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| Human World | Space on Sep 08, 2014

What is a supermoon?

The definition for “supermoon” is so generous that there are typically 4-6 of them each year.

What is a supermoon? We in astronomy used to call them perigean new moons or perigean full moons, that is, new or full moons closely coinciding with perigee – the moon’s closest point to Earth in its orbit. But, like almost everyone else, now we enjoy calling them supermoons. The name supermoon was coined by an astrologer, Richard Nolle, over 30 years ago. It was popularized and came to be the accepted term for most people only in the past few years. Are supermoons hype? In our opinion … gosh, no, just modern folklore. And they can cause real physical effects, such as larger-than-usual tides. The year 2014 has a total of five supermoons. They are the two new moons of January, and the full moons of July, August and September. Follow the links below to learn about the supermoons of 2014.

What is a supermoon?

What did astronomers call these moons before we called them supermoons?

When are the next supermoons in 2014?

Spring tides will accompany the July, August and September supermoons.

Dates of closest full supermoons in past and future years

What is a Black Moon?

About three or four times a year, the new or full moon coincides closely in time with the perigee of the moon—the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' The difference between ‘perigean spring tide’ and normal tidal ranges for all areas of the coast is small. In most cases, the difference is only a couple of inches above normal spring tides.  Image and caption via NOAA.

About three or four times a year, the new or full moon coincides closely in time with the perigee of the moon—the point when the moon is closest to the Earth. These occurrences are often called ‘perigean spring tides.’ The difference between ‘perigean spring tide’ and normal tidal ranges for all areas of the coast is small. In most cases, the difference is only a couple of inches above normal spring tides. Image and caption via NOAA.

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.

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 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 and allows for many supermoons. 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 perigee full moon, or a perigee new moon. Perigee just means “near Earth.”

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 next supermoons in 2014? By Nolle’s 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, in order to be considered a supermoon.

By that definition, the year 2014 has a total of five supermoons. The first supermoon, for 2014, came on New Year’s Day, with the January 1 new moon. The second came with the new moon of January 30, 2014. The full moons of July, August and September will also be supermoons, by Nolle’s definition. Thus, the next supermoons – aka perigee full moons – in 2014 are:

Full moon of July 12 at 11:25 UTC

Full moon of August 10 at 18:09 UTC

Full moon of September 9 at 1:38 UTC

Is the October 8, 2014 full moon a supermoon?

The full moon on August 10, 2014, will present the closest supermoon of the year (356,896 kilometers or 221,765 miles).

However, the new moons on January 1 and January 30 were not far behind, featuring the year’s second-closest and third-closest supermoons, respectively. On January 1, the moon turned new less than 10 hours before reaching lunar perigee – the moon’s closest point to Earth in its orbit. On January 30, the moon turned new nearly 12 hours after reaching lunar perigee.

Want more detail? Okay. 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.”

Around each new moon (left) and full moon (right) – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are called spring tides. A supermoon – new or full moon at its closest to Earth – accentuates these tides. Image via physicalgeography.net

Spring tides will accompany the July, August and September supermoons. Will the tides be larger than usual at the July, August and September 2014 full moons? Yes, all full moons (and new moons) combine with the sun to create larger-than-usual tides, but perigee full moons (or perigee 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 July, August and September extra-close full moons will accentuate the spring tide, giving rise to what’s called a perigean spring tide. If you live along an ocean coastline, watch for high tides caused by these full moons.

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.

By the way, some astronomers will call all the full moons listed above proxigee full moons.

But, like you, we’ll have fun just calling ‘em supermoons.

What most call a Blue Moon isn't blue in color. It's only Blue in name. This great moon photo from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho.

What does a full supermoon look like? Many astronomers say you can’t discern any difference with your eye between a supermoon and any ordinary full moon. At least one observer says you can discern a different, with the eye alone. This great moon photo is from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho.

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 a month. January 2014, for example, had two new moons. 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 second-of-two-new-moons-in-one-calendar-month definition will occur on October 30, 2016. Sten Odenwald at astronomycafe.net lists some other names for the second new moon in a calendar 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 to occur in one season. In 2015, the new moon coming on February 18 will present the third of four new moons in between the December 2014 solstice and the March 2015 equinox. This February 18, 2015 Black Moon will also be the closest new moon supermoon of 2015, but, once again, a seasonal Black Moon doesn’t necessarily have to be a supermoon.

Bottom line: The term supermoon doesn’t come from astronomy. It comes from astrology, and the definition is so generous that there are 4 to 6 supermoons each year. This post explains what a supermoon is, how many occur in 2014, which moon is the most “super” of all the 2014 supermoons (August 10), and gives a list of upcoming full supermoons for the years ahead.

See a supermoon! Awesome photos of the June 2013 full supermoon

Learn more: Tides and the pull of the moon and sun

Discern a supermoon’s large size with the eye? An observer says yes