermoons – aka near-perigee full moons – in 2015:
Full moon of August 29 at 18:35 UTC
Full moon of September 28 at 2:50 UTC
Full moon of October 27 at 12:05 UTC
The full moon on September 28, 2015, will present the closest supermoon of the year (356,896 kilometers or 221,754 miles). What’s more, this September 28, 2015 full moon will stage a total lunar eclipse, concluding a series of Blood Moon eclipses that started with the total lunar eclipse of April 15, 2014.
However, the new moon supermoon on February 18 only lies about 200 kilometers farther away than the September 28 full moon supermoon. At a distance of 357,098 kilometers or 221,890 miles, the new moon of February 18 features the second-closest supermoon of 2015.
Want more detail? Okay. In 2015, the moon comes closest to Earth on September 28 (356,877 kilometers), and swings farthest away some two weeks before, on September 14 (406,464 kilometers). That’s a difference of 49,587 kilometers (406,464 – 356,877 = 49,587). Ninety percent of this 49,587-figure equals 44,628.3 kilometers (0.9 x 49,587 = 44,628.3). Presumably, any new or full moon coming closer than 361,863.1 kilometers (406,464 – 44,628.3 = 361,835.7) would be “at or near (within 90% of) its closest approach to Earth.”
Spring tides will accompany the supermoons. Will the tides be larger than usual at the January, February and March 2015 new moons and the August, September and October 2015 full moons? Yes, all full moons (and new moons) combine with the sun to create larger-than-usual tides, but closer-than-average full moons (or closer-than-average new moons) elevate the tides even more.
Each month, on the day of the new moon, the Earth, moon and sun are aligned, with the moon in between. This line-up creates wide-ranging tides, known as spring tides. High spring tides climb up especially high, and on the same day low tides plunge especially low.
The closest new moon of the year on February 18 and the year’s closest full moon on September 28 are bound to accentuate the spring tide all the more, giving rise to what’s called a perigean spring tide. If you live along an ocean coastline, watch for high tides caused by the September 27-28 perigean full moon.
Will these high tides cause flooding? Probably not, unless a strong weather system accompanies the perigean spring tide. Still, keep an eye on the weather, because storms do have a large potential to accentuate perigean spring tides.
Dates of closest full supermoons in past and future years. 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. So we can figure the dates of the closest full moons in recent and future years as:
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.
Looking further into the future, the perigee full moon will come closer than 356,500 kilometers for the first time in the 21st century (2001-2100) on November 25, 2034 (356,446 km). The closest full moon of the 21st century will fall on December 6, 2052 (356,425 km).
By the way, some astronomers will call all the full moons listed above proxigee full moons.
But, like many of you, we’ll have fun just calling ’em supermoons.
What is a Black Moon? We had never heard the term Black Moon until early 2014. It doesn’t come from astronomy, or skylore, either. Instead, according to David Harper, the term comes from Wiccan culture. It’s the name for the second of two new moons in one calendar month. January 2014, for example, had two new moon supermoons, the second of which was not only a supermoon, but a Black Moon. Does a Black Moon have to be a supermoon in order to be called Black? No. You can read more about Black Moons here.
The next Black moon by the above definition will occur on October 30, 2016. Sten Odenwald at astronomycafe.net lists some other names for the second new moon in a month: Spinner Moon, Finder’s Moon, Secret Moon.
However, we’ve also come across another definition for Black Moon: the third of four new moons in one season. This last happened with the new moon supermoon of February 18, 2015, because this particular new moon was the third of four new moons to take place between the December 2014 solstice and the March 2015 equinox. The next Black Moon by this definition will occur on August 21, 2017, to feature a Black Moon total solar eclipse in the United States.
Bottom line: The term supermoon doesn’t come from astronomy. It 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 2015, which moon is the most “super” of all the 2015 supermoons, and gives a list of upcoming full supermoons for the years ahead.