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, 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 supermoon, for 2014, came on New Year’s Day, with the January 1 new moon, and the second is on January 30. By this definition, according to Nolle:
There are 4-6 supermoons a year on average.How many 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.
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 present the closest supermoon of the year (356,896 kilometers or 221,765 miles).
However, the new moons on January 1 and January 30 are 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 will turn 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.”
Assuming I figured everything correctly, that gives us our 5 supermoons of 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!
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 Black Moon? We had never heard the term Black Moon until recently. It doesn’t come from astronomy, or skylore. 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. Does a Black Moon have to be a supermoon in order to be called Black? No. You can read more about Black Moons here.
Sten Odenwald at astronomycafe.net lists some other names for the second new moon in a month: Spinner Moon, Finder’s Moon, Secret Moon.
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 accompany January 2014′s supermoons. Will the tides be larger than usual at the January 1 and 30 new moons? Yes, all new moons (and full moons) combine with the sun to create larger-than-usual tides, but perigee new moons (or perigee full 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 January 1 and 30 extra-close new 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 the two January 2014 new moons – or supermoons.
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.
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 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.
Closest full supermoons in the year’s ahead 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, 2011There 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.
May 6, 2012
June 23, 2013
August 10, 2014
September 28, 2015
November 14, 2016
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.
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, which moon is the most “super” of all the 2014 supermoons, and gives a list of upcoming full supermoons for the years ahead. The first supermoon of 2014 is on January 1. The second is on January 30. We’re hearing people call that second new moon in a month a Black Moon, but that name doesn’t come from astronomy, either. The January 30 new supermoon can be expected to raise the height of the tides slightly.
- EarthSky
